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ARMY TM 5-818-1AIR FORCE AFM 88-3, CHAP. 7

SOILS AND GEOLOGYPROCEDURES FOR FOUNDATION DESIGN OF

BUILDINGS AND OTHER STRUCTURES(EXCEPT HYDRAULIC STRUCTURES)

DEPARTMENTS OF THE ARMY AND THE AIR FORCEOCTOBER 1983

This manual has been prepared by or for the Government and, except to the extent indicated below,is public property and not subject to copyright.

Copyrighted material included in this manual has been used with the knowledge and permission ofthe proprietors and is acknowledged as such at point of use. Anyone wishing to make further use of anycopyrighted material, by itself and apart from this text, should seek necessary permission directly from theproprietors.

Reprint or republication of this manual should include a credit substantially as follows: "JointDepartments of the Army and Air Force, USA, Technical Manual TM 5-818-1/AFM 88-3, Chapter 7, Soilsand Geology Procedures for Foundation Design of Buildings and Other Structures (Except HydraulicStructures), 21 October 1983."

If the reprint or republication includes copyrighted material, the credit should also state: "Anyonewishing to make further use of copyrighted material, by itself and apart from this text, should seeknecessary permission directly from the proprietors."

* TM 5-818-1* AFM 88-3. Chap. 7

TECHNICAL MANUAL HEADQUARTERSNo. 5-818-1 DEPARTMENTS OF THE ARMYAIR FORCE MANUAL AND THE AIR FORCENo. 88-3, Chapter 7 WASHINGTON, DC, 21 October 1983

SOILS AND GEOLOGYPROCEDURES FOR FOUNDATION DESIGN OF

BUILDINGS AND OTHER STRUCTURES(EXCEPT HYDRAULIC STRUCTURES)

Paragraph PageCHAPTER 1. INTRODUCTION

Purpose ....................................................................................................1-1 1-1Scope .......................................................................................................1-2 1-1Objectives of foundation investigations ....................................................1-3 1-1Report of subsurface and design investigations .......................................1-4 1-1

2. IDENTIFICATION AND CLASSIFICATION OF SOILAND ROCK

Natural soil deposits .................................................................................2-1 2-1Identification of soils .................................................................................2-2 2-1Index properties ........................................................................................2-3 2-1Soil classification ......................................................................................2-4 2-5Rock classification ....................................................................................2-5 2-5Rock properties for foundation design ......................................................2-6 2-5Shales.......................................................................................................2-7 2-13

3. ENGINEERING PROPERTIES OF SOIL AND ROCKScope .......................................................................................................3-1 3-1Compaction characteristics of soils ..........................................................3-2 3-1Density of cohesionless soils ....................................................................3-3 3-1Permeability ..............................................................................................3-4 3-1Consolidation ............................................................................................3-5 3-5Swelling, shrinkage, and collapsibility ......................................................3-6 3-15Shear strength of soils ..............................................................................3-7 3-16Elastic properties (E, µ) ............................................................................3-8 3-22Modulus of subgrade reaction ..................................................................3-9 3-22Coefficient of at-rest earth pressure .........................................................3-10 3-22Properties of intact rock ............................................................................3-11 3-22Properties of typical shales.......................................................................3-12 3-22

4. FIELD EXPLORATIONSInvestigational programs ..........................................................................4-1 4-1Soil boring program ..................................................................................4-2 4-2Field measurements of relative density and consistency .........................4-3 4-5Boring logs................................................................................................4-4 4-6Groundwater observations........................................................................4-5 4-6In situ load tests........................................................................................4-6 4-9Geophysical exploration ...........................................................................4-7 4-9Borehole surveying ...................................................................................4-8 4-12

5. SETTLEMENT ANALYSESSettlement problems.................................................................................5-1 5-1Loads causing settlement .........................................................................5-2 5-1Stress computations .................................................................................5-3 5-1Settlement of foundations on clay ............................................................5-4 5-4Consolidation settlement ..........................................................................5-5 5-4Settlement of cohesionless soils ..............................................................5-6 5-10Eliminating, reducing, or coping with settlement ......................................5-7 5-10

6. BEARING-CAPACITY ANALYSISBearing capacity of soils ...........................................................................6-1 6-1Shear strength parameters .......................................................................6-2 6-1Methods of analysis ..................................................................................6-3 6-1Tension forces ..........................................................................................6-4 6-5Bearing capacity of rock ...........................................................................6-5 6-5

* This manual supersedes TM 5--818-1/AFM 88-3, Chapter 7, 15 August 1961.

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TM 5-818-1 / AFM 88-3, Chap. 7

Paragraph PageCHAPTER 7. DEWATERING AND GROUNDWATER CONTROL

General.....................................................................................................7-1 7-1Foundation problems ................................................................................7-2 7-1

8. SLOPE STABILITY ANALYSISGeneral.....................................................................................................8-1 8-1Slope stability problems............................................................................8-2 8-1Slopes in soils presenting special problems .............................................8-3 8-1Slope stability charts.................................................................................8-4 8-2Detailed analyses of slope stability ...........................................................8-5 8-3Stabilization of slopes ...............................................................................8-6 8-3

9. SELECTION OF FOUNDATION TYPEFoundation-selection considerations ........................................................9-1 9-1Adverse subsurface conditions .................................................................9-2 9-2Cost estimates and final selection ............................................................9-3 9-2

10. SPREAD FOOTINGS AND MAT FOUNDATIONSGeneral.....................................................................................................10-1 10-1Adequate foundation depth ......................................................................10-2 10-1Footing design .........................................................................................10-3 10-1Mat foundations .......................................................................................10-4 10-1Special requirements for mat foundations ................................................10-5 10-5Modulus of subgrade reaction for footings and mats ................................10-6 10-5Foundations for radar towers....................................................................10-7 10-5

11. DEEP FOUNDATIONS INCLUDING DRILLED PIERSGeneral.....................................................................................................11-1 11-1Floating foundations ................................................................................11-2 11-1Settlements of compensated foundations ................................................11-3 11-1Underpinning ............................................................................................11-4 11-3Excavation protection ...............................................................................11-5 11-5Drilled piers ..............................................................................................11-6 11-9

12. PILE FOUNDATIONSGeneral.....................................................................................................12-1 12-1Design ......................................................................................................12-2 12-1

13. FOUNDATIONS ON EXPANSIVE SOILSGeneral.....................................................................................................13-1 13-1Foundations problems ..............................................................................13-2 13-1

14. RETAINING WALLS AND EXCAVATION SUPPORTSYSTEMS

Design considerations for retaining walls .................................................14-1 14-1Earth pressures ........................................................................................14-2 14-1Equivalent fluid pressures ........................................................................14-3 14-1Design procedures for retaining walls ......................................................14-4 14-5Crib wall ....................................................................................................14-5 14-9Excavation support systems .....................................................................14-6 14-9Struttedexcavations ..................................................................................14-7 14-11Stability of bottom of excavation ...............................................................14-8 14-17Anchored walls .........................................................................................14-9 14-19

15. FOUNDATIONS ON FILL AND BACKFILLINGTypes of fill ...............................................................................................15-1 15-1Foundations on compacted fills ................................................................15-2 15-1Compaction requirements ........................................................................15-3 15-2Placing and control of backfill ...................................................................15-4 15-2Fill settlements..........................................................................................15-5 15-4Hydraulic fills ............................................................................................15-6 15-4

16. STABILIZATION OF SUBGRADE SOILSGeneral.....................................................................................................16-1 16-1Vibrocompaction .......................................................................................16-2 16-1Vibrodisplacement compaction .................................................................16-3 16-10Grouting and injection...............................................................................16-4 16-12Precompression........................................................................................16-5 16-14Reinforcement ..........................................................................................16-6 16-16Miscellaneous methods ............................................................................16-7 16-16

17. DESIGN FOR EQUIPMENT VIBRATIONS AND SEISMICLOADINGS

Introduction...............................................................................................17-1 17-1Single degree of freedom, damped, forced systems ................................17-2 17-1Foundations on elastic soils .....................................................................17-3 17-2

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TM 5-818-1 / AFM 88-3, Chap. 7

Paragraph PageWave transmission, attenuation, and isolation .........................................17-4 17-9Evaluation of S-wave velocity in soils ......................................................17-5 17-9Settlement and liquefaction .....................................................................17-6 17-14Seismic effects on foundations ................................................................17-7 17-18

CHAPTER 18. FOUNDATIONS IN AREAS OF SIGNIFICANT FROSTPENETRATION

Introduction ..............................................................................................18-1 18-1Factors affecting design of foundations ...................................................18-2 18-3Site investigations ....................................................................................18-3 18-5Foundation design ...................................................................................18-4 18-6

APPENDIX A. REFERENCESLIST OF FIGURES

PageFigure 2-1. Distribution of natural soil deposits in the United States ...................................................... 2-4

2-2. Typical grain-size distribution curves .................................................................................... 2-62-3. Weight-volume relationships ............................................................................................... 2-82-4. Modified core recovery as in index of rock quality ............................................................... 2-152-5. Classification of shales ........................................................................................................ 2-163-1. Typical CE 55 compaction test data .................................................................................... 3-23-2. Relation between relative density and dry density (scaled to plot

as a straight line) ........................................................................................................... 3-43-3. Angle of friction versus dry density for coarse-grained soils ................................................ 3-43-4. Generalized curves for estimating emax and em,, from gradational

and particle shape characteristics ................................................................................. 3-53-5. A summary of soil permeabilities and method of determination ........................................... 3-63-6. Examples of laboratory consolidation test data .................................................................... 3-73-7. Analyses of consolidation test data ...................................................................................... 3-83-8. Approximate relation between liquidity index and effective overburden

pressure, as a function of the sensitivity o f the soil ....................................................... 3-93-9. Approximate relation between void ratio and effective overburden pres-

sure for clay sediments, as a function of the Atterberg limits ........................................ 3-103-10. Approximate correlations for swelling index of silts and clays ............................................. 3-133-11. Correlations between coefficient of consolidation and liquid limit ........................................ 3-143-12. Predicted relationship between swelling potential and plasticity index

for compacted soils. ...................................................................................................... 3-153-13. Guide to collapsibility, compressibility, and expansion based on in situ

dry density and liquid limit ............................................................................................ 3-163-14. Shear test apparatus and shearing resistan ce of soils ......................................................... 3-173-15. Remolded shear strength versus liquidity index relationships for differ-

entclays ........................................................................................................................ 3-183-16. Normalized variation of su/po ratio for overconsolidated clay .............................................. 3-193-17. General relationship between sensitivity, liquidity index, and effective

overburden pressure .................................................................................................. 3-213-18. Empirical correlation between friction angle and plasticity index from

triaxial compression tests on normally consolidated undisturbed ................................. 3-22clays

3-19. Relation between residual friction angle and plasticity index ............................................... 3-233-20. Chart for estimating undrained modulus of clay ................................................................... 3-243-21. Coefficient of earth pressure at rest (Ko) as a function of overconsolidat-

ed ratio and plasticity index ........................................................................................... 3-254-1. Relative density of sand from the standard penetration test ................................................ 4-54-2. Rough correlation between effective friction angle, standard blow,

count and effective overburden pressure ...................................................................... 4-64-3. Correction factor for vane strength ....................................................................................... 4-74-4. Typical log of boring ............................................................................................................. 4-84-5. Typical details of Casagrande piezometer and piezometer using well

screen............................................................................................................................ 4-94-6. Determination of permeability from field pumping test on a fully

pene-trating well in an artesian aquifer ........................................................................ 4-105-1. Vertical stress beneath a uniformly loaded rectangular area ............................................... 5-55-2. Vertical stress beneath a triangular distribution of load on a rectangular

area ............................................................................................................................... 5-65-3. Influence value for vertical stress under a uniformly loaded circular area ............................ 5-75-4. Example of settlement analysis ............................................................................................ 5-85-5. Time factors for various boundary conditions ....................................................................... 5-96-1. Ultimate bearing capacity of shallow foundations under vertical, eccen-

tric loads ........................................................................................................................ 6-3iii

TM 5-818-1 / AFM 88-3, Chap. 7

PageFigure 6-2. Ultimate bearing capacity with groundwater effect .............................................................. 6-4

6-3. Ultimate bearing capacity of shallow foundations under eccentric inclinedloads.............................................................................................................................. 6-5

6-4. Example of bearing capacity computation for inclined eccentric load onrectangular footing ............................................................................................................... 6-6

6-5. Ultimate bearing capacity of deep foundations .................................................................... 6-76-6. Bearing capacity factors for strip and circular footings on layered founda-

tions in clay ................................................................................................................... 6-88-1. Slope stability charts for ø = 0 soils ..................................................................................... 8-48-2. Reduction factors ( µq, µw and _’w) for slope stability charts for ø = 0

and ø > 0 soils .............................................................................................................. 8-58-3. Reduction factors (tension cracks, lt) for slope stability charts for ø = 0

and ø > 0 soils ............................................................................................................... 8-68-4. Examples of use of charts for slopes in soils with uniform strength and ø

= 0 ................................................................................................................................ 8-78-5. Slope stability charts for + > 0 soils ...................................................................................... 8-88-6. Stability charts for infinite slopes .......................................................................................... 8-98-7. Slope stability charts for t = 0 and strength increasing with depth ........................................ 8-108-8. Method of moments for = 0................................................................................................... 8-118-9. Example problem for ordinary method of slices ................................................................... 8-128-10. Example of use of tabular form for computing weights of slices ........................................... 8-138-11. Example of use of tabular form for calculating factor of safety by ordi-

nary method of slices .................................................................................................. 8-148-12. Example of simplified wedge analysis .................................................................................. 8-1510-1. Example of method for selecting allowable bearing pressure .............................................. 10-210-2. Proportioning footings on cohesionless soils ....................................................................... 10-310-3. Distribution of bearing pressures .......................................................................................... 10-411-1. Effect of pore pressure dissipation during excavation and settlement re-

sponse........................................................................................................................... 11-211-2. Excavation rebound versus excavation depth ..................................................................... 11-311-3. Probable settlements adjacent to open cuts ......................................................................... 11-411-4. Methods of underpinning ...................................................................................................... 11-514-1. Load diagrams for retaining walls ......................................................................................... 14-214-2. Active pressure of sand with planar boundaries ................................................................... 14-314-3. Passive pressure of sand with planar boundaries ................................................................ 14-414-4. Active and passive earth pressure coefficients according to Coulomb

theory. ........................................................................................................................... 14-514-5. Horizontal pressures on walls due to surcharge ................................................................... 14-614-6. Design loads for low retaining walls, straight slope backfil ................................................... 14-714-7. Design loads for low retaining walls, broken slope backfill ................................................... 14-814-8. Estimates of increased pressure induced by compaction .................................................... 14-914-9. Design criteria for crib and bin walls ..................................................................................... 14-1014-10. Types of support systems for excavation ............................................................................. 14-1114-11. Pressure distribution-complete excavation ........................................................................... 14-1714-12. Stability of bottom of excavation in clay ................................................................................ 14-1814-13. Typical tieback details .......................................................................................................... 14-1914-14. Methods of calculating anchor capacities in soil ................................................................... 14-2016-1. Applicable grain-size ranges for different stabilization methods ........................................... 16-816-2. Range of particle-size distributions suitable for densification by vibro-

compaction .................................................................................................................... 16-916-3. Sand densification using vibratory rollers ............................................................................. 16-1116-4. Relative density as a function of vibroflot hole spaci ngs ...................................................... 16-1216-5. Allowable bearing pressure on cohesionless soil layers stabilized by vibro-

flotation ................................................................................................................................. 16-1316-6. Soil particle sizes suitable for different grout types and several concen-

trations and viscosities shown .............................................................................................. 16-1517-1. Response spectra for vibration limits .................................................................................... 17-217-2. Response curves for the single-degree-of-freedom system with viscous

damping......................................................................................................................... 17-317-3. Six modes of vibration for a foundation ............................................................................... 17-317-4. Equivalent damping ratio for oscillation of rigid circular footing on elas-

tic half-space ................................................................................................................. 17-517-5. Examples of computations for vertical and rocking motions ................................................. 17-617-6. Coupled rocking and sliding motion of foundation ................................................................ 17-817-7. Distribution of displacement waves from a circular footing on the elastic

half-space...................................................................................................................... 17-10

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TM 5-818-1 / AFM 88-3, Chap. 7

PageFigure 17-8. Theoretical relation between shear velocity ratio VpN,V and Poisson’s ............................... 17-11

17-9. Idealized cyclic stress-strain loop for soil ............................................................................ 17-1217-10. Variation of shear modulus with cyclic strain amplitude; G.x = G at

= 1 to 3 x 10-’ percent; scatter in data up to about +0.1 on vertical scale ..................... 17-1517-11. Variation of viscous damping with cyclic strain amplitude; data scat -

ter up to about ± 50 percent of average damping values shown for any strain............................................................................................................................. 17-16

18-1. Frost and permafrost in North America ................................................................................ 18-218-2. Ground temperatures during freezing season in Limestone, Maine ..................................... 18-418-3. Ground temperatures during freezing season in Fairbanks, Alaska .................................... 18-418-4. Design alternatives ............................................................................................................... 18-718-5. Heave force tests, average tangential adfreeze bond stress versus time ,

and timber and steel pipe piles placed with silt-water slurry in dryexcavated holes. Piles were installed within annual frost zone only, overpermafrost, to depths from ground surface of 3.6 to 6.5 feet ............................................... 18-11

LIST OF TABLESPage

Table 2-1. A Simplified Classification of Natural Soil Deposits .............................................................. 2-22-2. Determination of the Consistency of Clays ........................................................................... 2-52-3. Unified Soil Classification System ........................................................................................ 2-72-4. Descriptive Soil Names Used in Local Areas (L) and Names Widely Used ......................... 2-92-5. A Simplified Classification for Rocks .................................................................................... 2-102-6. Descriptive Criteria for Rock ................................................................................................. 2-112-7. Engineering Classification of Intact Rock ............................................................................. 2-143-1. Typical Engineering Properties of Compacted Materials ...................................................... 3-33-2. Estimating Degree of Preconsolidation ................................................................................ 3-113-3. Compression Index Correlations .......................................................................................... 3-123-4. Value of Coefficient of Compressibility (mv) for Several Granular Soils

During Virgin Loading .................................................................................................... 3-123-5. Sensitivity of Clays ............................................................................................................... 3-203-6. Values of Modulus of Subgrade Reaction (ks) for Footings as a Guide

to Order of Magnitude.................................................................................................... 3-243-7. An Engineering Evaluation of Shales ................................................................................... 3-263-8. Physical Properties of Various Shales .................................................................................. 3-274-1. Types and Sources of Documentary Evidence..................................................................... 4-14-2. Recommended Undisturbed Sample Diameters .................................................................. 4-34-3. Recommended Minimum Quantity of Material for General Sample Lab -

oratory Testing............................................................................................................... 4-44-4. Surface Geophysical Methods.............................................................................................. 4-114-5. Borehole Surveying Devices................................................................................................. 4-135-1. Causes of Settlement ........................................................................................................... 5-25-2. Values of Angular Distortion (δ/ι) That Can Be Tolerated Without

Cracking ........................................................................................................................ 5-35-3. Empirical Correlations Between Maximum (∆) and Angular Distortion

(δ/ι) ................................................................................................................................ 5-45-4. Methods of Eliminating, Reducing, or Coping with Settlements ........................................... 5-116-1. Estimates of Allowable Bearing Pressur ............................................................................... 6-26-2. Allowable Bearing Pressure for Jointed Rock ...................................................................... 6-98-1. Methods of Stabilizing Slopes and Landslides ..................................................................... 8-169-1. Foundation Possibilities for Different Subsoil Conditions ..................................................... 9-19-2. Checklist for Influence of Site Characteristics on Foundation Selection

for Family Housing ........................................................................................................ 9-311-1. Excavation Protection ........................................................................................................... 11-611-2. Design Parameters for Drilled Piers in Clay ......................................................................... 11-1014-1. Types of Walls ...................................................................................................................... 14-1214-2. Factors Involved in Choice of a Support System for a Deep Excavation. ............................ 14-1414-3. Design Considerations for Braced and Tieback Walls ......................................................... 14-1515-1. A Summary of Densification Methods for Building Foundations ........................................... 15-315-2. Compaction Density as a Percent of CE 55 Laboratory Test Density .................................. 15-416-1. Stabilization of Soils for Foundations of Structures .............................................................. 16-216-2. Applicability of Foundation Soil Improvement for Different Structures

and Soil Types (for Efficient Use of Shallow Foundations) ............................................ 16-716-3. Vibroflotation Patterns for Isolated Footings fo r an Allowable Bearing

Pressure ........................................................................................................................ 16-13

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TM 5-818-1 / AFM 883, Chap. 7

PageTable 17-1. Mass Ratio, Damping Ratio, and Spring Constant for Rigid Circular Foot-

ing on the Semi-Infinite Elastic Body ................................................................. 17-417-2. Values of kL/k for Elastic Layer (k from Table 17-1) ................................................ 17-717-3. Attenuation Coefficients for Earth Materials............................................................. 17-1017-4. Values of Constant rY Used with Equation (17-23) to Estimate Cyclic

Shear Modulus at Low Strains........................................................................... 17-1317-5. Values of Constant K, Used with Equation (17-24) to Estimate Cyclic

Shear Modulus at Low Strains for Sands .......................................................... 17-1417-6. Criteria for Excluding Need for Detailed Liquefaction Analyses............................... 17-17

vi

TM 541-1I / AFM 883. Chap. 7

CHAPTER 1

INTRODUCTION

1-1. Purpose. This manual presents guidance forselecting and designing foundations and associatedfeatures for buildings, retaining structures, andmachinery. Foundations for hydraulic structures are notincluded. Foundation design differs considerably fromdesign of other elements of a structure because of theinteraction between the structure and the supportingmedium (soil and/or rock).

1-2. Scope. Information contained herein is directedtoward construction usually undertaken on militaryreservations, although it is sufficiently general to permitits use on a wide variety of construction projects.

a. This manual includes-(1) A brief summary of fundamental

volumetric - gravimetric relationships.(2) Summaries of physical and engineering

properties of soil and rock.(3) General descriptions of field and

laboratory investigations useful for foundation selectionand design.

(4) Design procedures for constructionaspects, such as excavated slopes and shoring.

(5) Empirical design equations andsimplified methods of analysis, including design charts,soil property-index correlations, and tabulated data.

(6) Selected design examples to illustrateuse of the analytical methods.

b. Since the user is assumed to have somefamiliarity with geotechnical engineering, design equa-tions and procedures are presented with a minimum oftheoretical background and no derivations. The topics ofdewatering and groundwater control, pile foundations,and foundations on expansive soils are covered ingreater depth in separate technical manuals and are onlytreated briefly in this manual.

1-3. Objectives of foundation investigations. Theobjectives of foundation investigations are to determinethe stratigraphy and nature of subsurface materials andtheir expected behavior under structure loadings and topermit savings in design and construction costs. Theinvestigation is expected to reveal adverse subsurfaceconditions that could lead to construction difficulties,excessive maintenance, or possible failure of thestructure. The scope of investigations depends on thenature and complexity of sub-surface materials and thesize, requirements for, and cost of the structure.

1-4. Report of subsurface and designinvestigations. The report should contain sufficientdescription of field and laboratory investigation, sub-surface conditions, typical test data, basic assumptions,and analytical procedures to permit detailed review ofthe conclusions, recommendations, and final design. Theamount and type of information to be presented shall beconsistent with the scope of the investigation. For somestructures, a cursory review of foundation conditions maybe adequate. For major structures, the following outlineshall be used as a guide:

a. A general description of the site, indicatingprincipal topographic features in the vicinity. A plan mapthat shows the surface contours, the location of theproposed structure, and the location of all borings shouldbe included.

b. A description of the general and localgeology of the site, including the results of the geologicalstudies.

c. The results of field investigations, includinggraphic logs of all foundation and borrow borings,locations of and pertinent data from piezometers, and ageneral description of subsurface materials, based on theborings. The information shall be presented inaccordance with Government standards. The boring logsshould indicate how the borings were made, type ofsampler used, split-spoon penetration resistance, andother field measurement data.

d. Groundwater conditions, including data onseasonal variations in groundwater level and results offield pumping tests, if performed.

e. A general description of laboratory testsperformed, range of test values, and detailed test data onrepresentative samples. Atterberg limits should beplotted on a plasticity chart, and typical grain-size curveson a grain-size distribution plot. Laboratory test datashould be summarized in tables and figures asappropriate. If laboratory tests were not performed, thebasis for determining soil or rock properties should bepresented, such as correlations or reference to pertinentpublications.

f. A generalized geologic profile used fordesign, showing properties of subsurface materials anddesign values of shear strength for each critical stratum.The profile may be described or shown graphically.

1-1

TM 5-818-1 / AFM 88-3, Chap. 7

g. Where alternative foundation designs areprepared, types of foundations considered, together withevaluation and cost data for each.

h. A table or sketch showing the final size anddepth of footings or mats and lengths and types of piles,if used.

I. Basic assumptions for loadings and thecomputed factors of safety for bearing-capacitycalculations, as outlined in chapter 6.

j. Basic assumptions, loadings, and results ofsettlement analyses, as outlined in chapters 5, 6, and 10;

also, estimated swelling of subgrade soils. The effects ofcomputed differential settlements, and also the ef- fectsof swell, on the structure should be discussed.

k. Basic assumptions and results of otheranalyses.

I. An estimate of dewatering requirements, ifnecessary. The maximum anticipated pumping rate andflow per foot of drawdown should be presented.

m. Special precautions and recommendationsfor construction. Possible sources for fill and backfillshould also be given. Compaction requirements shouldbe described.

1-2

TM 5-818-1/AFM 88-3. Chap. 7

CHAPTER 2

IDENTIFICATION AND CLASSIFICATION OF SOIL AND ROCK2-1. Natural soil deposits.

a. The character of natural soil deposits isinfluenced primarily by parent material and climate. Theparent material is generally rock but may include partiallyindurated materials intermediate between soil and rock.Soils are the results of weathering, mechanicaldisintegration, and chemical decomposition of the parentmaterial. The products of weathering may have thesame composition as the parent material, or they may benew minerals that have resulted from the action of water,carbon dioxide, and organic acids with mineralscomprising the parent material.

b. The products of weathering that remain inplace are termed residual soils. In relatively flat regions,large and deep deposits of residual soils mayaccumulate; however, in most cases gravity and erosionby ice, wind, and water move these soils to form newdeposits, termed transported soils. Duringtransportation, weathered material may be mixed withothers of different origin. They may be ground up ordecomposed still further and are usually sorted accordingto grain size before finally being deposited. The newlyformed soil deposit may be again subject to weathering,especially when the soil particles find themselves in acompletely different environment from that in which theywere formed. In humid and tropical climates, weatheringmay significantly affect the character of the soil to greatdepths, while in temperate climates it produces a soilprofile that primarily affects the character of surface soils.

c. The character of natural soil depositsusually is complex. A simplified classification of naturalsoil deposits based on methods of deposition is given intable 2-1, together with pertinent engineeringcharacteristics of each type. More complete descriptionsof natural soil deposits are given in geology textbooks.The highly generalized map in figure 2-1 shows thedistribution of the more important natural soil deposits inthe United States.

2-2. Identification of soils.a. It is essential to identify accurately materials

comprising foundation strata. Soils are identified byvisual examination and by means of their indexproperties (grain-size distribution, Atterberg limits, watercontent, specific gravity, and void ratio). A descriptionbased on visual examination should include color, odorwhen present, size and shape of grains, gradation, anddensity and consistency characteristics.- Coarsegrained

soils have more than 50 percent by weight retained onthe No. 200 sieve and are described primarily on thebasis of grain size and density. With regard to grain-sizedistribution, these soils should be described as uniform,or well-graded; and, if in their natural state, as loose,medium, or dense. The shape of the grains and thepresence of foreign materials, such as mica or organicmatter, should be noted.

b. Fine-grained soils have more than 50percent by weight finer than the No. 200 sieve.Descriptions of these soils should state the color, texture,stratification, and odor, and whether the soils are soft,firm, or stiff, intact or fissured. The visual examinationshould be accompanied by estimated or laboratory-determined index properties. A summary of expedienttests for identifying fine-grained soils is given in table 2-2.The important index properties are summarized in thefollowing paragraphs. Laboratory tests for determiningindex properties should be made in accordance withstandard procedures.

2-3. Index properties.a. Grain-size distribution. The grain-size

distribution of soils is determined by means of sievesand/or a hydrometer analysis, and the results areexpressed in the form of a cumulative semilog plot ofpercentage finer versus grain diameter. Typical grain-size distribution curves are shown in figure 2-2. Theknowledge of particle-size distribution is of particularimportance when coarse-grained soils are involved.Useful values are the effective size, which is defined asthe grain diameter corresponding to the 10 percent finerordinate on the grain-size curve; the coefficient ofuniformity, which is defined as the ratio of the D60 size tothe D, , size (fig 2-2); the coefficient of curvature, whichis defined as the ratio of the square of the Do3 size to theproduct of the D1o and D60 sizes (table 2-3); and the 15 and85 percent sizes, which are used in filter design.

b. Atterberg limits. The Atterberg limitsindicate the range of water content over which a cohesivesoil behaves plastically. The upper limit of this range isknown as the liquid limit (LL); the lower, as the plasticlimit (PL). The LL is the water content at which a soil willjust begin to flow when slightly jarred in a pre

2-1

TM 5-818-1 / AFM 88-3, Chap. 7

Table 2-1. A Simplified Classification of Natural Soil Deposits

2-2

TM 5-818-1 / AFM 88-3, Chap. 7

Table 2-1. A Simplified Classification of Natural Soil Deposits-Continued

U. S. Army Corps of Engineers

scribed manner. The PL is the water content at which thesoil will just begin to crumble when rolled into threads 1/8inch in diameter. Fat clays that have a high content ofcolloidal particles have a high LL, while lean clays havinga low content of colloidal particles have acorrespondingly low LL. A decrease in LL and PL aftereither oven- or air-drying usually indicates presence oforganic matter. The plasticity index (PI) is defined asthe difference between the LL and PL. The liquidityindex (LI) is defined as the natural water content wn,minus the PL, divided by the PI; i. e. , LI = (wn - PL)/PI.The LI is a measure of the consistency of the soils. Softclays have an LI approaching 100 percent; whereas, stiffclays have an LI approaching zero.

c. Activity. The activity, A, of a soil is definedas A = PI/(% < 0.002 mm). The activity is a usefulparameter for correlating engineering properties of soil.

d. Natural water content. The natural watercontent of a soil is defined as the weight of water in thesoil expressed as a percentage of dry weight of solidmatter present in the soil. The water content is basedon the loss of water at an arbitrary drying temperature of1050 to 1100C.

e. Density. The mass density of a soil materialis its weight per unit volume. The dry density of a soil isdefined as the weight of solids contained in the unitvolume of the soil and is usually expressed in pounds percubic foot. Various weight-volume relationships arepresented in figure 2-3.

f. Specific gravity. The specific gravity of thesolid constituents of a soil is the ratio of the unit weight ofthe solid constituents to the unit weight of water. Forroutine analyses, the specific gravity of sands and clayeysoils may be taken as 2. 65 and 2. 70, respectively.

g. Relative density. Relative density isdefined by the following equation:

DR(%) = emax - e X 100 (2-1)emax - emin

whereemax = void ratio of soil in its loosest state

e = void ratio in its natural stateemin = void ratio in its densest possible state

Alternatively,

DR(%) = yd - ydmin X ydmax X 100 (2-2)ydmax - ydmin yd

whereyd = dry unit weight of soil in its natural

stateydmin = dry unit weight of soil in its loosest

stateydmax = dry unit weight of soil in its densest

stateThus, DR = 100 for a very dense soil, and DR = 0 for avery loose soil. Methods for determining emax or cor-responding densities have been standardized. Relativedensity is significant only in the case of coarse-grainedsoils.

2-3

TM 5-818-1 / AFM 88-3, Chap. 7

Figure 2-1. Distribution of natural soil deposits In the United States.

2-4

TM 5-818-1 / AFM 88-3, Chap. 7

Table 2-2. Determination of the Consistency of Clays

UnconfinedCompressiveStrength, qu

tsf Field Identification Consistency

Less than 0.25 Easily penetrated several inches by fist Very soft

0.25 - 0.5 Easily penetrated several inches by Softthumb

0.5 - 1.0 Can be penetrated several inches by Mediumthumb with moderate effort

1.0 - 2.0 Readily indented by thumb but pene- Stifftrated only with great effort

2.0 - 4.0 Readily indented by thumbnail Very stiff

Over 4.0 Indented with difficulty by thumbnail Hard


h. Consistency. The consistency of anundisturbed cohesive soil may be expressedquantitatively by the unconfined compressive strengthqu. Qualitative expressions for the consistency of clays interms of qµ are given in table 2-2. If equipment formaking unconfined compression tests is not available, arough estimate can be based on the simple fieldidentification suggested in the table; various smallpenetration or vane devices are also helpful.

2-4. Soil classification. The Unified Soil Classi-fication System, based on identification of soils ac-cording to their grain-size distribution, their plasticitycharacteristics, and their grouping with respect to be-havior, should be used to classify soils in connection withfoundation design. Table 2-3 summarizes the UnifiedSoil Classification System and also presents fieldidentification procedures for fine-grained soils or soilfractions. It is generally advantageous to include with thesoil classification anv regional or locally acceptedterminology as well as the soil name (table 2-4).2-5. Rock classification.

a. Geological classification. The geologicalclassification of rock is complex, and for mostengineering applications a simplified system of

classification, as shown in table 2-5, will be adequate.For any in-depth geology study, proper stratigraphicclassification by a qualified geologist should be made toensure that proper interpretation of profiles is beingmade. All the rock types in table 2-5 may exist in asound condition or may be fissured, jointed, or altered byweathering to an extent that will affect their engineeringbehavior. Descriptive criteria for the field classification ofrock is contained in table 2-6.

b. Classification of intact rock. An engineeringclassification of intact rock is contained in table 2-7. Theclassification is based on the uniaxial compressivestrength and the tangent modulus.

2-6. Rock properties for foundation design.a. The principal rock properties of concern for

foundation design are the structural features and shearstrength. Strength properties of rock are discussed inchapter 3. Structural features include-

(1) Types and patterns for rock defects (table2-6)-cracks, joints, fissures, etc.

(2) Bedding planes-stratification and slope(strike and dip).

(3) Foliation-a general term for a planararrange-

2-5

TM 5-818-1 / AFM 88-3, Chap. 7

Figure 2-2. Typicalgrain-size distribution curves.


2-6

TM 5-818-1 / AFM 88-3, Chap. 7

Table 2-3. Unified Soil Classification System


2-7

TM 5-818-1 / AFM 88-3, Chap. 7

Figure 2-3. Weight-volume relationships.


2-8

TM 5-818-1 / AFM 88-3. Chap. 7

Table 2-4. Descriptive Soil Names Used in Local Areas (L)and Names Widely Used

U. S. Army Corps of Engineers2-9

TM 5-818-1 / AFM 88-3, Chap. 7

Table 2-5. A Simplified Classification for Rocks


2-10

TM 5-818-1 / AFM 88-3, Chap. 7

Table 2-6. Descriptive Criter for Rock

1. Rock typea. Rock name (Generic)b. Hardness(1) Very soft: can be deformed by hand(2) Soft: can be scratched with a fingernail(3) Moderately hard: can be scratched easily with a knife(4) Hard: can be scratched with difficulty with a knife(5) Very hard: cannot be scratched with a knifec. Degree of weathering(1) Unweathered: no evidence of any mechanical or chemical alteration.(2) Slightly weathered: slight discoloration on surface, slight alteration along discontinuities, less than 10

percent of the rock volume altered, and strength substantially unaffected.(3) Moderately weathered: discoloring evident, surface pitted and altered with alteration penetrating well

below rock surfaces, weathering "halos" evident, 10 to 50 percent of the rock altered, and strengthnoticeably less than fresh rock.

(4) Highly weathered: entire mass discolored, alteration in nearly all of the rock with some pockets of slightlyweathered rock noticeable, some minerals leached away, and only a fraction of original strength (with wet strength usuallylower than dry strength) retained.

(5) Decomposed: rock reduced to a soil with relect rock texture (saprolite) and generally molded andcrumbled by hand.

d. Lithology (macro description of mineral components). Use standard adjectives, such as shaly, sandy, silty,and calcareous. Note inclusions, concretions, nodules, etc.

e. Texture and grain size(1) Sedimentary rocks

Texture Grain Diameter, mm Particle Name Rock Name* < - 80 Cobble Conglomerate* 5 -. 80 Gravel

Coarse grained 2 - 5Medium grained 0.4 - 2 Sand SandstoneFine grained 0.1 - 0.4

* Use clay-sand texture to describe conglomerate matrix.(Continued) (Sheet 1 of 3)

2-11

TM 5-818-1 / AFM 88-3, Chap. 7

Table 2-6. Descriptive Criteria for Rock-Continued

Texture Grain Diameter, mm Particle Name Rock Name

Very fine grained > - 0.1 Clay, Silt Shale, Claystone,Siltstone,Limestone

(2) Igneous and metamorphic rocks

Texture Grain Diameter, mmCoarse grained > - 5Medium grained 1 - 5Fine grained 0.1 - 1Aphanite < 0.1

(3) Textural adjectives. Use simple standard textural adjectives such as porphyritic, vesicular, pegmatitic, granular,and grains well developed. Do not use sophisticated terms such as holohyaline, hypidiomorphic granular, crystaloblastic,and cataclastic.

2. Rock structurea. Bedding(1) Massive: >3 ft thick(2) Thick bedded: beds from 1 to 3 ft thick(3) Medium bedded: beds from 4 in. to 1 ft thick(4) Thin bedded: beds less than 4 in. thickb. Degree of fracturing (jointing)(1) Unfractured: fracture spacing greater than 6 ft(2) Slightly fractured: fracture spacing from 3 to 6 ft(3) Moderately fractured: fracture spacing from 1 to 3 ft(4) Highly fractured: fracture spacing from 4 in. to 1 ft(5) Intensely fractured: fracture spacing less than 4 in.c. Shape of rock blocks(1) Blocky: nearly equidimensional(2) Elongated: rod-like(3) Tabular: flat or bladed

(Continued)(Sheet 2 of 3)

2-12

TM 5-818-1 / AFM 88-3, Chap. 7

Table 2-6. Descriptive Criteria for Rock-Continued

3. Discontinuitiesa. Joints(1) Type: bedding, cleavage, foliation, schistosity, extension(2) Separations: open or closed, how far open(3) Character of surface: smooth or rough; if rough, how much relief, average asperity angle(4) Weathering of clay products between surfacesb. Faults and shear zones(1) Single plane or zone: how thick(2) Character of sheared materials in zone(3) Direction of movement, slickensides(4) Clay fillingsc. Solution, cavities, and voids(1) Size(2) Shape: planar, irregular, etc.(3) Orientation (if applicable): developed along joints, bedding planes, at intersections of joints and bedding

planes, etc.(4) Filling: percentage of void volume and type of filling material (e.g. sand, silt, clay, etc.).

(Sheet 3 of 3)U. S. Army Corps of Engineers

ment of texture or structural features in any type of rock;e.g., cleavage in slate or schistosity in a metamorphicrock. The term is most commonly applied tometamorphic rock.

b. Samples that are tested in the laboratory(termed "intact" samples) represent the upper limit ofstrength and stress-strain characteristics of the rock andmay not be representative of the mass behavior of therock. Coring causes cracks, fissures, and weak planesto open, often resulting in a recovery of many rockfragments of varying length for any core barrel advance.Only samples (intact pieces) surviving coring and havinga length/diameter ratio of 2 to 2.5 are tested. RockQuality Designation (RQD) is an index or measure of thequality of the rock mass. RQD is defined as:

∑ Lengths of intactRQD = pieces > 4 in. long

Length of core advance

Referring to figure 2-4, with a core advance of 60 inchesand a sum of intact pieces, 4 inches or larger, of 34inches, the RQD is computed as:

34RQD = 60 = 0.57

Also shown in figure 2-4 is a qualitative rating of the rockmass in terms of RQD. RQD depends on the drillingtechnique, which may induce fracture as well as rockdiscontinuities. Fresh drilling-induced fractures may beidentified by careful inspection of the recovered sample.2-7. Shales.

a. Depending on climatic, geologic, andexposure conditions, shale may behave as either a rockor soil but must always be handled and stored as thoughit is soil. For these reasons, shale is consideredseparately from either soil or rock. Shale is a fine-grainedsedimentary rock composed essentially of compressed

2-13

TM 5-818-1 / AFM 88-3, Chap. 7

Table 2- 7. Engineering Classification of Intact Rock

I. On basis of strengh, σult :a

Class Description Uniaxial CompressiveA Very high strength Over 32,000B High strength 16,000 - 32,000C Medium strength 8,000 - 16,000D Low strength 4,000 - 8,000E Very low strength Less than 4,000

II. On basis of modulus ratio, Et / σult :a

Class Description Uniaxial CompressiveH High modulus ratio Over 500M Average (medium) ratio 200-500L Low modulus ratio Less than 200

a Rocks are classified by both strength and modulus ratio, such as AM, BL, BH, and CM.b Modulus ratio = Et/σult, where Et = tangent modulus at 50 percent ultimate strength and σult = uniaxial compressivestrength.

(Courtesy of K. G. Stagg and O. C. Zienkiewiez, RockMechanics in Engineering Practice, 1968, pp 4-5.Reprinted by permission of John Wiley & Sons, Inc, NewYork.)

and/or cemented clay particles. It is usually laminatedfrom the general parallel orientation of the clay particlesas distinct from claystone, siltstone, or mudstone, whichare indurated deposits of random particle orientation. Theterms "argillaceous rock" and "mudrock" are also used todescribe this type of rock. Shale is the predominatesedimentary rock in the earth’s crust.

b. Shale may be grouped as compactionshale, and cemented (rock) shale. Compaction shale is atransition material from soil to rock and can be excavatedwith usual large excavation equipment. Cemented shalegenerally requires blasting. Compaction shales havebeen formed by consolidation pressure and very littlecementing action. Cemented shales are formed by acombination of cementing and consolidation pressure.They tend to ring when struck by a hammer, do not slakein water, and have the general characteristics of

good rock. Compaction shales, being of an interme- diatequality, will generally soften and expand upon exposureto weathering when excavations are opened.

c. Dry unit weight of shale may range fromabout 80 pounds per cubic foot for poor-qualitycompaction shale to 160 pounds per cubic foot for high-quality cemented shale. Shale may have the appearanceof sound rock on excavation but will often deteriorate,during or after placement in a fill, into weak clay or silt, oflow shear strength. Figure 2-5 may be used as a guide inclassifying shale for foundation use.

d. Compaction shales may swell for years aftera structure is completed and require special studieswhenever found in subgrade or excavated slopes. Thepredicted behavior of shales cannot be based sofelyupon laboratory tests and must recognize localexperiences.

2-14

TM 5-818-1 / AFM 88-3, Chap. 7

Figure 2-4. Modified core recovery as in index of rock quality.

2-15

TM 5-818-1 / AFM 88-3, Chap. 7

Figure 2-5. Classification of shales.

2-16

TM 5-818-1/AFM 88-3, Chap. 7

CHAPTER 3

ENGINEERING PROPERTIES OF SOIL AND ROCK3-1. Scope. This chapter considers engineeringproperties of soil and rock useful in designingfoundations under static loading. Dynamic properties arediscussed in chapter 17.

a. Correlations. Tables and charts based oneasily determined index properties are useful for roughestimating or confirming design parameters. Testingprocedures employed by different soil laboratories haveinfluenced correlations presented to an unknown degree,and the scatter of data is usually substantial; cautionshould, therefore, be exercised in using correlationvalues. Undisturbed soil testing, either laboratory or field,or both, should be used for final design of majorfoundations. On smaller projects, an economic analysisshould determine if a complete soil exploration/laboratorytesting program is justified in lieu of a conservativedesign based on correlation data. Complex subsurfaceconditions may not permit a decision on solely economicgrounds.

b. Engineering properties. Properties ofparticular interest to the foundation engineer include-

(1) Compaction.(2) Permeability.(3) Consolidation-swell.(4) Shear strength.(5) Stress-strain modulus (modulus of

elasticity) and Poisson’s ratio.

3-2. Compaction characteristics of soils.The density at which a soil can be placed as fill or backfilldepends on the placement water content and thecompaction effort. The Modified Compaction Test (CE55) or comparable commercial standards will be used asa basis for control. The CE 55 test is described in TM 5-824-2/AFM 88-6, Chapter 2. (See app A for references.)Other compaction efforts that may be occasionally usedfor special projects include-

a. Standard compaction test: Three layers at25 blows per layer Hammer = 5.5 pounds with 12-inchdrop

b. Fifteen-blow compaction test:Three layers at 15 blows per layerHammer = 5.5 pounds with 12-inch drop

The results of the CE 55 test are represented bycompaction curves, as shown in figure 3-1, in which thewater content is plotted versus compacted dry density.The ordinate of the peak of the curve is the maximum drydensity, and the abscissa is the optimum water content

Wopt. Table 3-1 presents typical engineering propertiesof compacted soils; see footnote for compacted effortthat applies.

3-3. Density of cohesionless soils.a. Relative density of cohesionless soils has a

considerable influence on the angle of internal friction,allowable bearing capacity, and settlement of footings.An example of the relationship between relative densityand in situ dry densities may be conveniently determinedfrom figure 3-2. Methods for determining in situ densitiesor relative densities of sands in the field are discussed inchapter 4.

b. The approximate relationship among theangle of friction, +, DR, and unit weight is shown in figure3-3; and between the coefficient of uniformity, Cu, andvoid ratio, in figure 3-4.

c. The relative compaction of a soil is definedas

RC = ---------------- x 100(percent) (3-1)

where yfield = dry density in field and ymax (lab) = maximumdry density obtained in the laboratory. For soils where100 percent relative density is approximately the same as100 percent relative compaction based on CE 55, therelative compaction and the relative density are relatedby the following empirical equation:

RC = 80 + 0.2DR(DR > 40 percent) (3-2)

3-4. Permeability.a. Darcy’s law. The laminar flow of water

through soils is governed by Darcy’s law:q = kiA (3-3)

whereq = seepage quantity (in any time unit consistent

with k)k = coefficient of permeability (units of velocity)i = h/L = hydraulic gradient or head loss, h,

across the flow path of length, LA = cross-sectional area of flowb. Permeability of soil. The permeability -

depends primarily on the size and shape of the soilgrains, void ratio, shape and arrangement of voids,degree of saturation, and temperature. Permeability isdetermined in the laboratory by measuring the rate offlow of wa-

3-1

Y field

Y max (lab)

TM 5-818-1 / AFM 88-3, Chap. 7

ter through a specimen under known hydraulic gradient, i.Typical permeability values, empirical relationships, andmethods for obtaining the coefficient of permeability areshown in figure 3-5. Field pumping tests are the mostreliable means of determining the permeability of naturalsoil deposits (para 4-5). Permeability obtained in this

manner is the permeability in a horizontal direction. Thevertical permeability of natural soil deposits is affected bystratification and is usually much lower than thehorizontal permeability.


Figure 3-1. Typical CE 55 compaction test data.

3-2

TM 5-818-1 / AFM U-3. Chap. 7

Table 3-1. Typical Engineering Properties of Compacted Materials

Notes 1. All properties are for condition of "standard Proctor" maximum density. except values of k and CBH whichare for CE55 ,maximum density.

2. Typical strength characteristics are for effective strength envelopes and are obtained from ISBR data.3. Compression values are for vertical loading with complete lateral confinement.

*. 4. (>) indicates that typical property is greater than the value shown. ( ) indicates insufficient data available for anestimate.

(NAVFAC DM-7)

3-3

TM 5-818-1 / AFM 88-3. Chap. 7

Figure 3-2. Relation between relative density and dry density (scaled to plot as a straight line).

Figure 3-3. Angle of friction versus dry density for coarse-grained soils.

3-4

TM 5-818-1 / AFM 88-3, Chap. 7

c. Permeability of rock. Intact rock isgenerally impermeable, but completely intact rockmasses rarely occur. The permeability of rock masses iscontrolled by discontinuities (fissures, joints, cracks,etc.), and flow may be either laminar (Darcy’s lawapplies) or turbulent, depending on the hydraulicgradient, size of flow path, channel roughness, and otherfactors. Methods for determining the in situ permeabilityof rock are presented in chapter 4.

3-5. Consolidation. Consolidation is a time-de-pendent phenomenon, which relates change that occursin the soil mass to the applied load.

a. Consolidation test data. Consolidation orone-dimensional compression tests are made inaccordance with accepted standards. Results of tests(fig 3-6) are presented in terms of time-consolidationcurves and pressure-void ratio curves. The relationshipbetween void ratio and effective vertical stress, p, isshown on a semilogarithmic diagram in figure 3-6. The

test results may also be plotted as change in volumeversus effective vertical stress. Typical examples ofpressure - void ratio curves for insensitive and sensitive,normally loaded clays, and preconsolidated clays areshown in figure 3-7.

b. Preconsolidation pressure. Thepreconsolidation stress, pc, is the maximum effectivestress to which the soil has been exposed and mayresult from loading or drying. Geological evidence ofpast loadings should be used to estimate the order ofmagnitude of preconsolidation stresses before laboratorytests are performed. The Casagrande method ofobtaining the preconsolidation pressure fromconsolidation tests is shown in figure 3-7. Determiningthe point of greatest curvature

NOTE: THE MINIMUM VOID RATIOS WERE OBTAINED FROM SIMPLE SHEAR TESTS. CURVES AREONLY VALID FOR CLEAN SANDS WITH NORMAL TO MODERATELY SKEWED GRAIN-SIZEDDISTRIBUTIONS.

(Modified from ASTM STP 523 (pp 98-112). Copyright ASTM, 1916 Race St.,Philadelphia, PA. 19103. Reprinted/adapted with permission.)

Figure 3-4. Generalized curves for estimating emax, and emin from gradational and particle shape characteristics.

3-5

TM 5-818-1 / AFM 88-3, Chap. 7


Figure 3-5. A summary of soil permeabilities and methods of determination.

3-6

TM 5-818-1 / AFM 88-3, Chap. 7

EXAMPLES OF LABORATORY PRESSURE - VOID RATIO CURVES


Figure 3-6. Examples of laboratory consolidation test data.

3-7

TM 5-818-1 / AFM 88-3, Chap. 7

Figure 3- 7. Analyses of consolidation test data.

3-8

TM 5-818-1 / AFM 88-3, Chap. 7

requires care and judgment. Sometimes it is better toestimate two positions of this point-one as small as likely,and the other as large as plausible, consistent with thedata-and to repeat the construction for both cases. Theresult will be a range of preconsolidation stresses.Because the determination of pc involves someinevitable inaccuracy, the range of possible values maybe more useful than a single estimate which fallssomewhere in the possible range. The higher the qualityof the test specimen, the smaller is the range of possiblepc values. Approximate values of preconsolidationpressure may be estimated from figure 3-8 or 3-9. Table3-2 can be used to obtain gross estimates of sitepreconsolidation. This table and figures 3-8 and 3-9should be applied before consolidation tests areperformed to assure test loads sufficiently high to definethe virgin compression portion of e-log p plots.

c. Compression index. The slope of the virgincompression curve is the compression index Cc, definedin figure 3-6. Compression index correlations forapproximations are given in table 3-3. When volumechange is expressed as vertical strain instead of changein void ratio, the slope of the virgin compression part

of the £ versus log p curve is the compression ratio, CR,defined as

CR = ∆ε = Cc (3-4)log p2’ 1 + e0

p1’

where & is the change in vertical strain corresponding toa change in effective stress from p1’ to p2’, and e0 is theinitial (or in situ) void ratio. An approximate correlationbetween CR and natural water content in clays is givenby the following:

CR = 0.006 (w - 12)(3-5)

d. Coefficient of volume compressibility. Therelationship between deformation (or strain) and stressfor one-dimensional compression is expressed by thecoefficient of volume compressibility, m , which isdefined as

mv = ∆ε ∆ε = av (3-6)∆p ∆p (1 + e0) 1 + e0

mv = 0.434 Cc

(1 + e0)p’

Figure 3-8. Approximate relation between liquidity index and effective overburden pressure, as a function of the sensitivityof the soil

3-9

TM 5-818-1 / AFM 88-3, Chap. 7

where∆ε = change in vertical strain∆p = P2 - p; = corresponding change in

effective vertical stressav = Ae/Ap = coefficient of

compressibilityp’ = average of initial and final

effective vertical stress

The units of mv are the reciprocal of constrainedmodulus. Table 3-4 gives typical values of mv forseveral granular soils during virgin loading.

e. Expansion and recompression. Ifoverburden pressure is decreased, soil undergoesvolumetric expansion (swell), as shown in figure 3-7.The semilogarithmic, straight-line (this may have to beapproximated) slope of the swelling curve is expressedby the swelling index, Cs, as

Cs = ∆e (3-8)log p2’

p1’

where Ae is the change in void ratio (strictly a signapplies to Cc, Cs., Cr, and mv; however, judgment isusually used in lieu of signs). The swelling index isgenerally from one-fifth to one-tenth the compressionindex. Approximate values of Cs may be obtained fromfigure 3-10. The slope of the recompression curve isexpressed by the recompression index, Cr, as follows:

Cr = ∆e (3-9)log p2’

p1’

The value of Cr is equal to or slightly smaller than Cs.High values of Cr/Cs are associated withoverconsolidated clays containing swelling clay minerals.

(Courtesy of T. W. Lambe and R. V. Whitman,Soils Mechanics, 1969, p 320. Reprinted bypermission of John Wiley & Sons, Inc., New York.)

Figure 3-9. Approximate relation between void ratio and effective overburden pressure for clay sediments, as a function ofthe Atterberg limits.

3-10

TM 5-818-1 / AFM 88-3, Chap. 7

Table 3-2. Estimating Degree of Preconsolidation

Method RemarksSurface topography Soil below alluvial valley filling should generally have a preconsolidation stress at

least corresponding to elevation of abutments. In wide river valleys withterraces at several elevations, an elevation corresponding to previous surfaceelevation in the river valley may be several miles distant

Geological evidence Ask geologist for estimate of maximum preconsolidation stress. Erosion mayhave removed hundreds of feet of material even in abutment area

Water content If natural water content is near PL or below it, anticipate high preconsolidationstress. A high natural water content is not, itself, a suitable indicator ofabsence of overconsolidation

Standard penetration resistance If blow counts are high, anticipate high preconsolidation stress. From blowcounts, estimate undrained shear strength, su , in tons per square foot asapproximately 1/15 of blow count. If estimated value is substantially morethan corresponds to a su/po ratio of about 0.25, anticipate highpreconsolidation stresses

Undisturbed sampling If soil was too hard to sample with piston sampler, and a Denison or similarsampler was required, suspect high preconsolidation stress

Laboratory shear strengths If higher than those corresponding to a su/po ratio of about 0.3, anticipate highpreconsolidation stress

Compression index from consoli- If compression index appears low for Atterberg limits of soil, suspect that testdations tests loads were not carried high enough to determine virgin compression curve

and correct preconsolidation stress. Expected values for compression indexcan be estimated from correlations with water content and Atterberg limits(table 3-3)

Liquidity index and sensitivity Estimate preconsolidation stress from figures 3-8 and 3-9 (pc values may be low)


3-11

TM 5-818-1 / AFM 88-3, Chap. 7

f. Coefficient of consolidation. The soilproperties that control the drainage rate of pore waterare combined into the coefficient of consolidation, Cv,defined as follows:

Cv = k(1 + e0) = k (3-10)ywav’ ywmv

alternatively,Cv = TH2 (3-11)

t

wherek = coefficient of permeability in a vertical

directione0 = initial void ratioyy = unit weight of waterav = ∆e/∆p = coefficient of compressibility,

vertical deformationmv = coefficient of volume compressibilityT = Time factor (para 5-5) that depends on

percent consolidation and assumedpore pres-

Table 3-3. Compression Index Correlations

ClaysCc = 0.012 w , wn in percentCc = 0.01 (LL-13)

Sand, uniformCc = 0.03 , loose to Cc = 0.06, dense

Silt, uniformCc = 0.20


Table 3-4. Value of Coefficient of Compressibility (mv) for Several Granular Soils During Virgin Loading

mv x 10-4 per psiRelative Effective Pressure

Soil Density, DR 9 to 14 psi 28 to 74 psi

Uniform gravel 0 2.3 1.1

1 < D < 5 mm 100 0.6 0.4

Well-graded sand 0 5.0 2.7

0.02 < D < 1 mm 100 1.3 0.6

Uniform fine sand 0 4.8 2.0

0.07 < D < 0.3 mm 100 1.4 0.6

Uniform silt 0 25.0 4.0

0.02 < D < 0.07 mm 100 2.0 0.9

(Courtesy of T W. Lambe and R. V. Whitman, SoilsMechanics, 1969, p 155. Reprinted by permission ofJohn Wiley & Sons, Inc., New York.)

3-12

TM 5-818-1 / AFM 88-3, Chap. 7

Figure 3-10. Approximate correlations for swelling index of silts and clays.

3-13

TM 5-818-1 / AFM 88-3, Chap. 7

Figure 3-11. Correlations between coefficient of consolidation and liquid limit.

3-14

TM 5-818-1 / AFM 88-3, Chap. 7

sure distribution in soil caused by loadH = length of longest drainage path (lab or field)t = time at which the time factor is T for the

degree of consolidation that has occurred(generally, use t50 for T = 0.197 and 50percent consolidation)

Correlation between Cv and LL are shown in figure 3-11for undisturbed and remolded soil.

g. Coefficient of secondary compression. Thecoefficient of secondary compression, C∝, is strain εz =∆H/H0, which occurs during one log cycle of timefollowing completion of primary consolidation (fig 3-7).The coefficient of secondary compression is computedas

∆HC∝ = ∆εz = Hf (3-12)

log t_ log _t_tp tp

where tp is time to complete primary consolidation, andHf is total thickness of compressible soil at time tp. Soilswith high compressibilities as determined by thecompression index of virgin compression ratio willgenerally also have high values of C∝. Highly sensitiveclays and soils with high organic contents usually exhibithigh rates of secondary compression. Overconsolidationcan markedly decrease secondary compression.Depending on the degree of overconsolidation, the valueof C∝ is typically about one-half to one-third as large forpressures below the preconsolidation pressure as it is forthe pressures above the preconsolidation pressures. Formany soils, the value of C∝ approximately equals0.00015w, with w in percent.

h. Effects of remolding. Remolding ordisturbance has the following effects relative toundisturbed soil:

(1) e-log p curve. Disturbance lowers thevoid ratio reached under applied stresses in the vicinityof the preconsolidation stress and reduces the distinctbreak in the curve at the preconsolidation pressure (fig 3-7). At stresses well above the preconsolidation stress,the e-log p curve approaches closely that for goodundisturbed samples.

(2) Preconsolidation stress. Disturbancelowers the apparent preconsolidation stress.

(3) Virgin compression. Disturbancelowers the value of the compression index, but the effectmay not be severe.

(4) Swelling and recompression.Disturbance increases the swelling and recompressionindices.

(5) Coefficient of consolidation.Disturbance decreases the coefficient of consolidationfor both virgin compression and recompression (fig 3-11)in the vicinity of initial overburden and preconsolidationstresses. For good undisturbed samples, the value of

Cv decreases abruptly at the preconsolidation pressure.(6) Coefficient of secondary compression.

Disturbance decreases the coefficient of secondarycompression in the range of virgin compression.3-6. Swelling, shrinkage, and collapsibility.

a. The swelling potential is an index propertyand equals the percent swell of a laterally confined soilsample that has soaked under a surcharge of 1 poundper square inch after being compacted to the maximumdensity at optimum water content according to thestandard compaction test method. Correlation betweenswelling potential and PI for natural soils compacted atoptimum water content to standard maximum density isshown in figure 3-12.

b. The amount of swelling and shrinkagedepends on the initial water content. If the soil is wetterthan the shrinkage limit (SL), the maximum possibleshrinkage will be related to the difference between theactual water content and the SL. Similarly, little swellwill occur after-the water content has reached somevalue above the plastic limit.

(Courtesy of H. B. Seed, J. Woodward. J andLundgren, R., "Predication of Swelling Potential forSwelling Clay, " Journal, Soil Mechanics andFoundations Division, Vol 88, No. SM3, Part I. 1962,pp 53-87. Reprinted by permission of AmericanSociety of Civil Engineers, New York.)

Figure 3-12. Predicted relationship between swellingpotential and plasticity index for compacted soils.

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TM 5-818-1 / AFM 88-3, Chap. 7

c. Collapsible soils are unsaturated soils thatundergo large decreases in volume upon wetting with orwithout additional loading. An estimate of collapsibility(decrease in volume from change in moisture available)and expansion of a soil may be made from figure 3-13based on in situ dry density and LL.

3-7. Shear strength of soils.

a. Undrained and effective strengths. Theshear strength of soils is largely a function of theeffective normal stress on the shear plane, which equalsthe total normal force less the pore water pressure. Theshear strength, s, can be expressed in terms of the totalnormal pressure, σ, or the effective normal pressure, σ‘,by parameters determined from laboratory tests or,occasionally, estimated from correlations with indexproperties. The shear test apparatus is shown in figure3-14. The equations for shear strength are as follows:

s = c + σ tan φ (total shear strength parameters)s' = c' + (σ - u) tan φ' (effective shear

strength parameters)

The total (undrained) shear strength parameters, c andφ, are designated as cohesion and angle of internalfriction, respectively. Undrained shear strengths applywhere there is no change in the volume of pore water(i.e., no consolidation) and are measured in thelaboratory by shearing without permitting drainage. Forsaturated soils, φ = 0, and the undrained shear strength,c, is designated as su. The effective stress parameters,c' and φ', are used for determining the shear strengthprovided pore pressures, u, are known. Pore pressurechanges are caused by a change in either normal orshear stress and may be either positive or negative.Pore pressures are determined from piezometerobservations during and after construction or, for designpurposes, estimated on the basis of experience andbehavior of samples subjected to shear tests. Effectivestress parameters are computed from laboratory tests inwhich pore pressures induced during shear aremeasured or by applying the shearing load sufficientlyslow to result in fully drained conditions within the testspecimen.

(Courtesy of J. K. Mitchell and W. S. Gardner, "In SituMeasurement of Volume Change Characteristics, "Geotechnical Engineering Division Specialty Conference onIn Situ Measurement of Soil Properties, 1975. NorthCarolina State University, Raleigh, N. C. Reprinted bypermission of American Society of Civil Engineers, NewYork.)

Figure 3-13. Guide to collapsibility, compressibility, and expansion based on in situ dry density and liquid limit.

3-16

TM 5-818-1 / AFM 88-3, Chap. 7

Figure 3-14. Shear test apparatus and shearing resistance of soils.

3-17

TM 5-818-1-1 / AFM 88-3, Chap. 7

b. Undrained shear strength-cohesive soils.Approximate undrained shear strengths of fine-grainedcohesive soils can be rapidly determined on undisturbedsamples and occasionally on reasonably intact samplesfrom drive sampling, using simple devices such as thepocket penetrometer, laboratory vane shear device, orthe miniature vane shear device (Torvane). To establishthe reliability of these tests, it is desirable to correlatethem with unconfined compression tests. Unconfinedcompression tests are widely used because they aresomewhat simpler than Q triaxial compression tests, buttest results may scatter broadly. A more desirable test isa single Q triaxial compression test with the chamberpressure equal to the total in situ stress. Unconfinedcompression tests are appropriate primarily for testingsaturated clays that are not jointed or slickensided. The

Q triaxial compression test is commonly performed onfoundation clays since the in situ undrained shearstrength generally controls the allowable bearingcapacity. Sufficient unconfined compression and/or Qtests should be performed to establish a detailed profileof undrained shear strength with depth. Undrainedstrengths may also be estimated from the standardpenetration test, cone penetrometer soundings, and fieldvane tests, as discussed in chapter 4. For importantstructures, the effects of loading or unloading on theundrained shear strength should be determined by R(consolidated-undrained) triaxial compression tests onrepresentative samples of each stratum.

c. Strength parameters, cohesive soils. Theundrained shear strength of saturated clays can beexpressed as

(Courtesy of W. N. Houston and J. K. Mitchell, "PropertyInterrelationships in Sensitive Clays ," Journal, SoilMechanics and Foundations Division, Vol 95, No. SM4.1969, pp 1037-1062. Reprinted by permission ofAmerican Society of Civil Engineers. New York.)

Figure 3-15. Remoltded shear strength versus liquidity index relationships for different clays.

3-18

TM 5-818-1 / AFM 88-3, Chap. 7

(Courtesy of C C. Ladd and R. Foott, "New Design Procedure for Stability of Soft Clays , " Journal,Geotechnical Engineering Division , Vol 100, No. GT7, 1974, pp 763-786. Reprinted by permission

of American Society of Civil Engineers , New York.)

Figure 3-16. Normalized variation of su/p0 ratio for overconsolidated clay.

3-19

TM 5-818-1 / AFM 88-3, Chap. 7

su = cu φu = 0 (3-11)

su = 1 (σ1 - σ3) = qu (3-12)2 2

and is essentially independent of total normal stress.The undrained cohesion intercept of the Mohr-Cou-lombfailure envelope is cu.

(1) The undrained shear strength, s, , ofnormally consolidated cohesive soils is proportional tothe effective overburden pressure, po. An approximatecorrelation is as follows:

su = 0.11 + 0.0037 PI (3-13)p0

(2) A correlation between the remolded,undrained shear strength of clays and the liquidity indexis shown in figure 3-15.

(3) A correlation between the normalizedsu/p0 ratio of overconsolidated soils and theoverconsolidation ratio (OCR) is presented in figure 3-16.The value of p0 in (su/p0)OC is the effective presentoverburden pressure. Values of (su/p0) may beestimated from this figure when (su/po)NC and the OCRare known (NC signifies normally consolidated soils).

d. Sensitivity, cohesive soils. The sensitivityof a clay soil, St, is defined as follows:

St = Undisturbed compressive strengthRemolded compressive strength

(at same water content)

Terms descriptive of sensitivity are listed in table 3-5.Generalized relationships among sensitivity, liquidityindex, and effective overburden pressure are shown infigure 3-17. The preconsolidation pressure, rather thanthe effective overburden pressure, should be used foroverconsolidated soils when entering this figure.Cementation and aging cause higher values of sensitivitythan given in figure 3-17.

e. Effective strength parameters, cohesivesoils. As indicated in figure 3-14, the peak and residualstrengths may be shown as failure and postfailureenvelopes. Values of the peak drained friction angle fornormally consolidated clays may be estimated fromfigure 3-18. After reaching the peak shear strength,overconsolidated clays strain-soften to a residual valueof strength corresponding to the resistance to sliding onan established shear plane. Large displacements arenecessary to achieve this minimum ultimate strengthrequiring an annular shear apparatus or multiplereversals in the direct shear box. Typical values ofresidual angles of friction are shown in figure 3-19.

f. Shear strength, cohesionless soils.(1) In sandy soils, the cohesion is

negligible. Because of the relatively high permeability ofsands, the angle of internal friction is usually basedsolely on drained tests. The angle of internal friction ofsand is primarily affected by the density of the sand andnormally varies within the limits of about 28 to 46 de-

Table 3-5. Sensitivity of Clays

Sensitivity Descriptive Term0-1 Insensitive

1-2 Low sensitivity

2-4 Medium sensitivity

4-8 Sensitive

8-16 Extra sensitive

>16 Quick


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TM 5-818-1 / AFM 88-3, Chap. 7

grees (fig 3-3). Approximate values of φ are given asfollows:

φ = 30 + 0.15 DR for soils with less than 5percent fines(3-14)

φ = 25 + 0.15 DR for soils with more than 5percent fines(3-15)

Values of φ = 25 degrees for loose sands and φ = 35degrees for dense sands are conservative for mostcases of static loading. If higher values are used, theyshould be justified by results from consolidated- drainedtriaxial tests.

(2) Silt tends to be dilative or contractivedepending upon the consolidation stresses applied.

Thus, the back-pressure saturated, consolidated-undrained triaxial test with pore pressure measurementsis used. If the silt is dilative, the strength is determinedfrom the consolidated-drained shear test. The strengthdetermined from the consolidated-undrained test is usedif the silt is contractive. Typical values of the angle ofinternal friction from consolidated-drained testscommonly range from 27 to 30 degrees for silt and siltysands and from 30 to 35 degrees for loose and denseconditions. The consolidated-undrained tests yield 15 to25 degrees. The shear strength used for design shouldbe that obtained from the consolidated-drained tests.

(Courtesy of W. N. Houston and J. K. Mitchell, "PropertyInterrelationships in Sensitive Clays , " Journal, SoilMechan- ics and Foundations Division, Vol 95, No. SM4,1969, pp 1037-1062. Reprinted by permission ofAmerican Society of Civil Engineers, New York.)

Figure 3-17. General relationship between sensitivity, liquidity index, and effective overburden pressure.

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TM 5-818-1 / AFM 88-3, Chap. 7


Figure 3-18. Empirical correlation between friction angleand plasticity index from triaxial compression tests on

normally consolidated undisturbed clays.

3-8. Elastic properties (E, µ). The elastic modulusand Poisson’s ratio are often used in connection with theelasticity theory for estimating subsoil deformations.Both of these elastic properties vary nonlinearly withconfining pressure and shear stress. Typical valuesgiven below refer to moderate confining pressures andshear stresses corresponding to a factor of safety of 2 ormore.

a. In practical problems, stresses beforeloading are generally anisotropic. It is generallyconsidered that the modulus of elasticity is proportionalto the square root of the average initial principal stress,which may usually be taken as

σv’ 1 + 2K01/2 (3-16)

3

where K0 is the coefficient of at-rest earth pressure (para3-10) and δv’ is the effective vertical stress. Thisproportionality holds for 0.5 < K0 < 2, when workingstresses are less than one-half the peak strength.

b. The undrained modulus for normallyconsolidated clays may be related to the undrained shearstrength, su, by the expression

E = 250 to 500 (3-17)su

where su is determined from Q tests or field vane sheartests. The undrained modulus may also be estimatedfrom figure 3-20. Field moduli may be double thesevalues.

c. Poisson's ratio varies with strain and may

be as low as 0.1 to 0.2 at small strains, or more than 0.5.3-9. Modulus of subgrade reaction.

a. The modulus of subgrade reaction, ks, isthe ratio of load intensity to subgrade deformation, or:

ks = q (3-18)∆

whereq = intensity of soil pressure, pounds or

kips per square foot∆ = corresponding average settlement,

feetb. Values of k. may be obtained from general

order of decreasing accuracy:(1) Plate or pile load test (chaps 4 and 12).(2) Empirical equations (additional discussion

in chap 10).(3) Tabulated values (table 3-6).

3-10. Coefficient of at-rest earth pressure. Thestate of effective lateral stress in situ under at-restconditions can be expressed through the coefficient ofearth pressure at rest and the existing verticaloverburden pressure. This ratio is termed Ko and givenby the following:

Ko = σh’ (3-19)σv’

The coefficient of at-rest earth pressure applies for acondition of no lateral strain. Estimate values of K0 asfollows:

Normally consolidated soilSand:

K0 = 1 - sin φ‘ (3-20)Clay:

K0 = 0.95 - sin φ' (3-21)Figure 3-21 may be used for estimates of K0 for bothnormally consolidated and overconsolidated soils interms of PI. For overconsolidated soils, this figureapplies mainly for unloading conditions, and reloadingmay cause a large drop in K0 values. For soils thatdisplay high overconsolidation ratios as a result ofdesiccation, K0 will be overestimated by the relationshipshown in figure 3-21.

3-11. Properties of intact rock. The modulus ratioand uniaxial compressive strength of various intact rocksare shown in table 2 -7.

3-12. Properties of typical shales. Behavioralcharacteristics of shales are summarized in table 3-7;and physical properties of various shales, in table 3-8.Analyses of observed in situ behavior provide the mostreliable means for assessing and predicting the behaviorof shales.

( )

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TM 5-818-1 AFM 88-3, Chap. 7

Figure 3-19. Relation between residual friction angle and plasticity index.

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TM 5-818-1 / AFM 88-3, Chap. 7


Figure 3-20. Chart for estimating undrained modulus of clay.

Table 3-6. Values of Modulus of Subgrade Reaction (ks) for Footings as a Guide to Order of Magnitude

Soil Type Range of ks, kcfa

Loose sand 30-100

Medium sand 60-500

Dense sand 400-800

Clayey sand (medium) 200-500

Silty sand (medium) 150-300

Clayey soil

qu < 4 ksf 75-150

4 < qu < 8 150-300

8 < qu >300

a Local values may be higher or lower.


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TM 5-818-1 / AFM 88-3, Chap. 7

(Reproduced by permission of the National ResearchCouncil of Canadafrom the Canadian GeotechnicalJournal, Volume 2, pp 1-15, 1965.)

Figure 3-21. Coefficient of earthpressure at rest (K0) as a function of overconsolidated ratio and pasticity index.

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TM 5-818-1 / AFM 88-3, Chap. 7

Table 3- 7. An Engineering Evaluation of Shales

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TM 5-818-1 / AFM 88-3, Chap. 7

Table 3-8. Physical Properties of Various Shales

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TM 5-818-1 / AFM 88-3, Chap. 7

CHAPTER 4

FIELD EXPLORATIONS

4-1. Investigational programs. Fieldinvestigations can be divided into two major phases, asurface examination and a subsurface exploration:

Surface Examination Subsurface ExplorationDocumentary evidence PreliminaryField reconnaissance DetailedLocal experience

a. Documentary evidence. The logical andnecessary first step of any field investigation is thecompilation of all pertinent information on geological andsoil conditions at and in the vicinity of the site or sites

under consideration, including previous excavations,material storage, and buildings. Use table 4-1 as aguide to sources and types of documentation.

b. Field reconnaissance. A thorough visualexamination of the site and the surrounding area by thefoundation engineer is essential. This activity may becombined with a survey of local experience. The fieldreconnaissance should include an examination of thefollowing items as appropriate:

(1) Existing cuts (either natural or man-made). Railway and highway cuts, pipeline trenches, and

Table 4-1. Types and Sources of Documentary EvidenceTypes Sources Descriptions

Topographic, soil, drift Local, state, Federal, and uni- These maps provide information on lay(overburden), and bedrock versity geologic and agricul- of land, faulting (tectonics), andmaps tural organizations material types

Surface and subsurface U. S. Bureau of Mines and State Such data help locate subsurfacemining data, present and mining groups shafts and surface pits. The pres-past ence of cavities in the foundation

must be known. Current and even oldworkings may represent materialsources for construction. In addi-tion, surface pits near site may pro-vide opportunity to observe stratifi-cation of foundation and allow takingof disturbed samples

Aerial photographs, Conti- U. S. Government Printing Office Aerial photos offer a valuable means ofental U. S. establishing some insight into the

nature of foundation soils (2) andalso expedite familiarization withthe lay of the land

Aerial photographs, county U. S. Soil Conservation Service,and state areas local or district office

Local experience Technical Journals and published May include considerable boring data,reports; professional soci- test data, and descriptions ofeties, universities, and state problems in constructionagencies

Boring logs, water-well State Building Commission, City Some of these types of information canrecords, and construction Hall, County Court House, pri- usually be obtained for existingrecords vate concerns public buildings and facilities.

Private firms may cooperate in pro-viding limited data

Hydrological and tidal data State agencies, river boards, Flood history, groundwater levels, andU. S. Coast and Geodetic Sur- tidal data indicate protective meas-vey, and National Weather ures required during and after con-]Service struction.Any groundwater informa-

tion may aid in dewatering facilitiesand safe excavation slopes


TM 5-818-1 / AFM 88-3, Chap. 7

walls of river or stream valleys may reveal stratigraphyand offer opportunities to obtain general samples forbasic tests, such as Atterberg limits and grain-sizeanalysis for classification.

(2) Evidence of in situ soil performance. Astudy of landslide scars contributes greatly to the designof excavation slopes; it may indicate need for bracing orsuggest slope maintenance problems because ofgroundwater seepage. Evidence of general or localizedsubsidence suggests compressible subsoils, subsurfacecavities, or ongoing sink-hole formations as in areas oflimestone formations or abandoned mine cave-ins. Faultscarps or continuous cracks suggest bedrockmovements or mass soil movements.

(3) Existing structures. Carefulobservation of damage to existing structures, such ascracks in buildings (or poor roof alignment), misalignedpower lines, pavement conditions, corrosion on pipelines,or exposed metal and/or wood at water lines, maysuggest foundation problems to be encountered oravoided.

(4) Groundwater. The extent ofconstruction dewatering may be anticipated from factorssuch as the general water level in streams, spring lines,marshy ground, and variations in vegetal growth. Theeffects of lowering the water table during dewatering onsurrounding structures, as well as potentialenvironmental effects, should be appraised in apreliminary manner. Drainage problems likely to beencountered as a result of topography, confined workingspace, or increased runoff onto adjacent property shouldbe noted.

(5) Availability of construction materials.The availability of local construction material and water isa major economic factor in foundation type and design.Possible borrow areas, quarries and commercialmaterial sources, and availability of water should benoted.

(6) Site access. Access to the site fordrilling and construction equipment should be appraised,including the effects of climate during the constructionseason.

(7) Field investigation records.Considering the value and possible complexity of a fieldinvestigation, a well-kept set of notes is a necessity. Acamera should be used to supplement notes and toenable a better recall and/or information transfer todesign personnel.

c. Local experience. Special attention shouldbe given to the knowledge of inhabitants of the area.Farmers are generally well informed about seasonalchanges in soil conditions, groundwater, and streamflood frequencies. Owners of adjacent properties maybe able to locate filled areas where old ponds, lakes, orwells have been filled, or where foundation ofdemolished structures are buried.

d. Preliminary subsurface exploration. The

purpose of preliminary subsurface explorations is toobtain approximate soil profiles and representativesamples from principal strata or to determine bedrock orstratigraphic profiles by indirect methods. Auger orsplitspoon borings are commonly used for obtainingrepresentatives samples. Geophysical methods togetherwith one to several borings are often used in preliminaryexploration of sites for large projects, as they are rapidand relatively cheap. Procedures for geophysicalexploration are described in standard textbooks ongeotechnical engineering. Borings are necessary toestablish and verify correlations with geophysical data.Preliminary reconnaissance explorations furnish data forplanning detailed and special exploration of sites forlarge and important projects. The preliminary explorationmay be sufficient for some construction purposes, suchas excavation or borrow materials. It may be adequatealso for foundation design of small warehouses,residential buildings, and retaining walls located inlocalities where soil properties have been reasonablywell established as summarized in empirical rules of thelocal building code.

e. Detailed subsurface explorations. Forimportant construction, complex subsurface conditions,and cases where preliminary subsurface explorationsprovide insufficient data for design, more detailedinvestigations are necessary. The purpose is to obtaindetailed geologic profiles, undisturbed samples andcores for laboratory testing, or larger and fairlycontinuous representative samples of possibleconstruction materials. Test pits and trenches can beused to depths of 15 to 25 feet by using front-endloaders or backhoes at a cost that may comparefavorably with other methods, such as auger borings.Test pits allow visual inspection of foundation soils; also,high-quality undisturbed block samples may be obtained.Continuous (2 1/2 - to 5-foot intervals) sampling bymeans of opendrive, piston, or core-boring samplers isused for deeper explorations. Penetration, sounding orin situ tests, such as vane shear, or pressuremeter testsmay be conducted depending on sampling difficulty ordesired information.4-2. Soil boring program.

a. Location and spacing. Borings spaced in arigid pattern often do not disclose unfavorablesubsurface conditions; therefore, boring locations shouldbe selected to define geological units and subsurfacenonconformities. Borings may have to be spaced at 40feet or less when erratic subsurface conditions areencountered, in order to delineate lenses, boulders,bedrock irregularities, etc. When localized buildingfoundation areas are explored, initial borings should belocated near building corners, but locations should allowsome final shifting on the site. The number of boringsshould never be less than three and preferably five-oneat each corner and one at the center, unless subsurfaceconditions are known to be uniform and the

4-2

TM 5-818-1 / AFM 88-3, Chap. 7

foundation area is small. These preliminary boringsmust be supplemented by intermediate borings asrequired by the extent of the area, location of criticalloaded areas, subsurface conditions, and local practice.

b. Depth of exploration. The required depth ofexploration may be only 5 to 10 feet below grade forresidential construction and lightly loaded warehousesand office buildings, provided highly compressible soilsare known to not occur at greater depths. For importantor heavily loaded foundations, borings must extend intostrata of adequate bearing capacity and should penetrateall soft or loose deposits even if over- lain by strata ofstiff or dense soils. The borings should be of sufficientdepth to establish if groundwater will affect construction,cause uplift, or decrease bearing capacity. Whenpumping quantities must be estimated, at least twoborings should extend to a depth that will define theaquifer depth and thickness. Borings may generally bestopped when rock is encountered or after a penetrationof 5 to 20 feet into strata of exceptional stiffness. Toassure that boulders are not mistaken for bedrock, rockcoring for 5 to 10 feet is required. When an importantstructure is to be founded on rock, core boring shouldpenetrate the rock sufficiently to determine its quality andcharacter and the depth and thickness of the weatheredzone. Rock coring is expensive and slow, and theminimum size standard core diameter should be usedthat will provide good cores. NX or larger core sizes maybe required in some rock strata. Core barrels canremove cores in standard 5-, 10-, and 20-foot lengths(actual core may be much fractured, however; see para

2-6). Detailed exploration should be carried to a depththat encompasses all soil strata likely to be significantlyaffected by structure loading. If the structure is notfounded on piles, the significant depth is about 1 1/2 to 2times the width of the loaded area. An estimate of therequired depth can be made using the stress influencecharts in chapter 5 to find the depth such that

∆q <= 0.1q0 (4-1)where ∆q represents an increase in strata stress and q0

is the foundation contact pressure. Note that in the caseof a pile foundation, stresses are produced in the groundto an appreciable depth below the tips of the piles.Procedures to obtain ∆q apply as for other foundations.This depth criterion may not be adequate for complexand variable subsurface conditions.

c. Plugging borings. All borings should becarefully plugged with noncontaminating material if-

(1) Artesian water is present or will bewhen the excavation is made.

(2) Necessary to avoid pollution of theaquifer from surface infiltration, leaching, etc.

(3) Necessary to preserve a perchedwater table (avoid bottom drainage through borehole).

(4) Area is adjacent to stream or riverwhere flood stage may create artesian pressure throughthe borehole.

d. Sample requirements. Table 4-2 may beused as a guide for required sizes of undisturbedsamples, and table 4-3 for general samples. Thesampling program

Table 4-2. Recommended Undisturbed Sample Diameters

Test Minimum Sample Diameter, in.

Unit weight 3.0

Permeability 3.0

Consolidation 5.0

Triaxial compressiona 5.0

Unconfined compression 3.0

Direct shear 5.0

a Triaxial test specimens are prepared by cutting a short section of 5-in.-diam sample axially into fourquadrants and trimming each quadrant to the proper size. Three quadrants provide for three testsrepresenting the same depth; the fourth quadrant is preserved for a check test.


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TM 5-818-1 / AFM 88-3, Chap. 7

may depend on drilling equipment available andlaboratory facilities where tests will be performed.

(1) Undisturbed samples. Any method oftaking and removing a sample results in a stress change,possible pore water change, and some structurealteration because of displacement effects of thesampler. Careful attention to details and use of properequipment can reduce disturbance to a tolerable amount.Sample disturbance is related to the area ratio A, ,defined as follows:

Ar = D02 - D1

2 X 100 percent (4-2)D1

2

whereD0 = outside diameter of sampler tubeD1 = internal diameter of the cutting shoe

through which the sample passes (commonly the cuttingedge is swedged to a lesser diameter than the insidetube wall thickness to reduce friction)

The area ratio should be less than 10 percent forundisturbed sampling. Undisturbed samples arecommonly taken by thin-wall seamless steel tubing from2 to 3 inches in diameter and lengths from 2 to 4 feet.Undisturbed samples for shear, triaxial, andconsolidation testing are commonly 3 inches in diameter,but 5-inch-diameter samples are much preferred. Anindication of sample quality is the recovery ratio, Lr ,defined as follows:

Lr = Length of recovered sample (4-3)Length sample tube pushed

A value for Lr < 1 indicates that the sample wascompressed or lost during recovery, and Lr > 1 indicatesthat the sample expanded during recovery or the excesssoil was forced into the sampler.

(2) Representative samples. Samples can beobtained by means of auger or drive-sampling methods.Thick-wall, solid, or split-barrel drive samplers can beused for all but gravelly soils. Samples taken with a

Table 4-3. Recommended Minimum Quantity of Material for General Sample Laboratory Testing

Minimum Sample RequiredTest lb (Dry Weight)a

Water content 0.5Atterberg limits 0.2Shrinkage limits 0.5Specific gravity 0.2Grain-size analysis 0.5Standard compaction 30.0Permeability 2.0Direct shear 2.04-in.-diam consolidation 2.01.4-in.-diam triaxial (4 points) 2.02.8-in.-diam triaxial (4 points) 8.06-, 12-, or 15-in.-diam Discuss with laboratory

triaxial (4 Points)Vibrated density Discuss with laboratory

a Fine grained (all minus No. 4 sieve). For material containing plus No. 4 sieve sizes, the sampling requirements shouldbe discussed with the laboratory. In the final analysis, it is the responsibility of the engineer requesting the tests toensure that adequate size samples are obtained. Close coordination with the testing laboratories is essential.


4-4

TM 5-818-1 / AFM 88-3, Chap.7

drive sampler should be not less than 2 inches, andpreferably 3 inches or more in diameter. Where loosesands or soft silts are encountered, a special samplerwith a flap valve or a plunger is usually required to holdthe material in the barrel. A bailer can be used to obtainsands and gravel samples from below the water table.Split-spoon samples should be used to obtainrepresentative samples in all cases where piles are to bedriven or the density of cohesionless materials must beestimated.

4-3. Field measurements of relative density andconsistency.

a. Standard Penetration Test (SPT). This testis of practical importance as it provides a roughapproximation of the relative density or consistency offoundation soils and should always be made when pilesare to be driven. The split spoon is usually driven a totalof 18 inches; the penetration resistance is based on thelast 12 inches-the first 6 inches being to seat the samplerin undisturbed soil at the bottom of the boring. "Refusal"is usually taken at a blow count of 50 per 6 inches.(Commercial firms will usually charge an increased priceper foot of boring when the blow count (N-value) rangesfrom greater than 50 to 60 blows per foot of penetrationdue both to reduced daily footage of drilling and wear ofequipment.) An approximate correlation of results with

density for cohesionless soils is shown in figure 4-1, andwith φ in figure 4-2; but φ values above 35 degreesshould not be used for design on the basis of thesecorrelations. There is no unique relationship between N-values and relative density (DR) that is valid for allsands. The SPT data should be correlated with tests onundisturbed samples on large projects.

b. Cone penetration tests. In this test, a cone-shaped penetrometer is pushed into the soil at a slowconstant rate; the pressure required to advance the coneis termed the penetration resistance. The Dutch cone isthe most popular. The penetration resistance had beencorrelated with relative density of sands and undrainedshear strength of clays.

c. Vane tests. The in situ shear strength ofsoft to medium clays can be measured by pushing asmall four-blade vane, attached to the end of a rod, intothe soil and measuring the maximum torque necessaryto start rotation (shearing of a cylinder of soil ofapproximately the dimensions of the vane blades). Theundrained shear strength, su, is computed from thistorque, T, as follows:

T = suΠ d2h + d3 (4-4)2 4

(Courtesy of the American Society of Civil Engineers . "TaskCommittee for Foundation Design Manual of the Committee onShallow Foundations " Journal, Soil Mechanics andFoundations Division, No. SM6. 1972.)

Figure 4-1. Relative density of sand from the standard penetration test.

( )

4-5

TM 5-818-1 /AFM 88-3, Chap. 7

whered = diameter of vanesh = height of vaneϕ = 2/3 for uniform end-shear (usual

assumption) distribution= 3/5 for parabolic end-shear

distribution= 1/2 for triangular end-shear

distributionThe vane shear is best adapted to normally consolidated,sensitive clays having an undrained shear strength ofless than 500 pounds per square foot. The device is notsuitable for use in soils containing sand layers, manypebbles, or fibrous organic material. Vane tests shouldbe correlated with unconfined compression tests beforethey are used extensively in any area. Strength valuesmeasured using field vane shear tests should becorrected for the effects of anisotropy and strain rateusing Bjerrum’s correction factor, λ, shown in figure 4-3.This value represents an average and should bemultiplied by 0.8 to obtain a lower limit. The correction isbased upon field failures.

d. Borehole pressuremeter test. Apressuremeter can be used to obtain the in situ shearmodulus and/or K0. Several versions of the device existincluding self-boring equipment, which tends to avoid theloss of Ko conditions caused by soil relaxation when a

hole is pre-drilled and then the device is inserted. Themethod is subject to wide interpretation and should notnormally be employed in conventional investigations.

4-4. Boring logs. The results of the boring programshall be shown in terms of graphic logs of boring. Thelogs of borings shall be prepared in accordance withgovernmental standards. A typical log of boring is shownin figure 4-4.

4-5. Groundwater observations. In many types ofconstruction it is necessary to know the position of thegroundwater level, its seasonal variations, how it isaffected by tides, adjacent rivers or canals, or the waterpressures in pervious strata at various depths. Possiblefuture changes in groundwater conditions, such as thoseresulting from irrigation or reservoir construction, shouldbe anticipated.

a. Boreholes. With many fine-grained soils itmay be necessary to wait for long time periods beforewater table equilibrium is reached in boreholes.Observations made in a borehole during or shortly afterdrilling may be misleading. Even with pervious soil, awater level

(Courtesy of J. H. Schmertmann, “The Measurement of In SituShear Strength, " Proceedings, Conference on In SituMeasurement of Soil Properties. Raleigh. N. C., Vol 2, 1975,pp 57-180. Reprinted by permission of the American Societyof Civil Engineers. New York.)

Figure 4-2. Rough correlation between effective friction angle, standard blow count, and effective overburden pressure.

4-6

TM 5-818-1 / AFM 88-3, Chap. 7

reading should be taken 24 hours or more after drilling isstopped. Water level readings obtained in drill holesshould be shown on the boring log with the date of thereading and the date when the drill hole was made.

b. Piezometers. Piezometers provide anaccurate means for determining the groundwater levelover a period of time. In pervious strata, a temporarypiezometer may consist of a section of riser pipe, theopen bottom end of which is placed in a bag (filter) ofcoarse sand or gravel. The annular space between thepiezometer riser pipe and the drill hole immediatelyabove the stratum in which the water level is to be

determined should be sealed off with well-tamped clay orcement, or chemical grout. In granular soils where amore permanent system is desired, a 2-foot section ofwell-point screen can be attached to the bottom of thepipe. A well-point screen should be selected that willprevent entrance of foundation materials into the screen,or else a suitable filter material should be used. For allpiezometers, seal the top several feet below groundsurface around the riser pipe to prevent infiltration ofsurface water. In granular soils, the riser pipe is normallyabout 1 1/4-inches inside diameter and generally madeof plastic. In cohesive soils, a Casa-

(Courtesy of L Bjerrum, "Embankment on Soft Ground, "Proceedings, Conference on Performance of Earth and Earth-Supported Structures. Purdue University. Lafayette, Ind.,Vol2, 1975. Reprinted by permission of the American Societyof Civil Engineers, New York.)

Figure 4-3. Correction factor for vane strength.

4-7

TM 5-818-1 / AFM 88-3, Chap. 7

Figure 4-4. Typical log of boring.U. S. Army Corps of Engineers

4-8

TM 5-818-1 / AFM 88-3, Chap. 7

grande-type piezometer (fig 4-5) is recommended. Thewater level in the piezometer is determined by means ofa plumb line or sounding device. If the piezometric levelis above ground surface, a manometer or a Bourdongage can be connected to the riser pipe to greatlydecrease the time for equilibrium to be achieved. Ifrapidly changing pore water pressures in clay are to bedetermined, use closed system piezometers.

c. Field pumping tests. Where accurateknowledge of the permeability of the foundation soils isnecessary, field pumping tests offer the most reliablemeans. A rough estimate of the average permeability ofthe material around the bottom of a cased drill hole maybe obtained by lowering or raising the water level in thecasing and observing the rise and fall of the water levelas a function of time with respect to the stabilizedpiezometric water level (TM 5-818-5/AFM 88-5, Chapter6, para 38). A field pumping test is best performed bypumping from a well in which a constant flow ismaintained until the drawdown has stabilized, and whengroundwater levels are measured at several remoteborings or piezometers. It is desirable that well screensfully penetrate the strata for which the permeability is tobe measured. Formulas for computing the overallpermeability of a pervious stratum exhibiting gravity orartesian flow are shown in figure 3-5. The formula forthe special case of a fully penetrating well and artesianaquifer is given as an example in figure 4-6. Methods for

performing field pumping tests are described in TM 5-818-5/AFM 88-5, Chapter 6.

4-6. In situ load tests. Load tests are commonlymade on test piles to confirm design capacity and mayoccasionally be used to determine bearing capacity andsettlement characteristics. In general, specializedequipment and procedures are required to perform loadtests and considerable judgment and expertise must beemployed to interpret results. Plate load tests areoccasionally used for bearing capacity determinations.

4-7. Geophysical exploration. Geophysicalmethods of subsurface exploration are well suited forlarge sites due to the increasing cost of borings. Table4-4 summarizes those geophysical methods mostappropriate for site exploration. These methods areuseful for interpolation between borings. Geophysicaldata must be used in conjunction with borings andinterpreted by qualified experienced personnel, ormisleading information is almost certain to result. Thetwo most applicable geophysical methods for exploringfoundations currently in use are seismic refraction andelectrical resistivity. Information secured by seismicrefraction is primarily depth to bedrock and delineation ofinterfaces between zones of differing velocities. Anelectrical resistivity survey is superior in differ-


Figure 4-5. Typical details of Casagrande piezometer and piezometer using uwell screen.

4-9

TM 5-818-1 / AFM 88-3, Chap. 7

NOTE. WELL SHOULD BE PUMPED AT A CONSTANTRATE OF FLOW UNTIL THE RATE OF DRAWDOWNIN WELL AND PIEZOMETERS IS ESSENTIALLYCONSTANT.

DRAWDOWN AT EQUILIBRIUM VERSUSDISTANCE FROM WELL PLOTS AS A STRAIGHTLINE ON SEMI- LOGARITHMIC PLOT.

Figure 4-6. Determination of permeability from field pumping test on a fully penetrating well in an artesian aquifer.

4-10

TM 5-818-1 / AFM 88-3, Chap. 7

Table 4-4. Surface Geophysical Methods

Name of Method

Seismic methods:

Refraction

Continuous vibration

Electrical methods:

Resistivity

Drop in potential

Acoustic method

Procedure or Principle Utilized

Based on time required for seismic waves totravel from source of blast to point on groundsurface, as measured by geophones spaced atintervals on a line at the surface. Refraction ofseismic waves at the interface betweendifferent strata gives a pattern of arrival timesvs distance at a line of geophones.

The travel time of transverse or shear wavesgenerated by a mechanical vibrator consistingof a pair of eccentrically weighted disks isrecorded by seismic detectors placed atspecific distances from the vibrator.

Based on the difference in electricalconductivity or resistivity of strata. Resistivity ofsubsoils at various depths is determined bymeasuring the potential drop and currentflowing between two current and two potentialelectrodes from a battery source. Rerirstivityirs correlated to material type.

Based on the determination of the ratio ofpotential drops between three potentialelectrodes as a function of the current imposedon two current electrodes.

The time of travel of sound waves reflectedfrom the mud line beneath a body of water and

a lower rock surface is computed bypredetermining the velocity of sound in the

various media.

Applicability

Utilized to determine depth to rock or otherlower stratum substantially different in wavevelocity than the overlying material. Used onlywhere wave velocity in successive layersbecomes greater with depth. Used to determinerock type, rock and soil stratification, depth ofweathered zone, etc.

Velocity of wave travel end natural period ofvibration gives some indication of soil type.Travel time plotted as a function of distanceindicates depths or thicknesses of surfacestrata. Useful in determining dynamic modulusof subgrade reaction and obtaining informationon the natural period of vibration for design offoundations of vibrating structures.

Used to determine horizontal extent and depthof subsurface strata. Principal applications forinvestigating foundations of dams and otherlarge structures, particularly in exploringgranular river channel deposits or bedrocksurfaces, sources of construction material,potential infiltration and seepage zones, and incavity detection.

Similar to resistivity methods but gives sharperindication of vertical or steeply inclinedboundaries and more accurate depthdeterminations. More susceptible thanresistivity method to surface interference andminor irregularities in surface soils.

Currently used in shallow underwaterexploration to determine position of mud lineand depth to hard stratum underlying mud.Method has been used in water depths greaterthan 100 feet with penetrations of 850 feet tobedrock. Excellent display of subsurfacestratification. Used most efficiently in waterdepths up to 50 feet with penetrations ofadditional 350 feet to bedrock.

Note: Instrumentation for methods listed above currently available at WES. May be furnished as a service to Districtsand Divisions on request. Table adapted from "Design Manual, Soil Mechanics, Foundations, and EarthStructures, " Department of the Navy. Bureau of Yards and Docks.

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TM 5-818-1 / AFM 88-3, Chap. 7

entiating between sand and clays. Both methods requiredistinct differences in properties of foundation stratamaterials to be effective. The resistivity method requiresa high-resistivity contrast between materials beinglocated. The seismic method requires that the contrastin wave transmission velocities be high and that anyunderlying stratum transmit waves at a higher velocity(more dense) than the overlying stratum. Somedifficulties arise in the use of the seismic method if thesurface terrain and/or layer interfaces are steeply slopingor irregular instead of relatively horizontal and smooth.

4-8. Borehole surveying.a. Downhole surveying devices can be used in

correlating subsurface soil and rock stratification and inproviding quantitative engineering parameters, such asporosity, density, water content, and moduli. Once aboring has been made, the cost of using these tools inthe borehole is relatively modest. Different devicescurrently in use are summarized in table 4-5.

b. These devices can allow cost savings to bemade in the exploration program without lessening thequality of the information obtained.

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TM 5-818-1 / AFM 88-3, Chap. 7

Table 4-5. Borehole Surveying Devices

Devicea Measurement Obtained Primary UseElectric logging

Spontaneous potential Natural voltages between Differentiating be-(SP) fluids in materials of tween sands and

dissimilar lithology clays

Single-point Resistance of rock adja- With SP log providesresistivity cent to hole good indication of

subsurface strati-fication and soiltype

Multiple-point Resistivity of Determination of mudresistivity formations infiltration and

effective porosityRadiation logging

Gamma Natural gamma radiation Identification ofof materials clay seams, loca-

tion of radioactivetracers, and withSP and resistivitylogs provides in-formation on rela-tive porosity

Neutron Hydrogen atom Determination ofconcentration moisture content

and porosity belowzone of saturationporosity

Gamma-gamma Gamma radiation Correlates with bulkabsorption density and useful

to determine poros-ity if grain spe-cific gravity isknown

Sonic loggingAcoustic Travel time of primary With caliper and den-

velocity and shear wave sity logs determinevelocities dynamic elastic and

shear moduli of insitu rock

(Continued)

a All devices currently available at WES. May be furnished as a service to Districts and Divisions on request.(Sheet 1 of 3)

4-13

TM 5-818-1 / AFM 88-3, Chap. 7

Table 4-5. Borehole Surveying Devices-Continued

Devicea Measurement Obtained Primary Use

Sonic logging(Continued)

Acoustic Reflected acoustic Locate fractures andImagery energy voids and strike

and dip of joints,faults, beddingplanes, etc.

Fluid loggingTemperature Temperature gradient Determine geothermal

in borehole gradient and defi-nition of aquifers

Fluid resistivity Electrical resistance of With temperature databorehole fluids the determination

of dissolvedsolids--locatezones of water lossor gain

Trace ejector Controlled ejection of Groundwater flowtrace elements patterns

Fluid sampler Borehole or formation Uncontaminated sam-fluid from predeter- ples for waterminded depths quality studies

Visual logging

Borescope Visual image of the Cheap, rapid examina-sidewalls of a bore- tion of boreholehole to depths of 100 wallsft or less--a peri-scopic instrument

Borehole camera Photograph of a 360 deg Examination of bore-sweep of the borehole hole conditions,wall taken approxi- bedding, joints,mately at right angles etc.to the wall. Expo-sures timed so thatslight overlap of eachphotograph is obtained


4-14

TM 5-818-1 / AFM 88-3. Chap. 7

Table 4-5. Borehoe Surveying Devices-Continued

Devicea Measurement Obtained Primary Use

Visual logging(Continued)

Downhole camera Photograph of the side- Examination of bore-walls of the borehole hole conditions,taken from the bottom bedding, joints,of the camera.Several etc.feet of hole below de-vice taken with eachexposure

Borehole television Image of the sidewalls Rapid examination ofof the borehole dis- borehole conditionsplayed at the time of as the device isexposure on a surface lowered or raisedmonitor

Miscellaneous loggingCaliper Borehole size Washouts, fractures,

etc., needed forinterpretation ofother logs

Borehole surveyor Downhole directional Precise location andsurvey attitude of fea-

tures recorded onother logging tools

(Sheet 3 of 3)


4-15

TM 5-818-1 / AFM 88-3, Chap. 7

CHAPTER 5

SETTLEMENT ANALYSES

5-1. Settlement problems.a. Significant aspects of the settlement of

structures are total settlement-magnitude of downwardmovements- and differential settlement-difference insettlements at different locations in the structure. Table5-1 lists conditions that cause settlements which occurduring construction and result in only minor problemsand postconstruction settlements which occur after astructure is completed or after critical features arecompleted. Differential settlements distort a structure. Astructure can generally tolerate large uniform, or nearlyuniform, settlements.

b. Differential settlement can have a numberof undesirable results:

(1) Tilting is unsightly. A tilt of 1/250 canbe distinguished by the unaided eye.

(2) Moderate differential settlementcauses cracking and architectural damage. Withincreasing differential settlement, doors and windowsmay become distorted and not open and close properly.Larger differential settlements may cause floors andstairways to become uneven and treacherous andwindows to shatter. At this point, the usefulness of thebuilding has been seriously impaired.

(3) Severe differential settlements mayimpair structural integrity and make structuressusceptible to collapse during an earthquake or othermajor vibration.

(4) If a structure settles relative to thesurrounding ground, or the ground settles relative to thestructure, entryways may be disrupted, and utility linesmay be damaged where they enter the structure.

c. Even if settlements are uniform or nearlyso, large total settlements can also result in problems:

(1) Sites located near a river, lake, orocean may flood during periods of high water.

(2) Surface drainage may be disrupted. Ifwater ponds around and beneath structures, they maybecome inaccessible and subject to mildew and woodrot.

d. Experience provides a basis for estimatingthe magnitudes of differential settlement that causecracking of architectural finishes, such as plaster,stucco, and brick facing. Differential settlement is bestexpressed as the angular distortion in radians betweentwo points. Angular distortion, which alwaysaccompanies settlement of a building, is determined bythe uniformity of foundation soils, the stiffness of the

structure and its foundation, and the distribution of loadwithin the building. In conventional settlement analysesof the type described in this manual, the stiffness of thebuilding and foundation are not considered. Tolerableangular distortions are listed in table 5-2, and empiricalcorrelations that may be used to estimate probableangular distortions based on calculated maximumsettlements are summarized in table 5-3. Because ofthe natural variability of soils, differential settlement willoccur though total settlements are calculated to beuniform. An indirect means for controlling differentialsettlement is to limit total settlement to 3 inches forstructures on clay and to 1 1/2 inches for structures onsand.

5-2. Loads causing settlement.a. Loads causing settlement always include

the estimated dead load and a portion or all of the liveload. For office buildings, about 50 percent of theestimated building live load may be assumed to causesettlement. For heavily loaded warehouses and similarstructures, the full live load should be used.

b. For many purposes, settlements need becomputed only for the maximum dead load plussettlement-causing live load. Occasionally, settlementsoccurring during a part of the construction period mustbe computed. This may require additional stresscomputations for partial loading conditions.

c. Loads that are less than thepreconsolidation stress cause minor settlementsbecause only recompression of soil occurs. Theincrement of loading that exceeds the preconsolidationstress causes relatively large settlements and occursalong the virgin compression portion of laboratoryconsolidation curves. A careful estimate ofpreconsolidation stresses is essential for settlementanalyses. Means for estimating such stresses are givenin chapter 3.

5-3. Stress computations.a. One of the first steps in a settlement

analysis is computation of effective overburden stressesin the soil before and after loading. The initial stress p1’at any depth is equal to the effective weight of overlyingsoils and may be determined by multiplying the effectiveunit weight of the soil by its thickness. It is customary toconstruct a load-depth diagram by plotting

5-1

TM 5-818-1 / AFM 88-3, Chap. 7

Table 5-1. Causes of Settlement

Cause CommentCompression of foundation Soft, normally consolidated clays and

soils under static loads. peaty soils are most compressible.Loose silts, sands, and gravels arealso quite compressible.

Compression of soft clays Increased effective stress causesdue to lowering ground- settlement with no increase in sur-water table. face load.

Compression of cohesionless Loose sands and gravels are most sus-soils due to vibrations. ceptible.Settlement can be caused by

machine vibrations, earthquakes, andblasts.

Compression of foundation Loose silty sands and gravels are mostsoil due to wetting. susceptible.Settlements can be

caused by rise in groundwater tableor by infiltration.

Shrinkage of cohesive soils Highly plastic clays are most suscept-caused by drying. ible.Increase in temperature under

buildings containing ovens or fur-naces may accelerate drying. Wettingof highly plastic clays can causeswelling and heave of foundations.

Loss of foundation support Waterfront foundations must extend belowdue to erosion. maximum erosion depth.

Loss of foundation support Most pronounced in soft, saturated clays.due to excavation ofadjacent ground.

Loss of support due to Lateral shifting may result from land-lateral shifting of the slides, slow downhill creep, or move-adjacent ground ment of retaining structures.

Loss of support due to Soils overlying cavernous limestone andformation of sinkhole. broken conduits are susceptible.

Loss of support due to thaw- Permafrost should be insulated froming of permafrost.foundation heat.

Loss of support due to Loose, saturated sands are mostpartial or complete susceptible.liquefaction.

Downdrag on piles driven Loading on piles is increased by nega-through soft clay. tive skin friction if soil around

upper part of pile settles.


TM 5-818-1 / AFM 88-3, Chap. 7

stress versus depth, using average unit moist weights ofsoil above the water table and average unit submergedweights below the water table.

b. The final stress p at any depth is equal tothe effective overburden stress after the structure iscompleted plus the stress resulting from the structureload. If the structure is founded on individual footings,the final stress is the sum of stresses imposed by allfootings.

c. Foundation stresses caused by appliedloads are generally computed assuming the foundationto consist of an elastic, isotropic, homogeneous mass ofsemiinfinite extent, i.e., the Boussinesq case. Theincrement of stress at various depths is determined bymeans of influence values, such as shown in figures 5-1and 5-2, which give the vertical stress beneath arectangular area for uniform and triangular distributionsof load, respectively. Influence values for vertical

Table 5-2. Value of Angular Distortion (δ/ϑ) That Can Be Tolerated Without Cracking

AllowableType of Building L/H δ/ϑ

Steel frame with flexible siding -- 0.008

Steel or reinforced concrete frame -- 0.002 towith insensitive finish such as 0.003dry wall, glass, or moveablepanels

Steel or reinforced concrete frame > 5 0.002with brick, block, plaster, orstucco finish < 3 0.001

Load-bearing brick, tile, or > 5 0.0008concrete block walls < 3 0.0004

Circular steel tanks on flexible -- 0.008base, with fixed top

Circular steel tanks on flexible 0.002 tobase, with floating top 0.003

Tall slender structures, such as -- 0.002stacks, silos, and water tanks,with rigid mat foundations


5-3

TM 5-818-1 / AFM 88-3, Chap. 7

stress beneath a circular area are shown in figure 5-3. Ifthe foundation consists of a large number of individualfootings, influence charts based on the Boussinesq casewill greatly facilitate the computation of stresses.Programs for digital computers and programmablecalculators are also available.

d. A structure excavation reduces stress infoundation subsoils. The decrease in vertical stressescaused by the weight of excavated material is computedin the manner described in the previous paragraph. Thebottom of the excavation is used as a reference; verticalstresses produced by the weight of excavated materialare subtracted algebraically from the original overburdenpressure to compute final foundation stresses.

5-4. Settlement of foundations on clay.a. When a load is applied over a limited area

on clay, some settlement occurs immediately. Thisimmediate settlement, ∆Hi, , has two components: thatcaused by distortion or change of shape of the claybeneath the loaded area, and that caused by immediatevolume change in unsaturated soils. In saturated clays,there is little or no immediate volume change becausetime is required for water to drain from the clay.

b. Immediate settlements can be estimatedusing methods given in chapter 10. Values of undrainedmodulus determined from the slopes of stress-strain

curves from unconsolidated-undrained laboratorycompression tests are frequently only one-half or one-third as large as the in situ modulus. This difference isdue to disturbance effects, and the disparity may beeven more significant if the amount of disturbance isunusually large. The undrained modulus of the clay maybe estimated from figure 3-20. The values of the K inthis figure were determined from the field measurementsand, therefore, are considered to be unaffected bydisturbance. The value of Poisson’s ratio is equal to 0.5for saturated clays. For partly saturated clays, a value of0.3 can be assumed. Because immediate settlementsoccur as load is applied and are at least partially includedin results of laboratory consolidation tests, they are oftennot computed and only consolidation settlements areconsidered to affect a structure.

5-5. Consolidation settlement. Consolidationsettlement of cohesive soil is normally computed frompressure-void ratio relations from laboratoryconsolidation tests on representative samples. Typicalexamples of pressure-void ratio curves for insensitiveand sensitive, normally loaded clays, andpreconsolidated clays are shown in figure 3-7.Excavation results in a rebound of foundation soils andsubsequent recompression when structure loads areadded. This

Table 5-3. Empirical Correlations Between Maximum (∆) and Angular Distortion (δ/ϑ)

Approximate Valueof δ/ϑ fora

Type of Foundation ∆= 1 in.

Mats on sand 1/750(0.0013)

Rectangular mats on varved silt 1/1000 to 1/2000(0.001 to 0.0005)

Square mats on varved silt 1/2000 to 1/3000(0.0005 to 0.0003)

Mats on clay 1/1250(0.0008)

Spread footings on sand 1/600(0.0017)

Spread footings on varved silt 1/600(0.0017)

Spread footings on clay 1/1000(0.0010)a δ/ϑ increases roughly in proportion with ∆. For ∆ = 2 in., values of δ/ϑ would be about

twice as large as shown, for ∆ = 3 inches, three times as large, etc.

(Courtesy ofJ. P. Gould and J. D. Parsons, "Long - Term Performance of Tall Buildings of New York CityVaried Silts, " Proceedings, International Conference on Planning and Design of Tall Buildings , LehighUniversity, Bethlehem, Pa., 1975. Reprinted by permission of American Society of Civil Engineers, NewYork.)

5-4

TM 5-818-1 / AFM 88-3, Chap. 7

sequence should be simulated in consolidation tests byloading the specimen to the existing overburdenpressure po, unloading to the estimated stress afterexcavation pexc, and reloading the specimen to define thep-e curve at loads in excess of overburden and

preconsolidation stresses. Curves designated Ku infigure 3-7 are laboratory p-e curves. Soil disturbanceduring sampling affects laboratory p-e curves so that itusually


Figure 5-1. Vertical stress beneath a uniformly loaded rectangular area.

5-5

TM 5-818-1 AFM 88-3, Chap. 7

Figure 5-2. Vertical stress beneath a triangular distribution of load on a rectangular area.

5-6

TM 5-818-1 AFM 88-3, Chap. 7

becomes necessary to construct a field p-e curvesimulating consolidation in the field. These constructionsare also shown in figure 3-7. They are based on theassumption that the straight lower branch of the field p-ecurve, K, intersects the laboratory curve at e = 0.4e0.Furthermore, the field curve must pass through point a,corresponding to the present overburden pressure andthe natural void ratio. The field compression index, Cc, istaken as the slope of the straight lower branch of K onthe semilogarithmic diagram.

a. Settlement computations. The totalsettlement, ∆H, of a foundation stratum is computedaccording to the following formula:

∆H = e1 - e2 H (5-1)1 + e0

wheree1, e2 = initial and final void ratios,

respectively, from the fieldpressure-void curves,corresponding to the initial andfinal effective foundationpressures

eo = average initial void of the stratumH = total thickness of the compressible

stratum This formula also may beused to estimate foundationrebound due to excavation. Whenthe lower portion of

Figure 5-3. Influence value for vertical stress under a uniformly loaded circular area.

5-7

TM 5-818-1 / AFM 88-3, Chap. 7

the pressure void ratio curve is fairly straight, it may beconvenient to work with the compression index, in whichcase the formula for settlement is as follows:

∆H = Cc H log10 p2’ (5-2)1 + e0 p1’

An example of a settlement analysis in which therebound of the foundation and subsequentrecompression under the building load are determined isshown in figure 5-4 for a normally consolidatedfoundation.

b. Rate of settlement. The rate of settlementis determined by means of the theory of consolidation.This

U. S. Army Corps of engineersFigure 5-4. Example of settlement analysis.

5-8

TM 5-818-1 / AFM 88-3, Chap. 7

theory relates the degree of consolidation and timesubsequent to loading according to the followingexpression:

U(%) = f(T) (5-3)with

T = cv t (5-4)H2

whereU = degree of consolidation or ratio of

settlement that has occurred at a giventime to the ultimate settlement

T = dimensionless number called the timefactor that depends upon loading andboundary conditions

cv = property of the soil known as thecoefficient of consolidation

H = length of the drainage path, which inthe case of a specimen or stratumdraining from top and bottom would behalf the thickness of the specimen orstratum

t = time corresponding to U

The relation between time factor and percentconsolidation for various boundary conditions is shown infigure 5-5. If the values of cv and H are known for astratum of clay with given boundary conditions, thetheoretical curve can be replotted in the form of apercent consolidation-time curve; if the ultimatesettlement of the layer has been computed, the curvecan be further modified into a settlement-time curve. Inorder to compute, cv, it is necessary to transform thelaboratory time-consolidation curve for the loadincrement in question into the theoretical curve. Amethod for ad- justing the laboratory curve in order tocompute the


Figure 5-5. Time factors for various boundary conditions.

5-9

TM 5-818-1 / AFM 88-3, Chap. 7

coefficient of consolidation, cv, is shown in figure 3-7.Thus, the actual time required for the variouspercentages of consolidation to occur in the field can bedetermined by the following formula:

tf = T H2 (5-5)cv

wheretf = time for U(%) consolidation in the

field stratumH = length of the drainage path in the

fieldT = time factor corresponding to U(%)

consolidation

When settlement occurring during the constructionperiod may be of interest, the values of T and U alsocan be obtained from figure 5-5.

c. Secondary compression. For refinedestimates and special purposes, settlement resultingfrom secondary compression may have to be evaluated.The amount ∆Hs can be calculated as follows:

∆Hs = CαH log tsc (5-6)tp

wheretsc = tp + time interval during which

secondary compressionsettlement is to be calculated

tp = time to complete primaryconsolidation

H = total thickness of compressible soilOther terms have been previously defined. Secondarycompression settlements may be important whereprimary consolidation occurs rapidly, soils are highlyplastic or organic, and allowable settlements areunusually small.

5-6. Settlement of cohesionless soils. Thepermeability of cohesionless soils is usually sufficientlygreat that consolidation takes place during theconstruction period. For important projects, estimatesettlement using consolidation tests on undisturbedsamples or samples remolded at natural density.Alternately, settlements may be estimated from platebearing tests described in chapter 4. Design of footingson cohesionless soils, based on settlementconsiderations using the Standard Penetration Test, isdescribed in chapter 10.

5-7. Eliminating, reducing, or coping withsettlement. Design techniques for amelioratingsettlement problems are summarized in table 5-4.Differential settlements beneath existing structures canbe corrected by releveling by jacks, grouting (i.e., mudjacking) beneath slab foundations, or underpinning.These techniques are expensive, to varying degrees,and require specialists.

5-10

TM 5-818-1 / AFM 88-3, Chap. 7

Table 5-4. Methods of Eliminating, Reducing, or Coping With Settlements.

Method Comment

Use of piles, piers, or Differential settlements betweendeep footings. buildings and surrounding ground

can cause problems.

Excavate soft soil and Can be very costly if the com-replace with clean pressible layer extends togranular fill. large depth.

Displace soft soil with Difficult to control.Pockets ofweight of granular entrapped soft soil can causefill or by blasting. large differential settlements.

Reduce net load by Weight of one story building isexcavation. equal to weight of one or two

feet of soil.

Surcharge or preload Settlement is reduced by amountsite before which occurs before construc-construction. tion. Preload may be limited

by stability considerations.

Delay construction of Settlement which occurs beforebuildings to be built construction does not affecton fills. building.Fill settlement can

be accelerated using sanddrains.

Use a stiff foundation Can greatly reduce differentialwith deep grade beams. settlements.

Install leveling jacks Building can be releveledbetween the foundation periodically as foundationand the structure settles.

Select a building type Steel frames, metal siding, andwhich has a large tol- asphalt floors can withstanderance for differential large settlements and remainsettlement. serviceable.


5-11

TM 5-818-1 / AFM 88-3, Chap. 7

CHAPTER 6

BEARING-CAPACITY ANALYSIS

6-1. Bearing capacity of soils. Stressestransmitted by a foundation to underlying soils must notcause bearing-capacity failure or excessive foundationsettlement. The design bearing pressure equals theultimate bearing capacity divided by a suitable factor ofsafety. The ultimate bearing capacity is the loadingintensity that causes failure and lateral displacement offoundation materials and rapid settlement. The ultimatebearing capacity depends on the size and shape of theloaded area, the depth of the loaded area below theground surface, groundwater conditions, the type andstrength of foundation materials, and the manner inwhich the load is applied. Allowable bearing pressuresmay be estimated from table 6-1 on the basis of adescription of foundation materials. Bearing-capacityanalyses are summarized below.

6-2. Shear strength parameters.a. Appropriate analyses. Bearing-capacity

calculations assume that strength parameters forfoundation soils are accurately known within the depth ofinfluence of the footing. The depth is generally about 2to 4 times the footing width but is deeper if subsoils arehighly compressible.

(1) Cohesionless soils. Estimate φ’ fromthe Standard Penetration Test (table 4-5) or the conepenetration resistance. For conservative values, use φ’ =30 degrees.

(2) Cohesive soils. For a short-termanalysis, estimate su, from the Standard PenetrationTest (table 4-5) or the vane shear resistance. For long-term loadings, estimate φ’ from correlations with indexproperties for normally consolidated soils.

b. Detailed analyses.(1) Cohesionless soils. Determine φ’ from

drained (S) triaxial tests on undisturbed samples fromtest pits or borings.

(2) Cohesive soils. For a short-termanalysis, determine s, from Q triaxial tests onundisturbed samples with 03 equal to overburdenpressure. For a long-term analysis, obtain φ’ fromdrained direct shear (S) tests on undisturbed samples.For transient loadings after consolidation, obtain φ and cparameters from consolidated-undrained (R) triaxial testswith pore pressure measurements on undisturbedsamples. If the soil is dilative, the strength should bedetermined from drained S tests.

6-3. Methods of analysis.a. Shallow foundations.

(1) Groundwater level (GWL). Theultimate bearing capacity of shallow foundationssubjected to vertical, eccentric loads can be computedby means of theformulas shown in figure 6-1. For agroundwater level well below the bottom of the footing,use a moist unit soil weight in the equations given infigure 6-1. If the groundwater level is at ground surface,use a submerged unit soil weight in the equations.

(2) Intermediate groundwater levels.Where the groundwater level is neither at the surface norso deep as not to influence the ultimate bearing capacity,use graphs and equations given in figure 6-2.

(3) Eccentric or inclined footing loads. Inpractice, many structure foundations are subjected tohorizontal thrust and bending moment in addition tovertical loading. The effect of these loadings isaccounted for by substituting equivalent eccentric andlorinclined loads. Bearing capacity formulas for thiscondition are shown in figure 6-3. An example of themethod for computing the ultimate bearing capacity foran eccentric inclined load on a footing is shown in figure6-4.

(4) Loading combinations and safetyfactors. The ultimate bearing capacity should bedetermined for all combinations of simultaneousloadings. A distinction is made between normal andmaximum live load in bearing capacity computations.The normal live load is that part of the total live load thatacts on the foundation at least once a year; themaximum live load acts only during the simultaneousoccurrence of several exceptional events during thedesign life of the structure. A minimum factor of safety of2.0 to 3.0 is required for dead load plus normal live load,and 1.5 for dead load plus maximum live load. Safetyfactos selected should be based on the extent ofsubsurface investigations, reliability of estimatedloadings, and consequences of failure. Also, high safetyfactors should be selected if settlement estimates are notmade. In general, separate settlement analysis shouldbe made.

b. Deep foundations. Methods for computingthe ultimate bearing capacity of deep foundations aresummarized in figure 6-5. These analyses areapplicable to the design of deep piers and pilefoundations, as subsequently described. When the baseof the foundation is located below the ground surface ata depth greater

6-1

TM 5-818-1 / AFM 88-3. Chap. 7

Table 6-1. Estimates of Allowable Bearing Pressure


6-2

TM 5-818-1 / AFM 88-3, Chap. 7

Figure 6-1. Ultimate hearing capacity of shallow foundations under vertical, eccentric loads.

6-3

TM 5-818-1 / AFM 88-3, Chap. 7


Figure 6-2. Ultimate bearing capacity with groundwater effect.

6-4

TM 5-818-1 / AFM 88-3, Chap. 7


Figure 6-3. Ultimate bearing capacity of shallow foundations under eccentric inclined loads.

than the width of the foundation, the factor of safetyshould be applied to the net load (total weight ofstructure minus weight of displaced soil).

c. Stratified subsoils. Where subsoils arevariable with depth, the average shear strength within adepth below the base equal to the width of the loadedarea controls the bearing capacity, provided the strengthat a depth equal to the width of the loaded area or loweris not less than one-third the average shear strength ofthe upper layer; otherwise, the bearing capacity isgoverned by the weaker lower layer. For stratifiedcohesive soils, calculate the ultimate bearing capacityfrom the chart in figure 6-6. The bearing pressure on theweaker lower layer can be calculated by distributing thesurface load to the lower layer at an angle of 30 degreesto the vertical.

6-4. Tension forces. Footings subjected to asustained uplift force, Tu, should be designed with aminimum factor of safety of 1.5 with respect to weightforces resisting pullout expressed as

W > 1.5 (6-1)Tu

where W is the total effective weight of soil and concretelocated within the prism bounded by vertical lines at thebase of the footing. Use total unit weights above thewater table and the buoyant unit weight below. If theshear resistance on the vertical sides of the prismdefined above is considered, a minimum safety factor of2 should be used. The lateral earth pressure on thevertical sides of the prism should not exceed earthpressure at rest and should be considered as activeearth pressure if the soil is not well compacted.

6-5. Bearing capacity of rock. For a structurefounded on rock, adequate exploration is necessary todetermine the number and extent of defects, such asjoints, shear zones, and solution features. Estimates ofthe allowable bearing pressure can be obtained fromtable 6-1. Conservative estimates of the allowablebearing pressure can be obtained from the followingexpression:

qa = 0.2 qu (6-2)

Allowable bearing pressures for jointed rock can also beestimated from RQD values using table 6-2. Localexperience should always be ascertained.

6-5

TM 5-818-1 / AFM 88-3, Chap. 7


Figure 6-4. Example of bearing capaclty computation for inclined eccentric load on rectangular footing.

6-6

TM 5-818-1 / AFM 88-3, Chap. 7


Figure 6-5. Ultimate bearing capacity of deep foundations.

6-7

TM 5-818-1 / AFM 88-3, Chap. 7


Figure 6-6. Bearing capacity factors for strip and circular footings on layered foundations in clay.

6-8

TM 5-818-1 / AFM 88-3, Chap. 7

Table 6-2. Allowable Bearing Pressure for Jointed Rock

qa

RQD tsf

100 300

90 200

75 120

50 65

25 30

0 10

a If tabulated value exceeds unconfined compressive strength of intact samples of therock, allowable pressure equals unconfined compressive strength.


6-9

TM 5-818-1 / AFM 88-3, Chap. 7

CHAPTER 7

DEWATERING AND GROUNDWATER CONTROL

7-1. General. The following topics concerning d. Control of surface waters.foundation design using dewatering and groundwater e. Sheet-pile cofferdams.control techniques are discussed in the latest revision f. Foundation underdrainage andof TM 5-818-5 / NAVFAC P-418 / AFM 88-5, Chap. 6: waterproofing.

a. Excavations requiringdrainage. 7-2. Foundotion problems. The problems con-cerning dewatering or groundwater control should be

b. Seepage control. referred to the above-mentioned manual.

c. Seepage cutoffs.

7-1

TM 5-818-1 / AFM 88-3. Chap. 7

CHAPTER 8

SLOPE STABILITY ANALYSIS

8-1. General. This chapter is concerned withcharacteristics and critical aspects of the stability ofexcavation slopes; methods of designing slopes,including field observations and experience, slopestability charts, and detailed analyses; factors of safety;and methods of stabilizing slopes and slides. Theemphasis in this chapter is on simple, routineprocedures. It does not deal with specialized problems,such as the stability of excavated slopes duringearthquakes.

8-2. Slope stability problems. Excavation slopeinstability may result from failure to control seepageforces in and at the toe of the slope, too steep slopes forthe shear strength of the material being excavated, andinsufficient shear strength of subgrade soils. Slopeinstability may occur suddenly, as the slope is beingexcavated, or after the slope has been standing for sometime. Slope stability analyses are useful in sands, silts,and normally consolidated and overconsolidated clays,but care must be taken to select the correct strengthparameter. Failure surfaces are shallow in cohesionlessmaterials and have an approximately circular or slidingwedge shape in clays.

a. Cohesionless slopes resting on firm soil orrock. The stability of slopes consisting of cohesionlesssoils depends on the angle of internal friction φ’, theslope angle, the unit weight of soil, and pore pressures.Generally, a slope of 1 vertical (V) on 1 1/2 horizontal (H)is adequate; but if the slope is subjected to seepage orsudden drawdown, a slope of 1V on 3H.is commonlyemployed. Failure normally occurs by surface raveling orshallow sliding. Where consequences of failure may beimportant, required slopes can be determined usingsimple infinite slope analysis. Values of φ’ for stabilityanalyses are determined from laboratory tests orestimated from correlations (para 3-6). Pore pressuredue to seepage reduces slope stability, but static waterpressure, with the same water level inside and outsidethe slopes, has no effect. Benches, paved ditches, andplanting on slopes can be used to reduce runoffvelocities and to retard erosion. Saturated slopes incohesionless materials may be susceptible toliquefaction and flow slides during earthquakes, while dryslopes are subject to settlement and raveling. Relativedensities of 75 percent or larger are required to ensureseismic stability, as discussed in Chapter 17.

b. Cohesive slopes resting on firm soil or rock.The stability of slopes consisting of cohesive soils

depends on the strength of soil, its unit weight, the slopeheight, the slope angle, and pore pressures. Failureusually occurs by sliding on a deep surface tangent tothe top of firm materials. For relatively high slopes thatdrain slowly, it may be necessary to analyze the stabilityfor three limiting conditions:

(1) Short-term or end-of-constructioncondition. Analyze this condition using total stressmethods, with shear strengths determined from Q testson undisturbed specimens. Shear strengths fromunconfirmed compression tests may be used butgenerally may show more scatter. This case is often theonly one analyzed for stability of excavated slopes. Thepossibility of progressive failure or large creepdeformations exists for safety factors less than about1.25 to 1.50.

(2) Long-term condition. If the excavationis open for several years, it may be necessary toanalyze this condition using effective stress methods,with strength parameters determined from S tests or Rtests on undisturbed specimens. Pore pressures aregoverned by seepage conditions and can be determinedusing flow nets or other types of seepage analysis. Bothinternal pore pressures and external water pressuresshould be included in the analyses. This case generallydoes not have to be analyzed.

(3) Sudden drawdown condition, or otherconditions where the slope is consolidated under oneloading condition and is then subjected to a rapid changein loading, with insufficient time for drainage. Analyzethis condition using total stress methods, with shearstrengths measured in R and S tests. Shear strengthshall be based on the minimum of the combined R and Senvelopes. This.case is not normally encountered inexcavation slope stability.

c. Effect of soft foundation strata. The criticalfailure mechanism is usually sliding on a deep surfacetangent to the top of an underlying firm layer. Short-termstability is usually more critical than long-term stability.The strength of soft clay foundation strata should beexpressed in terms of total stresses and determinedusing Q triaxial compression tests on undisturbedspecimens or other methods described in chapter 4.

8-3. Slopes In soils presenting special problems.

a. Stiff-fissured clays and shales. Theshearing resistance of most stiff-fissured clays andshales may be

8-1

TM 5-818-1 / AFM 88-3, Chap. 7

far less than suggested by the results of shear tests onundisturbed samples. This result is due, in part, to priorshearing displacements that are much larger than thedisplacement corresponding to peak strength. Slopefailures may occur progressively, and over a long periodof time the shearing resistance may be reduced to theresidual value-the minimum value that is reached only atextremely large shear displacements. Temporary slopesin these materials may be stable at angles that aresteeper than would be consistent with the mobilization ofonly residual shear strength. The use of localexperience and empirical correlations are the mostreliable design procedures for these soils.

b. Loess. Vertical networks of interconnectedchannels formed by decayed plant roots result in a highvertical permeability in loess. Water percolatingdownward destroys the weakly cemented bonds betweenparticles, causing rapid erosion and slope failure.Slopes in loess are frequently more stable when cutvertically to prevent infiltration. Benches at intervals canbe used to reduce the effective slope angle. Horizontalsurfaces on benches and at the top and bottom of theslope must be sloped slightly and paved or planted toprevent infiltration. Ponding at the toe of a slope must beprevented. Local experience and practice are the bestguides for spacing benches and for protecting slopesagainst infiltration and erosion.

c. Residual soils. Depending on rock type andclimate, residual soils may present special problems withrespect to slope stability and erosion. Such soils maycontain pronounced structural features characteristic ofthe parent rock or the weathering process, and theircharacteristics may vary significantly over shortdistances. It may be difficult to determine design shearstrength parameters from laboratory tests.Representative shear strength parameters should bedetermined by back-analyzing slope failures and by usingempirical design procedures based on local experience.

d. Highly sensitive clays. Some marine claysexhibit dramatic loss of strength when disturbed and canactually flow like syrup when completely remolded.Because of disturbance during sampling, it may bedifficult to obtain representative strengths for such soilsfrom laboratory tests. Local experience is the best guideto the reliability of laboratory shear strength values forsuch clays.

e. Hydraulic fills. See Chapter 15.8-4. Slope stability charts.

a. Uniform soil, constant shear strength, φ =0, rotational failure.

(1) Groundwater at or below toe of slope.Determine shear strength from unconfined compression,or better, from Q triaxial compression tests. Use theupper diagram of figure 8-1 to compute the safety factor.If the center and depth of the critical circle are desired,obtain them from the lower diagrams of figure 8-1.

(2) Partial slope submergence, seepage

surcharge loading, tension cracks. The effect of partialsubmergence of a slope is given by a factor µw in figure8-2; seepage is given by a factor µw’ in figure 8-2;surcharge loading is given by a factor µq in figure 8-2;and tension cracks is given by a factor µt in figure 8-3.Compute safety factor from the following:

F = µw µw’ µq µt N0 C (8-1)γH + q - γwHw’

whereγ = total unit weight of soilq = surcharge loadingN0 = stability number from figure 8-1

If any of these conditions are absent, their correspondingi factor equals 1.0; if seepage out of the slope does notoccur, H. equals IH.

b. Stratified soil layers, φ = O, rotationalfailure.

(1) Where the slope and foundationconsist of a number of strata, each having a constantshear strength, the charts given in figures 8-1 through 8-3 can be used by computing an equivalent averageshear strength for the failure surface. However, aknowledge of the location of the failure surface isrequired. The coordinates of the center of the circularfailure surface can be obtained from the lower diagramsof figure 8-1. The failure surface can be constructed,and an average shear strength for the entire failuresurface can be computed by using the length of arc ineach stratum or the number of degrees intersected byeach soil stratum as a weighing factor.

(2) It may be necessary to calculate thesafety factor for failure surfaces at more than one depth,as illustrated in figure 8-4.

c. Charts for slopes in uniform soils with φ > 0.(1) A stability chart for slopes in soils with

φ > 0 is shown in figure 8-5. Correction factors forsurcharge loading at the top of the slope, submergence,and seepage are given in figure 8-2; and for tensioncracks, in figure 8-3.

(2) The location of the critical circle can beobtained, if desired, from the plot on the right side offigure 8-5. Because simple slopes in uniform soils with φ> 0 generally have critical circles passing through the toeof the slope, the stability numbers given in figure 8-5were developed by analyzing toe circles. Where subsoilconditions are not uniform and there is a weak layerbeneath the toe of the slope, a circle passing beneaththe toe may be more critical than a toe circle.

d. Infinite slopes. Conditions that can beanalyzed accurately using charts for infinite slopeanalyses shown in figure 8-6 are-

8-2

TM 5-818-1 / AFM 88-3, Chap. 7

(1) Slopes in cohesionless materialswhere the critical failure mechanism is shallow sliding orsurface raveling.

(2) Slopes where a relatively thin layer ofsoil overlies firmer soil or rock and the critical failuremechanism is sliding along a plane parallel to the slope,at the top of the firm layer.

e. Shear strength increasing with depth and φ= 0. A chart for slopes in soils with shear strengthincreasing with depth and + = 0 is shown in figure 8-7.

8-5. Detailed analyses of slope stability.If the simple methods given for estimating slope stabilitydo not apply and site conditions and shear strengthshave been determined, more detailed stability analysesmay be appropriate. Such methods are described inengineering literature, and simplified versions arepresented below.

a. The method of moments for φ = 0. Thismethod is simple but useful for the analysis of circularslip surfaces in φ = 0 soils, as shown in figure 8-8.

b. The ordinary method of slices. This simpleand conservative procedure for circular slip surfaces canbe used in soils with φ > 0. For flat slopes with high porepressures and φ > 0, the factors of safety calculated by

this method may be much smaller than values calculatedby more accurate methods. An example is presented infigures 8-9 through 8-11. Various trial circles must beassumed to find the critical one. If φ large and c is small,it may be desirable to replace the circular sliding surfaceby plane wedges at the active and passive extremities ofthe sliding mass.

c. The simplified wedge method. This methodis a simple and conservative procedure for analyzingnoncircular surfaces. An example is shown in 8-12.Various trial failure surfaces with different locations foractive and passive wedges must be assumed. The baseof the central sliding wedge is generally at the bottom ofa soft layer.

8-6. Stabilization of slopes. If a slide is beingstabilized by flattening the slope or by using a buttress orretaining structure, the shear strength at time of failurecorresponding to a factor of safety of 1 should becalculated. This strength can be used to evaluate thesafety factor of the slope after stabilization. Methods forstabilizing slopes and landslides are summarized in table8-1. Often one or more of these schemes may be usedtogether. Schemes I through V are listed approximatelyin order of increasing cost.

8-3

TM 5-818-1 / AFM 88-3, Chap. 7


Figure 8-1. Slope stability charts for φ = 0 soils.

8-4

TM 5-818-1 / AFM 88-3, Chap. 7


Figure 8-2. Reduction factors (µq, µw, µw’) for slope stability charts for φ =0 and φ >0 soils.

8-5

TM 5-818-1 / AFM 88-3, Chap. 7


Figure 8-3. Reduction factors (tension cracks, µt) for slope stability charts for φ = 0 and φ > O soils.

8-6

TM 5-818-1 / AFM 88-3, Chap. 7


Figure 8-4. Example of use of charts forslopes in soils with uniform strength and φ = 0.

8-7

TM 5-818-1 / AFM 88-3, Chap. 7


Figure 8-5. Slope stabdilty charts for φ > 0 soils.

8-8

TM 5-818-1 / AFM 88-3, Chap. 7


Figure 8-6. Stability charts for infinite slopes.

8-9

TM 5-818-1 / AFM 88-3. Chap. 7


Figure 8- 7. Slope stability charts for φ = 0 and strength increasing with depth.

8-10

TM 5-818-1 / AFM 88-3. Chap. 7


Figure 8-8. Method of moments for φ = 0.

8-11

TM 5-818-1 / AFM 88-3, Chap. 1


Figure 8-9. Example problem for ordinary method of slices.

8-12

TM 5-818-1 / AFM 88-3, Chap. 7


Figure 8-10. Example of use of tabular form for computing weights of slices.

8-13

TM 5-818-1 / AFM 88-3, Chap. 7

Figure 8-11. Example of use of tabular form for calculating factor of safety by ordinary method of slices.

8-14

TM 5-818-1 / AFM 88-3, Chap. 7

Figure 8-12. Example of simplified wedge analysis.

8-15

TM 5-818-1 / AFM 88-3, Chap. 7

Table 8-1. Methods of Stabilizing Slopes and Landslides

8-16

TM 5-818-1 / AFM 88-3, Chap. 7

CHAPTER 9

SELECTION OF FOUNDATION TYPE

9-1. Foundation - selection considerations.Selection of an appropriate foundation depends upon thestructure function, soil and groundwater conditions,construction schedules, construction economy, value ofbasement area, and other factors. On the basis ofpreliminary information concerning the purpose of thestructure, foundation loads, and subsurface soilconditions, evaluate alternative types of foundations forthe bearing capacity and total and differentialsettlements. Some foundation alternatives

for different subsoil conditions are summarized in table9-1.

a. Some foundation alternatives may not beinitially obvious. For example, preliminary plans may notprovide for a basement, but when cost studies show thata basement permits a floating foundation that reducesconsolidation settlements at little or no increase inconstruction cost, or even at a cost reduction, the valueof a basement may be substantial. Benefits of basementareas include needed garage space, office or stor-

Table 9-1. Foundation Possibilities for Different Subsoil Conditions

Foundation PossibilitiesSubsoil Conditions Light, Flexible Structure Heavy, Rigid Structure

Deep compact or Footing foundations 1. Footing foundationsstiff deposit 2. Shallow mat

Deep compressible 1. Footing foundations 1. Deep mat with possi-strata on compacted granular ble rigid construc-

zonea tion in basement2. Shallow mata 2. Long piles or cais-3. Friction piles sions to by-pass

3. Friction pilesSoft or loose 1. Bearing piles or 1. Bearing piles or

strata overly- piers piersing firm strata 2. Footing foundations 2. Deep mat

on compacted granularzonea

3. Shallow mat2

Compact or stiff 1. Footing foundationsa 1. Deep mat (floatinglayer overlyinga type)soft deposit 2. Shallow mata 2 Long piles or cais-

sons to by-pass softdeposit

Alternating soft 1. Footing foundationsa 1. Deep matand stiff 2. Shallow mata 2. Piles or caissons tolayers underlying firm

stratum to providesatisfactoryfoundation

a Consider possible advantages of site preloading, with and without vertical sand drains to accelerateconsolidation.

(Courtesy of L. J. Goodman and R. H. Karol. Theory and Practice of Foundation Engineering,1968, p 312. Reprinted by permission of Macmillan Company, Inc New York.)

9-1

TM 5-818-1 / AFM 88-3, Chap. 7

age space, and space for air conditioning and otherequipment. The last item otherwise may requirevaluable building space or disfigure a roofline.

b. While mat foundations are more expensiveto design than individual spread footings, they usuallyresult in considerable cost reduction, provided the totalarea of spread footings is a large percentage of thebasement area. Mat foundations may decrease therequired excavation area, compared with spreadfootings.

c. The most promising foundation typesshould be designed, in a preliminary manner, for detailedcost comparisons. Carry these designs far enough todetermine the approximate size of footings, length andnumber of piles required, etc. Estimate the magnitude ofdifferential and total foundation movements and theeffect on structure. The behavior of similar foundationtypes in the area should be ascertained.

d. Final foundation design should not bestarted until alternative types have been evaluated. Also,the effect of subsurface conditions (bearing capacity andsettlement) on each alternative should be at leastqualitatively evaluated.

e. A checklist of factors that could influencefoundation selection for family housing is shown in table9-2.

9-2. Adverse subsurface conditions. If poor soilconditions are encountered, procedures that may beused to ensure satisfactory foundation performanceinclude the following:

a. Bypass the poor soil by means of deepfoundations extending to or into a suitable bearingmaterial (chap. 11).

b. Design the structure foundations toaccommodate expected differential settlements.Distinguish between settlements during construction thataffect a structure and those that occur duringconstruction before a structure is affected by differentialsettlements.

c. Remove the poor material, and either treatand replace it or substitute good compacted fill material.

d. Treat the soil in place prior to constructionto improve its properties. This procedure generallyrequires considerable time. The latter two proceduresare carried out using various techniques of soilstabilization described in chapter 16.

9-3. Cost estimates and final selection.a. On the basis of tentative designs, the cost

of each promising alternative should be estimated.Estimate sheets should show orderly entries of items,dimensions, quantities, unit material and labor costs, andcost extensions. Use local labor and material costs.

b. The preliminary foundation designs that arecompared must be sufficiently completed to include allrelevant aspects. For example, the increased cost ofpiling may be partially offset by pile caps that are smallerand less costly than spread footings. Similarly, mat orpile foundations may require less excavation.Foundation dewatering during construction may be alarge item that is significantly different for somefoundation alternatives.

c. The most appropriate type of foundationgenerally represents a compromise betweenperformance, construction cost, design cost, and time.Of these, design cost is generally the least important andshould not be permitted to be a controlling factor. If alower construction cost can be achieved by an alternativethat is more expensive to design, construction costshould generally govern.

d. Foundation soils pretreatment byprecompression under temporary surcharge fill,regardless of whether vertical sand drains are providedto accelerate consolidation, requires a surcharge loadingperiod of about 6 months to a year. The time requiredmay not be available unless early planning studiesrecognized the possible foundation cost reduction thatmay be achieved. Precompression is frequentlyadvantageous for warehouses and one-story structures.Precompression design should be covered as a separatedesign feature and not considered inherent in structuredesign.

9-2

TM 5-818-1 / AFM 88-3, Chap. 7

Table 9-2. Checklist for Influence of Site Charateristics on Foundation Selection for Family Housing


9-3

TM 5-818-1 / AFM 88-3, Chap. 7

CHAPTER 10

SPREAD FOOTINGS AND MAT FOUNDATIONS

10-1. General. When required footings cover morethan half the area beneath a structure, it is oftendesirable to enlarge and combine the footings to coverthe entire area. This type of foundation is called a raft ormat foundation and may be cheaper than individualfootings because of reduced forming costs and simplerexcavation procedures. A mat foundation also may beused to resist hydrostatic pressures or to bridge oversmall, soft spots in the soil, provided the mat isadequately reinforced. Although mat foundations aremore difficult and more costly to design than individualspread footings, they can be used effectively.

10-2. Adequate foundation depth. The foundationshould be placed below the frost line (chap 18) becauseof volume changes that occur during freezing andthawing, and also below a depth where seasonal volumechanges occur. The minimum depth below whichseasonal volume changes do not occur is usually 4 feet,but it varies with location. If foundation soils consist ofswelling clays, the depth may be considerably greater,as described in TM 5-818-7. On sloping ground, thefoundation should be placed at a depth such that it willnot be affected by erosion.

10-3. Footing design.a. Allowable bearing pressures. Procedures

for determining allowable bearing pressures arepresented in chapter 6. In many instances, theallowable bearing pressure will be governed by theallowable settlement. Criteria for determining allowablesettlement are discussed in chapter 5. The maximumbearing pressure causing settlement consists of deadload plus normal live load for clays, and dead load plusmaximum live loads for sands. Subsoil profiles shouldbe examined carefully to determine soil stratacontributing to settlement.

b. Footings on cohesive soils.(1) If most of the settlement is anticipated

to occur in strata beneath the footings to a depth equal tothe distance between footings, a settlement analysisshould be made assuming the footings are independentof each other. Compute settlements for the maximumbearing pressure and for lesser values. An example ofsuch an analysis is shown in figure 10-1. If significantsettlements can occur in strata below a depth equal tothe distance between footings, the settlement analysisshould consider all footings to determine the settlementat selected footings. Determine the vertical stresses

beneath individual footings from the influence chartspresented in chapter 5. The footing size should beselected on the basis of the maximum bearing pressureas a first trial. Depending on the nature of soilconditions, it may or may not be possible to proportionfootings to equalize settlements. The possibility ofreducing differential settlements by proportioning footingareas can be determined only on the basis of successivesettlement analyses. If the differential settlementsbetween footings are excessive, change the layout ofthe foundation, employ a mat foundation, or use piles.

(2) If foundation soils are nonuniform in ahorizontal direction, the settlement analysis should bemade for the largest footing, assuming that it will befounded on the most unfavorable soils disclosed by theborings and for the smallest adjacent footing. Structuraldesign is facilitated if results of settlement analyses arepresented in charts (fig 10-1) which relate settlement,footing size, bearing pressures, and column loads.Proper footing sizes can be readily determined from suchcharts when the allowable settlement is known. After afooting size has been selected, compute the factor ofsafety with respect to bearing capacity for dead load plusmaximum live load condition.

c. Footings on cohesionless soils. Thesettlement of footings on cohesionless soils is generallysmall and will take place mostly during construction. Aprocedure for proportioning footings on sands to restrictthe differential settlement to within tolerable limits formost structures is given in figure 10-2.

d. Foundation pressures. Assume a planardistribution of foundation pressure for the structuralanalysis of a footing. This assumption is generallyconservative. For eccentrically loaded footings, thedistribution of the bearing pressure should be determinedby equating the downward load to the total upwardbearing pressure and equating the moments of theseforces about the center line in accordance withrequirements of static equilibrium. Examples of thebearing pressure distribution beneath footings are shownin figure 10-3.

10-4. Mat foundations.a. Stability. The bearing pressure on mat

foundations should be selected to provide a factor ofsafety of

10-1

TM 5-818-1 / AFM 88-3, Chap. 7

at least 2.0 for dead load plus normal live load and 1.5for dead load plus maximum live load. By lowering thebase elevation of the mat, the pressure that can beexerted safely by the building is correspondinglyincreased (chap 11), and the net increase in loading isreduced. The bearing pressure should be selected sothat the settlement of the mat foundation will be withinlimits that the structure can safely tolerate as a flexiblestructure. If settlements beneath the mat foundation are

more than the rigidity of the structure will permit, aredistribution of loads takes place that will change thepressure distribution beneath the structure, assubsequently described. The bearing capacity of loosesands, saturated silts, and low-density loess can bealtered significantly as a result of saturation, vibrations,or shock. Therefore, the allowable bearing pressure andsettlement of these soils cannot be determined in theusual manner for the foundation soils


Figure 10-1. Example of method for selecting allowable bearing pressure.

10-2

TM 5-818-1 / AFM 88-3, Chap. 7

may be subject to such effects. Replace or stabilizesuch foundation soils, as discussed in chapter 16, ifthese effects are anticipated.

b. Conventional analysis. Where thedifferential settlement between columns will be small,design the mat as reinforced concrete flat slab assumingplanar soil pressure distribution. The method is generallyapplicable where columns are more or less equallyspaced. For analysis, the mat is divided into mutuallyperpendicular strips.

c. Approximate plate analysis. When thecolumn loads differ appreciably or the columns areirregularly spaced, the conventional method of analysisbecomes seriously in error. For these cases, use an

analysis based on the theory for beams or plates onelastic foundations. Determine the subgrade modulus bythe use of plate load tests. The method is suitable,particularly for mats on coarse-grained soils whererigidity increases with depth.

d. Analysis of mats on compressible soils. Ifthe mat is founded on compressible soils, determinationof the distribution of the foundation pressures beneaththe mat is complex. The distribution of foundationpressures varies with time and depends on theconstruction sequence and procedure, elastic andplastic deformation properties of the foundation concrete,and

Figure 10-2. Proportioning footings on cohesionless soils.

10-3

TM 5-818-1 / AFM 88-3, Chap. 7

Figure 10-3. Distribution of bearing pressures.

10-4

TM 5-818-1 / AFM 88-3, Chap.7

time-settlement characteristics of foundation soils. As aconservative approach, mats founded on compressiblesoils should be designed for two limiting conditions:assuming a uniform distribution of soil pressure, andassuming a pressure that varies linearly from a minimumof zero at the middle to twice the uniform pressure at theedge. The mat should be designed structurally forwhichever distribution leads to the more severeconditions.

10-5. Special requirements for mat foundations.a. Control of groundwater. Exclude

groundwater from the excavation by means of cutoffs,and provide for temporary or permanent pressure reliefand dewatering by deep wells or wellpoints as describedin TM 5-818-5/AFM 88-5, Chapter 6. Specifypiezometers to measure drawdown levels duringconstruction. Specify the pumping capacity to achieverequired drawdown during various stages ofconstruction, including removal of the temporary systemat the completion of construction. Consider effects ofdrawdown on adjoining structures.

b. Downdrag. Placement of backfill againstbasement walls or deep raft foundations constructed inopen excavations results in downdrag forces if weight ofbackfill is significant with respect to structural loading.Estimate the downdrag force on the basis of data inchapter 14.

10-6. Modulus of subgrade reaction for footingsand mats.

a. The modulus of subgrade reaction can bedetermined from a plate load test (para 4-6) using a 1- by1- foot plate.

ksf = ksl B (10-1)

whereksf = the modulus of subgrade reaction for

the prototype footing of width Bksl = the value of the 1- by 1-foot plate in

the plate load testThe equation above is valid for clays and assumes noincrease in the modulus with depth, which is incorrect,and may give k1, which is too large.For footings or mats on sand:

ksf = ksl B + 1 2 (10-2)2B

For a rectangular footing or mat of dimensions of B xmB:

( 15m5)(10-3)

with a limiting value of ksf = 0.667ksl.b. ks may be computed as

ks = 36qa. (kips per square foot)(10-4)

which has been found to give values about as reliable asany method. This equation assumes qa (kips per squarefoot) for a settlement of about 1 inch with a safety factor,F ≅ 3. A typical range of values of ks is given in table 3-7.

10-7. Foundations for radar towers.a. General. This design procedure provides

minimum footing dimensions complying with criteria fortilting rotations resulting from operational wind loads.Design of the footing for static load and survival windload conditions will comply with other appropriatesections of this manual.

b. Design procedure. This design procedureis based upon an effective modulus of elasticity of thefoundation. The effective modulus of elasticity isdetermined by field plate load tests as described insubparagraph d below. The design procedure alsorequires seismic tests to determine the S-wave velocityin a zone beneath the footing at least 1 1/2 times themaximum size footing required. Field tests on existingradar towers have shown that the foundation performsnearly elastically when movements are small. Therequired size of either a square or a round footing toresist a specific angle of tilt, α, is determined by thefollowing:

B3 = 4320(F) M 1 - M2 (squareα Es footing) (10-5)

D3 = 6034(F) M 1 - M2 (roundα Es footing) (10-6)

whereB, D = size and diameter of footing,

respectively, feetF = factor of safety (generally use 2.0)M = applied moment at base of footing

about axis of rotation, foot-poundsα = allowable angle of tilt about axis of

rotation, angular mils (1 angular mil =0.001 radian)

Es = effective modulus of elasticity offoundation soil, pounds per cubic foot

The design using equations (10-5) and (10-6) is onlyvalid if the seismic wave velocity increases with depth.,If the velocity measurements decrease with depth,special foundation design criteria will be required. Thediscussion of these criteria is beyond the scope of thismanual.

c. Effective modulus of elasticity of foundationsoil (Es). Experience has shown that the design modulusof elasticity of in-place soil ranges from 1000 to 500, kipsper square foot. Values less than 1000 kips per squarefoot will ordinarily present severe settlement problemsand are not satisfactory sites for radar towers. Values inexcess of 5000 kips per square foot may be encounteredin dense gravel or rock, but such values are not used indesign.

( )

( )

( )

10-5

TM 5-818-1 / AFM 88-3, Chap. 7

(1) Use equations (10-5) and (10-6) tocompute-

(a) Minimum and maximum footing sizesusing Es = 1000 and 5000 kips per square foot,respectively.

(b) Two intermediate footing sizes usingvalues intermediate between 1000 and 5000 kips persquare foot.Use these four values of B or D in the followingequations to compute the increase (or pressure change)in the live load, ∆L.

square footing ∆L = 17.0M (pounds perB3 square foot) (10-7)

round footing ∆L = 20.3M (pounds perD3 square foot) (10-8)

(2) The Es value depends on the depth of thefooting below grade, the average dead load pressure onthe soil, and the maximum pressure change in the liveload, ∆L, on the foundation due to wind moments. Adetermination of the E, value will be made at theproposed footing depth for each footing size computed.

(3) The dead load pressure, q0, is computed asthe weight, W, of the radar tower, appurtenances, andthe footing divided by the footing area, A.

q0 = ΣW (10-9)A

The selection of loadings for the field plate load test willbe based on qo and ∆L.

d. Field plate load test procedure. Thefollowing plate load test will be performed at the elevationof the bottom of the footing, and the test apparatus willbe as described in TM 5-824-3/AFM 88-6, Chapter 3.

(1) Apply a unit loading to the plate equalto the smallest unit load due to the dead load pressureq0. This unit loading will represent the largest sizefooting selected above.

(2) Allow essentially full consolidationunder the dead load pressure increment. Deformationreadings will be taken intermittently during and at the endof the consolidation period.

(3) After consolidation under the deadload pressure, perform repetitive load test using the liveload pressure ∆L computed by the formulas in paragraph

10-7c. The repetitive loading will consist of the deadload pressure, with the live load increment applied for 1minute. Then release the live load increment and allowto rebound at the dead pressure for 1 minute. Thisprocedure constitutes one cycle of live load pressureapplication. Deformation readings will be taken at threepoints: at the start, after the live load is applied for 1minute, and after the plate rebounds under the dead loadpressure for 1 minute. Live load applications will berepeated for 15 cycles.

(4) Increase the dead load pressure, q0, tothe second lowest value, allow to consolidate, and thenapply the respective live load increment repetitively for 15cycles.

(5) Repeat step 4 for the remaining twodead load pressure increments.

(6) An uncorrected modulus of elasticityvalue is computed for each increment of dead and liveload pressure as follows:

Es’ = 25.5 ∆L (1 - µ2 )S

Es’ = uncorrected effective modulus ofelasticity for the loading condition used, pounds persquare foot

S = average edge deformation of the platefor the applied load, determined from the slope of the lastfive rebound increments in the repetitive load test, inches

µ = Poisson's ratio (see table 3-6).

(7) The above-computed uncorrectedmodulus of elasticity will be corrected for bending of theplate as described in TM 5-824-3/AFM 88-6, Chapter 3,where E' is defined above, and E, is the effectivemodulus of elasticity for the test conditions.

e. Selection of required footing size. Therequired footing size to meet the allowable rotationcriteria will be determined as follows:

(1) Plot on log-log paper the minimum andthe maximum footing size and the two intermediatefooting sizes versus the required (four assumed values)effective modulus of elasticity for each footing size.

(2) Plot the measured effective modulus ofelasticity versus the footing size corresponding to theloading condition used for each test on the same chartas above.

(3) These two plots will intersect. Thefooting size indicated by their intersection is the minimumfooting size that will resist the specified angle of tilt.

10-6

TM 5-818-1 / AFM 88-3, Chap. 7

CHAPTER 11

DEEP FOUNDATIONS INCLUDING DRILLED PIERS

11-1. General. A deep foundation derives its supportfrom competent strata at significant depths below thesurface or, alternatively, has a depth to diameter ratiogreater than 4. A deep foundation is used in lieu of ashallow foundation when adequate bearing capacity ortolerable settlements cannot be obtained with a shallowfoundation. The term deep foundation includes piles,piers, or caissons, as well as footings or mats set into adeep excavation. This chapter discusses problems ofplacing footings and mats in deep excavations anddesign of drilled piers. Drilled piers (or caissons) aresimply large-diameter piles, but the design process issomewhat different. An arbitrary distinction between apile and pier is that the caisson is 30 inches or more indiameter.

11-2. Floating foundations. A foundation set into adeep excavation is said to be compensated or floating ifthe building load is significantly offset by the load of soilremoved during excavation. The foundation is fullycompensated if the structural load equals the loadremoved by excavation, partially compensated if thestructural load is greater, and overcompensated if thestructural load is less than the weight of the excavatedsoil. A compensated foundation requires a study ofexpected subsoil rebound and settlement, excavationsupport systems, means to maintain foundation subsoilor rock integrity during excavation, and allowable bearingpressures for the soil or rock.

11-3. Settlements of compensated foundations.a. The sequence of subsoil heave during

excavation and subsequent settlement of a deepfoundation is illustrated in figure 11-1(a). If effectivestresses do not change in the subsoils upon the initialexcavation, i.e., the soil does not swell due to anincrease in water content, and if no plastic flow occurs,then only immediate or elastic rebound from change instress occurs. If the structural load is fully compensated,the measured settlement of the foundation would consistonly of recompression of the elastic rebound, generally asmall quantity, provided subsoils are not disturbed byexcavation.

b. If the negative excess pore pressures setup during excavation "dissipate, " i.e., approach staticvalues, before sufficient structural load is applied,foundation swell occurs in addition to elastic rebound.(The original effective stresses will decrease.) The

foundation load recompresses the soil, and settlement ofthe foundation consists of elastic and consolidationcomponents as shown in figure 11-1(b). Consolidationoccurs along the recompression curve until thepreconsolidation stress is reached, whereupon itproceeds along the virgin compression curve. Calculatethe foundation heave and subsequent settlement usingprocedures outlined in chapter 5.

c. If the depth with respect to the type andshear strength of the soil is such that plastic flow occurs,loss of ground may develop around the outside of theexcavation with possible settlement damage tostructures, roads, and underground utilities.

d. The rate and amount of heave may beestimated from the results of one-dimensionalconsolidation tests; however, field evidence shows thatthe rate of heave is usually faster than predicted. Astudy of 43 building sites found that the field heaveamounted to about one-third the computed heave.Where excavations are large and are open forsubstantial time before significant foundation loadingsare applied, the actual heave may be close to thecomputed heave. Figure 11-2 is a plot of a series of fieldresults of heave versus excavation depth, in which theheave increases sharply with the depth of excavation.An example of heave and subsequent settlementcalculations for a compensated foundation is shown infigure 5-4.

e. The yielding of the excavation bottom canbe .caused by high artesian water pressures under theexcavation or by a bearing capacity failure resulting fromthe overburden pressure on the soil outside theexcavation at subgrade elevation. Artesian pressure canbe relieved by cutoffs and dewatering of the underlyingaquifer using deep wells. The pumped water may be putback in the aquifer using recharge wells outside theexcavation perimeter to avoid perimeter settlements or topreserve the groundwater table for environmentalreasons, but this operation is not simple and should bedone only when necessary.

f. The likelihood of bearing capacity failureexists primarily in clayey soils and should be analyzed asshown in chapter 14. A factor of safety, Fs > 2, shouldideally be obtained to minimize yielding and possiblesettlement problems. A large plastic flow may cause thebottom of the excavation to move upwards with re-

11-1

TM 5-818-1 / AFM 88-3, Chap. 7

suiting loss of ground. To avoid this possibility,investigate-

(1) Potential for plastic flow, i.e.,relationship between shear stress and shear strength.

(2) Sequence of placing wall bracing.(3) Depth of penetration of sheeting below

base of excavation.g. Two commonly used procedures to control

bottom heave are dewatering and sequential excavationof the final 5 feet or more of soil. Groundwater loweringincreases effective stresses and may reduce heave.Where subsoil permeabilities are not large, a deep andeconomical lowering of the groundwater to minimize

heave can sometimes be achieved by an educator-typewellpoint system. Permitting a controlled rise of thegroundwater level as the building loan is applied acts toreduce effective stresses and counteracts the effect ofthe added building load. Sequential excavation isaccomplished by removing soil to final grade via a seriesof successive trenches. As each trench is opened, thefoundation element is poured before any adjacent trenchis opened. This procedure recognizes the fact that moreheave occurs in the later excavation stages than inearlier stages and is frequently used in shales.

h. The tilting of a compensated foundation canoccur if structural loads are not symmetrical or if soil


Figure 11-1. Effect of pore pressure dissipation during excavation and settlement response.

11-2

TM 5-818-1 / AFM 88-3, Chap. 7

conditions are nonuniform. Tilting can be estimated fromsettlement calculations for different locations of theexcavation. Control of tilt is not generally necessary butcan be provided by piles or piers, if required. Bearingcapacity is not usually important unless the building ispartially compensated and founded on clay. The factorof safety against bearing failure is calculated (see chap 6for quit) and compared with the final total soil stressusing the building load, qo, less the excavation stressas follows:

FS = qult (11-1)q0 - γD

The factor of safety should be between 2.5 and 3.0 fordead load plus normal live load.

i. Settlement adjacent to excavationsdepends on the soil type and the excavation supportsystem method employed (chap 14). With properlyinstalled strutted or anchored excavations in

cohesionless soils, settlement will generally be less than0.5 percent of excavation depth. Loss of ground due touncontrolled seepage or densification of loosecohesionless soils will result in larger settlements.Surface settlements adjacent to open cuts in soft to firmclay will occur because of lateral yielding and movementof soil beneath the bottom of the cut. Figure 11-3 can beused to estimate the magnitude and extent of settlement.

11-4. Underpinning.a. Structures supported by shallow

foundations or short piles may have to be underpinned iflocated near an excavation. Techniques forunderpinning are depicted in figure 11-4. The mostwidely accepted methods are jacked down piles or piers,which have the advantage of forming positive contactwith the building foundation since both can beprestressed. The use of drilled piers is of more recentvintage and is more economical where it can be used. Insandy soils, chemical


Figure 11-2. Excavation rebound versus excavation depth.

11-3

TM 5-818-1 / AFM 88-3. Chap. 7


Figure 11-3. Probable settlements adjacent to open cuts.

11-4

TM 5-818-1 / AFM 88-3, Chap. 7

injection stabilization (chap 16) may be used to underpinstructures by forming a zone of hardened soil to supportthe foundation.

b. In carrying out an underpinning operation,important points to observe include the following:

(1) Pits opened under the building must beas small as possible, and survey monitoring of thebuilding must be carried out in the areas of each pit todetermine if damaging movements are occurring.

(2) Care must be taken to preventsignificant lifting of local areas of the building duringjacking.

(3) Concrete in piers must be allowed toset before any loading is applied.

(4) Chemically stabilized sands must notbe subject to creep under constant load.

c. The decision to underpin is a difficult onebecause it is hard to estimate how much settlement a

building can actually undergo before being damaged.The values given in tables 5-2 and 5-3 may be used asguidelines.

11-5. Excavation protection. During foundationconstruction, it is important that excavation subsoils beprotected against deterioration as a result of exposure tothe elements and heavy equipment. Difficulties canoccur as a result of slaking, swelling, and piping of theexcavation soils. Also, special classes of soils cancollapse upon wetting (chap 3). Methods for protectingan excavation are described in detail in table 11-1 alongwith procedures for identifying problem soils. If thesemeasures are not carried out, soils likely will be subjectto a loss of integrity and subsequent foundationperformance will be impaired.


Figure 11-4. Methods of underpinning.

11-5TM 5-818-1 / AFM 88-3, Chap. 7

Table 11-1. Excavation Protection.

Soil Type Identification Problems and Mechanism Preventative Measures

Overconsolidated (1) Stiff plastic clays, (1) One dimensional (1) Rapid collection of sur-clays natural water content heave, maximum face water, or grading

near plastic limit heave at excava- around excavation(2) See figure 3-14 for tion center (2) Deep pressure relief to

swelling potential (2) Swelling dependent minimize reboundon plasticity (3) Place 4" - 6" working mat

(3) Usually fractured of lean concrete immedi-and fissured. Ex- ately after exposing sub-cavation opens grade (mat may be placedthese, causing over underseepage andsoftening and pressure relief systemsstrength loss placed in sand blanket)

(4) If sloped walls, use as-phalt sealer on verticalwalls, burlap with rubbersheeting or other mem-brane on flatter slopes

Chemically (1) Lab testing (1) Swelling, slaking (1) Protect from wetting andinert, unce- (2) Soil and geologic maps and strength loss drying by limiting areamented clay- (3) Local experience if water open at subgradestone or shale infiltration (2) Concrete working mat

(2) Rebound if over- (3) Paving, impervious mate-consolidated rials to avoid water in-(generally the filtration; place sealingcase) coats immediately after

(3) Cracking if dry exposureor evaporation (4) Reduce evaporation andallowed drying to prevent

swelling on resaturation(Continued)

(Sheet 1 of 3)

11-6

TM 5-818-1 / AFM 88-3, Chap. 7

Table 11-1. Excavation Protection-Continued


Limestone (1) Geologic maps and Open cavities and cav- (1) Avoid infiltration; col-local experience erns; soft infilling lect surface runoff and

(2) Borings and soundings in joints, cavities. convey it to a pointto determine cavity Dewatering can cause where its infiltrationlocations cavity collapse or will not affect

settlements of in- excavationfilling materials (2) Eliminate leaks from util-

ity or industrial pipingand provide for inspec-tion to avoid infiltra-tion continuing aftercorrection

(3) Avoid pumping whichcauses downward seepageor recharging

Collapsing soils (1) See figure 3-16, (1) When infiltrated (1) Drainage and collection(primarily Yd vs WL plot with water, sudden system to avoid waterlbess and vol- (2) Yd < 85 PCF decrease in bulk infiltrationcanic ash) (3) Large open "structure" volume; may or may (2) Preload excavation area,

with temporary source not require load- and pond with water toof strength ing in conjunction cause collapse prior to

(4) Liquid limit < 45% with seepage excavationplasticity index 0-25% (2) Water action re-

(5) Natural water content duces temporarywell below 100% source ofsaturation strength, usually

(6) Optimum water content capillary tensionfor collapse between or root structure13-39%(3) Low erosion

resistance


11-7

TM 5-818-1 / AFM 88-3, Chap. 7

Table 11-1. Excavation Protection-Continued.


Soft, normally (1) Recently deposited, or (1) Primarily low Provide support, and preventconsolidated no geologic loading strength, unable remolding: place timberor sensitive and unloading to support const. beams under heavyclays (2) Leached marine clays equipment equipment; cover

(3) Natural water content (2) Remolded by con- excavation bottom with 1’ -near or above liquid struction activ- 2’ of sand and gravel filllimit ity, causing and/or 6" - 8" of lean

strength loss concrete

(Sheet 3 of 3)

11-8

TM 5-818-1 / AFM 88-3, Chap. 7

11-6. Drilled piers. Drilled piers (also drilled shafts ordrilled caissons) are often more economical than pileswhere equipment capable of rapid drilling is readilyavailable, because of the large capacity of a pier ascompared with a pile.

a. Pier dimension and capacities. Drilled pierscan support large axial loads, up to 4, 000 kips or more,although typical design loads are on the order of 600 to1, 000 kips. In addition, drilled piers are used underlightly loaded structures where subsoils might causebuilding heaving. Shaft diameters for high-capacity piersare available as follows:

From 2- 1/2 feet by 6-inch incrementsFrom 5 feet by 1-foot increments

Also available are 15- and 2-foot-diameter shafts.Commonly, the maximum diameter of drilled piers isunder 10 feet with a 3- to 5-foot diameter very common.Drilled piers can be belled to a maximum bell size ofthree times the shaft diameter. The bells may behemispherical or sloped. Drilled piers can be formed to amaximum depth of about 200 feet. Low capacity drilledpiers may have shafts only 12 to 18 inches in diameterand may not be underreamed.

b. Installation. The drilled pier is constructedby drilling the hole to the desired depth, belling ifincreased bearing capacity or uplift resistance isrequired, placing necessary reinforcement, and filling thecavity with concrete as soon as possible after the hole isdrilled. The quantity of concrete should be measured toensure that the hole has been completely filled.Reinforcement may not be necessary for vertical loads;however, it will always be required if the pier carrieslateral loads. A minimum number of dowels will berequired for unreinforced piers to tie the superstructure tothe pier. Reinforcement should be used only ifnecessary since it is a construction obstruction.Consideration should be given to an increased shaftdiameter or higher strength concrete in lieu ofreinforcement. In caving soils and depending on localexperience, the shaft is advanced by:

(1) Drilling a somewhat oversize hole andadvancing the casing with shaft advance. Casing maybe used to prevent groundwater from entering the shaft.When drilling and underreaming is completed, thereinforcing steel is placed, and concrete is placedimmediately. The casing may be left in place orwithdrawn while simultaneously maintaining a head ofconcrete. If the casing is withdrawn, the potential existsfor voids to be formed in the concrete, and specialattention should be given to the volume of concretepoured.

(2) Use of drilling mud to maintain the shaftcavity. Drilling mud may be used also to prevent waterfrom entering the shaft by maintaining a positive headdifferential in the shaft, since the drilling fluid has a

higher density than water. The reinforcing steel can beplaced in the slurry-filled hole. Place concrete by tremie.

(3) Use of drilling mud and casing. Theshaft is drilled using drilling mud, the casing is placed,and the drilling mud is bailed. Core barrels and otherspecial drilling tools are available to socket the pier shaftinto bedrock. With a good operator and a drill in goodshape, it is possible to place 30- to 36-inch cores intosolid rock at a rate of 2 to 3 feet per hour. Underreamsare either hemispherical or 30- or 45-degree bell slopes.Underreaming is possible only in cohesive soils such thatthe underslope can stand without casing support, as nopractical means currently exists to case the bell.

c. Estimating the load capacity of a drilled pier.Estimate the ultimate capacity, Q, of a drilled pier asfollows:

Qu = Qus(skin resistance) + Qup(point) (11-2)

The design load based on an estimated 1-inchsettlement is:

Qd = Qus + Qup (11-3)3

(1) Drilled piers in cohesive soil. The skinresistance can be computed from the following:

HQus =αavg ∫ C cz dz

0where

αavg = factor from table 11-2H = shaft lengthC = shaft circumferencecz = undrained shear strength at depth z

Use table 11-2 for the length of shaft to be considered incomputing H and for limiting values of side shear. Thebase resistance can be computed from the following:

Qup = Nc cB AB (11-5)where

NC = bearing capacity factor of 9 (table 11-2)

cB = undrained shear strength for distanceof two diameters below tip

AB = base area(2) Drilled piers in sand. Compute skin

resistance from the following:H

Qus = αavg C ∫ Pz tan φ dz (11-6)0

where∝avg = 0.7, for shaft lengths less than 25

feet∝avg = 0.6, for shaft lengths between 25 and

40 feet∝avg = 0.5, for shaft lengths more than 40

feet

11-9TM 5-818-1 / AFM 88-3, Chap. 7

Table 11-2. Design Parameters for Drilled Piers in Clay

Side Resistance TipLimit on side Resistance

Design Category ∝avg shear - tsf Nc Remarks

A. Straight-sided shafts in either homo-geneous or layered soil with no soilof exceptional stiffness below thebase1. Shafts installed dry or by theslurry displacement method 0.6 2.0 92. Shafts installed with drilling (a) aavg may be in-

mud along some portion of the creased to 0.6 and sidehole with possible mud entrapment 0.3(a) 0.5(a) 9 shear increased to 2.0

B. Belled shafts in either homogeneous tsf for segments drilledor layered clays with no soil of dryexceptional stiffness below the base1. Shafts installed dry or by the

slurry displacement methods 0.3 0.5 92. Shafts installed with drilling (b) aavg may be in-

mud along some portion of the creased to 0.3 and sidehole with possible mud entrapment 0.15(b) 0.3(b) 9 shear increased to 0.5

tsf for segments drilledC. Straight-sided shafts with basedry

resting on soil significantlystiffer than soil around stem 0 0 9

D. Belled shafts with base restingon soil significantly stiffer thansoil around stem 0 0 9

Note: In calculating load capacity, exclude (1) top 5 ft of drilled shaft, (2) periphery of bell, and (3) bottom 5 ft ofstraight shaft and bottom 5 ft of stem of shaft above bell.


11-10

TM 5-818-1 / AFM 88-3, Chap. 7

pz = effective overburden pressure at depthz

φ = effective angle of internal frictionArching develops at the base of piers in sand similar topiers in clay; thus, the bottom 5 feet of shaft should notbe included in the integration limits of the aboveequations. The base resistance for a settlement of about1 inch can be computed from the following:

Qup = AB qt = 1.31 Bqt (11-7)0.6B

whereAB = base areaB = base diameterqt = 0 for loose sandqt = 32,000 pounds per square foot for

medium sandqt = 80,000 pounds per square foot for

dense sand

11-11

TM 5-818-1 / AFM 88-3, Chap. 7

CHAPTER 12

PILE FOUNDATIONS

12-1. General. Bearing piles are deep foundationsused to transmit foundation loads to rock or soil layershaving adequate bearing capacity to support thestructure and to preclude settlement resulting fromconsolidation of soil above these layers. When thebearing strata are below the groundwater table, andwhen offshore structures are being built, piles may bethe most economical type of deep foundation available

because they do not require dewatering of the site. Pilesalso may be used to compact cohesionless soils and toserve as anchorages against lateral thrust and verticaluplift.

12-2. Design. The selection, design, and placementof pile foundations are discussed in detail in the latestrevision of TM 5-809-7.

12-1

TM 5-818-1 / AFM 88-3, Chap. 7

CHAPTER 13

FOUNDATIONS ON EXPANSIVE SOILS

13-1. General. Natural and man-made deposits ofsoils that contain substantial proportions of clay mineralshave a potential for swelling or shrinking with change inwater content. Certain engineering aspects such as sitestudies, heave predictions, and foundation types arediscussed in the latest revision of TM 5-818-7.

13-2. Foundation problems. The problems related toexpansive soils should be referred to the above-mentioned manual.

13-1

TM 5-818-1 / AFM 88-3, Chap. 7

CHAPTER 14

RETAINING WALLS AND EXCAVATION SUPPORT SYSTEMS

14-1. Design considerations for retaining walls.a. General. Retaining walls must be designed

so that foundation pressures do not exceed allowablebearing pressures, wall settlements are tolerable, safetyfactors against sliding and overturning are adequate, andthe wall possesses adequate structural strength.Methods for evaluating earth pressures on retaining wallsand design procedures are summarized herein forcohesionless backfill materials, which should be usedwhenever practicable.

b. Forces acting on retaining walls. Forcesinclude earth pressures, seepage and uplift pressures,surcharge loads, and weight of the wall. Typical loaddiagrams for principal wall types are shown in figure 14-1. The magnitude and distribution of active and passiveearth pressures are developed from the earth theory forwalls over 20 feet high and from semiempirical curves forlower walls. The subgrade reaction along the base isassumed linearly distributed.

14-2. Earth pressures.a. Earth pressure at rest. For cohesionless

soils, with a horizontal surface, determine the coefficientof earth pressure at rest, K>, from the following:

K0 = 1 - sin φ (14-1)

b. Active earth pressure. Formulas forcalculating the coefficient of active earth pressure for acohesionless soil with planar boundaries are presentedin figure 14-2.

c. Passive earth pressure. Formulas forcalculating the coefficient of passive earth pressure for acohesionless soil with planar boundaries are presentedin figure 14-3.

d. Earth pressure charts. Earth pressurecoefficients based on planar sliding surfaces arepresented in figure 14-4. The assumption of a planarsliding surface is sufficiently accurate for the majority ofpractical problems. A logarithmic spiral failure surfaceshould be assumed when passive earth pressure iscalculated and the angle of wall friction, δ, exceeds φ’/3.Earth pressure coefficients based on a logarithmic spiralsliding surface are presented in textbooks ongeotechnical engineering. Passive pressure should notbe based on Coulomb theory since it overestimatespassive resistance. Because small movements mobilizeδ and concrete walls are relatively rough, the wall frictioncan be considered when estimating earth pressures. Ingeneral, values of δ for active earth pressures should not

exceed φ’/2 and for passive earth pressures should notexceed φ’/3. The angle of wall friction for walls subjectedto vibration should be assumed to be zero.

e. Distribution of earth pressure. Apresentation of detailed analyses is beyond the scope ofthis manual. However, it is sufficiently accurate toassume the following locations of the earth pressureresultant:

(1) For walls on rock:0.38H above base for horizontal ordownward sloping backfill

0.45H above base for upward slopingbackfill

(2) For walls on soil:0.33H above base of horizontal backfill0.38H above base of upward slopingbackfill

Water pressures are handled separately.f. Surcharge loads. Equations for

concentrated point and line load are presented in figure14-5. For uniform or nonuniform surcharge pressureacting on an irregular area, use influence charts basedon the Boussinesq equations for horizontal loads anddouble the horizontal pressures obtained.

g. Dynamic loads. The effects of dynamicloading on earth pressures are beyond the scope of thismanual. Refer to geotechnical engineering textbooksdealing with the subject.

14-3. Equivalent fluid pressures. The equivalentfluid method is recommended for retaining walls lessthan 20 feet high. Assign available backfill material to acategory listed in figure 14-6. If the wall must bedesigned without knowledge of backfill properties,estimate backfill pressures on the basis of the mostunsuitable material that may be used. Equivalent fluidpressures are shown in figure 14-6 for the straight slopebackfill and in figure 14-7 for the broken slope backfill.Dead load surcharges are included as an equivalentweight of backfill. If the wall rests on a compressiblefoundation and moves downward with respect to thebackfill, pressures should be increased 50 percent forbackfill types 1, 2, 3, and 5. Although equivalent fluidpressures include seepage effects and time-conditionedchanges in the backfill material, adequate drainageshould be provided.

14-1

TM 5-818-1 / AFM 88-3, Chap. 7


Figure 14-1. Load diagrams for retaining walls.

14-2

TM 5-818-1 / AFM 88-3, Chap. 7


Figure 14-2. Active pressure of sand with planar boundaries.

14-3

TM 5-818-1 / AFM 88-3, Chap. 7


Figure 14-3. Passive pressure of sand with planar boundaries.

14-4

TM 5-818-1 / AFM 88-3, Chap. 7

14-4. Design procedures for retaining walls.a. Criteria forselecting earth pressures.

(1) The equivalent fluid method should beused for estimating active earth pressures on retainingstructures up to 20 feet high, with the addition to earthpressures resulting from backfill compaction (fig 14-8).

(2) For walls higher than 20 feet, charts,equations, or graphical solutions should be used forcomputing lateral earth pressures, with the addition ofearth pressures resulting from backfill compaction.

(3) Use at-rest pressures for rigid retainingstructures resting on rock or batter piles. Designcantilever walls founded on rock or restrained fromlateral movement for at-rest pressures near the base ofthe wall, active pressures along the upper portions of thewall, and compaction-induced earth pressures from thetop to the depth at which they no longer increase lateral

earth pressures (fig 14-8). Generally, a linear variationin earth pressure coefficients with depth may beassumed between the sections of wall.

(4) Consider passive pressures in thedesign if applied loads force the structure to moveagainst the soil. Passive pressures in front of retainingwalls are partially effective in resisting horizontal sliding.

b. Overturning. Calculate the factor of safety,FS, against overturning, defined as the ratio of resistingmoments to the overturning moments. Calculate theresultant force using load diagrams shown in figure 14-1,as well as other loadings that may be applicable. Useonly half of the ultimate passive resistance in calculatingthe safety factor. The resultant of all forces acting on theretaining wall should fall within the middle third to providea safety factor with respect to overturning equal to orgreater than 1.5.

(Courtesy of R. B. Peck, W. E. Hanson, and T. H.Thornburn, Foundation Engineering, 1974, p 309. Reprintedby permission of John Wiley & Sons. Inc., New York.)14-5

Figure 14-4. Active and passive earth pressure coefficients according to Coulomb theory.

14-5

TM 5-818-1 / AFM 88-3, Chap. 7

c. Sliding.(1) The factor of safety against sliding,

calculated as the ratio of forces resisting movement tothe horizontal component of earth plus water pressure onthe back wall, should be not less than 2.0. If soil in frontof the toe is disturbed or loses its strength because ofpossible excavation, ponding, or freezing and thawing,passive resistance at the toe, Pp, should be neglectedand the minimum factor of safety lowered to 1.5; but if

the potential maximum passive resistance is small, thesafety factor should remain at 2.0 or higher.

(2) For high walls, determine the shearingresistance between the base of wall and soil fromlaboratory direct shear tests in which the adhesionbetween the concrete and the undisturbed soil ismeasured. In the absence of tests, the coefficient offriction between


Figure 14-5. Horizontal pressures on walls due to surcharge.

14-6

TM 5-818-1 / AFM 88-3, Chap. 7

concrete and soil may be taken as 0.55 forcoarsegrained soils without silt, 0.45 for coarserainedsoils with silt, and 0.35 for silt. The soil in a layerbeneath the base may be weaker, and the shearingresistance between the base of wall and soil shouldnever be assumed to exceed the soil strength. Considermaximum uplift pressures that may develop beneath thebase.

(3) If the factor of safety against sliding isinsufficient, increase resistance by either increasing thewidth of the base or lowering the base elevation. If thewall is founded on clay, the resistance against slidingshould be based on su for short-term analysis and φ’ forlong-term analysis.

Figure 14-6. Design loads for low retaining walls, straight slope backfill.

14-7

TM 5-818-1 / AFM 88-3, Chap. 7

d. Bearing capacity. Calculate from thebearing capacity analysis in chapter 6. Consider localbuilding codes or experience where applicable.

e. Settlement and tilting. When a highretaining wall is to be founded on compressible soils,estimate total and differential settlements using

procedures outlined in chapter 5. Reduce excessivetotal settlement by enlarging the base width of the wall orby using lightweight backfill material. Reduce tiltinginduced by differential settlement by proportioning thesize of the base such that the resultant force falls close

Figure 14-7. Design loads for low retaining walls, broken slope backfill.

14-8

TM 5-818-1 / AFM 88-3, Chap. 7

to the center of the base. Limit differential settlement tothe amount of tilting that should not exceed 0.05H. Ifsettlements are excessive, stabilize compressible soilsby surcharge loading or a support wall on piles.

f. Deep-seated failure. Check the overallstability of the retaining wall against a deep-seatedfoundation failure using methods of analysis outlined inchapter 8. Forces considered include weight of retainingwall, weight of soil, unbalanced water pressure,equipment, and future construction. The minimum safetyfactor is 1.5.

g. Use of piles. When stability against bearingcapacity failure cannot be satisfied or settlement isexcessive, consider a pile foundation. Use batter piles ifthe horizontal thrust of the lateral earth pressure is high.

14-5. Crib wall. Design criteria of crib walls arepresented in figure 14-9.

14-6. Excavation support systems. The use ofsteep or vertical slopes for a deep excavation is oftennecessitated by land area availability or economics.


Figure 14-8. Estimates of increased pressure induced by compaction.

14-9

TM 5-818-1 / AFM 88-3, Chap. 7

Such slopes are commonly supported by a cantileverwall (only for shallow excavations), a braced wall, or atieback wall (fig 14-10). In some cases, it may beeconomical to mix systems, such as a free slope and atieback wall or a tieback wall and a braced wall. Table14-1 summarizes the wall types with their typical

properties and advantages and disadvantages. Table14-2 lists factors for selecting wall support system sfor adeep excavation (>20 feet). Table 14-3 gives designparameters, such as factors of safety, heave problems,and supplemental references.

Figure 14-9. Design criteria for crib and bin walls.

14-10

TM 5-818-1 / AFM 88-3, Chap. 7

14-7. Strutted excavations.a. Empirical design earth pressure diagrams

developed from observations are shown in figure 14-11.In soft to medium clays, a value of m = 1.0 should beapplied if a stiff stratum is present at or near the base ofthe excavation. If the soft material extends to a sufficientdepth below the bottom of the excavation and significantplastic yielding occurs, a value of 0.4 should be used for

m. The degree of plastic yielding beneath an excavationis governed by the stability number N expressed as

N = γH/su (14-2)

where γ, H, and su, are defined in figure 14-11. If Nexceeds about 4, m < 1.0.


Figure 14-10. Types of support systems for excavation.

14-11

TM 5-818-1 / AFM 88-3, Chap. 7

Table 14-1. Types of Walls

Typical EIValues per foot

Name Section ksf Advantages Disadvantages

Steel sheeting 900-90, 000 (1) Can be impervious (1) Limited stiffness(2) Easy to handle and (2) Interlocks can be

construct lost in hard driv-(3) Low initial cost ing or in gravelly

soils

Soldier pile 2,000-120,000 (1) Easy to handle and (1) Wall is perviousand lagging construct (2) Requires care in

(2) Low initial cost placement of(3) Can be driven or lagging

augered

Cast-in-place 288, 000- (1) Can be impervious (1) High initial costconcrete 2, 300, 000 (2) High stiffness (2) Specialty contractorslurry wall (3) Can be part of required to

permanent constructstructure (3) Extensive slurry

disposal needed(4) Surface can be

rough.

Precast con- 288,000- (1) Can be impervious (1) High initial costcrete slurry 2,300,000 (2) High stiffness (2) Specialty contractor

(3) Can be part of required topermanent constructstructure (3) Slurry disposal

(4) Can be prestressed needed(4) Very large and

heavy members mustbe handled for deepsystems

(5) Permits someyielding of subsoils

(Continued)

14-12

TM 5-818-1 / AFM 88-3, Chap. 7

Table 14-1. Types of Walls-Continued

Typical EIValues per foot

Name Section ksf Advantages Disadvantages

Cylinder pile 115,000- (1) Secant piles (1) High initial costwall 1,000,000 impervious (2) Secant piles require

(2) High stiffness special equipment(3) Highly specialized

equipment notneeded for tangentpiles

(4) Slurry not needed

14-13

TM 5-818-1 / AFM 88-3, Chap. 7

b. For stiff-fissured clays, diagram (c) of figure14-11 applies for any value of N. If soft clays, diagram(b) applies except when the computed maximumpressure falls below the value of the maximum pressurein diagram (c). In these cases, generally for N < 5 or 6,diagram (c) is used as a lower limit. There are no designrules for stiff intact clays and for soils characterized byboth c and φ such as sandy clays, clayey sands, orcohesive silts.

c. The upper tier of bracing should always beinstalled near the top of the cut, although computationsmay indicate that it could be installed at a greater depth.Its location should not exceed 2su below the top of thewall.

d. Unbalanced water pressures should beadded to the earth pressures where the water can movefreely through the soil during the life of the excavation.Buoyant unit weight is used for the soil below water.Where undrained behavior of a soil is considered to ap-

ply, the use of total unit weights in calculating earthpressures automatically accounts for the loads producedby groundwater (fig 14-11). Pressures due to thesurcharge load are computed as indicated in previoussections and added to the earth and water pressures.

e. Each strut is assumed to support an areaextending halfway to the adjacent strut (fig 14-11). Thestrut load is obtained by summing the pressure over thecorresponding tributary area. Temperature effects, suchas temperature increase or freezing of the retainedmaterial, may significantly increase strut loads.

f. Support is carried to the sheeting betweenthe struts by horizontal structural members (wales). Thewale members should be designed to support auniformly distributed load equal to the maximumpressure determined from figure 14-11 times the spacingbetween the wales. The wales may be assumed to besimply supported (pinned) at the struts.

Table 14-2. Factors Involved in Choice of a Support System for a Deep ExcavationLends Itself To Use Downgrades Utility

Requirement Of Of Comment

1. Open excavation area Tiebacks or rakers or Crosslot strutscantilever walls (shal-low excavation)

2. Low initial cost Soldier pile or sheetpile Diaphragm walls, cyl- Depends somewhat on 3walls; combined soil inder pile wallsslope with wall

3. Use as part of per- Diaphragm or cylinder Sheetpile or soldier Diaphragm wall most com-manent structure pile walls pile walls mon as permanent

wall4. Deep, soft clay sub- Strutted or raker sup- Tiebacks, flexible Tieback capacity not

surface conditions ported diaphragm or walls adequate in soft clayscylinder pile walls

5. Dense, gravelly sand Soldier pile, diaphragm Sheetpile walls Sheetpile walls lose in-or clay subsoils or cylinder pile terlock on hard driving

6. Deep, overconsoli- Struts, long tiebacks Short tiebacks High lateral stresses aredated clays or combination tie- relieved in O.C. soils

backs and struts and lateral movements(fig. 14-10) may be large and

extend deep into soil7. Avoid dewatering Diaphragm walls, pos- Soldier pile wall Soldier pile wall is

sibly sheetpile walls perviousin soft subsoils

8. Minimize movements High preloads on stiff Flexible walls Analyze for stability ofstrutted or tied-back bottom of excavationwall

9. Wide excavation Tiebacks or rakers Crosslot struts Tiebacks preferable ex-(greater than 20m cept in very soft claywide) subsoils

10. Narrow excava- Crosslot struts Tiebacks or rakers Struts more economicaltion (less than but tiebacks still may20m wide) be preferred to keep

excavation open


14-14

TM 5-818-1 / AFM 88-3, Chap. 7

Table 14-3. Design Considerations for Braced and Tieback Walls

Design Factor Comments

1. Earth loads For struts, select from the semiempirical diagrams (fig. 14-10); for wallsand wales use lower loads - reduce by 25 percent from strut loading.Tiebacks may be designed for lower loads than struts unless preloaded tohigher values to reduce movements

2. Water loads Often greater than earth load on impervious wall.Should consider possi-ble lower water pressures as a result of seepage through or under wall.Dewatering can be used to reduce water loads

3. Stability Consider possible instability in any berm or exposed slope.Sliding po-tential beneath the wall or behind tiebacks should be evaluated. Deepseated bearing failure under weight of supported soil to be checked insoft cohesive soils (fig. 14-12)

4. Piping Loss of ground caused by high groundwater tables and silty soils.Difficulties occur due to flow beneath wall, through bad joints in wall,or through unsealed sheetpile handling holes. Dewatering may berequired.

5. Movements Movements can be minimized through use of stiff impervious wall supportedby preloaded tieback or braced system. Preloads should be at the levelof load diagrams (fig. 14-11) for minimizing movements

6. Dewatering - recharge Dewatering reduces loads on wall systems and minimizes possible loss ofground due to piping. May cause settlements and will then need to re-charge outside of support system. Not applicable in clayey soils

7. Surcharge Storage of construction materials usually carried out near wall systems.Allowance should always be made for surcharge, especially in uppermembers

(Continued)

14-15

TM 5-818-1 / AFM 88-3, Chap. 7

Table 14-3. Design Considerations for Braced and Tieback Walls-Continued

Design Factor Comments

8. Preloading Useful to remove slack from system and minimize soil movements.Preloadup to the load diagram loads (fig. 14-10) to minimize movements

9. Construction sequence Sequence used to build wall important in loads and movements of system.Moments in walls should be checked at every major construction stagefor maximum condition. Upper struts should be installed early

10. Temperature Struts subject to load fluctuation due to temperature loads; may be impor-tant for long struts

11. Frost penetration In very cold climates, frost penetration can cause significant loading onwall system. Design of upper portion of system should be conservative.Anchors may have to be heated

12. Earthquakes Seismic loads may be induced during earthquake. Local codes often govern

13. Codes For shallow excavations, codes completely specify support system.Variesfrom locality to locality. Consult OSHA requirements

Minimum Design14. Factors of safety Item Factor of Safety

Earth Berms 2.0Critical Slopes 1.5Noncritical Slopes 1.2Basal Heave 1.5General Stability 1.5

14-16

TM 5-818-1 / AFM 88-3, Chap. 7

14-8. Stability of bottom of excavation.a. Piping in sand. The base of an excavation

in sand is usually stable unless an unbalancedhydrostatic head creates a "quick" condition. Among themethods to eliminate instability are dewatering,application of a surcharge load at the bottom of theexcavation, and deeper penetration of the piling.

b. Heaving in clays. The stability againstheave of the bottom of an excavation in soft clay may beevaluated from figure 14-12. If the factor of safety is lessthan 1.5, the piling should be extended below the base ofthe excavation. Heave may also occur because of

unrelieved hydrostatic pressures in a permeable layerlocated below the clay.

c. Care of seepage. Small amounts ofseepage into the excavation can be controlled bypumping from sumps. Such seepage can be expected ifthe excavation extends below the water table intopermeable soils. If the soils consist of fine sands andsilts, the sumps should be routinely monitored forevidence of fines being washed from the soil byseepage. If large quantities of fine-grained materials arefound in the sumps, precautionary steps should be takento make the lagging or sheeting watertight to avoidexcessive settlements adjacent to the excavation.


Figure 14-11. Pressure distribution-complete excavation.

14-17

TM 8-818-1 / AFM 88-3, Chap. 7


Figure 14-12. Stability of bottom of excavation in clay.

14-18

TM 5-818-1 / AFM 88-3, Chap. 7

14-9. Anchored walls.a. Tiebacks have supplanted both strut and

raker systems in many instances to support wideexcavations. The tieback (fig 14-13) connects the wall toan anchorage located in a zone where significant soilmovements do not occur. The anchorage may be in soilor rock; soft clays probably present the only conditionwhere an anchorage in soil cannot be obtained reliably.In figure 14-13, the distance Lub should extend beyondthe "Rankine" zone some distance. This distance isnecessary, in part, to obtain sufficient elongation inanchored length of rod La during jacking so that soilcreep leaves sufficient elongation that the design load isretained in the tendon. After jacking, if the soil iscorrosive and the excavation is open for a long time, thezone Lub may be grouted. Alternatively, the length oftendon Lub is painted or wrapped with a greaseimpregnated wrapper (prior to placing in position).

b. The tieback tendon may be either a singlehigh-strength bar or several high-strength cables (fy on

the order of 200 to 270 kips per square inch) bunched. Itis usually inclined so as to reach better bearing material,to avoid hole collapse during drilling, and to pass underutilities. Since only the horizontal component of thetendon force holds the wall, the tendon should beinclined a minimum.

c. Tieback anchorages may be drilled usingcontinuous flight earth augers (commonly 4 to 7 inches indiameter) and may require casing to hold the hole untilgrout is placed in the zone La of figure 14-13, at whichtime the casing is withdrawn. Grout is commonly usedunder a pressure ranging from 5 to 150 pounds persquare inch. Underreaming may be used to increase theanchor capacity in cohesive soil. Belling is not possiblein cohesionless soils because of hole caving. Typicalformulas that can be used to compute the capacity oftieback anchorages are given in figure 14-14.

d. Exact knowledge of the anchor capacity isnot


Figure 14-13. Typical tieback details.

14-19

TM 5-818-1 / AFM 88-3, Chap. 7


Figure 14-14. Methods of calculating anchor capacities in soil.

14-20

TM 5-818-1 / AFM 88-3, Chap. 7

needed as all the anchors are effectively "proof-tested"(about 120 to 150 percent of design load) when thetendons are tensioned for the design load. One or moreanchors may be loaded to failure; however, as the costof replacing a failed anchor is often two to three timesthe cost of an initial insertion, care should be taken not

to fail a large number of anchors in any-test program. Ifthe tieback extends into the property of others,permission, and possibly a fee, will be required. Thetieback tendons and anchorages should normally be leftin situ after construction is completed. See table 14-3 foradditional design considerations.

14-21

TM 5-818-1 / AFM 88-3, Chap. 7

CHAPTER 15

FOUNDATIONS ON FILL AND BACKFILLING

15-1. Types of fill.a. Fills include conventional compacted fills;

hydraulic fills; and uncontrolled fills of soils or industrialand domestic wastes, such as ashes, slag, chemicalwastes, building rubble, and refuse. Properly placedcompacted fill will be more rigid and uniform and havegreater strength than most natural soils. Hydraulic fillsmay be compacted or uncompacted and are aneconomical means of providing fill over large areas.Except when cohesionless materials, i.e., clean sandsand gravels, are placed under controlled conditions sosilty pockets are avoided and are compacted as they areplaced, hydraulic fills will generally require some type ofstabilization to ensure adequate foundations.

b. Uncontrolled fills are likely to provide avariable bearing capacity and result in a nonuniformsettlement. They may contain injurious chemicals and,in some instances, may be chemically active andgenerate gases that must be conducted away from thestructure. Foundations on fills of the second and thirdgroups (and the first group if not adequately compacted)should be subjected to detailed investigations todetermine their suitability for supporting a structure, orelse they should be avoided. Unsuitable fills often canbe adequately stabilized.

15-2. Foundations on compacted fills.a. Compacted fill beneath foundations.

Compacted fills are used beneath foundations where it isnecessary to raise the grade of the structure aboveexisting ground or to replace unsatisfactory surface soils.Fills constructed above the natural ground surfaceincrease the load on underlying soils, causing largersettlements unless construction of the structure ispostponed until fill-induced settlements have takenplace. Settlements beneath a proposed fill can becomputed using methods outlined in chapter 5. Ifcomputed settlements are excessive, considersurcharging and postponing construction until theexpected settlement under the permanent fill loading hasoccurred. Extend the fill well beyond the loading area,except where the fill is placed against a cut slope.Where the fill is relatively thick and is underlain by softmaterials, check its stability with respect to deep sliding.If the fill is underlain by weaker materials, found thefootings on the fill unless settlement is excessive. If thefill is underlain by a stronger material, the footings maybe founded on the fill or on the stronger material.

b. Foundations partially on fill. Where aSloping ground surface or variable foundation depthswould result in supporting a foundation partially onnatural soil, or rock, and partially on compacted fill,settlement analyses are required to estimate differentialsettlements. In general, a vertical joint in the structureshould be provided, with suitable architectural treatment,at the juncture between the different segments offoundations. The subgrade beneath the portions offoundations to be supported on natural soils or rockshould be undercut about 3 feet and replaced bycompacted fill that is placed at the same time as the fillfor the portions to be supported on thicker compacted fill.

c. Location of borrow. Exploratoryinvestigations should be made to determine the suitablesources of borrow material. Laboratory tests todetermine the suitability of available materials includenatural water contents, compaction characteristics, grain-size distribution, Atterberg limits, shear strength, andconsolidation. Typical properties of compacted materialsfor use in preliminary analyses are given in table 3-1.The susceptibility to frost action also should beconsidered in analyzing the potential behavior of fillmaterial. The scope of laboratory testing on compactedsamples depends on the size and cost of the structure,thickness and extent of the fill, and also strength andcompressibility of underlying soils. Coarse-grained soilsare preferred for fill; however, most fine-grained soils canbe used advantageously if attention is given to drainage,compaction requirements, compaction moisture, anddensity control.

d. Design of foundations on fill. Foundationscan be designed on the basis of bearing capacity andsettlement calculations described in chapter 10. Thesettlement and bearing capacity of underlying foundationsoils also should be evaluated. Practically all types ofconstruction can be founded on compacted fills,provided the structure is designed to tolerate anticipatedsettlements and the fill is properly placed andcompacted. Good and continuous field inspection isessential.

e. Site preparation. The site should beprepared by clearing and grubbing all grass, trees,shrubs, etc. Save as many trees as possible forenvironmental considerations. The topsoil should bestripped and stockpiled for later landscaping of fill andborrow areas. Placing and compacting fills shouldpreferably be done when

15-1

TM 5-818-1 / AFM 88-3, Chap. 7

the area is still unobstructed by footings or otherconstruction. The adequacy of compacted fills forsupporting structures is dependent chiefly on theuniformity of the compaction effort. Compactionequipment generally can be used economically andefficiently only on large areas. Adverse weatherconditions may have a pronounced effect on the cost ofcompacted fills that are sensitive to placement moisturecontent, i.e., on materials having more than 10 to 20percent finer than the No. 200 sieve, depending ongradation.

f. Site problems. Small building areas orcongested areas where many small buildings or utilitylines surround the site present difficulties in regard tomaneuvering large compaction equipment. Backfillingadjacent to structures also presents difficulties, andpower hand-tamping equipment must be employed, withconsiderable care necessary to secure uniformcompaction. Procedures for backfilling around structuresare discussed in TM 5-818-4 / AFM 88-5, Chapter 5.

15-3. Compaction requirements.a. General. Guidelines for selecting

compaction equipment and for establishing compactionrequirements for various soil types are given in table 15-1. If fill materials have been thoroughly investigated andthere is ample local experience in compacting them, it ispreferable to specify details of compaction procedures,such as placement water content, lift thickness, type ofequipment, and number of passes. When the source ofthe fill or the type of compaction equipment is not knownbeforehand, specifications should be based on thedesired compaction result, with a specified minimumnumber of coverages of suitable equipment to assureuniformity of compacted densities.

b. Compaction specifications. For mostprojects the placement water content of soils sensitive tocompaction moisture should be within the range of -1 to+ 2 percent of optimum water content for the fieldcompaction effort applied. Each layer is compacted tonot less than the percentage of maximum densityspecified in table 15-2. It is generally important tospecify a high degree of compaction in fills understructures to minimize settlement and to ensure stabilityof a structure. In addition to criteria set forth in TM 5-818-4/AFM 88-5, Chapter 5, the following factors shouldbe considered in establishing specific requirements:

(1) The sensitivity of the structure to totaland differential settlement as related to structural designis particularly characteristic of structures to be foundedpartly on fill and partly on natural ground.

(2) If the ability of normal compactionequipment to produce desired densities in existing orlocally available materials within a reasonable range ofplacement water content is considered essential, specialequipment should be specified.

(3) The compaction requirements forclean, cohesionless, granular materials will be generallyhigher than those for cohesive materials, becausecohesionless materials readily consolidate, or liquify,when subjected to vibration. For structures with unusualstability requirements and settlement limitations, theminimum density requirements indicated in table 15-2should be increased. For coarse-grained, well-graded,cohesionless soils with less than 4 percent passing theNo. 200 sieve, or for poorly graded cohesionless soilswith less than 10 percent, the material should becompacted at the highest practical water content,preferably saturated. Compaction by vibratory rollersgenerally is the most effective procedure. Experienceindicates that pervious materials can be compacted to anaverage relative density of 85 + 5 percent with nopractical difficulty. For cohesionless materials, stipulatethat the fill be compacted to either a minimum density of85 percent relative density or 95 percent of CE 55compaction effort, whichever gives the greater density.

(4) If it is necessary to use fill materialhaving a tendency to swell, the material should becompacted at water contents somewhat higher thanoptimum and to no greater density than required forstability under proposed loadings (table 15-2). Thebearing capacity and settlement characteristics of the fillunder these conditions should be checked by laboratorytests and analysis. Swelling clays can, in someinstances, be permanently transformed into soils of lowerplasticity and swelling potential by adding a smallpercentage of hydrated lime (chap 16).

c. Compacted rock. Compacted crushed rockprovides an excellent foundation fill. Vibratory rollers arepreferable for compacting rock. Settlement of fill underthe action of the roller provides the most usefulinformation for determining the proper loose liftthickness, number of passes, roller type, and materialgradation. Compaction with a 10-ton vibratory roller isgenerally preferable. The rock should be kept watered atall times during compaction to obviate collapsesettlement on loading and first wetting. As generalcriteria for construction and control testing ofrock fill are not available, test fills should be employedwhere previous experience is inadequate and for largeimportant rock fills.

15-4. Placing and control of backfill. Backfill shouldbe placed in lifts no greater than shown in table 15-1,preferably 8 inches or less and dependingon the soil andtype of equipment available. No backfill should beplaced that contains frozen lumps of soil, as laterthawing will produce local soft spots. Backfill should notbe placed on muddy, frozen, or frost-cov-

15-2

TM 5-818-1 / AFM 88-3, Chap. 7

Table 15-1. A summary of Density Methods for Building Foundation


15-3

TM 5-818-1 / AFM 88-3, Chap. 7

Table 15-2. Compaction Density as a Percent of CE 55 Labortory Test Density

CE 55 Maximum Density, %Cohesive Cohesionless

Soils SoilsFill, embankment, and backfillUnder proposed structures, building 90 95a

slabs, steps, and paved areas

Under sidewalks and grassed areas 85 90

Subgrade

Under building slabs, steps, and paved 90 95areas, top 12 in.

Under sidewalks, top 6 in. 85 90

a May be 85% relative density, whichever is higher.

ered ground. Methods of compaction controf duringconstruction are described in TM 5-818-4/AFM 88-5,Chapter 5.

15-5. Fill settlements. A fill thickness of even 3 feetis a considerable soil load, which will increase stressesto a substantial depth (approximately 2B, where B =smallest lateral dimension of the fill). Stress increasesfrom the fill may be larger than those from structurefootings placed on the fill. Use procedures outlined inchapter 10 to obtain expected settlements caused by fillloading. Many fills are of variable thickness, especiallywhere an area is landscaped via both cutting and fillingto obtain a construction site. In similar cases, attentionshould be given to building locations with respect tocrossing cut and fill lines so that the proper type of-building settlement can be designed (building may act asa cantilever, or one end tends to break off, or as a beamwhere the interior sags). Proper placing of reinforcingsteel in the wall footings (top for cantilever action orbottom for simple beam action) may help control buildingcracks where settlement is inevitable; building joints canbe provided at critical locations if necessary. Thecombined effect of structure (one- and two-storyresidences) and fill loading for fills up to 10 feet inthickness on sound soil and using compaction controlshould not produce a differential settlement of either asmooth curved hump or sag of 1 inch in 50 feet or auniform slope of 2 inches in 50 feet.

15-6. Hydraulic fills. Hydraulic fills are placed onland or underwater by pumping material through apipeline from a dredge or by bottom dumping frombarges. Dredge materials vary from sands to silts andfine-grained silty clays and clays. Extensivemaintenance dredging in the United States has resultedin disposal areas for dredge materials, which are

especially attractive from an economic standpoint fordevelopment purposes. Dikes are usually required toretain hydraulic fills on land and may be feasible forunderwater fills. Underwater dikes may be constructedof large stones and gravel.

a. Pervious fills. Hydraulically placed perviousfills with less than 10 percent fines will generally be at arelative density of 50 to 60 percent but locally may belower. Controlled placement is necessary to avoid siltconcentrations. Compaction can be used to producerelative densities sufficient for foundation support (table15-1). Existing uncompacted hydraulic fills of perviousmaterials in seismic areas are subject to liquefaction,and densification will be required if important structuresare to be founded on such deposits. Rough estimates ofrelative density may be obtained using standardpenetration resistance. Undisturbed borings will berequired to obtain more precise evaluation of in situdensity and to obtain undisturbed samples for cyclictriaxial testing, if required. For new fills, the coarsestmaterials economically available should be used. Unlessspecial provisions are made for removal of fines, borrowcontaining more than 10 percent fines passing the No.200 sieve should be avoided, and even then controlledplacement is necessary to avoid local silt concentrations.

b. Fine-grained.fills. Hydraulically placedoverconsolidated clays excavated by suction dredgesproduce a fill of clay balls if fines in the washwater arepermitted to run off. The slope of such fills will beextremely flat ranging from about 12 to 16H on 1V.

(1) These fills will undergo largeimmediate con-

15-4

TM 5-818-1 / AFM 88-3, Chap. 7

solidation for about the first 6 months until the clay ballsdistort to close void spaces. Additional settlements for a1-year period after this time will total about 3 to 5 percentof the fill height.

(2) Maintenance dredgings andhydraulically placed normally consolidated clays willinitially be at water contents between 4 and 5 times theliquid limit. Depending on measures taken to inducesurface drainage, it will take approximately 2 yearsbefore a crust is formed sufficient to support lightequipment and the water content of the underlyingmaterials approaches the liquid limit. Placing 1 to 3 feetof additional conhesionless borrow can be used toimprove these areas rapidly so that they can supportsurcharge fills, with or without vertical sand drains toaccelerate consolidation. After consolidation, substantialone- or two-story buildings and spread foundations canbe used without objectionable settlement. Considerablecare must be used in applying the surcharge so that theshear strength of the soil is not exceeded (i.e., use lightequipment).

c. Settlements of hydraulic fills. If thecoefficient of permeability of a hydraulic fill is less than0.0002 foot per minute, the consolidation time for the fillwill be long and prediction of the behavior of thecompleted fill will be difficult. For coarse-grainedmaterials with a larger coefficient of permeability, fillconsolidation and strength buildup will be relatively rapidand reasonable strength estimates can be made. Wherefill and foundation soils are fine-grained with a lowcoefficient of permeability, piezometers should be placedboth in the fill and in the underlying soil to monitor

pore pressure dissipation. It may also be necessary toplace settlement plates to monitor the settlement.Depending on the thickness of the fill, settlement platesmay be placed both on the underlying soil and within thefill to observe settlement rates and amounts.

d. Compaction of hydraulic fills. Dike-landhydraulic fills can be compacted as they are placed byuse of-

(1) Driving track-type tractors back andforth across the saturated fill. (Relative densities of 70 to80 percent can be obtained in this manner forcohesionless materials.)

(2) Other methods such as vibratoryrollers, vibroflotation, terraprobing, and compaction piles(chap 16). Below water, hydraulic fills can be compactedby use of terraprobing, compaction piles, and blasting.

e. Underwater hydraulic fills. For structural fillplaced on a dredged bottom, remove the fines dispersedin dredging by a final sweeping operation, preferably withsuction dredges, before placing the fill. To preventextremely flat slopes at the edge of a fill, avoid excessiveturbulence during dumping of the fill material by placingwith clamshell or by shoving off the sides of deck barges.To obtain relatively steep slopes in underwater fill, usemixed sand and gravel. With borrow containing aboutequal amounts of sand and gravel, underwater slopes assteep as 1V on 2H may be achieved by carefulplacement. Uncontrolled bottom dumping from bargesthrough great depths of water will spread the fill over awide area. To confine such fill, provide berms or dikes ofthe coarsest available material or stone on the fillperimeter.

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TM 5-818-1 / AFM 88-3, Chap. 7

CHAPTER 16

STABILIZATION OF SUBGRADE SOILS

16-1. General.a. The applicability and essential features of

foundation soil treatments are summarized in tables 16-1and 16-2 and in figure 16-1. The depth of stabilizationgenerally must be sufficient to absorb most of thefoundation pressure bulb.

b. The relative benefits of vibrocompaction,vibrodisplacement compaction, and precompressionincrease as load intensity decreases and size of loadedarea increases. Soft, cohesive soils treated in place aregenerally suitable only for low-intensity loadings. Soilstabilization of wet, soft soils may be accomplished byaddition of lime; grout to control water flow intoexcavations to reduce lateral support requirements or toreduce liquefaction or settlement caused by adjacent piledriving; seepage control by electroosmosis; andtemporary stabilization by freezing. The range of soilgrain sizes for which each stabilization method is mostapplicable is shown in figure 16-1.

16-2. Vibrocompaction. Vibrocompaction methods(blasting, terraprobe, and vibratory rollers) can be usedfor rapid densification of saturated cohesionless soils (fig16-1). The ranges of grain-size distributions suitable fortreatment by vibrocompaction, as well as vibroflotation,are shown in figure 16-2. The effectiveness of thesemethods is greatly reduced if the percent finer than theNo. 200 sieve exceeds about 20 percent or if more thanabout 5 percent is finer than 0.002 millimeter, primarilybecause the hydraulic conductivity of such materials istoo low to prevent rapid drainage following liquefaction.The usefulness of these methods in partly saturatedsands is limited, because the lack of an increase of porewater pressure impedes liquefaction. Lack of completesaturation is less of a restriction to use of blastingbecause the high-intensity shock wave accompanyingdetonation displaces soil, leaving depressions that latercan be backfilled.

a. Blasting.(1) Theoretical design procedures for

densification by blasting are not available and continuouson-site supervision by experienced engineers havingauthority to modify procedures as required is esential ifthis treatment method is used. A surface heave of about6 inches will be observed for proper charge sizes andplacement depths. Surface cratering should be avoided.Charge masses of less than 4 to more than 60 pounds

have been used. The effective radius of influence forcharges using (M = lb) 60 percent dynamite is as follows:

R = 3M1/3 (feet) (16-1)Charge spacings of 10 to 25 feet are typical. The centerof charges should be located at a depth of about two-thirds the thickness of the layer to be densified, andthree to five successive detonations of several spacedcharges each are likely to be more effective than a singlelarge blast. Little densification is likely to result aboveabout a 3-foot depth, and loosened material may remainaround blast points. Firing patterns should beestablished to avoid the "boxing in" of pore water. Free-water escape on at least two sides is desirable.

(2) If blasting is used in partly saturatedsands or loess, preflooding of the site is desirable. Inone technique, blast holes about 3 to 3/2 inches indiameter are drilled to the desired depth of treatment,then small charges connected by prima cord, or simplythe prima cord alone, are strung the full depth of thehole. Each hole is detonated in succession, and theresulting large diameter holes formed by lateraldisplacement are backfilled. A sluiced-in cohesionlessbackfill will densify under the action of vibrations fromsubsequent blasts. Finer grained backfills can bedensified by tamping.

b. Vibratingprobe (terraprobe).(1) A 30-inch-outside-diameter, open-

ended pipe pile with 3/, -inch wall thickness is suspendedfrom a vibratory pile driver operating at 15 Hz. A probelength 10 to 15 feet greater than the soil depth to bestabilized is used. Vibrations of 7%- to 1-inch amplitudeare in a vertical mode. Probes are made at spacings of3 to 10 feet. After sinkage to the desired depth, theprobe is held for 30 to 60 seconds before extraction.The total time required per probe is typically 21/2 to 4minutes. Effective treatment has been accomplished atdepths of 12 to 60 feet. Areas in the range of 450 to 700square yards may be treated per machine per 8-hourshift.

(2) Test sections about 30 to 60 feet on aside are desirable to evaluate the effectiveness andrequired probe spacing. The grain-size range of treatedsoil should fall within limits shown in figure 16-2. Asquare pattern is often used, with a fifth probe at thecenter of each square giving more effective increaseddensification than a reduced spacing. Saturated soil

16-1

TM 5-818-1 / AFM 88-3, Chap. 7

Table 16-1. Stabilization of Soils for Foundations of Structures

16-2

TM 5-818-1 / AFM 88-3, Chap. 7

Table 16-1. Stabilization of Soils for Foundations of Structures-Continued

16-3

TM 5-818-1 / AFM 88-3, Chap. 7


16-4

TM 5-818-1 / AFM 88-3, Chap. 7


16-5

TM 5-818-1 / AFM 88-3, Chap. 7



16-6

TM 5-818-1 / AFM 88-3, Chap. 7


Table 16-2. Applicability of Foundation Soil Improvement for Different Structures and Soil Types (for efficient use of Shallow Foundations)

16-7

TM 5-818-1 / AFM 88-3, Chap. 7

(Courtesy of J. K. Mitchell, "Innovations in Ground Stabilization," Chicago Soil Mechanics LectureSeries, Innovations in Foundation Construction, Illinois Section, 1972. Reprinted by permission of TheAmerican Society of Civil Engineers, New York.)

Figure 16-1. Applicable grain-size ranges for different stabilization methods.

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TM 5-818-1 / AFM 88-3, Chap. 7

(Courtesy of J. K. Mitchell, "Innovations in Ground Stabilization," Chicago Soil Mechanics LectureSeries, Innovations in Foundation Construction, Illinois Section, 1972. Reprinted by permission of TheAmerican Society of Civil Engineers, New York.)

Figure 16-2. Range of particle-size distributions suitable for densification by vibrocompaction.

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TM 5-818-1 / AFM 88-3, Chap. 7

conditions are necessary. Underlying soft clay layersmay dampen vibrations.

c. Vibratory rollers. Where cohesionlessdeposits are of limited thickness, e.g., less than 6 feet, orwhere cohesionless fills are being placed, vibratoryrollers are likely to be the best and most economicalmeans for achieving high density and strength. Use withflooding where a source of water is available. Theeffective depth of densification may be 6 feet or more forthe heaviest vibratory rollers (fig 16-3a). For a fill placedin successive lifts, a density-depth distribution similar tothat in figure 16-3b results. It is essential that the liftthickness, soil type, and roller type be matched. Properlymatched systems can yield compacted layers at arelative density of 85 to 90 percent or more.

16-3. Vibrodisplacement compaction. The methodsin this group are similar to those described in thepreceding section except that the vibrations aresupplemented by active displacement of the soil and, inthe case of vibroflotation and compaction piles, bybackfilling the zones from which the soil has beendisplaced.

a. Compaction piles. Partly saturated or freelydraining soils can be effectively densified andstrengthened by this method, which involves drivingdisplacement piles at close spacings, usually 3 to 6 feeton centers. One effective procedure is to captemporarily the end of a pipe pile, e.g., by a detachableplate, and drive it to the desired depth, which may be upto 60 feet. Either an impact hammer or a vibratory drivercan be used. Sand or other backfill material isintroduced in lifts with each lift compacted concurrentlywith withdrawal of the pipe pile. In this way, not only isthe backfill compacted, but the compacted column hasalso expanded laterally below the pipe tip forming acaisson pile.

b. Heavy tamping (dynamic consolidation).(1) Repeated impacts of a very heavy

weight (up to 80 kips) dropped from a height of 50 to 130feet are applied to points spaced 15 to 30 feet apart overthe area to be densified. In the case of cohesionlesssoils, the impact energy causes liquefaction followed bysettlement as water drains. Radial fissures that formaround the impact points, in some soils, facilitatedrainage. The method has been used successfully totreat soils both above and below the water table.

(2) The product of tamper mass andheight of fall should exceed the square of the thicknessof layer to be densified. A total tamping energy of 2 to 3blows per square yard is used. Increased efficiency isobtained if the impact velocity exceeds the wave velocityin the liquefying soil. One crane and tamper can treatfrom 350 to 750 square yards per day. Economical useof the method in sands requires a minimum treatmentarea of 7500 square yards. Relative densities of 70 to 90

percent are obtained. Bearing capacity increases of 200to 400 percent are usual for sands and marls, with acorresponding increase in deformation modulus. Thecost is reported as low as one-fourth to one-third that ofvibroflotation.

(3) Because of the high-amplitude, low-frequency vibrations (2-12 Hz), minimum distancesshould be maintained from adjacent facilities as follows:

Piles or bridge abutment 15-20 feetLiquid storage tanks 30 feetReinforced concrete buildings 50 feetDwellings 100 feetComputers (not isolated) 300 feetc. Vibroflotation.

(1) A cylindrical penetrator about 15inches in diameter and 6 feet long, called a vibroflot, isattached to an adapter section containing lead wires andhoses. The whole assembly is handled by a crane. Arotating eccentric weight inside the vibroflot develops ahorizontal centrifugal force of about 10 tons at 1800revolutions per minute. Total weight is about 2 tons.

(2) To sink the vibroflot to the desiredtreatment depth, a water jet at the tip is opened and actsin conjunction with the vibrations so that a hole can beadvanced at a rate of about 3.6 feet per minute; then thebottom jet is closed, and the vibroflot is withdrawn at arate of about 0.1 foot per minute. Newer, heaviervibroflots operating at 100 horsepower can be withdrawnat twice this rate and have a greater effective penetrationdepth. Concurrently, a cohesionless sand or gravelbackfill is dumped in from the ground surface anddensified. Backfill consumption is at a rate of about 0.7to 2 cubic yards per square yard of surface. In partlysaturated sands, water jets at the top of the vibroflot canbe opened to facilitate liquefaction and densification ofthe surrounding ground. Liquefaction occurs to a radialdistance of 1 to 2 feet from the surface of the vibroflot.Most vibroflotation applications have been to depths lessthan 60 feet, although depths of 90 feet have beenattained successfully.

(3) A relationship between probablerelative density and vibroflot hold spacings is given infigure 16-4. Newer vibroflots result in greater relativedensities. Figure 16-5 shows relationships betweenallowable bearing pressure to limit settlements to 1 inchand vibroflot spacing. Allowable pressures for"essentially cohesionless fills" are less than for cleansand deposits, because such fills invariably containsome fines and are harder to densify.

(4) Continuous square or triangularpatterns are often used over a building site.Alternatively, it may be desired to improve the soil only atthe locations of individual spread footings. Patterns andspacings required for an allowable pressure of 3 tons persquare foot and square footings are given in table 16-3.

16-10

TM 5-818-1 / AFM 88-3, Chap. 7

a. DENSITY VERSUS DEPTH FOR DIFFERENTNUMBERS OF ROLLER PASSES

b. DENSITY VERSUS DEPTH RELATIONSHIP FOR ASERIES OF 2-FT LIFTS

Figure 16-3. Sand densification using vibratory rollers.

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TM 5-818-1 / AFM 88-3, Chap. 7

16-4. Grouting and injection. Grouting is a high-costsoil stabilization method that can be used where there issufficient confinement to permit required injectionpressures. It is usually limited to zones of relatively smallvolume and to special problems. Some of the moreimportant applications are control of groundwater duringconstruction; void filling to prevent excessive settlement;strengthening adjacent foundation soils to protect againstdamage during excavation, pile driving, etc.; soilstrengthening to reduce lateral support requirements;stabilization of loose sands against liquefaction;foundation underpinning; reduction of machinefoundation vibrations; and filling solution voids incalcareous materials.

a. Grout types and groutability. Grouts can beclassified as particulate or chemical. Portland cement isthe most widely used particulate grouting material.Grouts composed of cement and clary are also widelyused, and lime-slurry injection is finding increasingapplication. Because of the silt-size particles in thesematerials, they cannot be injected into the pores of soilsfiner than medium to coarse sand. For successfulgrouting of soils, use the following guide

(D15)soil > 25(D85)grout

Type I portland cement, Typeportland cement, and TypeIII portland cement, Type III portland cement, andprocessed bentonite cannot be used to penetrate soilsfiner than 30, 40, and 60 mesh sieve sizes, respectively.Different types of grouts may be combined to bothcoarse- and fine-grained soils.

b. Cement and soil-cement grouting. See TM5-818-6/AFM 88-32 for discussion of planning andimplementation of foundation grouting with cement andsoil-cement.

c. Chemical grouting. To penetrate the voidsof finer soils, chemical grout must be used. The mostcommon classes of chemical grouts in current use aresilicates, resins, lignins, and acrylamides. The viscosityof the chemical-water solution is the major factorcontrolling groutability. The particle-size ranges over


Figure 16-4. Relative density as a function of vibroflot hole spacings.

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TM 5-818-1 / AFM 88-3, Chap. 7


Figure 16-5. Allowable bearing pressure on cohesionless soil layers stabilized by vibroflotation.

Table 16-3. Vibroflotation Patterns for Isolated Footings for an Allowable Bearing Pressure.

Square Footing Vibroflotation Center to CenterSize, ft Points Spacing, ft Pattern

4.0 1 --- ---4.5-5.5 2 6.0 Line

6-7 3 7.5 Triangle7.5-9.5 4 6.0 Square10-12 5 7.5 Square + 1

@ Center


16-13

TM 5-818-1 / AFM 88-3, Chap. 7

which each of these grout types is effective is shown infigure 16-6.

16-5. Precompression.a. Preloading. Earth fill or other material is

placed over the site to be stabilized in amounts sufficientto produce a stress in the soft soil equal to thatanticipated from the final structures. As the timerequired for consolidation of the soft soil may be long(months to years), varying directly as the square of thelayer thickness and inversely as the hydraulicconductivity, preloading alone is likely to be suitable onlyfor stabilizing thin layers and with a long period of timeavailable prior to final development of the site.

b. Surcharge fills. If the thickness of the fillplaced for pre-loading is greater than that required toinduce stresses corresponding to structure-inducedstresses, the excess fill is termed a surcharge fill.Although the rate of consolidation is essentiallyindependent of stress increase, the amount ofconsolidation varies approximately in proportion to thestress increase. It follows, therefore, that the preloadingfill plus surcharge can cause a given amount ofsettlement in shorter time than can the preloading fillalone. Thus, through the use of surcharge fills, the timerequired for preloading can be reduced significantly.

(1) The required surcharge and loadingperiod can be determined using conventional theories ofconsolidation. Both primary consolidation and most ofthe secondary compression settlements can be takenout in advance by surcharge fills. Secondarycompression settlements may be the major part of thetotal settlement of highly organic deposits or old sanitarylandfill sites.

(2) Because the degree of consolidationand applied stress vary with depth, it is necessary todetermine if excess pore pressures will remain at anydepth after surcharge removal. If so, further primaryconsolidation settlement under permanent loadingswould occur. To avoid this occurrence, determine theduration of the surcharge loading required for pointsmost distant from drainage boundaries.

(3) The rate and amount of preload maybe controlled by the strength of the underlying soft soil.Use berms to maintain foundation stability and place fillin stages to permit the soil to gain strength fromconsolidation. Predictions of the rates of consolidationstrength and strength gain should be checked during fillplacement by means of piezometers, borings, laboratorytests, and in situ strength tests.

c. Vertical drains.(1) The required preloading time for most

soft clay deposits more than about 5 to 10 feet thick willbe large. The consolidation time may be reduced byproviding a shorter drainage path by installing verticalsand drains. Sand drains are typically 10 to 15 inches

in diameter and are installed at spacings of 5 to 15 feet.A sand blanket or a collector drain system is placed overthe surface to facilitate drainage. Other types of drainsavailable are special cardboard or combination plastic-cardboard drains. Provisions should be made to monitorpore pressures and settlements with time to determinewhen the desired degree of precompression has beenobtained.

(2) Both displacement andnondisplacement methods have been used for installingsand drains. Although driven, displacement drains areless expensive than augered or "bored" nondisplacementdrains; they should not be used in sensitive deposits or instratified soils that have higher hydraulic conductivity inthe horizontal than in the vertical direction. Verticaldrains are not needed in fibrous organic depositsbecause the hydraulic conductivity of these materials ishigh, but they may be required in underlying soft clays.

d. Dynamic consolidation (heavy tamping).Densification by heavy tamping has also been reportedas an effective means for improving silts and clays, withpreconstruction settlements obtained about 2 to 3 timesthe predicted construction settlement. The time requiredfor treatment is less than for surcharge loading with sanddrains. The method is essentially the same as that usedfor cohesionless soils, except that more time is required.Several blows are applied at each location followed by a1- to 4-week rest period, then the process is repeated.Several cycles may be required. In each cycle thesettlement is immediate, followed by drainage of porewater. Drainage is facilitated by the radial fissures thatform around impact points and by the use of horizontaland peripheral drains. Because of the necessity for atime lapse between successive cycles of heavy tampingwhen treating silts and clays, a minimum treatment areaof 18,000 to 35,000 square yards (4 to 8 acres) isnecessary for economical use of the method. Thismethod is presently considered experimental insaturated clays.

e. Electroosmosis. Soil stabilization byelectroosmosis may be effective and economical underthe following conditions: (1) a saturated silt or silty claysoil, (2) a normally consolidated soil, and (3) a low porewater electrolyte concentration. Gas generation anddrying and fissuring at the electrodes can impair theefficiency of the method and limit the magnitude ofconsolidation pressures that develop. Treatment resultsin nonuniform changes in properties between electrodesbecause the induced consolidation depends on thevoltage, and the voltage varies between anode andcathode. Thus, reversal of electrode polarity may bedesirable to achieve a more uniform stress condition.Electroosmosis may also be used to accelerate theconsolidation under a preload or surcharge fill. Themethod is relatively expensive.

16-14

TM 5-818-1 / AFM 88-3, Chap. 7


Figure 16-6. Soil particle sizes suitable for different grout types and several concentrations and viscosities shown.

16-15

TM 5-818-1 / AFM 88-3, Chap. 7

16-6. Reinforcement. The supporting capacity ofsoft, compressible ground may be increased andsettlement reduced through use of compressionreinforcement in the direction parallel to the appliedstress or tensile reinforcement in planes normal to thedirection of applied stress. Commonly usedcompression reinforcement elements include mix-in-place piles and walls. Strips and membranes are usedfor tensile reinforcement, with the latter sometimes usedto form a moisture barrier as well.

a. Mix-in-place piles and walls. Severalprocedures are available, most of them patented orproprietary, which enable construction of soil-cement orsoil-lime in situ. A special hollow rod with rotating vanesis augered into the ground to the desired depth.Simultaneously, the stabilizing admixture is introduced.The result is a pile of up to 2 feet in diameter. Cement,in amounts of 5 to 10 percent of the dry soil weight, isbest for use in sandy soils. Compressive strengths inexcess of 200 kips per square foot can be obtained inhese materials. Lime is effective in both expansiveplastic clays and in saturated soft clay. Compressivestrengths of about 20 to 40 kips per square foot are to beexpected in these materials. If overlapping piles areformed, a mix-in-place wall results.

b. Vibroreplacement stone columns. Avibroflot is used to make a cylindrical, vertical hole underits own weight by jetting to the desired depth. Then, 1/2-to 1- cubic yard coarse granular backfill, usually gravel orcrushed rock 3/4 to 1 inch is dumped in, and the vibroflotis used to compact the gravel vertically and radially intothe surrounding soft soil. The process of backfilling andcompaction by vibration is continued until the densifiedstone column reaches the surface.

c. Strips and membranes.(1) Low-cost, durable waterproof

membranes, such as polyethylene, polypropolyleneasphalt, and polyester fabric asphalt, have hadapplication as moisture barriers. At the same time, thesematerials have sufficient tensile strength that when usedin envelope construction, such as surrounding a well-compacted, fine-grained soil, the composite structurehas a greater resistance to applied loads thanconventional construction with granular materials. Thereason is that any deformation of the enveloped soil layercauses tension in the membrance, which in turnproduces additional confinement on the soil and thusincreases its resistance to further deformation.

(2) In the case of a granular soil wheremoisture infiltration is not likely to be detrimental tostrength, horizontally bedded thin, flat metal or plasticstrips can act as tensile reinforcing elements.Reinforced earth has been used mainly for earthretaining structures; however, the feasibility of usingreinforced earth slabs to improve the bearing capacity ofgranular soil has been demonstrated.

(3) Model tests have shown that theultimate bearing capacity can be increased by a factor of2 to 4 for the same soil unreinforced. For these tests,the spacing between reinforcing layers was 0.3 times thefooting width. Aggregate strip width was 42 percent ofthe length of strip footing.

d. Thermal methods. Thermal methods offoundation soil stabilization, freezing or heating, arecomplex and their costs are high.

(1) Artificial ground freezing. Frozen soilis far stronger and less pervious than unfrozen ground.Hence, artificial ground freezing has had application fortemporary underpinning and excavation stabilization.More recent applications have been made to back-freezing soil around pile foundations in permafrost andmaintenance of frozen soil under heated buildings onpermafrost. Design involves two classes of problems;namely, the structural properties of the frozen ground toinclude the strength and the stress-strain-time behavior,and thermal considerations to include heat flow, transferof water to ice, and design of the refrigeration system.

(2) Heating. Heating fine-grained soils tomoderate temperatures, e.g., 1000C+, can cause dryingand accompanying strength increase if subsequentrewetting is prevented. Heating to higher temperaturescan result in significant permanent propertyimprovements, including decreases in water sensitivity,swelling, and compressibility; and increases in strength.Burning of liquid or gas fuels in boreholes or injection ofhot air into 6- to 9-inch-diameter boreholes can produce4- to 7-foot-diameter strengthened zones, aftercontinuous treatment for about 10 days. Dry or partlysaturated weak clayey soils and loess are well suited forthis type of treatment, which is presently regarded asexperimental.

16-7. Miscellaneous methods.a. Remove and replace. Removal of poor soil

and replacement with the same soil treated bycompaction, with or without admixtures, or by a higherquality material offer an excellent opportunity forproducing high-strength, relatively incompressible,uniform foundation conditions. The cost of removal andreplacement of thick deposits is high because of theneed for excavation and materials handling, processing,and recompaction. Occasionally, an expensivedewatering system also may be required. Excludinghighly organic soils, peats and sanitary landfills, virtuallyany inorganic soil can be processed and treated so as toform an acceptable structural fill material.

b. Lime treatment. This treatment of plasticfine-

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TM 5-818-1 / AFM 88-3, Chap. 7

grained soils can produce high-strength, durablematerials. Lime treatment levels of 3 to 8 percent byweight of dry soil are typical.

c. Portland cement. With treatment levels of 3to 10 percent by dry weight, portland cement isparticularly well suited for low-plasticity soils and sandsoils.

d. Stabilization using fills.(1) At sites underlain by soft, compressible

soils and where filling is required or possible to establishthe final ground elevation, load-bearing structural fills canbe used to distribute the stresses from light structures.Compacted sands and gravels are well suited for thisapplication as are also fly ash, bottom ash, slag, andvarious lightweight aggregates, such as expended shale,clam and oyster shell, and incinerator ash. Admixturestabilizers may be incorporated in these materials toincrease their strength and stiffness.

(2) Clam and oyster shells as a structuralfill material over soft marsh deposits represent a newdevelopment. The large deposits of clam and oyster orreef shells that are available in the Gulf States coastalareas can be mined and tasportd short distanceseconomically. Clam shells are 1/4, to 1/2 inch indiameter; whereas, oyster shells, which are coarser andmore elongated, are 2 to 4 inches in size. Whendumped over soft ground, the shells interlock; if there arefinmes and water present, some cementation developsowing to the high calcium carbonate (>90 percent)content. In the loose state, the shell unit weight is about63 pounds per square foot; after construction, it is about95 pounds per square foot. Shell embenkments "float"over very soft ground; whereas, conventional fills wouldsink out of sight. About a 5-foot-thick layer is required tobe placed in a single lift. The only compaction used isfrom the top of the lift, so the upper several inches aremore tightly knit and denser than the rest of the layer.

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TM 5-818-1 / AFM 88-3, Chap. 7

CHAPTER 17

DESIGN FOR EQUIPMENT VIBRATIONS AND SEISMIC LOADINGS

17-1. Introduction.a. Vibrations caused by steady state or transient

loads may cause settlement of soils, excessive motionsof foundations or structures, or discomfort or distress topersonnel. Some basic design factors for dynamicloading are treated in this section. Design of afoundation system incorporates the equipment loading,subsurface material properties, and geometricalproportions in some analytical procedure.

b. Figure 17-1 shows some limiting values ofvibration criteria for machines, structures, and personnel.On this diagram, vibration characteristics are describedin terms of frequency and peak amplitudes ofacceleration, velocity, or displacement. Values offrequency constitute the abscissa of the diagram andpeak velocity is the ordinate. Values of peakdisplacement are read along one set of diagonal linesand labelled in displacement (inches), and peakacceleration values are read along the other set ofdiagonal lines and labelled in various amounts of g, theacceleration of gravity. The shaded zones in the upperright-hand corner indicate possible structural damage towalls by steady-state vibrations. For structural safetyduring blasting, limit peak velocity to 2.0 inches persecond and peak acceleration to 0.10g for frequenciesexceeding 3 cycles per second. These limits mayoccasionally have to be lowered to avoid beingexcessively annoying to people.

c. For equipment vibrations, limiting criteriaconsist of a maximum velocity of 1.0 inch per second upto a frequency of about 30 cycles per second and a peakacceleration of 0.15g above this frequency. However,this upper limit is for safety only, and specific criteriamust be established for each installation. Usually,operating limits of equipment are based on velocitycriteria; greater than 0.5 inch per second indicatesextremely rough operation and machinery should be shutdown; up to 0.10 inch per second occurs for smooth,well-balanced equipment; and less than 0.01 inch persecond represents very smooth operation.

d. Figure 17-1 also includes peak velocitycriteria for reaction of personnel to steady-statevibrations. Peak velocities greater than 0.1 inch persecond are "troublesome to persons," and peakvelocities of 0.01 inch per second are just "barelynoticeable to persons." It is significant that persons andmachines respond to equivalent levels of vibration.

Furthermore, persons may notice vibrations that areabout 1/100 of the value related to safety of structures.

17-2. Single degree of freedom, damped, forcedsystems.

a. Vibrations of foundation-soil systems canadequately be represented by simple mass-spring-dashpot systems. The model for this simple systemconsists of a concentrated mass, m, supported by alinear elastic spring with a spring constant, k, and aviscous damping unit (dashpot) having a dampingconstant, c. The system is excited by an external force,e.g., Q = Qo sin (ωt), in which Qo is the amplitude of theexciting force, ω = 2πfo is the angular frequency (radiansper second) with fo the exciting frequency (cycles persecond), and t is time in seconds.

b. If the model is oriented as shown in theinsert in figure 17-2(a), motions will occur in the verticalor z direction only, and the system has one degree offreedom (one coordinate direction (z) is needed todescribe the motion). The magnitude of dynamic verticalmotion, Az, depends upon the magnitude of the externalexcitation, Q, the nature of Qo, the frequency, fo, and thesystem parameters m, c, and k. These parameters arecustomarily combined to describe the "natural frequency"as follows:

1 kfn = 2π m (17-1)

and the "damping ratio" asc

D= 2√km (17-2)

c. Figure 17-2(a) shows the dynamic responseof the system when the amplitude of the exciting force,Qo, is constant. The abscissa of the diagram is thedimensionless ratio of exciting frequency, fo, divided bythe natural frequency, fn, in equation (17-1). Theordinate is the dynamic magnification factor, Mz, which isthe ratio of Az to the static displacement, Az = (Qo/k).Different response curves correspond to different valuesof D.

d. Figure 17-2(b) is the dynamic response ofthe system when the exciting force is generated by arotating mass, which develops:

Qo = me ( e ) 4π2fo2 (17-3)

17-1

TM 5-818-1 / AFM 88-3, Chap. 7

where

me =the total rotating masse = the eccentricityfo = the frequency of oscillation, cycles per

second

e. The ordinate Mz. (fig 17-2(b)) relates thedynamic displacement, Az, to me e/m. The peak value ofthe response curve is a function of the damping ratio andis given by the following expression:

1Mz(max) or Mz = 2D√1-D2 (17-4)

For small values of D, this expression becomes 1/2D.These peak values occur at frequency ratios of

fofn=√1-D2 (fig. 17-2a)

or (17-5)fo 1fn= √1-2D2 (fig. 17-2b)

17-3. Foundations on elastic soils.a. Foundations on elastic half-space. For very

small deformations, assume soils to be elastic materialswith properties as noted in paragraph 3-8. Therefore,theories describing the behavior of rigid foundationsresting on the surface of a semi-infinite, homogeneous,isotropic elastic body have been found useful for study ofthe response of real footings on soils. The theoreticaltreatment involves a circular foundation of radius, ro, onthe surface of the ideal half-space. This foundation hassix degrees of freedom: (1-3) translation in the vertical(z) or in either of two horizontal (x and y) directions; (4)torsional (yawing) rotation about the vertical (z) axis; or(5-6) rocking (pitching) rotation about either of the twohorizontal (x and y) axes. These vibratory motions areillustrated in figure 17-3.

(1) A significant parameter in evaluatingthe dynamic response in each type of motion is theinertia reaction of the foundation. For translation, this issimply the mass, m = (W/g); whereas in the rotational

(Courtesy) of F. E Richart, Jr., J R. Hall. Jr., and R. D.Woods, Vibrations of Soils and Foundations, 1970, p 316.Reprinted by permission of Prentice-Hall, Inc., EnglewoodCliffs, N. J.)

Figure 17-1. Response spectra for tibraton limits.

17-2

TM 5-818-1 / AFM 88-3, Chap. 7

(Courtesy of F. E. Richart, Jr., J. R. Hall, Jr., and R. D. Woods,Vibrations of Soils and Foundations,1970, pp 383-384. Reprinted bypermission of Prentice-Hall, Inc., Englewood Cliffs, N. J.)

Figure 17-2. Response curves for the single-degree-of-freedom system with viscous damping.

Figure 17-3. Six modes of vibration for a foundation.U. S. Army Corps of Engineers

Figure 17-3. Six modes of vibration for a foundation.

17-3

TM 5-818-1 / AFM 88-3, Chap. 7

modes of vibration, it is represented by the massmoment of inertia about the axis of rotation. For torsionaloscillation about the vertical axis, it is designated as Ιθ;whereas for rocking oscillation, it is Ιψ, (for rotation aboutthe axis through a diameter of the base of thefoundation). If the foundation is considered to be a rightcircular cylinder of radius ro, height h, and unit weight y,expressions for the mass and mass moments of inertiaare as follow:

π ro2 h y

m = g (17-6)

π ro4 h y

Is = 2g (17-7)

πro2hy r0

2 + h2 (17-8)Ιψ= g 4 3

(2) Theoretical solutions describe themotion magnification factors M, or ML, for example, interms of a "mass ratio" Bz and a dimensionlessfrequency factor ao Table 17-1 lists the mass ratios,damping ratios, and spring constants corresponding tovibrations of

the rigid circular footing resting on the surface of anelastic semi-infinite body for each of the modes ofvibration. Introduce these quantities into equations givenin paragraph 17-2 to compute resonant frequencies andamplitudes of dynamic motions. The dimensionlessfrequency, ao, for all modes of vibration is given asfollows:

2πforo p (17-9)ao = Vs = ωro G

The shear velocity, Vs, in the soil is discussed inparagraph 17-5.

(3) Figure 17-4 shows the variation of thedamping ratio, D, with the mass ratio, B, for the fourmodes of vibration. Note that D is significantly lower forthe rocking mode than for the vertical or horizontaltranslational modes. Using the expression M = 1/(2D)for the amplitude magnification factor and theappropriate D, from figure 17-4, it is obvious that M, canbecome large. For example, if Bψ = 3, then Dψ = 0.02and Mψ = 1/(2 x 0.02) = 25.

Table 17-1. Mass ratio, Damping Ration, and Spring Constant for Rigid Circular Footing on the Semi-Infinite Elastic Body


)(

17-4

TM 5-818-1 / AFM 88-3, Chap. 7

(Courtesy of F. E. Richart, Jr. J. R. Hall, Jr., and R. D.Woods, Vibrations of Soils and Foundations, 1970. p226. Reprinted by permission of Prentice-Hall, Inc.,Englewood Cliffs, N. J.)

Figure 17-4. Equivalent damping ratio for oscillation ofrigid circular footing on elastic half-space.

b. Effects of shape of foundation. Thetheoretical solutions described above treated a rigidfoundation with a circular contact surface bearing againstthe elastic half-space. However, foundations are usuallyrectangular in plan. Rectangular footings may beconverted into an equivalent circular footing having aradius ro determined by the following expressions:

For translation in z- or x-directions:ro = 4cd (17-10)

πFor rocking:

ro =4 16cd3 (17-11)3n

For torsion:ro =4 16cd(c2 + d2) (17-12)

6πIn equations (17-10), (17-11), and (17-12), 2c is thewidth of the rectangular foundation (along the axis ofrotation for rocking), and 2d is the length of thefoundation (in the plane of rotation for rocking). Twovalues of ro are obtained for rocking about both x and yaxes.

c. Computations. Figure 17-5 presentsexamples of computations for vertical motions (Example1) and rocking motions (Example 2).

d. Effect of embedment. Embedment offoundations a distance d below the soil surface maymodify the dynamic response, depending upon the soil-foundation contact and the magnitude of d. If the soilshrinks away from the vertical faces of the embeddedfoundation, no beneficial effects of embedment mayoccur. If the basic evaluation of foundation response isbased on a rigid circular footing (of radius ro) at thesurface, the effects of embedment will cause an increase

in resonant frequency and a decrease in amplitude ofmotion. These changes are a function of the type ofmotion and the embedment ratio d/ro.

(1) For vertical vibrations, both analyticaland experimental results indicate an increase in thestatic spring constant with an increase in embedmentdepth. Embedment of the circular footing a distance d/ro

< 1.0 produces an increase in the embedded springconstant kzd’ which is greater than kz (table 17-1) by kzd/kz

≅ (1 + 0.6 d/ro). An increase in damping also occurs, i.e.,Dzd/Dz (1 + 0.6 d/ro). These two approximate relationslead to an estimate of the reduction in amplitude ofmotion because of embedment from Azd / Az = 1 / Dzd / Dz

x kzd/kz). This amount of amplitude reduction requirescomplete soil adhesion at the vertical face, and test datahave often indicated less effect of embedment. Testdata indicate that the resonant frequency may beincreased by a factor up to (1 + 0.25 d/ro) because ofembedment.

(2) The influence of embedment oncoupled rocking and sliding vibrations depends on theratio Bω/Bx (table 17-1). For Bω/Bx ≅ 3.0, the increase innatural frequency due to embedment may be as much as(1 + 0.5 d/ro). The decrease in amplitude is stonglydependent upon the soil contact along the vertical face ofthe foundation, and each case should be evaluated onthe basis of local soil and construction conditions.

e. Effect of finite thickness of elastic layer.Deposits of real soils are seldom homogeneous tosignificant depths; thus theoretical results based on theresponse of a semi-infinite elastic media must be usedwith caution. When soil layers are relatively thin, withrespect to foundation dimensions, modifications to thetheoretical half-space analyses must be included.

(1) Generally, the effect of a rigid layerunderlying a single elastic layer of thickness, H, is toreduce the effective damping for a foundation vibrating atthe upper surface of the elastic layer. This conditionresults from the reflection of wave energy from the rigidbase back to the foundation and to the elastic mediumsurrounding the foundation. For vertical or torsionalvibrations or a rigid circular foundation resting on thesurface of the elastic layer, it has been established that avery large amplitude of resonant vibrations can occur if

Vs

> 4H(17-13)fo

In equation (17-13), V, is the shear wave velocity in theelastic layer and fo is the frequency of footing vibrations.When the conditions of equation (17-4) occur, the naturalfrequency (equation (17-1)) becomes the importantdesign criterion because at that frequen-

17-5

TM 5-818-1 / AFM 88-3, Chap. 7


Figure 17-5. Examples of computations for vertical and rocking motions.

17-6

TM 5-818-1 / AFM 88-3, Chap. 7

cy excessive dynamic motion will occur. To restrict thedynamic oscillation to slightly larger than the staticdisplacement, the operating frequency should bemaintained at one half, or less, of the natural frequency(fig 17-2).

(2) The relative thickness (expressed byH/ro) also exerts an important influence on foundationresponse. If H/ro is greater than about 8, the foundationon the elastic layer will have a dynamic responsecomparable to that for a foundation on the elastic half-space. For H/ro < 8, geometrical damping is reduced,and the effective spring constant is increased. Thevalues of spring constant, k, in table 17-1 are taken asreference values, and table 17-2 indicates the increasein spring constant associated with a decrease inthickness of the elastic layer. Values of the increase inspring constants for sliding and for rocking modes ofvibration will tend to fall between those given for verticaland torsion for comparable H/ro conditions.

f. Coupled modes of vibration. In general,vertical and torsional vibrations can occur independentlywithout causing rocking or sliding motions of thefoundation. To accomplish these uncoupled vibrations,the line of action of the vertical force must pass throughthe center of gravity of the mass and the resultant soilreaction, and the exciting torque and soil reaction torquemust be symmetrical about the vertical axis of rotation.Also, the center of gravity of the foundation must lie onthe vertical axis of torsion.

(1) When horizontal or overturningmoments act on a block foundation, both horizontal(sliding) and rocking vibrations occur. The couplingbetween these motions depends on the height of thecenter of gravity of. the machine-foundation about theresultant soil reaction. Details of a coupled rocking andsliding analysis are given in the example in figure 17-6.

(2) A "lower bound" estimate of the firstmode of coupled rocking and sliding vibrations can beobtained from the following:

1 = 1 + 1 (17-14)fo

2 fx2 +fψ

2

In equation (17-14), the resonant frequencies in thesliding x and rocking ψ motions can be determined by

introducing values from table 17-1 into equations (17-1)and (17-5). (Note that equation (17-14) becomes lessuseful when Dx is greater than about 0.15). The firstmode resonant frequency is usually most important froma design standpoint.

g. Examples. Figure 17-5, Example 1,illustrates a procedure for design of a foundation tosupport machine-producing vertical excitations. Figure17-5, Example 2, describes the analysis of uncoupledhorizontal and rocking motion for a particular foundationsubjected to horizontal excitations. The designprocedure of Example 1 is essentially an iterativeanalysis after approximate dimensions of the foundationhave been established to restrict the static deflection to avalue comparable to the design criterion.

(1) In figure 17-5, Example 1 shows thatrelatively high values of damping ratio D are developedfor the vertical motion of the foundation, and Example 2illustrates that the high damping restricts dynamic mo-tions to values slightly larger than static displacementcaused by the same force. For Example 2, establishingthe static displacement at about the design limit valueleads to satisfactory geometry of the foundation.

(2) Example 2 (fig 17-5) gives thefoundation geometry, as well as the analysis needed toascertain whether the design criterion is met. It isassumed that the 400-pound horizontal force is constantat all frequencies and that a simple superposition of thesingledegree-of-freedom solutions for horizontaltranslation and rocking will be satisfactory. Because thehorizontal displacement is negligible, the rocking motiondominates, with the angular rotation at resonanceamounting to (Mψ x ψs) or Aψ = 5.6 x 0.51 x 10-6 = 2.85 x10-6 radians. By converting this motion to horizontaldisplacement at the machine center line, it is found thatthe design conditions are met.

(3) In figure 17-6, the foundation ofExample 2 (fig. 17-5) is analyzed as a coupled systemincluding both rocking and sliding. The response curvefor angular rotation shows a peak motion of Aψ = 2.67 x10-6 radians, which is comparable to the value found byconsidering rocking alone. The coupled dynamicresponse of any rigid foundation, e.g., a radar tower, can

Table 17-2. Values of kL/L for Elastic Layer (k from Table 17-1)

H/ro 0.5 1.0 2.0 4.0 8.0 ∞

Vertical 5.0 2.2 1.47 1.23 1.10 1.0

Torsion -- 1.07 1.02 1.009 -- 1.0


17-7

TM 5-818-1 / AFM 88-3, Chap. 7


Figure 17-6. Coupled rocking and sliding motion of foundation.

17-8

TM 5-818-1 / AFM 88-3, Chap. 7

be evaluated by the procedure illustrated in figure 17-6.

17-4. Wave transmission, attenuation, andisolation. Vibrations are transmitted through soils bystress waves. For most engineering analyses, the soilmay be treated as an ideal homogeneous, isotropicelastic material to determine the characteristics of thestress waves.

a. Half-space. Two types of body waves maybe transmitted in an ideal half-space, compression (P-)waves and shear (S-) waves; at the surface of thehalfspace, a third wave known as the Rayleigh (R-) waveor surface wave will be transmitted. The characteristicsthat distinguish these three waves are velocity, wavefrontgeometry, radiation damping, and particle motion.Figure 17-7 shows the characteristics of these waves asthey are generated by a circular footing undergoingvertical vibration on the surface of an ideal half-spacewith is µ = 0.25. The distance from the footing to eachwave in figure 17-7 is drawn in proportion to the velocityof each wave. The wave velocities can be computedfrom the following:ρ

P-wave velocity:vc= λ+2G (17-15)

pS-wave velocity:

vs = G (17-16)p

R-wave velocity:vR = Kvs (17-17)

whereλ = 2µG and G are Lame’s E

1-2µ constants; G =2(1 + j)p = y/G= mass density of soily = moist or saturated unit weightK = constant, depending on Poisson’s ratio

0.87 < K < 0.98 for 0 <_< 0.5(1) The P- and S-waves propagate radially

outward from the source along hemispherical wavefronts, while the R-wave propagates outward along acylindrical wave front. All waves encounter anincreasingly larger volume of material as they traveloutward, thus decreasing in energy density withdistance. This decrease in energy density and itsaccompanying decrease in displacement amplitude iscalled geometrical damping or radiation damping.

(2) The particle motions are as follows: forthe P-wave, a push-pull motion in the radial direction; forthe S-wave, a transverse motion normal to the radialdirection; and for the R-wave, a complex motion, whichvaries with depth and which occurs in a vertical planecontaining a radius. At the surface, R-wave particlemotion describes a retrograde ellipse. The shadedzones along the wave fronts in figure 17-7 represent the

relative particle amplitude as a function of inclinationfrom vertical.

b. Layered media.(1) In a layered medium, the energy

transmitted by a body wave splits into four waves at theinterface between layers. Two waves are reflected backinto the first medium, and two waves are transmitted orrefracted into the second medium. The amplitudes anddirections of all waves can be evaluated if the propertiesof both media and the incident angle are known. If alayer containing a lower modulus overlies a layer with ahigher modulus within the half-space, another surfacewave, known as a Love wave, will occur. This wave is ahorizontally oriented S-wave whose velocity is betweenthe S-wave velocity of the layer and of the underlyingmedium.

(2) The decay or attenuation of stresswaves occurs for two reasons: geometric or radiationdamping, and material or hysteretic damping. Anequation including both types of damping is the following:

A2 = A1 r1 C exp[-α (r2 - r1 )] (17-18)r2

whereA2 = desired amplitude at distance r2A, = known or measured amplitude at

radial distance r, from vibration source

C = constant, whichdescribesgeometrical damping

= 1 for body (P- or S-) waves= 0.5 for surface or R-waves

α = coefficient of attenuation, whichdescribes material damping (valuesin table 17-3)

c. Isolation. The isolation of certain structuresor zones from the effects of vibration may sometimes benecessary. In some instances, isolation can beaccomplished by locating the site at a large distancefrom the vibration source. The required distance, r2, iscalculated.from equation (17-18). In other situations,isolation may be accomplished by wave barriers. Themost effective barriers are open or void zones liketrenches or rows of cylindrical holes. Somewhat lesseffective barriers are solid or fluid-filled trenches orholes. An effective barrier must be proportioned so thatits depth is at least two-thirds the wavelength of theincoming wave. The thickness of the barrier in thedirection of wave travel can be as thin as practical forconstruction considerations. The length of the barrierperpendicular to the direction of wave travel will dependupon the size of the zone to be isolated but should be noshorter than two times the maximum plan dimension ofthe structure or one wavelength, whichever is greater.17-5. Evaluation of S-wave velocity In soils. Thekey parameter in a dynamic analysis of a

17-9

TM 5-818-1 / AFM 88-3, Chap. 7

(Courtesy of F. E. Richart, Jr., J. R. Hall, Jr., and R. D. Woods.Vibrations of Soils and Foundations, 1970, p 91. Reprinted bypermission of Prentice-Hall, Inc., Englewood Cliffs, N. J.)

Figure 17-7. Distribution of displacement waves from a circular footing on the elastic half-space.

soil-foundation system is the shear modulus, G. Theshear modulus can be determined in the laboratory orestimated by empirical equations. The value of G canalso be computed by the field-measured S-wave velocityand equation (17-16).

a. Modulus at low strain levels. The shearmodulus and damping for machine vibration problemscorrespond to low shear-strain amplitudes of the order of1 to 3 x 10-4 percent. These properties may bedetermined from field measurements of the seismic

wave velocity through soil or from special cycliclaboratory tests.

b. Field wave velocity tests. S-wave velocitytests are preferably made in the field. Measurementsare obtained by inducing a low-level seismic excitation atone location and measuring directly the time required forthe induced S-wave to travel between the excitation andpickup unit. Common tests, such as uphole, downhole,or crosshole propagation, are described in geotechnicalengineering literature.

Table 17-3. Attenuation Coefficients for Earth Materials

Materials a (1/ft) @ 50 HZa

Sand Loose, fine 0.06Dense, fine 0.02

Clay Silty (loess) 0.06Dense, dry 0.003

Rock Weathered volcanic 0.02Competent marble 0.00004

a α is a function of frequency. For other frequencies, f, compute αf = (f/50) x α50


17-10

TM 5-818-1 / AFM 88-3, Chap. 7

(1) A problem in using seismic methods toobtain elastic properties is that any induced elastic pulse(blast, impact, etc.) develops three wave types previouslydiscussed, i.e., P-, S-, and R-waves. Because thevelocity of all seismic waves is hundreds of feet persecond and the pickup unit detects all three wave pulsesplus any random noise, considerable expertise isrequired to differentiate between the time of arrival of thewave of interest and the other waves. The R-wave isusually easier to identify (being slower, it arrives last;traveling near the surface, it contains more relativeenergy). Because R- and S-wave velocities are relativelyclose, the velocity of the R-wave is frequently used incomputations for elastic properties.

(2) Because amplitudes in seismic surveyare very small, the computed shear and Young’s moduliare considerably larger than those obtained fromconventional laboratory compression tests.

(3) The shear modulus, G, may becalculated from the S- (approximately the R-wave) wavevelocity as follows:

G = ρVs2 (17-19)

whereρ = y/32.2 = mass density of soil using

wet or total unit weightVs = S-wave velocity (or R-wave), feet per

secondThis equation is independent of Poisson’s ratio. The Vs

value is taken as representative to a depth ofapproximately one-half wavelength. Alternatively, theshear modulus can be computed from the P-wavevelocity and Poisson’s ratio from:

G = p( 1 - 2µ)Vp2 (17-20)

2(1- µ)The use of this equation is somewhat limited becausethe velocity of a P-wave in water is approximately 5000feet per second (approximately the velocity in manysoils) and Poisson’s ratio must be estimated. Forsaturated or near saturated soils, µ - 0.5. The theoreticalvariation of the ratio Vs/Vp with µ is shown in figure 17-8.


Figure 17-8. Theoretical relation between shear velocity ratio Vp/Vs and Poisson’s ratio.

17-11

TM 5-818-1 / AFM 88-3, Chap. 7

c. Laboratory measurement of dynamicstress-strain properties. Low shear-strain amplitude, i.e.less than 10-2 percent, shear modulus data may beobtained from laboratory tests and usually involveapplying some type of high-frequency forced vibration toa cylindrical sample of soil and measuring an appropriateresponse. Some types of tests allow the intensity level ofthe forced vibration to be varied, thus yielding moduli atdifferent shear strains.

(1) High strain-level excitation, i.e. 0.01 to1.0 percent, may be achieved by low-frequency, cyclicloading triaxial compression tests on soil samples. Themodulus, damping, and strain level for a particular testare calculated directly from the sample response data.The usual assumption for calculating the modulus anddamping from forced cyclic loading tests on laboratorysamples is that at any cyclic strain amplitude the soilbehaves as a linear elastic, viscous, damped material.A typical set of results may take the form of a hysteresisloop as shown in figure 17-9. Either shear or normalstress cyclic excitation may be used. The shear modulusis calculated from the slope of the peak-to-peak secantline. The damping is computed from the area of the

hysteresis loop, and the strain level is taken as thesingle-amplitude (one-half the peak-to-peak amplitude ororigin to peak value) cyclic strain for the condition duringthat cycle of the test. Note that the equations for modulusand damping shown in figure 17-9 assume the soilbehaves as an equivalent elastic viscous, dampenedmaterial, which is linear within the range of strainamplitude specified. This assumption is usually made inmost soil dynamics analyses because of the low-vibration amplitudes involved. If the cyclic hysteresisloops are obtained from triaxial test specimens, theresulting modulus will be the stress-strain modulus, E. Ifthe tests involve simple shear or torsion shear such thatshear stresses and strains are measured, the resultingmodulus will be the shear modulus, G. In either case,the same equations apply.

(2) The shear modulus, G, can becomputed from the stress strain modulus and Poisson’sratio as follows:

G = E (17-21)2(1+µ)


Figure 17-9. Idealized cyclic stress-strain loop for soil.

17-12

TM 5-818-1 / AFM 88-3, Chap. 7

The shear strain amplitude, AΕ, may be computed fromthe axial strain amplitude, Ε,and Poisson’s ratio asfollows:

AΕ = Ε(1 + µ) (17-22)For the special case of saturated soils, Poisson’s ratio is0.5, which leads to the following:

G = E/3AΕ = 1.5Ε

d. Correlations.(1) Empirical correlations from many sets

of data have provided several approximate methods forestimating the S-wave velocity and shear modulus forsoils corresponding to low-strain excitation. For manyundisturbed cohesive soils and sands:G =1230(21973 - e)2

(OCR)" (o)0.5 (pounds 1 + persquare inch) (17-23)

wheree = void ratioη = empirical constant, which depends on

the PI of cohesive soils (table 17-4). For sands, PI = 0and η = 0, so OCR term reduces to 1.0. For clays, themaximum value is η = 0.5 for PI > 100.

σ0 = 1/3 (σ1 + σ2 + σ3) = mean normaleffective stress, pounds per square inch

(2) For sands and gravels, calculate thelow-strain shear modulus as follows:G = 1000(K2)(σ0)

0.5 (pounds per square foot) (17-24)where

K2 =empirical constant (table 17-5)=90 to 190 for dense sand, gravel, and cobbles

with little clayσ0 = mean normal effective stress as in equation

(17-23) (but in units of pounds per square foot)

(3) For cohesive soils as clays and peat,the shear modulus is related to Su as follows:

G = K2su (17-25)For clays, K2 ranges from 1500 to 3000. For peats, K2

ranges from 150 to 160 (limited data base).(4) In the laboratory, the shear modulus of

soil increases with time even when all other variables areheld constant. The rate of increase in the shear modulusis approximately linear as a function of the log of timeafter an initial period of about 1000 minutes. The changein shear modulus, ∆G, divided by the shear modulus at1000 minutes, G1000, is called the normalized secondaryincrease. The normalized secondary increases rangefrom nearly zero percent per log cycle for coarse sandsto more than 20 percent per log for sensitive clays. Forgood correlation between laboratory and fieldmeasurements of shear modulus, the age of the in situdeposit must be considered, and a secondary timecorrection applies to the laboratory data.

e. Damping in low strain levels. Criticaldamping is defined as

cc = 2 √km (17-26)where k is the spring constant of vibrating mass and mrepresents mass undergoing vibration (W/g). Viscousdamping of all soils at low strain-level excitation isgenerally less than about 0.01 percent of critical dampingfor most soils or:

D = c/c, < 0.05 (17-27)It is important to note that this equation refers only tomaterial damping, and not to energy loss by radiationaway from a vibrating foundation, which may also beconveniently expressed in terms of equivalent viscousdamping. Radiation damping in machine vibrationproblems is a function of the geometry of the problemrather than of the physical properties of the soil.

Table 17-4. Values of Constant r Used with Equation (17-23) to Estimate Cyclic Shear Modulus at Low Strains

Plasticity Index K0 020 0.1840 0.3060 0.4180 0.48>100 0.50

(Courtesy of 0. Hardin and P. Drnevich. "ShearModulus and Damping in Soils: Design Equations andCurves," Journal., Soil Mechanics and FoundationsDivision. Vol 98. No. SM7. 1972, pp 667-692. Reprintedby permission of American Society of Civil Engineers,New York.)

17-13

TM 5-818-1 / AFM 88-3, Chap. 7

Table 17-5. Values of Constant K2 Used with Equation (17-24) to Estimate Cyclic Shear Modulus at Low Strains for Sands

e K2 Dr(%)0.4 70 900.5 60 750.6 51 600.7 45 450.8 39 400.9 33 30

(Courtesy of H. B. Seed and L M. Idriss, "Simplified Procedures forEvaluating Liquefaction Potential," Journal, Soil Mechanics andFoundations Division Vol 97, No. SM9. 1971, pp 1249-1273. Reprinted bypermission of American Society of Civil Engineers, New York.)

f. Modulus and damping at high strain levels.The effect of increasingly higher strain levels is to reducethe modulus (fig 17-10) and increase the damping of thesoil (fig 17-11). Shear modulus and damping values athigh strains are used mainly in computer programs foranalyzing the seismic response of soil under earthquakeloading conditions. The various empirical relations formodulus and damping pertain to sands and soft,normally consolidated clays at low-to-medium effectiveconfining pressures, in the range of about 100 feet oroverburden. Stiff overconsolidated clays and all soils athigh effective confining pressure exhibit lower values ofdamping and higher values of modulus, especially athigh strain levels. As a maximum, the modulus anddamping values for stiff or strong soils at very higheffective confining pressures correspond to valuespertaining to crystalline or shale-type rock.

17-6. Settlement and liquefaction.a. Settlement. Repeated shearing strains of

cohesionless soils cause particle rearrangements.When the particles move into a more compact position,settlement occurs. The amount of settlement dependson the initial density of the soil, the thickness of thestratum, and the intensity and number of repetitions ofthe shearing strains. Generally, cohesionless soils withrelative densities (Dr) greater than about 75 percentshould not develop settlements. However, under 106 or107 repetitions of dynamic loading, even dense sandsmay develop settlements amounting to 1 to 2 percent ofthe layer thickness. To minimize settlements that mightoccur under sustained dynamic loadings, the soilbeneath and around the foundation may beprecompacted during the construction process byvibroflotation, multiple blasting, pile driving, or vibratingrollers acting at the surface. The idea is to subject thesoil to a more severe dynamic loading condition during

construction than it will sustain throughout the designoperation.

b. Liquefaction of sands. The shearingstrength of saturated cohesionless soils depends uponthe effective stress acting between particles. Whenexternal forces cause the pore volume of a cohesionlesssoil to reduce the amount V, pore water pressures areincreased during the time required to drain a volume V ofwater from the soil element. Consequently, porepressure increases depend upon the time rate of changein pore volume and the drainage conditions (permeabilityand available drainage paths). When conditions permitthe pore pressure, u, to build up to a value equal to thetotal stress, σn, on the failure plane, the shear strength isreduced to near zero and the mixture of soil grains andwater behaves as a liquid. This condition is trueliquefaction, in which the soil has little or no shearingstrength and will flow as a liquid. Liquefaction or flowfailure of sands involves a substantial loss of shearingstrength for a sufficient length of time that large de-formations of soil masses occur by flow as a heavyliquid.

c. Liquefaction due to seismic activity. Soildeposits that have a history of serious liquefactionproblems during earthquakes include alluvial sand,aeolian sands and silts, beach sands, reclaimed land,and hydraulic fills. During initial field investigations,observations that suggest possible liquefaction problemsin seismic areas include low penetration resistance;artesian heads or excess pore pressures; persistentinability to retain granular soils in sampling tubes; andany clean, fine, uniform sand below the groundwatertable. The liquefaction potential of such soils forstructures in seismic areas should be addressed unlessthey meet one of the criteria in table 17-6. In the eventthat

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TM 5-818-1 / AFM 88-3, Chap. 7

(Courtesy of H. B. Seed and L M. Idriss, "Simplified Procedures forEvaluating Liquefaction Potential," Journal, Soil Mechanics andFoundations Division Vol 97, No. SM9. 1971, pp 1249-1273. Reprintedby permission of American Society of Civil Engineers, New York.)

Figure 17-10. Variation of shear modulus with cyclic strain amplitude; Gmax = G at Ε = 1 to 3 x 10-4 percent; scatter in dataup to about ± 0.1 on vertical scale.

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TM 5-818-1 / AFM 88-3, Chap. 7

(Courtesy of H. B Seed and 1. M Idrisv. "SimplifiedProcedure for Evaluating Soil Liquefaction Potential."Journal, Soul Mechanics and Foundations Division. Vol97, No. SM9. 1971, pp 1249-1273. Reprinted bypermission of the American Society, of Civil Engineers.Newt York.)

Figure 17-11. Variation of viscous damping with cyclic strain amplitude, data scatter up to about ±50 percent of averagedumping values shown for any strain.

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TM 5-818-1 / AFM 88-3, Chap. 7

Table 17-6. Criteria for Excluding Need for Detailed Liquefaction Analyses

1. CL, CH, SC, or GC soils.2. GW or GP soils or materials consisting of cobbles, boulders, uniform rock fill, which have free-draining

boundaries that are large enough to preclude the development of excess pore pressures.3. SP, SW, or SM soils which have average relative density equal to or greater than 85 percent, provided that

the minimum relative density is not less than 80 percent.4. ML or SM soils in which the dry density is equal to or greater than 95 percent of the modified Proctor (CE 55)

density.5. Soils of pre-Holocene age, with natural overconsolidation ratio equal to or greater than 16 and with relative

density greater than 70 percent.6. Soils located above the highest potential groundwater table.7. Sands in which the "N" value is greater than three times the depth in feet, or greater than 75; provided that 75

percent of the values meet this criterion, that the minimum "N" value is not less than one times the depth in feet, that thereare no consistent patterns of low values in definable zones or layers, and that the maximum particle size is not greaterthan 1 in. Large gravel particles may affect "N" values so that the results of the SPT are not reliable.

8. Soils in which the shear wave velocity is equal to or greater than 2000 fps. Geophysical survey data and sitegeology should be reviewed in detail to verify that the possibility of included zones of low velocity is precluded.

9. Soils that, in undrained cyclic triaxial tests, under isotropically consolidated, stress-controlled conditions, andwith cyclic stress ratios equal to or greater than 0.45, reach 50 cycles or more with peak-to-peak cyclic strains not greaterthan 5 percent; provided that methods of specimen preparation and testing conform to specified guidelines.

Note: The criteria given above do not include a provision for exclusion of soils on the basis of grain-size distribution, andin general, grain-size distribution alone cannot be used to conclude that soils will not liquefy. Under adverse conditionsnonplastic soils with a very wide range of grain sizes may be subject to liquefaction.


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TM 5-818-1 / AFM 88-3, Chap. 7

none of the criteria is met and a more favorable sitecannot be located, the material in question should beremoved, remedial treatment applied as described inchapter 16, or a detailed study and analysis should beconducted to determine if liquefaction will occur.

Ground motions from earthquakes cause motions offoundations by introducing forces at the foundation-soilcontact zone. Methods for estimating ground motionsand their effects on the design of foundation elementsare discussed in TM 5-809-10 / AFM 88-3, Chapter 13.

17-7. Seismic effects on foundations.

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TM 5-818-1 / AFM 88-3, Chap. 7

CHAPTER 18

FOUNDATIONS IN AREAS OF SIGNIFICANT FROST PENETRATION

18-1. Introduction.a. Types of areas. For purposes of this

manual, areas of significant frost penetration may bedefined as those in which freezing temperatures occur inthe ground to sufficient depth to be a significant factor infoundation design. Detailed requirements of engineeringdesign in such areas are given in TM 5-818-2/AFM 88-6,Chapter 4, and the Arctic and Subarctic Constructionseries, TM 5-852-1 through 9/AFM 88-19, Chapters 1through 9, respectively. Areas of significant frostpenetration may be subdivided as follows:

(1) Seasonal frost areas.(a) Significant ground freezing occurs

in these areas during the winter season, but withoutdevelopment of permafrost.1 In northern Texas,significant seasonal frost occurs about 1 year in 10. Alittle farther north it is experienced every year. Depth ofseasonal freezing increases northward with decreasingmean annual and winter air temperatures untilpermafrost is encountered. With still further decrease ofair temperatures, the depth of annual freezing andthawing becomes progressively thinner.

(b) The layer extending through bothseasonal frost and permafrost areas in which annualfreeze-thaw cycles occur is called the annual frost zone.In permafrost areas, it is also called the active layer. It isusually not more than 10 feet thick, but it may exceed 20feet. Under conditions of natural cover in very coldpermafrost areas, it may be as little as 1 foot thick. Itsthickness may vary over a wide range even within asmall area. Seasonal changes in soil properties in thislayer are caused principally by the freezing and thawingof water contained in the soil. The water may bepermanently in the annual frost zone or may be drawninto it during the freezing process and released duringthawing. Seasonal changes are also produced byshrinkage and expansion caused by temperaturechanges.

(2) Permafrost areas.(a) In these areas, perennially frozen

ground is found below the annual frost zone. In NorthAmerica, permafrost is found principally north of latitudes55 to 65 degrees, although patches of permafrost arefound much farther south on mountains where the

1 Specialized terms relating to frozen groundareas are defined in TM 5-818-2/AFM 88-6, Chapter 4,and TM 5-852-1/AFM 88-19, Chapter 1.

temperature conditions are sufficiently low, includingsome mountains in the contiguous 48 States. In areas ofcontinuous permafrost, perennially frozen ground isabsent only at a few widely scattered locations, as at thebottoms of rivers and lakes. In areas of discontinuouspermafrost, permafrost is found intermittently in variousdegrees. There may be discontinuities in both horizontaland vertical extent. Sporadic permafrost is permafrostoccurring in the form of scattered permafrost islands. Inthe coldest parts of the Arctic, the ground may be frozenas deep as 2000 feet.

(b) The geographical boundariesbetween zones of continuous permafrost, discontinuouspermafrost, and seasonal frost without permafrost arepoorly defined but are represented approximately infigure 18-1.

b. General nature of design problems.Generally, the design of foundations in areas of onlyseasonal frost follows the same procedure as wherefrost is in- significant or absent, except that precautionsare taken to avoid winter damage from frost heave orthrust. In the spring, thaw and settlement of frost-heaved material in the annual frost zone may occurdifferentially, and a very wet, poorly drained groundcondition with temporary but substantial loss of shearstrength is typical.

(1) In permafrost areas, the same annualfrost zone phenomena occur, but the presence of theunderlying permafrost introduces additional potentiallycomplex problems. In permafrost areas, heat flow frombuildings is a fundamental consideration, complicatingthe design of all but the simplest buildings. Any changefrom natural conditions that results in a warming of theground beneath a structure can result in progressivelowering of the permafrost table over a period of yearsthat is known as degradation. If the permafrost containsice in excess of the natural void or fissure space of thematerial when unfrozen, progressive downward thawmay result in extreme settlements or overlying soil andstructures. This condition can be very serious becausesuch subsidence is almost invariably differential andhence very damaging to a structure. Degradation mayoccur not only from building heat but also from solarheating, as under pavements, from surface water andgroundwater flow, and from underground utility lines.Proper insulation will prevent degradation in somesituations, but where a con

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TM 5-818-1 / AFM 88-3, Chap. 7

tinuous source of heat is available, thaw will in mostcases eventually occur.

(2) The more intense the winter cooling ofthe frozen layer in the annual frost zone and the morerapid the rate of frost heave, the greater the intensity ofuplift forces in piles and foundation walls. The lower thetemperature of permafrost, the higher the bearingcapacity and adfreeze strength that can be developed,the lower the creep deformation rate under footings andin tunnels and shafts, and the faster the freeze-back ofslurried piles. Dynamic response characteristics offoundations are also a function of temperature. Both

natural and manufactured construction materialsexperience significant linear and volumetric changes andmay fracture with changes in temperature. Shrinkagecracking of flexible pavements is experienced in all coldregions. In arctic areas, patterned ground is widespread,with vertical ice wedges formed in the polygonboundaries. When underground pipes, power cables, orfoundation elements cross shrinkage cracks, rupturemay occur during winter contraction. During summerand fall, expansion of the warming ground may causesubstantial horizontal forces if the cracks have becomefilled with soil or ice.

U. S. Army Corps of EngineersFigure 18-1. Frost and permafrost in North America.

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(3) Engineering problems may also arisefrom such factors as the difficulty of excavating andhandling ground when it is frozen; soft and wet groundconditions during thaw periods; surface and subsurfacedrainage problems; special behavior and handlingrequirements for natural and manufactured materials atlow temperatures and under freeze-thaw action; possibleice uplift and thrust action on foundations; condensationon cold floors; adverse conditions of weather, cost, andsometimes accessibility; in the more remote locations,limited local availability of materials, support facilities,and labor; and reduced labor efficiency at lowtemperatures.

(4) Progressive freezing and frost heaveof foundations may also develop under refrigeratedware- houses and other facilities where sustained interiorbelow-freezing temperatures are maintained. Thedesign procedures and technical guidance outlined inthis chapter may be adapted to the solution of thesedesign problems.

18-2. Factors affecting design of foundations.a. Physiography and geology. Physiographic

and geologic details in the area of the proposedconstruction are a major factor determining the degree ofdifficulty that may be encountered in achieving a stablefoundation. For example, pervious layers in fine-grainedalluvial deposits in combination with copiousgroundwater supplies from adjacent higher terrain mayproduce very high frost-heave potential, but clean, free-draining sand and gravel terrace formations of greatdepth, free of excess ice, can provide virtually trouble-free foundation conditions.

b. Temperature. The most important factorscontributing to the existence of adverse foundationconditions in seasonal frost and permafrost regions arecold air temperatures and the continual changes oftemperature between summer and winter. Mean annualair temperatures usually have to be 2º to 8ºF belowfreezing for permafrost to be present, althoughexceptions may be encountered both above and belowthis range. Ground temperatures, depths of freeze andthaw, and thickness of permafrost are the product ofmany variables including weather, radiation, surfaceconditions, exposure, snow and vegetative cover, andinsulating or other special courses. The properties ofearth materials that determine the depths to whichfreezing-and-thawing temperatures will penetrate belowthe ground surface under given temperature differentialsover a given time are the thermal conductivity, thevolumetric specific heat capacity, and the volumetriclatent heat of fusion. These factors in turn vary with thetype of material, density, and moisture content. Figure18-2 shows how ground temperatures vary during thefreezing season in an area of substantial seasonalfreezing having a mean annual temperature of 37ºF(Limestone, Maine), and figure 18-3 shows similar data

for a permafrost area having a mean annual temperatureof 26ºF (Fairbanks, Alaska).

(1) For the computation of seasonal freezeor thaw penetration, freezing-and-thawing indexes areused based upon degree-days relative to 32 F. For theaverage permanent structure, the design indexes shouldbe those for the coldest winter and the warmest summerin 30 years of record. This criterion is more conservativethan that used for pavements because buildings andother structures are less tolerant of movement thanpavements. It is important to note that indexes foundfrom weather records are for air about 4.5 feet above theground; the values at ground surface, which determinefreeze-and-thaw effects, are usually different, beinggenerally smaller for freezer conditions and larger forthawing where surfaces are exposed to the sun. Thesurface index, which is the index determined fortemperature immediately below the surface, is n timesthe air index, where n is the correction factor. Turf,moss, other vegetative cover, and snow will reduce the nvalue for temperatures at the soil surface in relation to airtemperatures and hence give less freeze or thawpenetration for the same air freezing or thawing index.Values of n for a variety of conditions are given in TM 5-852-4/AFM 88-19, Chapter 4.

(2) More detailed information on indexesand their computation is presented in TM 5-852-6/AFM88-19, Chapter 6. Maps showing distribution of indexvalues are presented in TM 5-852-1/AFM 88-19, Chapter1, and TM 5-818-2/AFM 88-6, Chapter 4.

c. Foundation materials. The foundationdesign decisions may be critically affected by thefoundation soil, ice, and rock conditions.

(1) Soils.(a) The most important properties of

soils affecting the performance of engineering structuresunder seasonal freeze-thaw action are their frost-heavingcharacteristics and their shear strengths on thawing.Criteria for frost susceptibility based on percentage byweight finer than 0.02 millimeter are presented in TM 5-818-2/AFM 88-6, Chapter 4. These criteria have alsobeen developed for pavements. Heave potential at thelower limits of frost susceptibility determined by thesecriteria is not zero, although it is generally low tonegligible from the point of view of pavementapplications. Applicability of these criteria to foundationdesign will vary, depending upon the nature andrequirements of the particular construction. Relativefrost-heaving qualities of various soils are shown in TM5-818-2/AFM 88-6, Chapter 4.

(b) Permafrost soils cover the entirerange of types from very coarse, bouldery glacial drift toclays and organic soils. Strength properties of frozensoils

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Figure 18-2. Ground temperature during freezing season in Limestone, Maine.

U. S. Army Corps of EngineersFigure 18-3. Ground temperatures during freezing season In Fairbanks, Alaska.

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TM 5-818-1 / AFM 88-3, Chap. 7

are dependent on such variables as gradation, density,degree of saturation, ice content, unfrozen moisturecontent, temperature, dissolved soils, and rate ofloading. Frozen soils characteristically exhibit creep atstresses as low as 5 to 10 percent of the rupture strengthin rapid loading. Typical strength and creep relationshipsare described in TM 5-852-4/AFM 88-19, Chapter 4.

(2) Ice. Ice that is present in the ground inexcess of the normal void space is most obvious asmore or less clear lenses, veins or masses easily visiblein cores, and test pits or excavations, but it may also beso uniformly distributed that it is not readily apparent tothe unaided eye. In the annual frost zone, excess ice isformed by the common ice segregation process,although small amounts of ice may also originate fromfilling of shrinkage cracks; ice formations in this zonedisappear each summer. Below the annual frost zone,excess ice in permafrost may form by the same type ofice segregation process as above, may occur as verticalice wedges formed by a horizontal contraction-expansionprocess, or may be "fossil ice" buried by landslides orother events. Although most common in fine- grainedsoils, substantial bodies of excess ice are not uncommonin permanently frozen clean, granular deposits. Thepossible adverse effects of excess ice are discussed inparagraph 18-4a(2)(b).

(3) Rock. Bedrock subject to freezingtemperatures should never be assumed problem-free inabsence of positive subsurface information. In seasonalfrost areas, mud seams in bedrock or concentrations offines at or near the rock surface, in combination with theability of fissures in the rock to supply large quantities ofwater for ice segregation, frequently cause severe frostheave. In permafrost areas, very substantial quantitiesof ice are often found in bedrock, occurring in fissuresand cracks and along bedding planes.

d. Water conditions.(1) If free water drawn to developing ice

segregation can be easily replenished from an aquiferlayer or from a water table within a few feet of the planeof freezing, heave can be large. However, if a freezingsoil has no access to free water beyond that contained invoids of the soil immediately at or below the plane offreezing, frost heave will necessarily be limited.

(2) In permafrost areas, the supply ofwater available to feed growing ice lenses tends to belimited because of the presence of the underlyingimpermeable permafrost layer, usually at relativelyshallow depths, and maximum heave may thus be lessthan under otherwise similar conditions in seasonal frostareas. However, uplift forces on structures may behigher because of lower soil temperatures andconsequent higher effective tangential adfreeze strengthvalues.

(3) The water content of soil exerts asubstantial effect upon the depth of freeze or thawpenetration that will occur with a given surface freezingor thawing index. Higher moisture contents tend to

reduce penetration by increasing the volumetric latentheat of fusion as well as the volumetric specific heatcapacity. While an increase in moisture also increasesthermal conductivity, the effect of latent heat of fusiontends to be predominant. TM 5-852-6/AFM 88-19,Chapter 6, contains charts showing thermal conductivityrelationships.

e. Frost-heave forces and effect of surcharge.Frost- heave forces on structures may be quite large.For some engineering construction, complete preventionof frost heave is unnecessary and uneconomical, but formost permanent structures, complete prevention isessential. Under favorable soil and foundation loadingconditions, it may be possible to take advantage of theeffect of surcharge to control heave. It has beendemonstrated in laboratory and field experiments that therate of frost heaving is decreased by an increase ofloading on the freezing plane and that frost heaving canbe completely restrained if sufficient pressure is applied.However, heave forces normal to the freezing plane mayreach more than 10 tons per square foot. Detailedinformation on frost-heaving pressures and on the effectof surcharge is presented in TM 5-852-4/AFM 88-19,Chapter 4.

f. Type of structure. The type and uses of astructure affect the foundation design in frost areas as inother places. Applicable considerations are discussed inTM 5-852-4/AFM 88-19, Chapter 4.

18-3. Site investigations.a. General. In addition to the needed site

investigations and data described in the manuals fornonfrost conditions, design of foundations in areas ofsignificant frost penetration requires special studies anddata because of factors introduced by the special frost-related site conditions. Detailed site investigationprocedures applicable for arctic and subarctic areas aredescribed in TM 5-852-2/AFM 88-19, Chapter 2, and TM5-852-4/AFM 88-19, Chapter 4, and may be adapted orreduced in scope, as appropriate, in areas of less severewinter freezing. Methods of terrain evaluation in arcticand subarctic regions are given in TM 5-852-8.

b. Remote sensing and geophysicalinvestigations. These techniques are particularlyvaluable in selection of the specific site location, when achoice is possible. They can give clues to subsurfacefrozen ground conditions because of effects of groundfreezing upon such factors as vegetation, land wastage,and soil and rock electrical and accoustical properties.

c. Direct site investigations. The number andextent of direct site explorations should be sufficient toreveal in detail the occurrence and extent of frozen

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TM 5-818-1 / AFM 88-3, Chap. 7

strata, permafrost and excess ice including ice wedges,moisture contents and groundwater, temperatureconditions in the ground, and the characteristics andproperties of frozen materials and unfrozen soil and rock.

(1) The need for investigation of bedrockrequires special emphasis because of the possibilities offrost heave or ice inclusions as described in paragraph18-2c(3). Bedrock in permafrost areas should be drilledto obtain undisturbed frozen cores whenever iceinclusions could affect the foundation design orperformance.

(2) In areas of discontinuous permafrost,sites require especially careful exploration and manyproblems can be avoided by proper site selection. As anexample, the moving of a site 50 to 100 feet from itsplanned position may place a structure entirely on orentirely off permafrost, in either case simplifyingfoundation design. A location partly on and partly offpermafrost might involve an exceptionally difficult orcostly design.

(3) Because frozen soils may havecompressive strengths as great as that of a leanconcrete and because ice in the ground may be meltedby conventional drilling methods, special techniques arefrequently required for subsurface exploration in frozenmaterials. Core drilling using refrigerated drilling fluid orair to prevent melting of ice in the cores providesspecimens that are nearly completely undisturbed andcan be subjected to the widest range of laboratory tests.By this procedure, soils containing particles up to bouldersize and bedrock can be sampled, and ice formationscan be inspected and measured. Drive sampling isfeasible in frozen fine-grained soils above about 250Fand is often considerably simpler, cheaper, and faster.Samples obtained by this procedure are somewhatdisturbed, but they still permit ice and moisture contentdeterminations. Test pits are very useful in manysituations. For frozen soils that do not contain very manycobbles and boulders, truck-mounted power augersusing tungsten carbide cutting teeth will provide excellentservice where classification, gradation, and rough ice-content information will be sufficient. In both seasonalfrost and permafrost areas, a saturated soil condition iscommon in the upper layers of soil during the thawseason, so long as there is frozen, impervious soil stillunderlying. Explorations attempted during the thawseason are handicapped and normally require casedboring through the thawed layer. In permafrost areas, itis frequently desirable to carry out explorations duringthe colder part of the year, when the annual frost zone isfrozen, than during the summer.

(4) In subsurface explorations thatencounter frozen soil, it is important that the boundariesof frozen and thawed zones and the amount and modeof ice occurrence be recorded. Materials encounteredshould be identified in accordance with the Unified Soil

Classification System (table 2-3), including the frozen soilclassification system, as presented in TM 5-852-2/ AFM88-19, Chapter 2.

(5) In seasonal frost areas, the mostessential site date beyond those needed for nonfrostfoundation design are the design freezing index and thesoil frost-susceptibility characteristics. In permafrostareas, as described in TM 5-852-4/AFM 88-19, Chapter4, the date requirements are considerably more complex;determination of the susceptibility of the foundationmaterials to settlement on thaw and of the subsurfacetemperatures and thermal regime will usually be themost critical special requirements. Ground temperaturesare measured most commonly with copper-constantanthermocouples or with thermistors. (6)Special siteinvestigations, such as installation and testing of testpiles, or thaw-settlement tests may be required.Assessment of the excavation characteristics of frozenmaterials may also be a key factor in planning anddesign.18-4. Foundation design.

a. Selection of foundation type. Only sufficientdiscussion of the relationships between foundationconditions and design decisions is given below toindicate the general nature of the problems andsolutions. Greater detail is given in TM 5-852-4/AFM 88-19, Chapter 4.

(1) Foundations in seasonal frost areas.(a) When foundation materials within

the maximum depth of seasonal frost penetration consistof clean sands and gravels or other non-frost-susceptiblematerials that do not develop frost heave or thrust, orthaw weakening, design in seasonal frost areas may bethe same as for nonfrost regions, using conventionalfoundations, as indicated in figure 18-4. Effect of thefrost penetration on related engineering aspects, such assurface and subsurface drainage systems orunderground utilities, may need special consideration.Thorough investigation should be made to confirm thenonfrost susceptibility of subgrade soils prior to designfor this condition.

(b) When foundation materials withinthe annual frost zone are frost-susceptible, seasonalfrost heave and settlement of these materials may occur.In order for ice segregation and frost heave to develop,freezing temperatures must penetrate into the ground,soil must be frost-susceptible, and adequate moisturemust be available. The magnitude of seasonal heavingis dependent on such factors as rate and duration offrost penetration, soil type and effective pore size,surcharge, and degree of moisture availability. Frostheave in a freezing season may reach a foot or more insilts and some clays if there is an unlimited supply ofmoisture available. The frost heave may lift or tilt

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TM 5-818-1 / AFM 88-3, Chap. 7

*Permanent Construction - Construction incorporating the type and quality of materials and equipment, and details andmethods of construction, which results in a building or facility suitable to serve a specific purpose over a minimum lifeexpectancy of 25 years with normal maintenance.

**Temporary Construction - Construction incorporating the type and quality of materials and equipment, and details andmethods of construction, which results in a building or facility suitable to provide minimum accommodations at low firstcost to serve a specific purpose for a short period of time, 5 years or less, in which the degree of maintenance is not aprimary design consideration.

Figure 18-4. Design alternatives.

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TM 5-818-1 / AFM 88-3, Chap. 7

foundations and structures, commonly differentially, witha variety of possible consequences.

(c) When thaw occurs, the ice withinthe frostheaved soil is changed to water and escapes tothe ground surface or into surrounding soil, allowingoverlying materials and structures to settle. If the wateris released by thaw more rapidly than it can be drainedaway or redistributed, substantial loss in soil strengthoccurs. In seasonal frost areas, a heaved foundationmay or may not return to its before-heave elevation.Friction on lateral surface or intrusion of softened soilinto the void space below the heaved foundationmembers may prevent full return. Successive winterseasons may produce progressive upward movement.

(d) Therefore, when the soils withinthe maximum depth of seasonal frost penetration arefrost-susceptible, foundations in seasonal frost areasshould be supported below the annual frost zone, usingconventional foundation elements protected against upliftcaused by adfreeze grip and against frost overturning orsliding forces, or the structure should be placed oncompacted non-frost-susceptible fill designed to controlfrost effects (fig 18-4).

(2) Foundations in permafrost areas.Design on permafrost areas must cope with both theannual frost zone phenomena described in paragraph18-4a(l) and those peculiar to permafrost.

(a) Permafrost foundations notadversely affected by thaw. Whenever possible,structures in permafrost areas should be located onclean, non-frost-susceptible sand or gravel deposits orrock that are free of ground ice or of excess interstitialice, which would make the foundation susceptible tosettlement on thaw. Such sites are ideal and should besought whenever possible. Foundation design underthese conditions can be basically identical withtemperate zone practices, even though the materials arefrozen below the foundation support level, as has’ beendemonstrated in Corps of Engineers construction ininterior Alaska. When conventional foundation designsare used for such materials, heat from the structure willgradually thaw the foundation to progressively greaterdepths over an indefinite period of years. In 5 years, forexample, thaw may reach a depth of 40 feet. However, ifthe foundation materials are not susceptible tosettlement on thaw, there will be no effects on thestructure from such thaw. The possible effect ofearthquakes or other dynamic forces after thawingshould be considered.

(b) Permafrost foundations adverselyaffected by thaw. When permafrost foundation materialscontaining excess ice are thawed, the consequencesmay include differential settlement, slope instability,development of water-filled surface depressions thatserve to intensity thaw, loss of strength of frostloosenedfoundation materials under excess moisture conditions,development of underground uncontrolled drainagechannels in permafrost materials susceptible to bridging

or piping, and other detrimental effects. Often, theresults may be catastrophic. For permafrost soils androck containing excess ice, design should consider threealternatives, as indicated in figure 18-4: maintenance ofstable thermal regime, acceptance of thermal regimechanges, and modification of foundation conditions priorto construction. These approaches are discussed in TM5-852-4/AFM 88-19, Chapter 4. Choice of the specificfoundation type from among those indicated in figure 18-4 can be made on the basis of cost and performancerequirements after the development of details to thedegree needed for resolution.

b. Foundation freeze and thaw and techniquesfor control. Detailed guidance for foundation thermalcomputations and for methods of controlling freeze-and-thaw penetration is presented in TM 5-852-4 and TM 5-852-6/AFM 88-19, Chapters 4 and 6, respectively.

(1) Design depth of ordinary frostpenetration.

(a) For average permanentstructures, the depth of frost penetration assumed fordesign, for situations not affected by heat from astructure, should be that which will occur in the coldestyear in 30. For a structure of a temporary nature orotherwise tolerant of some foundation movement, thedepth of frost penetration in the coldest year in 10 oreven that in the mean winter may be used, as may bemost applicable. The design depth should preferably bebased on actual measurements, or on computations ifmeasurements are not available. When measurementsare available, they will almost always need to be adjustedby computations to the equivalent of the freezing indexselected as the basis for design, as measurements willseldom be available for a winter having a severityequivalent to that value.

(b) The frost penetration can becomputed using the design freezing index and thedetailed guidance given in TM 5-852-6/AFM 88-19,Chapter 6. For paved areas kept free of snow,approximate depths of frost penetration may beestimated from TM 5-818-2/AFM 88-6, Chapter 4, or TM852-3/AFM 88-19, Chapter 3, entering the appropriatechart with the air freezing index directly. A chart is alsopresented in TM 5-852-4/AFM 88-19, Chapter 4, fromwhich approximate depths of frost penetration may beobtained for a variety of surface conditions, using the airfreezing index in combination with the appropriatesurface index/air correction factor (n-factor).

(c) In the more developed parts ofthe cold regions, the building codes of most cities specifyminimum footing depths, based on many years of localexperience; these depths are invariably less than themaximum observed frost penetrations. The code

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TM 5-818-1 / AFM 88-3, Chap. 7

values should not be assumed to represent actual frostpenetration depths. Such local code values have beenselected to give generally suitable results for the types ofconstruction, soil moisture, density, and surface coverconditions, severity of freezing conditions, and buildingheating conditions that are common in the area.Unfortunately, the code values may be inadequate orinapplicable under conditions that differ from thoseassumed in formulating the code, especially for unheatedfacilities, insulated foundations, or especially coldwinters. Building codes in the Middle and North AtlanticStates and Canada frequently specify minimum footingdepths that range from 3 to 5 feet. If frost penetrationsof this order of magnitude occur with fine silt and clay-type soils, 30 to 100 percent greater frost penetrationmay occur in well-drained gravels under the sameconditions. With good soil data and a knowledge of localconditions, computed values for ordinary frostpenetration, unaffected by building heat, may beexpected to be adequately reliable, even though thefreezing index may have to be estimated from weatherdata from nearby stations. In remote areas, measuredfrost depths may be entirely unavailable.

(2) Design depth of ordinary thawpenetration. Estimates of seasonal thaw penetration inpermafrost areas should be established on the samestatistical measurement bases as outlined insubparagraph a(2)(b) above for seasonal frostpenetration. The air thawing index can be converted to asurface thawing index by multiplying it by the appropriatethawing-conditions n-factor from TM 5-852-4/AFM 88-19,Chapter 4. The thaw penetration can then be computedusing the detailed guidance given in TM 5-852-6/AFM88-19, Chapter 6. Approximate values of thawpenetration may also be estimated from a chart of the airthawing index versus the depth of thaw in TM 5-852-4/AFM 88-19, Chapter 4. Degradation of permafrost willresult if the average annual depth of thaw penetrationexceeds the average depth of frost penetration.

(3) Thaw or freeze beneath structures.(a) Any change from natural

conditions, which results in a warming of the groundbeneath a structure, can result in progressive lowering ofthe permafrost table over a period of years. Heat flowfrom a structure into underlying ground containingpermafrost can only be ignored as a factor in the long-term structural stability when the nature of the permafrostis such that no settlement or other adverse effects willresult. The source of heat may be not only the buildingheat but also the solar radiation, underground utilities,surface water, and groundwater flow. TM 5- 852-4 andTM 5-852-6/AFM 88-19, Chapters 4 and 6, respectively,provide guidance on procedures for estimating the depthof thaw under a heated building with time.

(b) The most widely employed,

effective and economical means of maintaining a stablethermal regime under a heated structure, withoutdegradation of permafrost, is by use of a ventilatedfoundation. Under this scheme, provision is made for thecirculation of cold water air between the insulated floorand the underlying ground. The same scheme can beused for the converse situation of a refrigerated facilitysupported on unfrozen ground. The simplest way ofproviding foundation ventilation is by providing an openspace under the entire building, with the structuresupported on footings or piling. For heavier floorloadings, ventilation ducts below the insulated floor maybe used. Experience has shown that ventilatedfoundations should be so elevated, sloped, oriented, andconfigured as to minimize possibilities for accumulationof water, snow, ice, or soil in the ducts. Guidance in thethermal analysis of ventilated foundations, including theestimation of depths of summer thaw in supportingmaterials and design to assure winter refreezing, is givenTM 5-852-4 and TM 5-852-6/AFM 88-19, Chapters 4 and6, respectively.

(c) Natural or forced circulationthermal piles or refrigeration points may also be used foroverall foundation cooling and control of permafrostdegradation.

(4) Foundation insulation. Thermalinsulation may be used in foundation construction in bothseasonal frost and permafrost areas to control frostpenetration, frost heave, and condensation, to conserveenergy, to provide comfort, and to enhance theeffectiveness of foundation ventilation. Unanticipatedloss of effectiveness by moisture absorption must beavoided. Cellular glass should not be used where it willbe subject to cyclic freezing and thawing in the presenceof moisture. Insulation thicknesses and placement maybe determined by the guidance given in TM 5-852-4 andTM 5-852-6/AFM 88-19, Chapters 4 and 6, respectively.

(5) Granular mats. In areas of significantseasonal frost and permafrost, a mat of non-frost-susceptible granular material may be used to moderateand control seasonal freeze-and-thaw effects in thefoundation, to provide drainage under floor slabs, toprovide stable foundation support, and to provide a dry,stable working platform for construction equipment andpersonnel. Seasonal freezing-and-thawing effects maybe totally or partially contained within the mat. Whenseasonal effects are only partially contained, themagnitude of seasonal frost heave is reduced throughboth the surcharge effect of the mat and the reduction offrost penetration into underlying frost-susceptible soils.TM 5-852-4/AFM 88-19, Chapter 4, provides guidance inthe design of mats.

(6) Solar radiation thermal effects. Thecontrol of summer heat input from solar radiation is veryimportant in foundation design in permafrost areas.Correc-

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TM 5-818-1 / AFM 88-3, Chap. 7

tive measures that may be employed include shading,reflective paint or other surface material, and sometimeslive vegetative covering. In seasonal frost areas, it maysometimes be advantageous to color critical surfacesblack to gain maximum effect of solar heat in reducingwinter frost problems. TM 5-852-4/AFM 88-19, Chapter4, provides guidance on the control of solar radiationthermal effects.

c. Control of movement and distortion. Theamount of movement and distortion that may betolerated in the support structure must be establishedand the foundation must be designed to meet thesecriteria. Movement and distortion of the foundation mayarise from seasonal upward, downward, and lateraldisplacements, from progressive settlement arising fromdegradation of permafrost or creep deflections underload, from horizontal seasonal shrinkage and expansioncaused by temperature changes, and from creep, flow,or slide of material on slopes. Heave may also occur ona nonseasonal basis if there is progressive freezing inthe foundation, as under a refrigerated building orstorage tank. If the subsurface conditions, moistureavailability, frost penetration, imposed loading, or otherfactors vary in the foundation area, the movements willbe nonuniform. Effects on the foundation and structuremay include various kinds of structural damage, jammingof doors and windows, shearing of utilities, and problemswith installed equipment.

(1) Frost-heave and thaw-settlementdeformations.

(a) Frost heave acts in the samedirection as the heat flow, or perpendicular to thefreezing plane. Thus, a slab on a horizontal surface willbe lifted directly upward, but a vertical retaining wall mayexperience horizontal thrust. Foundation members, suchas footings, walls, piles, and anchors, may also begripped on their lateral surfaces and heaved by frostforces acting in tangential shear. Figure 18-5 shows anexample of frost-heave forces developed in tangentialshear on timber and steel pipe piles restrained againstupward movement.

(b) In rivers, lakes, or coastal water bodies,foundation members to which floating ice may adheremay also be subject to important vertical forces as waterlevels fluctuate.

(c) Among methods that can be used to controldetrimental frost action effects are placing non-frostsusceptible soils in the depth subject to freezing toavoid frost heave or thrust; providing sufficientembedment or other anchorage to resist movementunder the lifting forces; providing sufficient loading on thefoundation to counterbalance upward forces; isolatingfoundation members from heave forces; battering ortapering members within the annual frost zone to reduceeffectiveness of heave grip; modifying soil frostsusceptibility; in seasonal frost areas only, takingadvantage of natural heat losses from the facility to

minimize adfreeze and frost heave; or cantileveringbuilding attachments, e.g., porches and stairs, to its mainfoundation.

(d) In permafrost areas, movementand distortion caused by thaw of permafrost can beextreme and should be avoided by designing for full andpositive thermal stability whenever the foundation wouldbe adversely affected by thaw. If damaging thawsettlement should start, a mechanical refrigerationsystem may have to be installed in the foundation or aprogram of continual jacking may have to be adopted forleveling of the structure. Discontinuance or reduction ofbuilding heat can also be effective. Detailed guidance isgiven in TM 5-852-4/AFM 88-19, Chapter 4.

(2) Creep deformation. Only very smallloads can be carried on the unconfined surface of ice-saturated frozen soil without progressive deformation.The allowable long-term loading increases greatly withdepth but may be limited by unacceptable creepdeformation well short of the allowable stress leveldetermined from conventional short-term test. Presentpractice is to use large footings with low unit loadings;support footings on mats of well-drained non-frost-susceptible granular materials, which reduce stresses onunderlying frozen materials to conservatively low values;or place foundations at sufficient depth in the ground sothat creep is effectively minimized. Pile foundations aredesigned to not exceed sustainable adfreeze bondstrengths. In all cases, analysis is based on permafrosttemperature at the warmest time of the year. For caseswhich require estimation of foundation creep behavior,see TM 5-852-4/AFM 88-19, Chapter 4.

d. Vibration problems and seismic effects.(1) Foundations supported on frozen

ground may be affected by high stress-type dynamicloadings, such as shock loadings from high-yieldexplosions, by lower stress pulse-type loadings as fromearthquakes or impacts, or by relatively low-stress,relatively low-frequency, steady-state vibrations. Ingeneral, the same procedures used for nonfrozen soilconditions are applicable to frozen soils. Design criteriaare given in TM 5-809-10/AFM 88-3, Chapter 13; TM 5-856-4; TM 5-852-4/AFM 88-19, Chapter 4; and chapter17 of this manual. These manuals also containreferences to sources of data on the general behaviorand properties of nonfrozen soils under dynamic loadand discuss types of laboratory and field tests available.However, design criteria, test techniques, and methodsof analysis are not yet firmly established for engineeringproblems of dynamic loading of foundations. Therefore,the Office, Chief of Engineers, ATTN: DAEN- ECE-T,WASH DC 20314, or HQUSAF/PREE, WASH DC20332, should be notified upon initiation of design andshould participate in establishing criteria and ap-

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TM 5-818-1 / AFM 88-3, Chap. 7

U.S. Army Corps of EngineersAVERAGE ADFREEZE STRESS vs TIME

Figure 18-5. Heave force tests, average tangential adfreeze bond stress versus time, and timber and steel pipe pilesplaced with silt-water slurry in dry excavated holes. Piles were installed within annual frost zone only, over permafrost, to

depths from ground surface of 3.6 to 6.5 feet.

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TM 5-818-1 / AFM 88-3, Chap. 7

proach and in planning field and laboratory tests.(2) All design approaches require knowledge of

the response characteristics of the foundation materials,frozen or nonfrozen, under the particular load involved.As dynamic loadings occur in a range of stresses,frequencies, and types (shock, pulse, steadystatevibrations, etc.), and the response of the soil variesdepending upon the load characteristics, the requireddata must be obtained from tests that produce the sameresponses as the actual load. Different design criteriaare used for the different types of dynamic loading, anddifferent parameters are required. Such properties asmoduli, damping ability, and velocity of propagation varysignificantly with such factors as dynamic stress, strain,

frequency, temperature, and soil type and condition. TM5-852-4/AFM 88-19, Chapter 4, discusses theseproperties for frozen ground.

e. Design criteria for various specificengineering features. In addition to the basicconsiderations outlined in the preceding paragraphs ofthis chapter, the design of foundations for frost andpermafrost conditions requires application of detailedcriteria for specific engineering situations. Guidance forthe design of various specific features, constructionconsideration, and monitoring of performance offoundation is presented in TM 5-852-4/AFM 88-19,Chapter 4.

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TM 5-818-1 / AFM 88-3, Chap. 7

APPENDIX A

REFERENCES

Government Publications

Departments of the Army, the Navy, and the Air Force

TM 5-809-7 Pile FoundationsTM 5-809-10/AFM 88-3, Chap. 13 Seismic Design for BuildingsTM 5-818-2/AFM 88-6, Chap. 4 Soils and Geology: Pavement Design for Frost ConditionsTM 5-818-4/AFM 88-5, Chap. 5 Soils and Geology: Backfill for Subsurface StructuresTM 5-818-5/AFM 88-5, Chap. 6 Dewatering and Groundwater Control for Deep ExcavationsTM 5-818-6/AFM 88-32 Grouting Methods and EquipmentTM 5-818-7 Foundations on Expansive SoilsTM 5-824-3/AFM 88-6, Chap. 3 Rigid Pavements for Airfields Other Than ArmyTM 5-852-1/AFM 88-19, Chap. 1 Arctic and Subarctic Construction: General ProvisionsTM 5-852-2/AFM 88-19, Chap. 2 Arctic and Subarctic Construction: Site Selection and

DevelopmentTM 5-852-3/AFM 88-19, Chap. 3 Arctic and Subarctic Construction: Runway and Road DesignTM 5-852-4/AFM 88-19, Chap. 4 Arctic and Subarctic Construction: Building FoundationsTM 5-852-5/AFM 88-19, Chap. 5 Arctic and Subarctic Construction: UtilitiesTM 5-852-6/AFM 88-19, Chap. 6 Arctic and Subarctic Construction: Calculation Methods for

Determination of Depths of Freeze and Thaw in SoilsTM 5-852-7/AFM 88-19, Chap. 7 Arctic and Subarctic Construction: Surface Drainage Design

for Airfields and Heliports in Arctic and Subarctic RegionsTM 5-852-8 Arctic and Subarctic Construction: Terrain Evaluation in

Arctic and Subarctic RegionsTM 5-852-9/AFM 88-19, Chap. 9 Arctic and Subarctic Construction: BuildingsTM 5-856-4 Design of Structures to Resist the Effects of Atomic Weapons:

Structural Elements Subjected to Dynamic LoadsNAVFAC DM-7 Soil Mechanics, Foundations, and Earth Structures

5-818-1 / AFM 88-3, Chap. 7

The proponent agency of this publication is the Office of the Chief of Engineers, United StatesArmy. Users are invited to send comments and suggested Improvements on DA Form 2028(Recommended Changes to Publications and Blank Forms) direct to HQDA (DAEN-ECE-G), WASHDC 20314.

By Order of the Secretaries of the Army and the Air Force:

JOHN A. WICKHAM, JR.General, United States Army

Official: Chief of StaffROBERT M. JOYCE

Major General, United States ArmyThe Adjutant General

CHARLES A. GABRIEL, General, USAFOfficial: Chief of Staff

JAMES H. DELANEY, Colonel, USAFDirector of Administration

Distribution:Army: To be distributed in accordance with DA Form 12-34B, requirements for TM 5-800 Series: Engineering

and Design for Real Property Facilities.AirForce: F

*US GOVERNMENT PRINTING OFFICE 1996 - 406-421/50226

PIN: 025929-000

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