site investigation, soil types and foundation design in relation to ground movement

8/11/2019 SITE INVESTIGATION, SOIL TYPES AND FOUNDATION DESIGN IN RELATION TO GROUND MOVEMENT

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SITE INVESTIGATION, SOIL TYPES AND FOUNDATION

DESIGN IN RELATION TO GROUND MOVEMENT

BY

ALBERT CAUCHI B.Sc.Eng., Eur.Ing., M.A.S.C.E., C.Eng.,

M.I.Struct.E.(Lond.), A. & C.E., M.I.M., C.L.J.


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CONTENTS

1.0 Introduction

1.1 Identification and Classification Table of Soils

1.2 Authors Experience of Different Soils and SiteInvestigations in Different Countries

2.0 Site Investigation and Soil Types

2.1 Information Required from Site Investigation

2.2 Site Investigation of foundation failures

2.3 Methods of Sub-surface exploration

2.4 Foundation Properties of Soil and Rock Types

3.0 Foundation Design in Relation to Ground Movement

3.1 Soil Movements

3.2 Ground movements due to Water Seepage and SurfaceErosion

3.3 Ground Movements due to Vibration

3.4 Ground Movements due to Hillside Creep

3.5 Ground Movements due to Mining Subsidence

3.6 Foundations on Filled Ground

3.7 Machinery Foundations

4.0 Derivation of Bearing Capacities of Soils from SiteInvestigations

5.0 The Control of Ground Water in Excavations

6.0 Shoring and Underpinning

7.0 Chemical Attack from Ground and Water

8.0 Tower of Pisa


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1.2 Authors experiences of different soils and site investigations indifferent countries

Hereunder are listed a number of Soil and Rock Types that Albert Cauchi has

encountered during his working life in different countries:

United Kingdom: 1969; 1972 1974; 1995

Clay Slopes, Gateshead Bypass Newcastle, Highway

Keuper Marl Erdington, Birmingham Block of apartments, 20storey. Use of split spoon samplers for disturbed samples and Pilconwayfarer rig, 30m deep boreholes

Quarry Fill- Birmingham Office Block, 6 storey

River Silt with high water table Heavy loading from clothingwarehouse and factory, existing foundations were old brick foundations1800s

Flat Clay reasonably uniform soil condition Trench fill foundation formental hospital West Birmingham

London Clay Eddies Disco

Mining subsidence Old coal mines Derbyshire, CLASP System

Sandstone/Keuper Marl Central Birmingham Old Victorian MedicalSchoolold train tunnel underneath

Sandstone-Spring foundations Repertory Theatre Birmingham,train under building.

Sultanate of Oman: 1979 1981

Silt/Igneous Rock differential settlement. RUWI, Prestigious villa

Red Layered, Fractured Igneous Rock Qurum Masqat Mountains

Barchan Sand Dune Qurum Masqat, Villas

Marl Sur

Granite Red/Black Marbat South Oman Dhofar low-costhousing project, 500 houses. Controlled explosives used for drainagetrenching

Gravel Sahalnawt (Pink) Clilffs, Military Camp.

Pink Limestone


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Kuwait: 1982 1986

Loose Sand high water table Kasr Al-Hassawi Mesila Beach,dewatering using sheet piles

Sand loose to compact high water table Al-Ahli Bank 40 storeys

Sand loose to compact Fly-over foundations GhazaliExpressway. Use of Pitcher samples for undisturbed samples, by truckmounted rig. 60m deep boreholes

Libya: 1987 1990

Gatch - Raguba

Sand Marsa Brega

Seaweed/Sand Marsa Brega Harbour

Sulphate Attack Hateiba

Mica/Sand Assamoud

Malta: 1966 1971; 1975 1979; 1990 2002

Upper Coralline Limestone Mtarfa

Clay Slopes Xemxija, Mellieha (Santa Marija Estate), Gozo

Coralline Limestone with caverns Preluna Hotel

Globigerina Limestone fissured Tal-Qroqq Hospital, geophysicalinvestigation soundwages generated by fired shots.

Sand with high water table Birzebbugia

7 metres deep

Clay St. Pauls Bay, White House

Marl Qormi, Cannon Road in Regional Road

Reclaimed Land Freeport

Turbazz Bugibba


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Vietnam: 1997

Mountainous and Paddy Field, Terrain: Igneous Rock and Clay HoaBinh, North Vietnam (4 hours south of Hanoi)

Flat Clay (next to canal)(Estuary of Mekong) Soc Trang SouthVietnam (7 hours south of Saigon, Ho Chi Minh City)

Saudi Arabia: 1976 1979

Sand Riyadh

Sandstone Mountains- Taif

Sand, high water table Jeddah Dammam/Dahran


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1.0 INTRODUCTION

A site investigation in one form or another is always required before the

commencement of a building or engineering structure. It is ideal that from theearliest time, definitely during the design stage, this starts taking place. Theinvestigation may range from a simple examination of the surface soil such asdigging your heel into the ground to a few shallow trial pits, to a detailed studyof the soil and ground water, including chemical analysis to a considerabledepth below the surface by means of boreholes and tests, in situ and/orlaboratory of the materials encountered.

Additional information about ground conditions may also be obtained byexamining open sewer trenches or shallow excavations for roadworks, orhand auger borings.

Contoured ordinance sheets and geological maps could also be useful.

When troublesome foundation conditions are encountered it would benecessary to sink deep boreholes, supplemented by soil tests. These wouldalso be necessary the larger the building or structure to be built is.

A detailed site investigation involving deep boreholes and laboratory testing ofsoils is always a necessity for heavy structures such as bridges, multi-storeybuildings, or industrial plants. Even if rock is known to be present at a shallowdepth it is advisable to excavate down to expose the rock in a few places toensure that there are no zones of deep weathering or heavily shattered orfaulted rock. Thorough investigations are equally necessary for engineeringstructures founded in deep excavations. As well as providing information forfoundation design, they provide essential information on the soil and groundwater conditions to contractors tendering for the work.


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2.0 SITE INVESTIGATION AND SOIL TYPES

2.1 Information Required from a Site Investigation

Assuming a fairly detailed study is required, the following information should

be obtained in the course of a site investigation for foundation engineeringpurposes:

a) The general topography of the site as it affects foundation design andconstruction, e.g. surface configuration, adjacent property, thepresence of watercourses, ponds, hedges, trees, rock out-crops, etc.,and the available access for construction vehicles and plant.

b) The location of buried services such as electric power and telephonecables, water mains, and sewers.

c) The general geology of the area with particular reference to the maingeological formations underlying the site and the possibility ofsubsidence from mineral extraction or other causes.

d) The previous history and use of the site including information of anydefects or failures of existing or former buildings attributable tofoundation conditions.

e) Any special features such as the possibility of earthquakes or climaticfactors such a flooding, seasonal swelling and shrinkage, permafrost or

soil erosion.

f) The availability and quality of local constructional materials such asconcrete aggregates, building and road stone, and water forconstructional purposes.

g) For maritime or river structures information on normal spring and neaptide ranges, extreme high and low tidal ranges and river levels,seasonal river levels and discharges, velocity of tidal and river currents,and other hydrographic and meteorological data.

h) A detailed record of the soil and rock strata and ground waterconditions within the zones affected by foundation bearing pressuresand construction operations, or of any deeper strata affecting the sideconditions in any way.

i) Results of laboratory tests on soil and rock samples appropriate to theparticular foundation design or constructional problems.

j) Results of chemical analyses on soil or ground water to determinepossible deleterious effects on foundation structures.

A more detailed list of other information required in connection with generalengineering design and construction is given in the British Standard Code ofPractice for Site Investigations(CP2001).


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Items (a) to (g) above can be obtained from a general reconnaissance of thesite and from a study of geological memoirs and maps and other publishedrecords. A close inspection given by walking over the site area will oftenshow significant indications of sub surface features. For example, concealedswallow holes (sink holes) in chalk or limestone formations are often revealed

by random depressions and marked irregularity in the ground surface; soilcreep is indicated by wrinkling of the surface on a hillside slope, or leaningtrees; abandoned mine workings are shown by old shafts or heaps of mineralwaste; glacial deposits may be indicated by mounts or hummocks (drumlins)in a generally flat topography; and river or lake deposits by flat low-lying areasin valleys. The surface indications of ground water are the presence ofsprings or wells, and marshy ground with reeds (indicating the presence of ahigh water table with poor drainage and the possibility of peat). Professionalgeological advice should be sought in the case of large projects coveringextensive areas.

On very extensive sites, aerial photography is a valuable aid in siteinvestigations. Skilled interpretations of aerial photographs can reveal muchof the geology and topography of a site. Geological mapping from aerialphotographs as practised by specialist firms is a well-established science.

Old maps as well as up-to-date publications should be studied since thesemay show the previous use of the site and are particularly valuable wheninvestigating backfilled areas. Museums or libraries in the locality oftenprovide much information in the form of maps, memoirs, and pictures orphotographs of a site in past times. Local authorities should be consulted fordetails of buried services. If particular information on the history of a site hasan important bearing on foundation design, for example the location of buriedpits or quarries, every endeavour should be made to cross-check sources ofinformation especially if they are based on memory or hearsay. Peoplesmemories are notoriously unreliable on these matters

Items (h), (i), and (j) of the list are obtained from boreholes or other methodsof sub-surface exploration, together with field and laboratory testing of soils orrocks. It is important to describe the type and consistency of soils in thestandard manner laid down in CP 2001. The standard descriptions are basedon internationally recognised soil classifications and are shown in Table 1.1.

Descriptions are given in the following sequence:

1. Consistency for cohesive soils, e.g. soft, stiff, etc.Density for non-cohesive soils, e.g. loose, dense, etc.

2. Structure (i.e. fissured, laminated, etc.)3. Colour.4. Particle size classification with the predominating type last.

As examples:

Stiff fissures brown clay;Firm laminated grey clayey silt;Loose yellow fine sand.


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In some cases it may be relevant to state the odour of the soil, such as thatcaused by organic matter or chemical wastes, to draw attention to possibleeffects on foundation concrete or steel.

The divisions between the various ranges of particle size as shown in Table

1.1, i.e. gravels, sands, silts and clays and their sub-divisions, are quitearbitrary but they generally accord with the behaviour of the soils in theengineering sense. For example, clays show quite different behaviour fromsands when subjected to foundation pressures, and their drainagecharacteristics when pumping from foundation excavations are also verydifferent. It is not unusual to describe soils by other forms of classification,e.g. the Unified System of the U.S. Corps of Engineers and the Bureau ofReclamation, when considering them as foundation soils; such classificationsare generally restricted to road and airfield work.

Rock types are given a simple geological classification descriptive of their

engineering characteristics. There may be some confusion in describing thehardne3ss of rocks when considering them in relation to soil consistency.Thus a weak sandstone will have a much higher bearing capacity than a softclay. To avoid confusion the terms friable or weakly-cemented should beused to describe weak rocks wherever these terms are applicable. Thecurrent revisions of CP 2001 adopt a new system of nomenclature for rockstrengths and substitute the terms weak and strong rocks to replace theprevious soft and hard. This system should help to avoid confusion withdescriptions of soil consistency. Examples of engineering descriptions of rocktypes are:

Friable coarse-grained red sandstone;Weakly-cemented porous shelly limestone;Strong fissured purple siltstone;Loose disintegrated grey shale;Weak red and white decomposed granite.


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2.2 Site Investigations of Foundation Failures

From time to time it is necessary to make investigations of failures or defectsin existing structures. The approach is somewhat different from that of normalsite investigation work, and usually takes the form of trial pits dug at various

points to expose the soil at foundation level and the foundations themselves,together with deep trial pits or borings to investigate the full depth of the soilaffected by bearing pressures. A careful note is taken of all visible crackingand movements in the superstructure since the patter of cracking is indicativeof the mode of foundation movement, e.g. by sagging or hogging. It is oftennecessary to make long-continued observations of changes in level and ofmovement of cracks by means of tell-tales. Glass or paper tell-tales stuck onthe cracks by cement pats are of little use and are easily lost or damaged.The tell-tales should consist of brass or bronze plugs cemented into holesdrilled in the wall on each side of the crack and so arranged that both verticaland horizontal movements can be measured by micrometer gauges.

Similarly, points for taking levels should be well-secured against removal ordisplacement. They should consist preferably of steel bolts or pins set in thefoundations and surrounded by a vertical pipe with a cover at ground level.The levels should be referred to a well-established datum point at somedistance from the affected structure; ground movements which may havecaused foundation failure should not cause similar movement of the levellingdatum.

A careful study should be make of adjacent structures to ascertain whetherfailure is of general occurrence, as in mining subsidence, or whether it is dueto localised conditions. The past history of the site should be investigatedwith particular reference to the former existence of trees, hedgerows, formbuildings, or waste dumps. The proximity of any growing trees should benoted, and information should be sought on the seasonal occurrence ofcracking, for example if cracks tend to open or close in winter or summer, orare worse in dry years or wet years. Any industrial plant in which forginghammers or presses cause ground vibrations should be noted, and inquiriesshould be made about any construction operations such as deep trenches,tunnels, blasting, or piling which may have been carried out in the locality.


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Shell and auger borings can be carried out in all types of soil since theboreholes can be lines where required with steel casing tubes, and a widevariety of tools are used for different soil and rock types. A shell and auger rigemploying a friction winch to raise and lower the boring tools is shown in Plate

I.

Boring in cohesive soils is effected by augers, clay-circle (an open-endedsteel tube with a flap-valve and cutting edge at its lower end). Sands andgravels are removed from the boreholes by the shell. When boring throughrock or boulders, chisels of various types are used to break up the rock andthe fragments are cleaned out by the shell. When boring through rock orboulders, chisels of various types are used to break up the rock and thefragments are cleaned out by the shell. In hard rock the progress is slow andit is preferable to employ rotary core drilling as described later in this chapter.The use of water poured in the hole is often unavoidable in shell and auger

boring, particularly when boring in gravels. However, it should always beused sparingly and the occasions when water is added should be noted onthe site borehole records.

Machines used for percussion boring loosen the soil by repeated blows of aheavy chisel or spud, and the resulting slurry is removed by shell or bywashing. The stroke of each blow is controlled mechanically be means of awalking beam. This method of drilling for site investigation purposes isdeprecated by some engineers on the grounds of possible deep disturbanceof the soil and the lack of sensitivity or feel in its operation. However, askilled operator can regulate the force of the blow to a degree at least equal tothat obtainable by the friction winch used in shell and auger boring.Therefore, due attention is paid to adequate soil sampling there is no reasonwhy borings made by the percussion drill should not yield reliable information.

In wash boring(Fig. 1) the soil is loosened and removed from the borehole bya stream of water or drilling mud issuing from the lower end of the wash pipe


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which is worked up and down or rotated by hand in the borehole. The wateror mud flow carries the soil up the annular space between the wash pipe andthe casing, and it overflows at ground level where the soil in suspension isallowed to settle in a pond or tank and the fluid is re-circulated or dischargedto waste as required. Samples of the settled-out soil can be retained for

identification purposes but this procedure is often unreliable as the cuttingsare mixed as they flow up the borehole and in the settling tank, and thestructure of the soil is destroyed. However, accurate identification can beobtained if frequent dry sampling is resorted to, using undisturbed sampletubes or the split-spoon sampler. Wash boring has the advantage that thestructure or density of the soil below the bottom of the borehole is notdisturbed by blows of the boring tools, but the method cannot be used in largegravels or soil containing boulders. It is best suited to uniform sands or clays.A variety of tools are used for fitting to the end of the wash pipe for differentsoil types. The use of mid instead of water allows the hole to remain uncasedin cohesionless soils.

Wash probing (Fig 2) is a simple method of determining the depth to aninterface between soft or loose soils and a stiff or compact layer. The wash

pipes delivering water at high pressure are worked up and down in anuncased hole. Thus there is no positive identification of the soil since there isoften no return of the wash-water. Dry sampling through the wash pipescannot easily be achieved and in many cases is impossible. However, ifample water is available and if the soil does not contain large cobbles orboulders, the method is rapid and cheap in establishing the level of a well-defined stratum which can be located by the feel of the wash pipes as theyare worked up and down. Wash probings must be correlated with boreholessunk by more positive methods and they should only be regarded as filling-indata between widely spaced boreholes. They are a convenient method of

rapid sub-bottom exploration for river or marine works; to investigate, forexample, the depth of sand or mud over bedrock on a piling or dredgingproject.


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Dynamic or static cone penetration equipment is used to determine thecharacteristics and stratification of soil deposits by measuring either thenumber of blows required to drive the cone a fixed distance (dynamic conetests) or the force required to push the cone into the soil at progressivelyincreasing depths (static cone tests). A record of penetration resistance with

depth is obtained from which it is possible, by correlation with boreholes, todeduce the stratification of the soils. However, such methods are of morevalue in determining the bearing characteristics of the soils by direct in-situmeasurement.


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SOIL SAMPLING

There are two main types of soil samples which can be recovered fromboreholes or trial pits.

Disturbed samples, as their name implies, are samples taken from the boringtools: examples are auger parings, the contents of the split-spoon sampler inthe standard penetration test, sludges from the shell or was-water return, orhand samples dug from trial pits. The structure of the natural soil may bedisturbed to a considerable degree by the action of the boring tools orexcavation equipment. The samples are placed in airtight jars or bags andlabelled to identify the locality, borehole number, depth of sample, and date ofsampling.

Undisturbed samples represent as closely as is practicable the true in-situstructure and water content of the soil. The usual method of obtaining

undisturbed samples is to drive a thin-walled tube for its full extent into the soiland then to withdraw the tube and its contents. It is important not to overdrivethe sampler as this compresses the contents. It should be recognised that nosample taken by driving a tube into the soil can be truly undisturbed. In fact insoft and sensitive soil the true in-situ shear strength as determined by vanetests has been shown to be three or four times the shear strength determinedby unconfined compression tests on undisturbed tube samples taken fromborings. The care in sampling procedure and the elaborateness of theequipment depends on the class of work which is being undertaken, theimportance of accurate results on the design of the works, and the fundsallowed for the investigation.


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2.4 Foundation Properties of Soil and Rock Types

In the following pages, some notes are given on the engineering properties ofvarious soils and rocks, with special reference to their bearing capacity andbehaviour during construction and under foundation loading. Reference to CP

2001 should be made for geological descriptions of these materials. Thephysical characteristics of the main groups and sub-groups of soils are givenin Table 1.1.

Non-cohesive Soils

Gravels in the form of alluvial deposits are usually mixed with sands to agreater or lesser degree. Examples of the range of particle-size distributionare the beach gravels of the south coast of England, which contain little or nosand. The sandy gravels which are wide-spread in the Thames Valley maycontain 60 per cent or more of sand.

Gravels and sandy gravels in a medium-dense or denser state have a highbearing capacity and low compressibility. Compact gravelly soils give rise todifficulty in driving piles through them. If deep penetration is required intogravel strata it is usually necessary to adopt steel piles which have a higherpenetrating ability than concrete or timber members.

Sandy gravels in a damp state but above the water table have some cohesionand can therefore be excavated to stand at very steep slopes provided thatthey are protected from erosion by flowing water (Plate XIV). Loose gravelswithout sand binder are unstable in the slopes of excavations and require tobe cut back to their angle of repose at about 30oto 35o.

Heavy pumping is required if deep excavations in open gravels are madebelow the water table, but the water table in sandy gravel can be lowered bywell points or deep wells with only moderate pumping. As an alternative toproviding large capacity pumping plant the permeability of slightly sandy orclean gravels can be substantially reduced by injecting cement, clay slurries,or chemicals.

Erosion or solution of fine material from the interstices of gravel deposits can

result in a very permeable and unstable formation . Open gravels caused bysolution are sometimes found in the alluvial deposits derived from limestoneformations.

Sandy soils have bearing capacity and compressibility characteristics similarto gravels, although very loosely deposited sands (e.g. dune sands) have ahigh compressibility requiring correspondingly low bearing pressures in orderto avoid excessive settlement of foundations.

Dense sands and cemented sands have a high resistance to the driving ofpiles and steel piles are required if deep penetrations are necessary.

Sands in their naturally deposited state above the water table are usuallydamp or cemented to a varying degree and thus will stand at a steep slope inexcavations. However support by timbering or sheet piling is necessary in


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deep and narrow excavations where a sudden collapse caused by dryingout of the sand or vibrations might endanger workmen.

Excavation in sands below the water table will result in slumping of the sidesor boiling of the bottom (Plate XV), unless a properly designed ground water

lowering system is used. This instability which is also known as quick orrunning sand condition, is due to the erosive action of water flowing towardsthe excavation. By providing a ground water lowering system to draw wateraway from the excavation towards filter wells or wellpoints a condition of highstability can be achieved. In particular circumstances it may be necessary tostabilise the sands by the injection of chemicals, or excavation undercompressed air in caissons may be required.

Loosely deposited sands are sensitive to the effects of vibrations whichinduce a closer state of packing of the particles. Therefore specialconsideration should be given to the design of machinery foundations on

loose to medium-dense sands, and it is necessary to take precautions againstthe settlement of existing structures due to vibrations arising from suchconstruction operations as blasting or pile driving.

In some arid parts of the world the structure of loose deposits of sand is liableto collapse upon wetting with consequent serious settlement of structuresfounded on these deposits. Wetting may be due to fracture of drains orleaking water pipes. Collapsing sands are found in some parts of SouthAfrica, Rhodesia, and Angola. Sand deposits can be formed as a result ofweathering and breakdown of calcareous formations. Examples of these arelimestone sands which are found on the coasts and islands of theMediterranean Sea; shelly sands and coral sands which are found on thecoasts and islands of the Persian Gulf and the Pacific Ocean, and on thesouth-eastern seaboard of U.S.A.; and gypsum sands which are found in Iraqand Persian Gulf territories. These deposits which are formed fromweathering are nearly always in a loose state except at the surface wherethey may be weakly cemented by silt or salt spray. Low foundation bearingpressures are required unless the loose deposits can be compacted byvibration or other methods.

Distinct from these products of weathering are the cemented calcareous

sands or sandstones which are formed by saline and lime-rich waters beingdrawn up by temperature effects and evaporation in the surface layers to forma hard crust. These soils are known in various parts of the world as caliche,tufa, or Steppen calcaire (French North Africa), havara (Cyprus), andkurkar (Israel and Jordan). The deposits occur widely in Australia wherethey are known as limestone rubble. A feature of their formation is theirregularity in thickness and distribution of the hardened crust. It may exist inseveral distinct layers of varying thickness separated by loose sands or softclay, or in irregular masses of varying degrees of cementation. Thus it isdifficult to design foundations to take full advantage of the high bearingcapacity of the cemented material. Disturbance of the cemented sands by

excavating machinery, construction traffic, or flowing water results in rapidbreakdown to a material having the texture of a sandy silt which is highlyunstable when wet. Cemented sands are highly abrasive to excavationmachinery.


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Cohesive Soils

The foundation characteristics of cohesive soils vary widely with theirgeological formation, moisture content, and mineral composition. It is

impossible in this chapter to cover all the types and combinations which existin nature, and the following notes are restricted to the characteristics of someof the well-known types.

Boulder Clays are generally stiff to hard. Because of their heavypreconsolidation in glacial times only small consolidation settlements willoccur under heavy foundation bearing pressures. Some boulder clays arehighly variable containing lenses (often water-bearing) of gravels, sands andsilts. In such conditions, foundation design must take account of the variablebearing capacity and compressibility at any particular locality. When carryingout deep excavations in variable glacial deposits, precautions should be taken

against inrushes from water-bearing pockets. Excavations in stiff to hardboulder clays will stand vertically without support for long periods (Plate XIII).The presence of random boulders or pockets of large gravel and cobbles cancause difficulties in driving sheet piles or bearing piles into boulder clays.Another type of glacial deposit is varved claywhich comprises layers of siltyclay separated by thinner layers of sand or silt. These intervening layers areoften water-bearing, which causes difficulties in bleeding of sand or silt intoexcavations. Varved clays are usually softer in consistency and morecompressible than boulder clays. Driving piles into varved clays may weakenthe strength of the clay layer to that of a soft slurry. Also, where varved claysare bordering a lake or river, fluctuations in the water level may becommunicated to the sand layers with a detrimental effect on their bearingcapacity. For these reasons varved clays are generally held to betroublesome soils in foundation engineering.

Clay-with-Flints exists widely in Southern England as a mantle over the chalkformations; it is partly a residual soil composed of the insoluble parts of chalkleft after solution of the calcareous material and partly of the clays, sands andpebbles of Tertiary age that once existed with chalk. Below the zone ofsurface weathering the clay-with flints is stiff to very stiff in consistency andhas a low compressibility. In some deposits there may be difficulties in

excavation due to the occurrence of masses of large flints in close contactand bound with hard dry clay. the clay fraction of clay-with-flints is a lean typeof clay and consequently does not show marked volume changes with varyingmoisture content.

Stiff Fissured Clays such as London Clay, Barton Clay (in Hampshire), theLias Clays of the Midlands, and the Weald and Gault Clays of South-easternEngland have a relatively high bearing capacity below their softenedweathered surface. Also, since they are preconsolidated clays, they have amoderate to low compressibility. They are fat clays and heavy structuresfounded on them show slow settlement over a very long period of years. Stiff

fissured clays show marked volume changes with varying moisture content.Thus foundations need to be taken down to a depth where there will be littleor no appreciable movement resulting from swell and shrinkage of the clay inalternating wet and dry seasons. For the same reason it is necessary to avoid


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accumulation of water at the bottom of excavations in order to avoid swellingand softening of the soil. Fissuring in these soils can cause a wide variationin shear strength determined by laboratory tests on samples taken by drivetubes, due to random distribution of fissures and their partial opening duringsampling. Thus it is difficult to assess the results when assessing bearing

capacity. Because of these difficulties, shear strengths are often determinedby in-situ tests such as the Dutch Cone or a small-diameter plate-loading testmade in a bore-hole.

The fissured structure of these clays cause difficulties, mainly unpredictable,in the stability of slopes or excavations, the stability of the walls of unlinedholes sunk by mechanical boring methods for deep piers or piles, and in thedesign of timbering or sheetpiling to excavations. Clays having similarcharacteristics to the British stiff fissured clays include the Fort Union shaleof Montana and the Bearpaw shale of Saskatchewan. Stiff fissured clay alsooccurs in Northern France, Denmark, and Trinidad.

Tropical Red Clays are principally residual soils resulting from physical andchemical weathering of igneous rocks. They are wide-spread in India, Africa,South America, Hawaii, West Indies, and Far Eastern countries. They areusually lean clays with a relatively high bearing capacity and lowcompressibility. However, in certain tropical conditions, leaching of the clayscan occur at shallow depths, leaving a porous material with a fairly highcompressibility. Vargas has described the settlement of heavy structures onporous red clay at Sao Paolo, Brazil. Tropical red clays do not show anymarked volume changes with varying moisture content and can be excavatedto vertical or steeply battered slopes with little risk of collapse.

Laterite is a term given to a ferruginous soil of clayey texture, which has aconcretionary appearance. It is essentially a product of tropical weather andoccurs widely in Central and South America, West and Central Africa, India,Malaya, the East Indies, and Northern Australia. Laterites arecharacteristically reddish-brown or yellow in colour. They exist in the form ofa stiff to hard crust 6m or more thick overlying rather softer clayey materialsfollowed by the parent rock. Laterites have a high bearing capacity and lowcompressibility. They do no present any difficult foundation engineeringproblems.

Tropical Black Claysare also developed on igneous rocks, examples beingthe black cotton soils of Sudan and Kenya, the vlei soils of SouthernRhodesia, and the adobe of South Western U.S.A. Black Clays are alsofound in India, Nigeria, and Australia. They are generally found in poorly-drained topography. Unlike the tropical red clays, black clays are verytroublesome in foundation engineering in that they show marked volumechanges with changes in moisture content, and because of their poordrainage characteristics they come impassable to construction traffic in thewet season. Because these clays exist in countries where there are markedwet and dry seasons, the soil movements brought about by alternate wetting

and drying are severe and extend to considerable depths. In many cases ithas been found necessary to construct even light buildings on piledfoundations to get below the zones of soil movement.


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Saline Calcareous Clays are widely distributed in the Near and Middle East.They are found in the Mesopotamian Plain of Iraq, the coastal plains of theLevant and South-west Iran, the coast of North Africa, the islands of theMediterranean, the limestone plateaux of Jordan, and in Utah and Nevada,U.S.A. These soils were formed by the deposition of clay minerals in saline or

lime-rich waters. The deposits are augmented by wind-blown sand and dust.The profile of calcareous silty clays is similar throughout the arid and semi-arid countries of the Near and Middle East. it comprises a surface crust about2m thick of hard to stiff desiccated clay overlapping soft moist clay. Thesurface crust is not softened to any appreciable depth by the winter rains.

The stiff crust has adequate bearing capacity to support light structures, butheavy structures requiring wide foundations which transmit pressures to theunderlying soft and compressible layers may suffer serious settlement unlesssupported by piles driven to less compressible strata. Calcareous clays showmarked volume changes with varying moisture content, and where there are

marked seasonal changes, as in the wet winter and dry summer of thecountries bordering the Mediterranean, the soil movements extend to a depthof 5m or more below ground level, and special precautions in foundationdesign are required. In regions where there are no marked differences inseasonal rainfall, as in Southern Iraq, soil movement is not a serious problem.In some regions the stiff crust is a weakly cemented agglomeration of sand orgravel-size particles of clayey material probably resulting from deposition bywinds. These soils may suffer collapse on inundation combined withfoundation loading.

Excavation in the stiff crust of calcareous clays is not a difficult problem,although there is likely to be some seepage of water from fissures.Excavation in the deeper soft deposits is likely to be troublesome due toslumping of the sides and heaving of the bottom.

Alluvial (including Marine) Clays are geologically recent materials formed bythe deposition of silty and clayey material in river valleys, estuaries and on thebed of the sea. They are normally consolidated, i.e. they have consolidatedunder their own weight and have not been subjected in their geological historyto a preconsolidation load as in the case of boulder clays and stiff-fissuredclays. Since they are normally consolidated they show a progressive increase

in shear strength with increasing depth ranging from very soft near the groundsurface to firm or stiff at depth. Atmospheric drying and the effects ofvegetation produce a stiff surface crust on alluvial clays. The thickness of thiscrust is generally 1 to 1.2m in Great Britain, but it is likely to be much greaterand liable to vary erratically in thickness in arid climates. Some regions showseveral layers of desiccation separated by soft, normally consolidated clayeylayers. Moderately high bearing pressures, with little or no accompanyingsettlement, can be adopted for narrow foundations in the surface crust whichdo not transmit stresses to the underlying soft and highly compressibledeposits. In the case of wide or deep foundations it is necessary to adoptvery low bearing pressures, or to use a special type known as the buoyancy

raft, or to support the structure on piles driven through the soft and firmalluvial clays to a satisfactory bearing stratum.


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Alluvial clays, especially marine clays, are sensitive to disturbance, i.e. ifthey are disturbed in sampling or in construction operations they show amarked loss in shear strength. The sensitivity*can range from 2 or 3 in thecase of estuarine clays of the Thames and the Firth of Forth to as much as150 in the post-glacial clays of Eastern Canada. The marine clays of Norway

and Sweden are also highly sensitive due to leaching of salts from the pore-water of the soil by the percolation of fresh water, leaving an open latticestructure which is readily broken down on disturbance.

Excavations below the dried-out surface crust require support by timbering orsheetpiling; open excavations require to be cut back to shallow slopes toavoid massive rotational slips. Excavations in soft clays exceeding a certaindepth width ratio are subject to failure by heaving of the bottom orappreciable inward yielding of the side supports.

As in the case of stiff fissured clays, precautions must be taken against the

effects on foundations of seasonal swelling and shrinkage and the dryingaction of the roots of vegetation.

Alluvial clays are frequently varved or laminated clays interbedded with layersof peat, sand and silt as in the Fens of East Anglia, and in major river deltas.

Siltsoccur as glacial or alluvial deposits, or as windblown deposits. Examplesof the latter are the brickearth of South-eastern England, and loess whichis found in widespread tracts in Mid-western and North-western U.S.A., China,India, Russia, and Israel. Glacial and alluvial silts are generally water-bearingand soft in consistency. They are among the most troublesome soils inexcavation work, since they are readily susceptible to slumping and boiling.Being retentive of water they cannot readily be dewatered by conventionalground water lowering systems. Silts are liable to frost heave.

Brickearths are generally firm to stiff and do not normally present any difficultproblem in foundation work. Similarly, loess soils are slightly cemented andhave a high bearing capacity. However, they are liable to collapse of theirstructure on wetting which may occur as a result of flooding or even brokenwater mains. Loess soils can stand with vertical faces to a great heightprovided they are protected from erosion by flowing water.

Peat consists of dead and fossilised organic matter. It is found in many partsof the world. Extensive deposits occur in Northern Europe, North Americaand U.S.S.R., where it is overlain by living vegetable matter in muskegterrain. Peat is permeable fibrous material and is highly compressible. Theusual procedure is to take foundations of structures below peat to lesscompressible strata unless heavy settlements can be tolerated. Lewis hasdescribed the settlement of a road embankment in Suffolk, where in a littleover four years 1.5m of peat and organic clay were compressed by nearly0.3m under the load of a 1.5m thick embankment. Another undesirablecharacteristic of peat is its wasting. The ground surface of the peat in the

Fen districts of East Anglia is slowly sinking through the years due to

Undisturbed shear strength*Sensitivity = -------------------------------------

Remoulded shear strength


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consolidation of the ground under its own weights (accelerated by drainage),to the fibrous pet blowing away in the wind, and to accidental or deliberateburning of the peat. Thus if foundations are taken below peat layers bymeans of piles or piers the surrounding ground surface sinks relative to thestructure with the result that over a long period of years the foundations

become exposed. Many instances of this can be seen in the Fens. Peatsmay contain organic acids which are aggressive to concrete.


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Rocks

Volcanic Rockssuch as granite, basalt, gabbro, gneiss, schist, and porphyry,normally have excellent bearing capacity unless they are heavily faulted orshattered.

In tropical countries, volcanic rocks are often deeply weathered by physicaland chemical action. In Hong Kong the granite is weathered to depths up to60m. Beneath a surface layer of red earth, the decomposed granite consistsof coarse quartz grains in a porous matrix of decomposed felspar and biotite.In foundations, it behaves as a free-draining course sand of lowcompressibility. Lumb states that decomposed granite has an angle ofshearing resistance of 35 when fully drained and that unsaturated soils donot show collapse phenomena when they become fully saturated. Bouldersof undecomposed granite, 3m or more in size, are found in deeper layers ofpartly weathered granite. Decomposed granite is stable in excavations and

will stand at a near vertical face over a long period of time.

Quartzite has excellent load bearing characteristics. A presumed bearingvalue of 4000 kN/m

2can be taken for hard sandstones in which quartzites

can be classed. Quartzites are highly abrasive causing heavy wear in drillingequipment and the teeth of excavator buckets or grabs. Quartzite bouldersare frequently found in glacial drift in Wales and Scotland.

Sandstones in unweathered state have good load bearing capacity;presumed bearing values of 4000 kN/m2 for strong sandstones and 2000kN/m2 for weak sandstones are permissible. However, they can causeconsiderable difficulty in foundation work due to deep and irregularweathering. The extent and type of weathering depends on the cementingmedium. Clayey cement leads to a rock with low strength which is liable tosoften on exposure to water or frost, and such sandstones may show frostweathering to a great depth as a result of ice action in glacial times. If thecement is calcareous it may have been dissolved out of the rock to anirregular extent, forming random pockets of loose sand within the sound rock.Great care is needed when undertaking exploratory drilling in sandstones.Percussion drilling is generally unsatisfactory since the thickness and extentof weathered layers or pocket is obscured as the rock is broken up by the

percussion tools. Rotary core drilling is the preferred method, but cases haveoccurred where fairly hard rock has been identified as a loose sand becauseof careless drilling techniques or the adoption of too small a core size. Amplecore sizes should be specified, for example Nor Hfor fairly strong sandstonesand up to Ksize for weak weathered rocks. Where exploration in sandstonesof the Coal Measures in Great Britain is being undertaken by trial pits withoutboreholes, it should not be assumed that a bed of hard sandstone found atthe bottom of the pit represents sound bedrock capable of carrying highbearing pressures. It is quite possible that the sandstone is underlain at nogreat depth by weaker mudstones or shale.

Mudstones or Siltstones have similar foundation characteristics to sandstonesbut are often found in a weathered state. Being structureless depositsmudstones are often difficult to break up by explosives in bulk excavations,since they do not shatter like the harder bedded or fissured rocks.


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Shalesare often found in a weathered state having the consistency of a softto firm clay, or glacial action may have caused frost-shattering producing deepdeposits of loose broken particles. When intact and unweathered, shaleshave a high bearing capacity, 2000 kN/m

2or more for hard shales and

mudstones. Some shales are liable to soften in contact with water. Theshaly slates (known locally as shillet) of south-west England arecharacterised by their steep and contorted dip which can vary in angle anddirection over short distances. The shaly slates sometimes contain thin layersweathered to a soft clay consistency, which may be the cause of creep andslips of hillsides and instability in slopes of excavations.

Shale mixed with coal particles is found in large dumps in coal-mining districtsin Britain. Usually the shale is found in these dumps in a burnt state due tospontaneous combustion. The reddish burnt shale is useful for filling.However, if the material contains unburnt or partially burnt shale it is liable to

swell in contact with water, leading to heavy pressures on retaining walls.Burnt or unburnt shale often contains sulphates in appreciable quantity.Cases have occurred of sulphate attack on foundation walls and floors due tothe use of shale filling below ground floors.

Marl. The Keuper Marls outcrop widely in the Triassic deposits of theMidlands and West of England. It is difficult to assess their bearing capacitydue to variable depth of weathering and interbedding with sandstones. Theyare often highly fissured and due to the percolation of water, softening takesplace about the fissures. Solution of calcareous nodules can also result infrequent small cavities. Near the surface the whole mass of the material maybe softened, but with increasing depth the softening is restricted to a narrowzone about the fissures. At greater depths the fissures become tightly closedand free of water, when the rock is in a strong unweathered state. Plateloading tests have shown the unweathered Marl to have an ultimate bearingcapacity of 4000 to 5000 kN/m2 . Chandler and Davies have establishedvalues of the deformation modulus of Keuper Marl appropriate to spreadfoundations where the anticipated settlements are in the range of 0.01 to 0.05per cent of the foundation width. The values are:

MN/m2

Zone I (unweathered) 26-250Zone II (slightly weathered) 9-70Zone III (moderately weathered) 2-48Zone IV (heavily weathered) 2-13

Rotary core drilling with large size cores (preferably K) should be used inKeuper Marl, possibly in conjunction with Denison or open drive samplers inthe softer weathered layers.

Heavy ground water flow may be encountered in foundation excavations in

fissured water-bearing Keuper Marls. The ground water is also liable to besulphate-bearing.


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Chalk varies widely in character from a soft crumbly deposit to a hard,massively bedded rock. In Southern England there are three types: theUpper Chalk which is generally white, crumbly and much-fissured, andcontains many flints; the Middle Chalk is white but more massive inappearance with fewer flints; the Lower Chalk is grey, very massive, and

fairly hard with relatively few fissures and bedding planes; it contains veryfew flints.

Limestonesdo not have many of the difficult characteristics of chalk (which ofcourse is a form of limestone). they are generally harder and less prone todeep weathering. A bearing pressure of 4000 kN/m2 can be used formassively bedded limestones. However the engineer should always be onthe look-out for very deep swallow holes or deep fissures filled with softenedmaterial in limestone formations. These are widespread in Derbyshire,Somerset and Glamorgan. They may be concealed by superficial depositsbut they are often indicated by depressions and irregularities in the ground

surface. On one site investigated in Derbyshire a mass of swallow holes werefound in a small area. The intact limestone occurred as irregularly shapedislands or promontories wholly or partially surrounded by sands and claysinfilling the swallow holes. Borings over 15m deep did not reach the bottom ofthe infilling material.

In some areas the overlying rocks may have collapsed into swallow holes inthe limestone, the collapsed material forming a mass of loose fragments.Piling is useless in such circumstances, and the usual expedient is to bridgeover the swallow holes or to cover them by a dome-shaped slab bearing on itsrim around the edge of the subsidence crater. If the swallow holes are verywide it is necessary to re-site the structure since they are always liable torenewed subsidence at quite unpredictable intervals of time.

Ground water flow may be very heavy in excavations taken into water-bearinglimestone especially if the rock is cavernous or heavily fissured. It is oftennecessary to resort to grouting with cement, cement/sand mixture, sawdust,or bitumen in order to reduce the quantity of water to be pumped.

Some limestone formations such as the Forest Marble or Lower (Blue) Liasare thinly bedded and interbedded with clays. Special consideration should

be given to foundations on such formations, especially at the investigationstages where the problems are similar to those described for Keuper Marlabove.


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3.0 FOUNDATION DESIGN IN RELATION TO GROUND MOVEMENTS

Ground movements which are independent of stresses imposed by thefoundation loading can occur. Examples of these are movements due to swelland shrinkage of the soil under varying moisture and temperature conditions,

frost heave, hillside creep, mining and regional subsidence and settlementsdue to shock and vibration.

It is necessary to take precautions against the effects of these movements onthe structure, either by deepening the foundations to place them on groundwhich is not susceptible to movement, or if this is not economically possible,to adopt special forms of construction which will allow appreciable movementwithout damaging the structure.

The various types of ground movement are described in this chapter and thefoundation designs appropriate to these movements are discussed.

3.1 Soil Movements

Wetting and Drying of Clay Soils

Some types of clay soil show marked swelling with increase of moisturecontent, followed b y shrinkage after drying out. The effect of this seasonalvolume change is to cause a rise and fall in the ground surface accompaniedby tension cracks in the soil in drying periods and closing of the cracks in thewet season. The results of measurements made by the Building ResearchStation at Garston, Hertfordshire, on plates buried at various depths below theground surface are shown in Fig. 3.

The movement at a depth of 1.2m below ground level was less than 6mm in a

year which was drier than normal for British conditions. It was concluded bythe Building Research Station that foundations placed at a depth of 1.1 to1.2m below ground level in shrinkable clay soils should be little affected byswell and shrinkage movement. The general practice is to place foundations


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of structures which are not particularly sensitive to movements, e.g. ordinarydwelling houses, at a depth of 0.9m below ground level. It is desirable to godeeper in the case of foundations of structures sensitive to small movements,say for buildings with expensive wall finishes.

Precautions are also necessary in the construction of ground floors onshrinkable clays. Although it is a reasonable precaution to take strip or padfoundations to a depth of 0.9 to 1.2m below ground level, it will beuneconomical to excavate to this depth for the full ground floor area of abuilding. The most effective and economical procedure is to allow freedom ofmovement between the foundation walls and ground floor slab. Following thevery dry summer of 1959, the ground floor slabs of houses in Hertfordshirewere lifted some 5 to 10 mm above the foundation brickwork due to swellingof the clay in the autumn rains. The concrete floors were cast against thebrickwork without any separating membrane, with the result that the brickworkwas lifted, forming wide horizontal cracks around the foundation walls. It is

also desirable to provide some form of drainage of hardcore or gravel fillingbelow solid ground floors on clay soils, since accumulations of water at theconstruction stage (before the building is roofed-in) will cause long-termswelling of the clay, resulting in upheaval of the floors and walls carried bythem.

CLIMATIC FACTORS

There are, however, two further factors which greatly increase the problem ofswell and shrinkage and which may necessitate special methods offoundation design. The first factor is the effect of a wide difference inseasonal rainfall and soil temperature conditions. These conditions are met inthe Sudan, the Levant coast of the Mediterranean, South Africa, and in thesouth-western parts of the United States of America.

Seasonal differences in the moisture content of the clay soil at Lydda airportin Israel are shown in Fig. 4 and measured ground movements at variousdepths at Leuhof, South Africa, are shown in Fig. 5.

In Israel and Jordan, the winter rainfall from October to April amounts to about600mm, while the months of May to September are practically rainless. themeasurements shown in Fig 4. were made by the Palestine Public WorksDepartment after well-built dwelling houses on the airport had cracked


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severely soon after construction. It will be noted that appreciable moisturecontent changes occur to a depth of 3m below ground level. Although the soiltype is a silty clay, it is not a particularly heavy clay, having a liquid limit ofabout 50 per cent. It should be noted that the severe movement is due almostwholly to the climatic conditions rather than to the type of clay.

EFFECTS OF VEGETATION

The second factor which aggravates the swell and shrinkage problem is theeffect of the roots of vegetation. The roots of trees and shrubs can extractconsiderable quantities of water from the soil. Cooling and Ward have notedthat root systems of isolated trees spread to a radius greater than the heightof the tree, and in southern England they have caused significant drying of fatclay soil to a depth of about 3 m. Differential movements of 100 mm havebeen recorded in houses 25 m from a row of black poplars.

The problems caused by root systems are two-fold. First there is the problemof foundations on sites which have recently been cleared of trees and hedges,and secondly there is the problem of cracking in existing structures caused bysubsequent planting of trees and shrubs close to them.

The pronounced shrinkage which accompanies removal of water from clay

soils can take place both vertically and horizontally. Thus, precautions mustbe taken not only against settlement but also against forces tending to tearthe foundations apart.

An example of movements following clearance of vegetation was the crackingof a block of flats in south-west London in 1952. The site of the buildings hadbeen crossed by a well-grown hedgerow which was grubbed up about onemonth before the foundation excavation was commenced. The brick footingsof the cracked block were constructed in June and July during a fairly long dryspell. August was a rainy month, and by September the dried-out clay soil inthe vicinity of the root system had absorbed moisture to such an extent that

the corner of the building nearest to the hedge line has risen by 25mm,causing severe cracking of the foundations and walls before construction hadbeen taken above first floor level. Levels taken along the ground floor areshown in Fig. 6.


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As already mentioned, foundations constructed at depths of 0.9 to 1.2m aresatisfactory for sites in shrinkable clays in Great Britain where the soil is notaffected by trees or shrubs. Where trees and hedgerows are cleared from asite swelling of the desiccated clay can continue for a very long period ofyears until the moisture content reaches equilibrium with the surroundingground. Samuels and Cheney observed swelling of London Clay continuingover a period of 20 years after cutting down mature trees. The soil beneathareas previously occupied by old and paved areas must also be allowed tocome to an equilibrium moisture content with the adjoining uncovered ground.

PILED FOUNDATIONS

Delays of 20 years or more after cutting down trees on a clay site are notusually admissible and the only satisfactory procedure is to adopt piledfoundations. it must be remembered that swelling takes place beneath theground floor of a building as well as beneath the foundations and to avoiddamage by uplift to the interior of the building it is essential to provide asuspended floor carried by the piles. A clear space should be left beneath the

floor and alongside ground beams. In South Africa and Israel bored piles areextensively used even for light single-storey structures. The design offoundations of this type must take into consideration appreciable uplift forceswhich result from the adhesion of the clay to the pile shaft when the soil isswelling in the rainy season. Reinforcement must be provided to preventtransverse cracking and lifting of the pile shaft. To economise in the shaftlength required to resist bearing and uplift forces it is usual to provide andunder-reamed bulb end on the bottom of the pile (Fig. 7).


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Piled foundations have been found to be more economical than rafts for thefoundations of houses in swelling clays in Western Canada. In thesedesiccated clays the swelling can occur at a rate of 25mm a year for severalyears. It is usually the result of watering gardens. Where there have been

plumbing leaks swelling has been at a rate of 75mm a year. Space isprovided beneath suspended floor and beams to allow for the rise in level ofthe ground surface.

Investigations made at Leuhof in South Africa showed that movements in aclay soil were significant to a depth of 5.5m below ground level, causingfailure in tension of 228mm diameter bored and cast-in-lace, concrete pilesreinforced with four in. vertical bars. The area of vertical steel was 1.24 percent of the cross-section. Failure occurred at a depth of 5.5m indicting anuplift of 138 kN on the pile shaft.

Provision must also be made for relative movement between the walls of thestructure (supported on piles) and the ground floor where it is constructeddirectly on the soil. It may be advantageous to provide a suspended groundfloor carried on the piles.

SHRINKAGE OF CLAYS DUE TO HIGH TEMPERATURES

Severe shrinkage of clay soils can be caused by the drying-out of the soilbeneath the foundations of boilers, kilns, and furnaces. Cooling and wardreported that the heat from a 61m by 30.5 m brick kiln had penetrated through

2.7 m of brick rubble filling and then for the full 19.5 m thickness of theunderlying Oxford Clay. The kiln was demolished and a cold process buildingwas erected in its place. Seven years later one corner of the new buildinghad settled 330mm, and elsewhere the building had risen 178mm. In thesame paper, Cooling and Ward quote the case of a battery of threeLancashire boilers 2.75 m diameter by 3 m long, where settlements up to150mm at the centre and 75mm at the sides had occurred after two winterseasons of filing. the temperature and moisture conditions in the London Claybeneath the boilers are shown in Fig. 8.


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Where furnaces, boiler houses, and the like are constructed on clay soils it isnecessary to provide an insulating air-gap between the source of heat and thefoundation concrete, or alternatively to provide sufficient depth of concrete orother material to ensure that the temperature at the bottom of the foundationis low enough to prevent appreciable drying of the soil. The procedure forcalculating the required thickness of insulation has been described by Wardand Sewell.

3.2 Ground Movements due to Water Seepage and Surface Erosion

Troubles with water seepage and erosion occur mainly in sandy soils.Internal erosion can result from ground water seeping into fractured sewers orculverts carrying with it fine soil particles. the consequent loss of ground frombeneath foundations may lead to collapse of structures. Trouble of this kind isliable to occur in mining subsidence areas where sewers and water mainsmay be broken. It can also occur as a result of careless technique in deepexcavations below the water table when soil particles are carried into theexcavation by flowing ground water.

In South Africa, Rhodesia, and the Luanda region of Angola dry loose sandshave been known to subside as a result of seepage of water from leakingmains or drainage pipes. it is often the practice to provide special forms offoundations as a safeguard against such contingencies (Jennings andKnight). Similar troubles have occurred with loess soils. Holtz and Hilf havedescribed the tilting of a grain elevator in Kansas due to the action of surfacewater in a loess foundation soil. They recommend various forms offoundation treatment including driving displacement piles into prewetted soil,prewetting accompanied by surcharge and the injection of silt slurry.

Surface erosion may take place as a result of loss of material in strong windsor erosion by flowing water. Fine sands, silts, and dry peat are liable toerosion by the wind. The possibility of undermining of foundations can readilybe provided for by a minimum foundation depth of about 0.3m, and byencouraging the growth of vegetation or by blanketing the erodible soil bygravel, crushed rock, or clay. Surface erosion by flowing water may besevere if structures are sited in the bottom of valleys, especially in regions oftropical rainstorms. Normal foundation depths (say 0.9 to 1.2m) areinadequate for cases of erosion by floodwaters, but this possibility can beprovided for by attention to the siting of structures, adequate drainage and

paving or other forms of surface protection, of paths taken by periodicaldischarges of flood water. Severe erosion can take place around thefoundations of bridges or other structures in waterways subjected to heavy


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flood discharges. The required depths of such foundations can be obtainedby hydraulic calculations and local observationsFrom time to time cases are reported of subsidence due to solution ofminerals from the ground as a result of water seepage. Subsidence and theformation of swallow holes are not uncommon in Britain in the Carboniferous

Limestone and Chalk districts. Beles and Stanculescu described an unusualcase of subsidence of a group of buildings constructed on a salt massif inRumania. Seepage of fresh water beneath the massif had caused solution ofthe salt, resulting in 750mm of settlement which was continuing at the rate of25mm per month. The rate of settlement was reduced to 1.5mm per monthby the construction of an impermeable barrier around three sides of theaffected building. The barrier was formed from a double row of boreholes at1m centres, with a dry mixture of clay and sulphite lye rammed down theboreholes into the water-bearing zone. Troubles with solution cavities canbest be avoided by careful geological investigations before any construction iscommenced.

3.3 Ground Movements due to Vibrations

The process of using vibrators for consolidating concrete or vibratory rollersand plates for compacting sandy or gravelly soils are well-known. If a pokervibrator is pushed into a mass of loosely placed concrete, the surface of theconcrete will subside as its density is increased by the vibrations transmittedto it. It has been found that high frequency in vibrating plant is more effectivethan low frequency for consolidating concrete or soils.

The same effects of consolidation and subsidence can occur if foundations onsands or sandy gravels (or if the soils themselves) are subjected to vibrationsfrom an external source. Thus, vibrations can be caused by out-of-balancemachinery, reciprocating engines, drop hammers, pile driving, rock blasting,or earthquakes. Damage to existing structures resulting from pile-drivingvibrations is not uncommon, and it is usual to take precautions against theseeffects when considering schemes for piled foundations in sands adjacent toexisting structures.

Experiments in the field and laboratory and records of damage have shownthat the most serious settlements due to vibrations are caused by high-

frequency vibrations in the range of 500 to 2,500 impulses per minute. This isalso within the range of steam turbines and turbogenerators. Terzaghi andPeck record a case of turbogenerator foundations on a fairly dense sand andgravel in Germany. The frequency of the machinery was 1,500 r.p.m. andsettlements exceeded 0.3m within a year of putting the plant to work.Terzaghi and Peck also mention long-continued traffic vibrations andearthquakes as causes of foundation settlement. If the foundations ofstructures carrying vibrating machinery cannot be taken down to a stratum notsensitive to vibrations (clays for example do not usually settle under vibratingloads), then special methods of mounting the machinery to damp down thevibrations must be adopted. Consolidation of sands beneath foundations by

vibration processes are described in Chapter 11. Methods of designingmachinery foundations to absorb or damp down vibrations are described laterin this chapter.


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3.4 Ground Movements due to Hillside Creep

Certain natural hillside slopes are liable to long-term movement which usuallytakes the form of a mass of soil on a relatively shallow surface of sliding orslipping downhill. Typical of such movements are hillsides in London Clay

with slopes of 10o

or steeper. Their effects can be seen in parallel ridges inthe ground surface and in leaning trees.

Normally, the weight of structures erected on these slopes is insignificant inrelation to the mass of the slipping ground. Consequently foundation loadinghas little or no influence on the factor of safety against slipping. However,other construction operations may have a serious effect on the slope stability,for example regarding operations involving terracing the slopes may changethe state of stress both in cut and fill areas, or the natural drainage of subsoilwater may be intercepted by retaining walls.

Instability of slopes may occur on rocky hillsides where the strata dip with theground surface, especially where bedding planes in shales or clayey marlsare lubricated by water. Again the risk of instability is increased by regradingoperations or alteration of natural drainage conditions rather than by thefoundation loading.

There is little that can be done to restore the stability of hillside slopes in clayssince the masses of earth involved are so large, and regrading operations onthe scale required are usually impossible. the best advice is to avoid buildingin such areas, or if this cannot be done, to design the foundations so that thewhole structure will move as one unit with provision for correcting the level asrequired. Suitable methods of construction are discussed in the next sectionof this chapter.

Local instability in rocky slopes can be corrected by grouting or by rockbolting.


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3.5 Ground Movement due to Mining Subsidence

Forms of Subsidence

The magnitude and lateral extent of subsidence due to mineral extraction

depends on the method used for winning the minerals from the groundwhether by mining, pumping, or dredging. The main problems in Great Britainarise from coal mine workings and these will be discussed in some detail onthe following pages. An early method of mining coal was by sinking bell-pits,practised in medieval times. a vertical shaft was sunk to the level of the coalseam, and mining then proceeded in all directions radially from the shaft. Thebottom of the shaft was belled-out as necessary to support the roof (Fig. 9)

and mining continued until the roof was in imminent danger of collapse or theaccumulation of ground water became too much for the primitive baling orpumping equipment. The shafts were filled with spoil from other workings orwere used as rubbish tips. Most of the traces of these workings were lostover the centuries but they still remain, notably in the NorthumberlandCoalfield, as a source of trouble in foundation design. Their presence can

sometimes be detected by depressions in the ground. Electrical resistivityand proton magnetometer equipment have been successfully used to tracethe whereabouts of concealed bell-pits and disused mine shafts. Havingestablished the location of workings of this type, structures can be re-sited asnecessary to void them, or they can be suitably bridged with beams or domedstructures or backfilled using injection techniques.

Similar workings were excavated in many coalfields throughout the country inthe 1926 coal strike. These workings in the form of shafts, drifts, or deeptrenches were excavated at or near the outcrops of coal seams. No recordsof their location were kept, but local inquiry will sometimes establish their

presence.


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PILLAR AND STALL WORKINGS

As mining techniques improved, in particular with the development of steampumping plant in the eighteenth and nineteenth centuries, workings wereextended to greater distances from the shafts. Support to the roof was given

by methods known variously as pillar and stall, room and pillar, or bordand pillar. Galleries were driven out from the shaft with cross-galleriesleaving rectangular pillars of unworked coal to support the roof (Fig. 10).

Only 30 to 50 per cent of the coal was extracted in this way in the firstadvance of the workings from the shaft. On the return workings towards theshaft the pillars were removed either in entirety, to allow full collapse of theroof, or partially, to given continued support. large pillars were left beneathchurches and similar public buildings and sometimes beneath the collieryheadworks.

In many coalfields in Britain the presence of these old pillar and stall workingswith partially worked pillars remain as a constantly recurring problem infoundation design where new structures are to be built over them. If the depthof cover of soil and rock overburden is large, the additional load of the buildingstructure is relatively insignificant and the risk of subsidence due to the newloading is negligible (Fig. 11(a)). If, however, the overburden is thin, andespecially if it consists of weak crumbly material, there is a risk that theadditional load imposed by the new structure will cause a breakdown in an

arched and partially collapsed roof, leading to coal subsidence (Fig. 11(b)).


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There is an increased risk of subsidence or renewal of past subsidence, ofpillar and stall workings if underlying seams are being worked by longwallmethods.

There is also a risk of subsidence if flooded workings are pumped dry, when

the effective weight of the overburden will be increased as a result ofremoving the supporting water pressure. This will increase the load on thepillars, possibly to the point of their collapse.

LONGWALL WORKINGS

The present-day method of coalmining is by longwall working whereby thecoalface is continuously advanced over a long front. The roof close to theface is supported by props, and the goaf or cavity left by the coal extractionis partially filled by waste material (stowage). As the props are removed orallowed to crush down, the roof subsides, resulting in slow settlement of the

ground surface. The surface subsidence takes the form of an advancingwave moving at the same rate as the advancing coalface (Fig. 12). Theamount of subsidence at ground level is usually less than the depth of theunderground gallery due to bulking of the collapsing strata. Solid packing ofthe gallery with crushed mine dirt may reduce the subsidence to only one-halfof that given by the unfilled gallery. The trough or basin of subsidence occursall round the area of coal extraction, and the affected area at ground level islarger than the area of extraction The angle between the vertical and a linedrawn from the coal face to intersect the ground surface at the edge of thesubsidence wave is known as the limit angle (Fig. 12); it is commonly 35

o.

Movement of the ground surface is not only vertical; horizontal strains arecaused as the subsidence wave advances. Thus the building atAin Fig. 13first experiences a tilting towards the trough accompanied by tensile strains inthe ground surface which tend to pull the building apart. As the wavecontinues to advance, the ground becomes concave. When the concave partof the subsidence wave reaches the building, the direction of tilting will havebeen reversed and the ground surface will be in a compression zone tendingto crush the building. As the wave advances further the building will finallyright itself and the horizontal strains will eventually die away. Vertical andhorizontal movements resulting from longwall mining are severe. The amountof tilting depends on the surface slope but it may vary from 1 in 50 or steeper

over shallow workings to practically nothing over deep workings. Horizontalstrains may be as much as 0.8 per cent for shallow workings, but are morecommonly 0.2 per cent or less.

The protection in one form or another of structures cannot be neglected. Itmust be appreciated that the movements are rarely uniform. It is possible topredict with reasonable accuracy the amount of settlement and the extent ofthe subsidence zone if the coal is horizontal or nearly so, and if theoverburden conditions are reasonably uniform. If, however, the coal seam isdipping steeply, no reasonable predictions can be made. Variations in theoverburden, especially in the depth of soil cover, can cause differential vertical

and horizontal movements across individual structures. The problem isfurther complicated if seams are worked at deeper levels, at different times,and in different directions of advance.


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OTHER FORMS OF SUBSIDENCE

Other forms of underground mineral extraction by mining or pumping methodsgive rise to similar subsidence problems. In Cheshire, brine is extracted by

pumping from salt-bearing rocks. In earlier years indiscriminate pumpingcaused heavy long-term subsidence over a wide area due to removal ofbuoyant support of the overburden strata, and the formation of large cavitiesdue to the solution of the salt in the rock. Similar subsidence is caused by theremoval, by pumping, ofmineral oil and natural gas.

The extent and depth of subsidence can be greatly reduced by carefullyplanned extraction accompanied by recharge or recuperation pumpingwhereby water or gas is pumped in simultaneously with the extraction of theminerals. In some brine-pumping areas of the North of England the quantitypumped from a borehole is limited and the boreholes are spaced at such

intervals that individual cavities are separated by pillars of salt of sufficientthickness to give support to the overburden.

Protection against Subsidence due to Longwall Mining

Schemes for protection against subsidence should be drawn up inconsultation with the local mine authorities. A great deal can be done toreduce the slope of the subsidence trough, and hence to reduce the tensileand compressive ground strain, by planning the extraction of the mineral insuccessive strips of predetermined width. The slopes can also be reducedconsiderably by concurrent mining of two or more seams beneath a site,

advancing the working face in different directions. The opinion of a consultingmining engineer or geologist with knowledge of the area is advisable. Themeasures to be taken depend to a great extent on the type and function of thestructure. Complete protection can be given by leaving a pillar of unworkedcoal beneath the structure (Fig. 14). This involves payments to the mineowners for the value of the unworked coal. Even for an isolated structure thisis liable to cost many tens of thousands of pounds, and the subsidence effectsaround the fringe of the pillar are increased in severity. Therefore, protectionby unworked pillars is only considered in the case of structures such as damswhere structural damage might have catastrophic effects. Measure forprotection of structures and full bibliographies on the subject are given in theInstitution of Civil Engineers Report on Mining Subsidence, and in theSubsidence Engineers Handbook,published by the National Coal Board.


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The general principles recommended in the report are:

(a) Rigid frame or similar statically indeterminate structures should beavoided. Simply supported spans and flexible super-structures should beused whenever possible.

(b) The shallow raft foundation is the best method of protection againsttension or compression strains in the ground surface.

(c) Large structures should be divided into independent units not exceeding18m by 18m in plan. Gaps between the units should be at least 50mm(or more for tall structures where tilting could cause closure of the gap).

(d) Bearing pressures beneath the foundations of heavy structures should beas highas possible.

Although the orientation of a structure in relation to the direction of advance ofthe subsidence wave has, theoretically, an effect on the distortion of thestructure (Fig. 15), the National Coal Board handbook states that there is little

point in trying to orient the structure in any particular direction relative to themine workings unless the mining is to be undertaken in the near future to adefinite plan. The handbook states that there is not enough differencebetween the maximum slope in a transverse profile and that in a longitudinalprofile materially to affect the design of a structure. The design should allowfor the maximum normal movements which are predicted for the seams to beworked.

Structures should not be sited within 15m of known geological faults, sincesubsidence is likely to be severe near fault planes.

PROTECTION BY RAFT FOUNDATIONS

Raft foundations should be as shallow as possible, preferably above theground, so that compressive strains can take place beneath them instead oftransmitting direct compressive forces to their edges, and they should beconstructed on a membrane so that they will slide as ground movements

occur beneath them. It is then only necessary to provide enoughreinforcement in the rafts to resist tensile and compressive stresses set up byfriction in the membrane. In the case of light structures such as dwellinghouses, it is not usually practicable to make the raft any smaller than the plan


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area of the building. However, in the case of heavy structures it is desirableto adopt the highest possible bearing pressures so that the plan dimensions ofthe raft are the smallest possible (Fig 16). By this means the total horizontaltensile and compressive forces acting on the underside of the raft are kept toa minimum and the lengths of raft acting as a cantilever (Fig 16 (a)) at the

hogging stage, or as a beam (Fig 16 (b)) at the sagging stage, are also aminimum. Mauntner has analysed the conditions of support shown in Figs. 16as follows:

_____ 4qb____qmax = 3(b 2l) for cantilevering

and qmax = _____ qb_____ for free support(b l )

where bis the length of the structure in the vertical plane under consideration,qis the uniformly assumed design pressure in undisturbed ground and lis theunsupported length for cantilevering or free support. The value of qmaxdepends on the length lwhich in turn depends on the ratio qmax . As soon as

qvalue qmax approaches the ultimate bearing capacity of the ground, yielding ofthe ground will occur, causing the structure to tilt in the case of the cantilever(Fig. 16 (a)) and to settle more or less uniformly in the free support face (Fig.16 (b). In both cases the effect is to increase the area of support given to the


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underside of the foundation, hence reducing the length of the cantilever, orthe span of the beam, and reducing the stresses in the foundation structure orsuperstructure. It is clear from the above equations that the smaller the ratioof the ultimate bearing capacity to the design bearing pressure, the less willbe the length of cantilever or the unsupported span length of the beam. In

other words the design bearing pressure should be kept as close as possibleto the ultimate bearing capacity qmax . The value of qmax can be determinedfrom soil mechanics tests or by plate bearing tests on the ground; hence thevalue of l can be estimated approximately. It can be assessed only roughlybecause the assumed straight line pressure distribution shown in Fig. 16 (a)or the uniform distribution at each end of the free support case in Fig. 16 (b) isnot necessarily true. It should also be noted that the alignment of the front ofthe subsidence wave in relation to the foundation plan is not known inadvance. It is therefore necessary to analyse various positions of thesubsidence wave and calculate the worst condition of support for the structure(Fig. 16 (c)). Because the design bearing pressure is made close to the

ultimate, the consolidation settlement may be severe if the ground iscompressible (e.g. a clay or loose sand). However, the magnitude of theconsolidation settlements will be small in relation to the mining subsidencemovements.

A typical design of a light raft for a dwelling house as recommended by theBritish Government authorities would include the following design features:

a. A 150mm layer of compacted sand or other suitable granular materialis placed on the ground surface.

b. A layer of waterproofed paper is placed over the granular sub-baselayer to act as a surface of sliding.

c. Reinforcement is provided to resist the frictional forces acting on theunderside of the slab as it slides over the sub-base.

d. The frictional forces may be in a transverse or longitudinal directionand may be taken as the product of half the weight of the structure andthe coefficient of friction between the slab and the granular material.

e. The coefficient of friction may be taken as 2/3 .f. The permissible tensile stress in the steel may be taken as 200 N/mm

2

and the permissible compressive stress on the concrete as 14 N/mm2.

g. Snow loads and wind loads on the building may be neglected and the

floor superload may be taken as 480 N/m2

.h. The reinforcement is placed in the centre of the slab to allow both forhogging and sagging of the ground surface, but the thickness of the raftand the percentage of reinforcement is such that the raft will deformunder vertical movements rather than remain in one rigid plane.

i. The design makes allowance for resistance to movement given by thesuperstructure, i.e. the windows and doors are arranged so as not toweaken the walls, internal load-bearing walls are tied with externalwalls, floors and roofs are secured to all walls, plasterboard (orfibreboard) is used for ceilings instead of plaster, and lime mortar isused for brickwork instead of c

site investigation, soil types and foundation design in relation to ground movement

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