1

AN INTRODUCTION TO BUILDING FOUNDATIONS AND SOIL IMPROVEMENT METHODS

SEAONC 2008 Spring SeminarSan Francisco, 16 April 2008

Hadi J. Yap, PhD, PE, GE

2

General Foundation Types

• Shallow Foundations– Spread footings: isolated, continuous– grid or waffle– Post-Tensioned Slabs (PT Slabs)– Mats

• Deep Foundations

3

Factors to be Considered in Selecting Foundation Type

• Subsurface conditions• Column loads and spacing, basements• Site constraints

– noise– vibrations– proximity to existing improvements, slope, channel

• Economics

4

Shallow Foundations

• Suitable where underlying material is strong

• Can be used in engineered fill if building load is light to moderate

• Mats can be used to span localized weak areas

• Mats can be used on weaker soil for structure with basements where net load (weight of structure minus weight of soil removed) is low

5

Sources of Settlement• Immediate Settlement (sand and clay)

Occurs as the load is applied• Consolidation (saturated clay)

A slow process of squeezing water out of the pores in soft clay when loaded

• Liquefaction (saturated sand) Temporary loss of shear strength in loose sand due to a rise in excess pore water pressure during cyclic loading such as seismic

• Seismic Densification (dry/moist sand)Densification of loose sand above the groundwater level due to ground shaking

6

Total and Differential Settlement

• Building can tolerate large total settlement if the differential settlement is within tolerable limits

• Where the total settlement is large, flexible connections should be provided to underground utilities where they enter the building

7

Allowable Differential Settlement for Buildings

Angular Distortion = Differential settlement/Distance

Angular Distortion Limits (Bjerrum, 1963):

• 1/500 – safe limit where cracking is not permissible• 1/300 – limit where first cracking in panel walls is to

be expected• 1/150 – limit where structural damage to general

buildings is to be feared

8

Design Parameters for Spread Footings

• Minimum width• Minimum embedment depth• Allowable bearing pressure• Allowable passive pressure• Allowable base friction coefficient

9

Spread Footing Excavations

10

When mat is to be considered

• When total footprint area of spread footings is more than, say, 50% of building footprint

• To reduce total and differential settlement

• To bridge areas of weak subgrade

11

Design Parameters for Mat Foundations

• Minimum embedment depth • Allowable bearing pressures• Allowable passive pressure• Allowable base friction coefficient• Subgrade modulus

12

Mat Subgrade and Mud Slab

13

Mat Rebars

14

Deep Foundation Types

• Drilled Piers/Cast-In-situ-Drilled-Hole [CIDH] Piles

• Driven Piles (Concrete, Steel H)• Tubex Piles• Auger Cast Piles• Torque Down Piles• Micropiles

15

Drilled Piers

• Can use one large diameter pier in lieu of several smaller, driven piles

• Lengths can be adjusted in the field – reduce waste/build-up

• Derive axial capacity mainly from skin friction• Need to use casing and/or drilling fluid if

groundwater and/or loose soil is present

16

Drilled Pier Installation

17

Driven Precast, Prestressed, Concrete Piles

• Economical in San Francisco Bay Area

• Can be used where soft soil, non-engineered fill, or high groundwater level, is present

• Fabricated at yard – good quality control

• Moderately high capacity – up to 344 kips for 14”square piles using 6,000 psi concrete

18

Concrete and Steel Piles

19

Driven Steel H-Piles• More expensive than driven concrete piles• Suitable where depth to bearing soil layer

varies; can conveniently be cut and spliced• Design must consider corrosion• Moderate to high capacity – up to 456 kips

for HP14X89 using 50 ksi steel• Lateral resistance varies with load direction

relative to pile axis

20

Soil Improvement

If poor soil conditions are encountered:

• Bypass poor soil, use deep foundations• Remove poor soil, replace with engineered fill• Improve soil properties in place

21

Factors to be Considered in Selecting Soil Improvement Method

• Soil type; fines content (silt- and clay-size)• Area and depth of treatment• Soil properties – strength, compressibility• Proposed structure and settlement criteria• Availability of skills, equipment, materials• Adjacent improvements• Economics

22

Primary objectives of soil improvement

CLAY– Increase bearing capacity or slope stability– Reduce foundation settlement

SAND- Reduce liquefaction potential- Increase bearing capacity- Reduce foundation settlement

23

Mechanisms of Soil Improvement for Clay

• Consolidation- Preloading

• Reinforcement- Soil-Cement Columns- Vibro-Replacement Stone Columns- Geopiers® and Vibro Piers™

• Mixing- Soil-cement columns

24

Mechanisms of Soil Improvement for Sand• By vibration

– Impacts at surface: Dynamic compaction– Depth vibrator: Vibro-compaction

• By vibration and displacement of backfill- Vibro-replacement stone columns- Vibro Piers™

• By displacement of backfill material- Compaction grouting

• By binding particles- Permeation grouting (e.g. ultra-fine cement)

• By mixing– Soil-cement columns

25

Soil Improvement Methods

XPermeation GroutingXCompaction GroutingXXGeopiers® and Vibro Piers™XXStone ColumnsXVibro-CompactionXXSoil-Cement ColumnsXDynamic Compaction

XPreloading

SandClayMethod

26

Preloading

• Performed by placing fill over soft clay• Improve foundation soil for buildings,

embankments, runways, bridge abutments• Type of preloads: earth fill, water, vacuum • Use prefabricated vertical (wick) drains to

reduce preloading time • Wick drains: plastic core wrapped in

geotextile; generally 4” wide and 1/8” to 3/8”thick

27

Wick Drain Installation

28

Preloading (cont’d)• Typical wick drain spacing is 3 to 6 feet,

depending on soil permeability and time available

• Typical preloading period is 3 to 6 months, depending on soil permeability and degree of consolidation to be achieved

• Construction monitoring: settlement (settlement plates/probes), pore water pressure (piezometers), lateral movement (inclinometers)

29

Source: ASCE, Geotechnical Special Publication No. 69, 1997

Preloading with Wick Drains and Instrumentation

30

Dynamic Compaction• Involves repeated dropping of heavy weights

onto ground surface• Effective for sand, waste, and rubble fills• Pounders: concrete blocks, steel plates, or

thick steel shells filled with concrete/sand• Typical weight of pounders: 6 to 30 tons,

depending on the depth of soil to be improved

• Typical drop heights: 40 to 100 feet

31

Dynamic CompactionSource: Hayward Baker

32

Dynamic Compaction (cont’d)

• Most effective for soil with less than 25% fines (silt- and clay-size particles; material passing #200 sieve [0.075 mm opening])

• Typical improvement depth is 10 to 30 feetD ≈ 0.5 √(WH)

where:D = improvement depth in mW = pounder weight in metric tonH = drop height in m

33

Dynamic Compaction (cont’d)

• Ground Vibrations< 0.5 inch/sec to prevent cracks in walls< 2.0 inch/sec to prevent structural damage

• Construction monitoring– Induced settlement– Ground vibration– Ground heave – Pore water pressure– Verification testing (SPT, CPT)

34

Scaled Energy Factor versus Particle Velocity

Source: FHWA, Dynamic Compaction, 1995

35

Soil-Cement Columns• Mixing in-situ soil with cementitious materials using

mixing shafts consisting of auger cutting heads, auger flights, or mixing paddles

• Produce soil-cement columns with higher strength, lower compressibility, and lower permeability than the native soil

• Used to improve bearing capacity and slope stability, and as shoring walls

• Typical compressive strength of cylinders ranges from 15 to 300 psi

• Typical permeability of mix ranges from 10-6 to 10-7

cm/sec

36

Soil-Cement ColumnInstallation

37

Soil-Cement Shoring Wall

38

Soil-Cement Wall Installation

39

Steel Beam Installation

40

Vibro-Compaction

• Densifying granular soil by inserting a vibrating probe into the ground

• Probe spacing ranges from 6 to 14 feet• Suitable for sand with less than 15% fines (silt- and

clay-size particles) • Vibrator is a torpedo shaped horizontally vibrating

probe, 10 to 15 feet long, and weighs about 2 tons. The probe penetrates to the design depth under its own weight assisted by water jetting

41

Vibrator and Water Jets

42

Vibro-Compaction (cont’d)

• The action of vibrator and water jetting reduce inter-granular forces between soil particles allowing them to become denser

• The vibrator starts at the bottom of the hole and raised to treat the next interval; the procedure is repeated as backfill sand is added

• If backfill is not added, craters with diameters of 10 to 15 feet can form around vibrator

43

Vibro-Replacement Stone Columns

• Extends the range of soil types that can be improved to silt and clay

• The probe is penetrated to design depth and gravel/crushed rock is placed in the hole as the probe is withdrawn in vertical increments of 2 to 5 feet

• A stone column is formed with the stone laterally compacted against the surrounding soil

44

Vibro-Replacement Stone Columns (Cont’d)

• Three primary methods– Wet, top feed method: hole is saturated with

jetting water – Dry, top feed method: hole formed by probe

remains open without water– Dry, bottom feed: stone backfill is fed through a

hopper and tube to the bottom of the hole• Construction Monitoring

– Settlement and ground heave– Amount of stone backfill used– Verification testing (SPT, CPT)

45

Wet Top Feed MethodSource: Bauer

46

Dry Bottom Feed Method

47

Geopiers® (Rammed Aggregate Piers)

• Locally installed by Farrell Design-Build Company• Typically 24 to 36 inches in diameter and 6 to 30 feet

deep, constructed by drilling and ramming crushed rock in 12-inch lifts

• The ramming equipment consists of excavators equipped with 2,000 to 4,000 lbs hydraulic hammers with beveled tampers

• The ultimate bearing capacity of a pier ranges from 100 to 300 kips in compression and 100-150 kips in uplift (with steel anchor). Allowable bearing capacity range from 5 to 8 ksf

48

Completed Geopiers®

49

Geopiers®

50

51

Vibro Piers™• Installed using displacement and vibratory energy to

depths of 10 to 30 feet by Hayward Baker• Installation methods:

– Dry top feed method: stone is placed in pre-augered hole, densified in 6 to 12-inch lifts with a vibrator

– Dry bottom feed method: for high groundwater level. The vibrator with tremie pipe attachment are penetrated to the design depth to install and densify the stone in place. Little or no waste results from this method.

52

Dry Top Feed Method

Source: Hayward Baker Inc.Dry Bottom Feed

53

Compaction Grouting

• To compact loose soil or to produce control displacement to lift structure

• Involves injection of low-slump (less than 2 inches) grout (soil-cement mixture) which does not enter soil pores but remain in a homogeneous mass

• Grout material may consist of fine sand mixed with 12% cement and water to produce stiff, mortar-like mixture

• Grout pipe is installed to maximum treatment depth and grout is injected at high pump pressure as the pipe is withdrawn incrementally, forming a column of interconnected grout bulb

54

Compaction Grouting

55

Compaction Grouting (cont’d)

• Can be performed “stage down” or, more commonly, in a “stage up” process, as follows:– Advancing the grout pipe to the bottom of

treatment depth– Injecting the grout until refusal criteria is

achieved, based on injected grout volume, injection pressure, or ground heave

– Extracting the grout pipe to the next depth interval and injecting the grout

– Repeat the process until reaching the upper limit of treatment zone

56

Compaction Grouting

Source: Hayward Baker

57

Compaction Grouting (cont’d)

• Construction monitoring:– Injected grout volume– Pressure loss/ground surface heave– Verification testing: pre- and post-grouting

SPT/CPT

58

Ultra-Fine Cement Grouting

• Uses micro-fine cement (particle size ranges from 1 to 10 microns); can penetrate fine sand

• Used to increase bearing capacity of sand under existing footings and/or reduce potential settlement

• Can be used to retain shallow excavation in loose sand

• Unconfined compressive strength can exceed 100 psi

59

Ultra-Cement Grouting

60

Elevator Pit Excavation