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17 FoundationsDesign

M J TomlinsonFICE, FIStructE, MConsE

Contents

17.1 General principles 17/317.1.1 The function of foundations 17/317.1.2 General procedure in foundation design 17/317.1.3 Foundation loading 17/317.1.4 The design of foundations to eliminate or

reduce total and differential settlements 17/4

17.2 Shallow foundations 17/417.2.1 Definitions 17/417.2.2 Foundation depths 17/417.2.3 Allowable bearing pressures 17/517.2.4 Description of types of shallow

foundations 17/517.2.5 Shallow foundations carrying eccentric

loading 17/717.2.6 The structural design of shallow

foundations 17/717.2.7 Ground treatment beneath shallow

foundations 17/9

17.3 Deep foundations 17/917.3.1 Definitions 17/917.3.2 The design of basements 17/1017.3.3 Buoyancy rafts (hollow box

foundations) 17/1317.3.4 Caisson foundations 17/13

17.4 Piled foundations 17/1817.4.1 General descriptions of pile types 17/1817.4.2 Details of some types of displacement

piles 17/1917.4.3 Types of replacement piles 17/2217.4.4 Raking piles to resist lateral loads 17/2317.4.5 Anchoring piles to resist uplift loads 17/2317.4.6 Pile caps and ground 17/2317.4.7 Testing of piles 17/24

17.5 Retaining walls 17/2417.5.1 General 17/2417.5.2 Gravity walls 17/2517.5.3 Cantilevered reinforced concrete walls 17/2517.5.4 Counterfort walls 17/2517.5.5 Buttressed walls 17/2617.5.6 Tied-back diaphragm walls 17/2617.5.7 Contiguous bored pile walls 17/2617.5.8 Materials and working stresses 17/2617.5.9 Reinforced soil retaining walls 17/26

17.6 Foundations for machinery 17/2617.6.1 General 17/2617.6.2 Foundations for vibrating machinery 17/2717.6.3 Foundations for turbo-generators 17/27

17.7 Foundations in special conditions 17/2717.7.1 Foundations on fill 17/2717.7.2 Foundations in areas of mining

subsidence 17/27

17.8 The durability of foundations 17/2917.8.1 General 17/2917.8.2 Timber 17/2917.8.3 Metals 17/3017.8.4 Concrete 17/3017.8.5 Brickwork 17/30

References 17/30

Bibliography 17/31

17.1 General principles

17.1.1 The function of foundations

Foundations have the function of spreading the load from thesuperstructure so that the pressure transmitted to the ground isnot of a magnitude such as to cause the ground to fail in shear,or to induce settlement of the ground that will cause distortionand structural failure or unacceptable architectural damage. Infulfilling these functions the foundation, substructure andsuperstructure should be considered as one unit. The tolerabletotal and differential settlement must be related to the type anduse of the structure and its relationship to the surroundings.Foundations should be designed to be capable of being con-structed economically and without risk of protracted delays.The construction stage of foundation work is not infrequentlysubjected to delays arising from unforeseen ground conditions.The latter cannot always be eliminated even after makingdetailed site investigations. Thus, elaborate and sophisticateddesigns and construction techniques which depend on an exactforeknowledge of the soil strata should be avoided. Designsshould be capable of easy adjustment in depth or lateral extentto allow for variations in ground conditions and should takeaccount of the need for dealing with groundwater.

Foundation designs must take into account the effects ofconstruction on adjacent property, and the effects on theenvironment of such factors as piledriving vibrations, pumpingand discharge of groundwater, the disposal of waste materialsand the operation of heavy mechanical plant.

Foundations must be durable to resist attack by aggressivesubstances in the sea and rivers, in soils and rocks and ingroundwaters. They must also be designed to resist or toaccommodate movement from external causes such as seasonalmoisture changes in the soil, frost heave, erosion and seepage,landslides, earthquakes and mining subsidence.

17.1.2 General procedure in foundation design

The various steps which should be followed in the design offoundations are as follows.

(1) A site investigation should be undertaken to determine thephysical and chemical characteristics of the soils and rocksbeneath the site, to observe groundwater levels and toobtain information relevant to the design of the founda-tions and their behaviour in service. The general principlesand procedures described in Chapter 11 should be fol-lowed.

(2) The magnitude and distribution of loading from the super-structure should be established and placed in the variouscategories, namely:(a) dead loading (permanent structure and self-weight of

foundations);(b) 'permanent' live loading, e.g. materials stored in silos,

bunkers or warehouses;(c) intermittent live loading, e.g. human occupancy of

buildings, vehicular traffic, wind pressures;(d) dynamic loading, e.g. traffic and machinery vibrations,

wind gusts, earthquakes.(3) The total and differential settlements which can be tolerated

by the structure should be established. The tolerable limitsdepend on the allowable stresses in the superstructure, theneed to avoid 'architectural' damage to claddings andfinishes, and the effects on surrounding works such asdamage to piped connections or reversal of fall in drainageoutlets. Acceptable differential settlements depend on thetype of structure; a framed industrial shedding with pin-jointed steel or precast concrete elements and sheet metal

cladding, for example, can withstand a much greaterdegree of differential settlement than a 'prestige' officebuilding with plastered finishes and tiled floors.

(4) The most suitable type of foundation and its depth belowground level should be established having regard to theinformation obtained from the site investigation and tak-ing into consideration the functional requirements of thesubstructure, e.g. a basement may be needed for storagepurposes or for parking cars.

(5) Preliminary values of the allowable bearing pressures (orpile loadings) appropriate to the type of foundation shouldbe determined from a knowledge of the ground conditionsand the tolerable settlements.

(6) The pressure distribution beneath the foundations shouldbe calculated based on an assessment of foundation widthscorresponding to the preliminary bearing pressures or pileloadings, and taking into account eccentric or inclinedloading.

(7) A settlement analysis should be made, and from the resultsthe preliminary bearing pressures or foundation depthsmay need to be adjusted to ensure that total and differen-tial settlements are within acceptable limits. The settlementanalysis may be based on simple empirical rules (seeChapter 9) or a mathematical analysis taking into accountthe measured compressibility of the soil.

(8) Approximate cost estimates should be made of alternativedesigns, from which the final design should be selected.

(9) Materials for foundations should be selected and concretemixes designed taking into account any aggressive sub-stances which may be present in the soil or groundwater, orin the overlying water in submerged foundations.

(10) The structural design should be prepared.(11) The working drawings should be made. These should take

into account the constructional problems involved and,where necessary, should be accompanied by drawingsshowing the various stages of construction and the designof temporary works such as cofferdams, shoring or under-pinning.

17.1.3 Foundation loading

A foundation is required to support the dead load of thesuperstructure and substructure, the live load resulting from thematerials stored in the structure or its occupancy, the weight ofany materials used in backfilling above the foundations, andwind loading.

When considering the factor of safety against shear failure ofthe soil (see Chapter 9) the dead loading together with themaximum live load may be either a statutory or code of practicerequirement, e.g. the requirements of the BS Code of practice forloading, BS 6399, or it may be directly calculated if the loads tobe applied are known with some precision.

With regard to wind loading the BS Code of practice forfoundations, BS 8004 states:

Where the foundation loading beneath a structure due to wind isa relatively small proportion of the total loading, it may bepermissible to ignore the wind loading in the assessment ofallowable bearing pressure, provided the overall factor of safetyagainst shear failure is adequate. For example, where individualfoundation loads due to wind are less than 25% of the loadingsdue to dead and live loads, the wind loads may be neglected inthis assessment. Where this ratio exceeds 25%, foundations maybe so proportioned that the pressure due to combined dead, liveand wind loads does not exceed the allowable bearing pressureby more than 25%.

When considering the long-term settlement of foundations, thelive load should be taken as the likely realistic applied load over

the early years of occupancy of the structure. Consolidationsettlements should not necessarily be calculated on the basis ofthe maximum live load.

Loadings on foundations from machinery are a special casewhich will be discussed in section 17.6.

17.1.4 The design of foundations to eliminate orreduce total and differential settlements

The amount of differential settlement which is experienced by astructure depends on the variation in compressibility of theground and the variation in thickness of the compressiblematerial below foundation level. It also depends on the stiffnessof the combined foundation and superstructure. Excessive dif-ferential settlement results in cracking of claddings and finishesand, in severe cases, to structural damage. Where the totalsettlements are expected to be small, cracking and structuraldamage can be avoided by limiting the total settlement. Forexample, if the total settlement of buildings on isolated padfoundations is limited to about 25 mm the differential settlementis unlikely to cause any significant damage. Buildings on raftscan usually tolerate somewhat greater total settlements. Wheretotal settlements are expected to be appreciably greater than25 mm the effects of differential settlement should be consideredin relation to the type and function of the structure. Theseeffects are discussed comprehensively by Padfield and Sharrock1

who tabulate acceptable deflection limits as shown in Table17.1.2

Differential settlement may be eliminated or reduced to atolerable degree by one or a combination of the followingmeasures:

(1) Provision of a rigid raft either as a thick slab, or with deepbeams in two directions, or in cellular construction.

(2) Provision of deep basements or buoyancy rafts to reduce thenet bearing pressure on the soil (see sections 17.3.2.1 and17.3.3).

(3) Transference of foundation loading to deeper and lesscompressible soil by basements, caissons, shafts or piles (asdescribed in sections 17.3 and 17.4).

(4) Provision of jacking pockets within the substructure, orbrackets on columns from which to re-level the superstruc-ture by jacking.

(5) Provision of additional loading on lightly loaded areas byballasting with kentledge or soil.

(6) Ground treatment processes to reduce the compressibility ofthe soil.

17.2 Shallow foundations

17.2.1 Definitions

British Standard 8004 defines shallow foundations as thosewhere the depth below finished ground level is less than 3 m andwhich include many strip, pad and raft foundations. The codestates that the choice of 3 m is arbitrary, and shallow founda-tions where the depth: breadth ratio is high may need to bedesigned as deep foundations.

(1) A pad foundation is an isolated foundation to spread aconcentrated load (Figure 17.1).

(2) A strip foundation is a foundation providing a continuouslongitudinal bearing (Figure 17.2).

(3) A raft foundation is a foundation continuous in two direc-tions, usually covering an area equal to or greater than thebase area of the structure (Figure 17.3).

17.2.2 Foundation depths

The first consideration is, of course, that the foundation shouldbe taken down to a depth where the bearing capacity of the soilis adequate to support the foundation loading without failure ofthe soil in shear or excessive consolidation of the soil. Theminimum requirement is thus to take the foundations belowloose or disturbed topsoil, or soil liable to erosion by wind orflood. Provided these considerations are met the object shouldthen be to avoid too great a depth to foundation level. A depthgreater than 1.2m will probably require support of the excava-tion to ensure safe working conditions for operatives fixing

Table 17.1 Limiting values of distortion and deflection of structures. (After Tomlinson (1986) Foundation design and construction (5th edn.).Longman Scientific and Technical)

Type of structure

Framed buildingsand reinforcedload-bearing walls

Unreinforcedload-bearing walls

Type of damage

Structural damage

Cracking in wallsand partitions

Cracking by sagging

Cracking by hogging

Limiting values

Values of relative rotation (angular distortion) , P

Skempton and Meyerhof 4

MacDonald3

1/150 1/250

1/300 (but 1/500 1/500recommended)

Values for deflection ratio A/Z.

Meyerhoff4 Polshin andTokar5

0.4xlO-3 LlH= 3:0.3 to0.4 X l O " 3

Polshin andTokar5

1/200

1/500(0.7/1000 to 1/1000for end bays)

Burland and Wroth7

At L///= 1:0.4 x l O - 3

At LjH= 5: 0.8 x IQ-3

AtL/#= 1:0.2 x 10~3

At L///= 5: 0.4 x l O - 3

Bjerrum6

1/150

1/500

Note: The limiting values for framed buildings are for structural members of average dimensions. Values may be much less for exceptionally large and stiff beams, or columnsfor which the limiting values of angular distortion should be obtained by structural analysis.

reinforcing steel or formwork, which adds to the cost of thework. If at all possible the foundations should be kept abovegroundwater level in order to avoid the costs of pumping, andpossible instability of the soil due to seepage of water into thebottom of an excavation. It is usually more economical to adoptwide foundations at a comparatively low bearing pressure, oreven to adopt the alternative of piled foundations, than toexcavate below groundwater level in a water-bearing gravel,sand or silt.

Apart from considerations of allowable bearing pressures,shallow foundations in clay soils are subject to the influences ofground movements caused by swelling and shrinkage (due toseasonal moisture changes or tree root action), in cohesive soilsand weak rocks to frost action, and in most ground conditionsto the effects of adjacent construction operations such asexcavations or pile-driving.

It is usual to provide a minimum depth of 500 mm for strip orpad foundations as a safeguard against minor soil erosion, theburrowing of insects or animals, frost heave (in British climaticconditions other than those sites subject to severe frost expo-sure), and minor local excavations and soil cultivation. Thisminimum depth is inadequate for foundations on shrinkableclays where swelling and shrinkage of the soil due to seasonalmoisture changes may cause appreciable movements of founda-tions placed at a depth of 1.2 m or less below the ground surface.A depth of 0.9 to 1 m is regarded as a minimum at which someseasonal movement will occur but is unlikely to be of amagnitude sufficient to cause damage to the superstructure orordinary building finishes.8

Movements of clay soils can take place to much greaterdepths where the soil is affected by the drying action of trees andhedges, and in countries where there is a wide difference betweenthe rainfall in the dry season and wet season.9 Permafrost(permanently frozen ground) has a considerable influence onfoundation depths.

Consideration should be given to the stability of shallowfoundations on stepped or sloping ground. Analyses as des-cribed in Chapter 9 should be made to ensure that there is anadequate safety factor against a shear slide due to loadingtransmitted to the slope from the foundations.

The depth of foundations in relation to mining subsidenceproblems is discussed in section 17.7.2.

17.2.3 Allowable bearing pressures

Allowable bearing pressures (see definition in Chapter 9) forshallow foundations may be based on experience, or for prelimi-

nary design purposes on simple tables of presumed bearingvalues for a standard range of soil and rock conditions.

Where appropriate, more precise allowable bearing pressuresfor shallow foundations on cohesionless soils may be obtainedfrom empirical relationships based on the results of in situ testsmade on the soils (Chapter 11). In the case of shallow founda-tions on cohesive soils, the allowable bearing pressures may beobtained by applying an arbitrary safety factor to the ultimatebearing capacity calculated from shear strength determinationson the soil (Chapter 9). Where settlements are a critical factor inthe design of foundations, detailed settlement analyses will berequired based on the measured compressibility of the soil(Chapter 9).

17.2.4 Description of types of shallow foundations

17.2.4.1 Pad foundations

Pad foundations (Figure 17.1) are suitable to support thecolumns of framed structures. Pad foundations supportinglightly loaded columns can be constructed using unreinforcedconcrete, in which case the depth is proportioned so that theangle of spread from the base of the column to the outer edge ofthe ground bearing does not exceed 1 vertical:! horizontal(Figure 17.4). The thickness of the foundation should not be lessthan the projection from the base of the column to its outeredge, and it should not be less than 150mm.

Pad foundations to be excavated by a powered rotary augershould be circular in plan, so providing a self-supportingexcavation in firm to stiff cohesive soils and weak rocks. Squareor rectangular foundations can be excavated by mechanicalgrabs or backacters. The designs should not require the bottomto be trimmed by hand to a regular profile (Figure 17.4). Thisnecessitates operatives working at the bottom of excavations inconfined conditions, and for safety reasons the sides of excava-tions deeper than 1.2 m may have to be supported.

Figure 17.4 Proportioning of unreinforced concrete foundations

Soil profile left by machineexcavation

Figure 17.1 Pad foundationFigure 17.2 Strip foundation

Figure 17.3 Raft foundation

ColumnBackfill

Load-bearing wallBackfill

Figure 17.5 Reinforced concrete strip foundation

17.2.4.2 Strip foundations

Strip foundations are suitable for supporting load-bearing wallsin brickwork or block work. The traditional form of stripfoundation is shown in Figure 17.6(a). The concrete-filledtrench foundation (Figure 17.6(b)) is suitable for stable soils inlevel ground conditions but should not be used where substan-tial swelling of clay soils may occur owing, say, to removal oftrees or hedges. The swelling is accompanied by horizontalthrust on the foundation followed by movement of the founda-tion and superstructure. Strip foundations are also an economi-cal method of supporting a row of closely spaced columns(Figure 17.7).

As a general rule, the thickness of unreinforced strip founda-tions should not be less than the projection from the base of thewall and not less than 150mm. Where foundations are laid atmore than one level, at each change of level the higher founda-

Figure 17.8 Stepping of strip foundations

The excavations for strip foundations are normally under-taken by a backacter machine, and it is usually possible to trimby the machine bucket to a rectangular bottom profile.

Reinforcement can be provided to strip foundations to enablesavings to be made in the volume of concrete and also infoundation depths owing to the lesser required thickness of thebase slab. Reinforcement is also necessary to enable the founda-tions to bridge over weak pockets of soil to minimize differentialsettlement due to variable loading conditions, e.g. when a stripfoundation is provided to support a row of columns carryingdifferent loads.

The procedure for the design of reinforced concrete founda-tions is described in section 17.2.6. In nonaggressive soil condi-tions a concrete mix consisting of 1 part of ordinary Portlandcement to 9 parts of combined aggregate is suitable for unrein-forced concrete strip foundations. The design of concrete mixessuitable for aggressive soil conditions is described in section17.8.4.

Figure 17.7 Strip foundation for closely spaced columns

tion should extend over and unite with the lower one for adistance of not less than the thickness of the foundation and notless than 300mm (Figure 17.8).

Savings in the volume of concrete can be obtained by provid-ing steel reinforcement for pad foundations where heavy col-umn loads are to be carried, and it may be advantageous to savedepth of excavation by adopting a relatively thin base slabsection (Figure 17.5). Reinforcement is also necessary for foun-dations carrying eccentric loading which may induce heavybending moments and shear forces in the base slab. Theprocedure for reinforced concrete design is described in section17.2.6.

Steel barreinforcement

50-75 mmblinding concrete

75 mm cover

Figure 17.6 Unreinforced concrete strip foundations forload-bearing walls, (a) Traditional; (b) concrete-filled trench

Compactedhardcore

Fine concretefilling

Polythenesheeting lapped withdamp-proof course

Fine concretefill ing

150mm(min)

Groundlevel

150mm(min)

(practical minimumfor bricklaying)

450mmMass concrete

375mm

Compactedbackfilling

Compactedhardcore

Groundlevel

Polythene \sheeting lapped withdamp-proof course

Drained cavi tyDamp-proof course

17.2.4.3 Raft foundations

Raft foundations are a means of spreading foundation loadsover a wide area thus minimizing bearing pressures and limitingsettlement. By stiffening the rafts with beams and providingreinforcement in two directions the differential settlements canbe reduced to a minimum.

Edge beams and internal beams can be designed as 'upstand'or 'downstand' projections (Figure 17.9). Downstand beamssave formwork and allow the rafts to be concreted in one pour.However, the required trench excavations may not be self-supporting in loose soils and there are difficulties in maintainingthe required profile in water-bearing ground. Upstand beamsare required where rafts are designed to allow horizontal groundmovements to take place beneath them, as in mining subsidenceareas (section 17.7.2.3).

Raft foundations, in order to function as load-spreadingsubstructures, must be reinforced and concrete mixes must be inaccordance with code of practice requirements for reinforcedconcrete (BS 8110). Special mixes may be required in aggressivesoil conditions.

17.2.5 Shallow foundations carrying eccentric loading

The soil adjacent to the sides of shallow foundations cannot berelied on to provide resistance to overturning moments causedby eccentric loading on the foundations. This is because in claysthe soil is likely to shrink away from the foundation in dryweather and, in the case of cohesionless soils, excavation andsubsequent backfilling will cause loose conditions around thesides. It is therefore necessary to check that the soil beneath thefoundation will not be overstressed or suffer excessive compres-sion under the unequal bearing pressures induced by the eccen-tric loading.

The pressure distribution beneath an eccentrically loadedfoundation is assumed to be linear. For the pad foundationshown in Figure 17.10(a) where the resultant of the overturningmoment M and the vertical load W falls within the middle thirdof the base:

Maximum pressure

<--&?For a centrally loaded pad foundation this becomes:

W 6A/ „„.q~ = BL + VL (17'2)

The minimum bearing pressure is given by:

= JK_6M (17.3)tfmin BL B2L

Figure 17.10 Eccentrically loaded foundations, (a) Resultantwithin middle third; (b) resultant outside middle third

When the resultant W and M falls outside the middle third ofthe base, Equation (17.3) indicates that tension theoreticallyoccurs beneath the base. However, tension cannot develop andredistribution of bearing pressure will occur as shown in Figure17.10(b). The maximum bearing pressure is then given by:

AW<--up=*) (17'4)

In Equations (17.1) to (17.4) W is the total axial load on thecolumn, M is the bending moment on the column, y is thedistance from the centroid of the pad to the edge, 7 is themoment of inertia of the plan dimensions of the pad, e is thedistance from the centroid of the pad to the line of action of theresultant loading.

The maximum bearing pressure qmM should not exceed theallowable bearing pressure appropriate to the depth and widthof the foundation, but the effective width for consideration ofsettlement in cohesionless soils (see Chapter 9) can be taken asone-third of the overall width for the pressure distributionshown in Figure 17.10(b) for a triangular distribution of pres-sure.

17.2.6 The structural design of shallow foundations

17.2.6.1 Pad and strip foundations

The following steps should be taken in the structural design of apad foundation.

(1) Calculate the base area of the foundation by dividing thetotal net load by the allowable bearing pressure on the soil,taking into account any eccentric loading.

(2) Calculate the required overall depth of the base slab at thepoint of maximum bending moment.

Figure 17.9 Reinforced concrete raft foundations, (a) Withupstand beam; (b) with downstand beam

Fabricreinforcement

Polythene membraneBlindingconcrete

Damp-proofcourse

Bar reinforcementFabric reinforcement

Stirrups

'Bar reinforcement

(3) Decide on either a simple slab base with horizontal uppersurface or a sloping upper surface, depending on the eco-nomics of construction.

(4) Check the calculated depth of the slab by computing thebeam shear stress at critical sections on the assumption thatdiagonal shear reinforcements should not be provided.

(5) Design the reinforcement.(6) Check the bond stress in the steel.

The main reinforcement, consisting of bars at the bottom of thebase slab, is designed on the assumption that the projectionbehaves as a cantilever with its critical section on the face of thecolumn (Line X-X in Figure 17.11), and with a loading on theunderside of the cantilever equal to net bearing pressure underthe worst conditions of loading, i.e. maximum eccentricity if theloading is not wholly axial. In Figure 17.11, the bendingmoment at the face of the column is given by:

Mb = «^ (17.5)

For pads of uniform thickness, the critical section of shear isalong a vertical section Y-Y extending across the full width ofthe pad at a distance from the face of the column as defined inclause 3.4.5.8 of BS 8110. It is also necessary to check thepunching shear along a critical peripheral section at a distance1.5 times the thickness of the pad from the faces of the column.If the shear stress or punching shear stress exceed permissiblelimits they should be reduced by increasing the effective depth ofthe pad. Shear reinforcement in the form of stirrups or inclinedbars should be avoided if at all possible.

Strip foundations are designed in the same manner, thecritical sections for bending moment and shear being as shownin Figure 17.11.

17.2.6.2 Raft foundations

Rafts are provided on compressible soils, and particularly onsoils of variable compressibility. Thus, wherever rafts areneeded from the aspect of soil compressibility, some settlementis inevitable, either in the form of dishing (on soils of uniformcompressibility) or hogging (where the compressibility of thesoil or the thickness of the compressible layer varies across theraft) or twisting where the compressibility conditions are irregu-lar.

Distortion of a raft will also occur as a result of variation inthe superimposed loading. The magnitude of dishing, hoggingor twisting, i.e. the angular distortion of the raft, will depend onthe stiffness of the raft and of the superstructure. Only in thecase of a uniformly loaded raft on a soil of uniform compressibi-lity can the raft be designed as an inverted floor, either in slaband beam construction or as a stiff slab (Figure 17.3). In allother cases the design is a complex process of redistributingcolumn load bending moments and shears by the amountcalculated from a consideration of the stiffness of the substruc-ture and superstructure and the settlement of the soil. Thestarting point is always the theoretical total and differentialsettlements calculated by the soil mechanics engineer on theassumption of a fully flexible foundation. Flexibility of the raftis desirable to keep bending moments and shears to a minimum,but if the raft is too flexible there will be excessive distortion ofthe superstructure.

Analysis of the complex interaction between the raft structureand a subgrade soil undergoing elastic or plastic deformationlends itself to computer methods for solution. A report by theInstitution of Structural Engineers10 discusses the problemsinvolved in computer analysis. Reference may also be made tothe work of Hooper11 and Poulos and Davis.12 Where settle-ments are expected to be fairly small, the complexities of raftdesign can be avoided by designing the substructure as a seriesof touching but not interconnected pad or strip foundations.

Column

Critical sectionfor bending

Critical sectionfor shear

Effect ive depth

75mm minimumcover to reinforcementBlinding concrete q

(bearing pressure)

Figure 17.11 Reinforced concrete pad foundations

This will greatly reduce the amount of reinforcement required toresist the high bending moments and shears which occur in theshort stiff members of a raft with close-spaced columns.

17.2.7 Ground treatment beneath shallow foundations

If the ground beneath a proposed structure is highly compres-sible it may be economical to adopt shallow foundations inconjunction with a geotechnical process to reduce the compres-sibility of the ground as an alternative to deep foundationstaken down to a stratum of lower compressibility. Geotechnicalprocesses which may be considered are:

(1) Preloading.(2) Injection of cement or chemicals.(3) Deep vibration.(4) Dynamic compaction.

(See also Chapter 9.)Preloading Preloading consists of applying a load to theground equal to, or greater than, the proposed foundationloading so that settlement of the ground will be complete beforethe structure is erected. The method is applicable to loosegranular soils or granular fills, where the settlement will berapid. It is generally unsuitable for soft clays where shear failuremay occur under rapid application of preload and, because ofthe long-term character of consolidation settlement, the pre-loading would have to be sustained over a long period to beeffective. Preloading is most economical over a large area wherethe granular material such as gravel or colliery waste can beprovided in bulk and moved progressively across a site usingearthmoving machinery.

The injection of cement or chemicals Injection of cement orchemicals is suitable for treatment of loose granular soils or fillswhere the particle size distribution of the materials is suitablefor the acceptance of grouts. The effect of injecting cement orchemicals is to replace the void spaces by relatively incompres-sible material, thus greatly reducing the overall compressibilityof the ground mass.

Cement or chemicals used for injection are costly and theprocess is not normally recommended for dealing with largefoundation areas or deep compressible strata. The process isusually restricted to small-scale application beneath importantstructures such as complex machinery installations. It is alsoemployed as a remedial treatment to arrest the excessive settle-ment of foundations.

Unslaked lime can be mixed with soft clays by rotary drillingequipment to form load-bearing columns of stabilized soil.13

These are suitable for the foundations of light buildings pro-vided that minor settlements are acceptable.

Deep vibration Deep vibration methods comprise the insertionof a large vibrating unit into the soil for the full depth requiredfollowed by its slow withdrawal. Granular material is fed intothe depression surrounding the vibration unit as it is withdrawn,and the unit is re-inserted several times to form a cylinder ofdensely compacted soil mixed with the imported material. Byadopting close-spaced insertions on a grid pattern beneathloaded areas or in single or double rows beneath strip founda-tions* the whole mass of compressible soil can be compacted to areasonably uniform state, thus reducing the total and differen-tial settlements beneath the applied loading.

In the 'vibroflotation' process the vibratory unit is assisted inits insertion by water jetting. During withdrawal the direction ofthe jets is reversed to consolidate the added materials. In the'vibro-replacement' process no water jetting is used, the vibra-tory unit resembling a large poker vibrator. Compressed air is

used to assist penetration of the vibratory unit in the vibro-displacement process.

The depth of treatment is limited to the maximum depth towhich the vibratory unit can be inserted which, with the mostpowerful units assisted by water jetting, is about 20 to 30 m. Theprocess has been used to advantage in compacting very looselyplaced brick rubble and building debris filling on urban redeve-lopment sites. Houses can then be built on conventional stripfoundations on the fill which has been compacted to a reasona-bly uniform state of density. The process may not be suitable ifthe debris contains a high proportion of timber or other organicor soluble materials which may decay or dissolve over a periodof years, resulting in further settlement of the fill.

Dynamic compaction This consists of dropping a heavy weighton to the surface of the soil to compact and consolidate theweaker upper layers. Commonly weights of 15 to 2Ot aredropped from heights of about 20 m to achieve useful compac-tion of the soil over a depth of about 10m. Tamping is usuallyundertaken on a rectangular grid at points spaced 5 to 10mapart. About five to ten blows of the tamper are applied to eachgrid point and the resulting craters are backfilled with granularmaterial. Successive passes are then applied to the same orintermediate grid points until the desired standard of compac-tion has been achieved. The process is suitable for free-drainingcoarse granular soils, rockfill, refuse tips and industrial wastetips. Fill material in waste tips should not contain appreciablequantities of biodegradable or soluble substances.

The deep vibration and dynamic compaction processes havebeen reviewed comprehensively by Greenwood and Kirsch.14

17.3 Deep foundations

17.3 Definitions

Deep foundations are required to carry loads from a structurethrough weak compressible soils or fills on to stronger and lesscompressible soils or rocks at depth, or for functional reasons.The types of deep foundations in general use are as follows.

(1) Basements.(2) Buoyancy rafts (hollow box foundations).(3) Caissons.(4) Cylinders.(5) Shaft foundations.(6) Piles.

Basements These are hollow substructures designed to provideworking or storage space below ground level. The structuraldesign is governed by their functional requirements rather thanfrom considerations of the most efficient method of resistingexternal earth and hydrostatic pressures. They are constructedin place in open excavations.

Buoyancy rafts (hollow box foundations) Buoyancy rafts arehollow substructures designed to provide a buoyant or semi-buoyant substructure beneath which the net loading on the soilis reduced to the desired low intensity. Buoyancy rafts can bedesigned to be sunk as caissons (see below): they can also beconstructed in place in open excavations.

Caissons Caissons are hollow substructures designed to beconstructed on or near the surface and then sunk as a single unitto their required level.

Cylinders Cylinders are small single-cell caissons.

Shaft foundations These are constructed within deep excava-tions supported by lining constructed in place and subsequentlyfilled with concrete or other prefabricated load-bearing units.

Piles Piles are relatively long and slender members constructedby driving preformed units to the desired founding level, or bydriving or drilling-in tubes to the required depth - the tubesbeing filled with concrete before or during withdrawal - or bydrilling unlined or wholly or partly lined boreholes which arethen filled with concrete. Piles form a large group within thegeneral classification of deep foundations and will be describedseparately in section 17.4.

17.3.2 The design of basements

17.3.2.1 General

Basements are constructed in place in open excavations. Thelatter can be excavated with sloping sides, or with groundsupport in the form of sheeting or sheet piling. The choice ofeither excavation method depends on the clear space availablearound the substructure and the need to safeguard existingstructures adjacent to the excavation. It may be economical touse the permanent retaining walls as the means of groundsupport as described in section 17.3.2.3. A circular shape to abasement can save construction costs where ground support isrequired, as cross-bracing to support the sheeted sides may notbe needed. A circular plan should always be considered forstructures such as underground pumping stations.

The walls of basements are designed as retaining wallssubjected to external earth pressure and water pressure. Themethods of calculating earth pressure on retaining walls aredescribed in Chapter 9. If no groundwater is encountered in siteinvestigation boreholes it must not be assumed that there willnot be any water pressure. For example, where backfill is placedbetween the walls of a basement and the sides of an excavationin clay soil a reservoir will be formed in which surface waterrunning across the site will collect and a head of water willprogressively rise around the walls. Such accumulations ofwater will not occur in permeable soil or rock formations inwhich the rate of downward seepage exceeds the inflow fromsurface water.

The floors of basements are designed to resist the upwardearth pressure and any water pressure. The basement slabs spanbetween the external walls or cross-walls or between groundbeams placed along the lines of the interior columns. Alterna-tively, they can be designed as flat slabs propped at column andwall positions. They act as raft foundations subjected to bend-ing moments and shears induced by differential settlements. Theresults of the site investigation will normally provide estimatesof total and differential settlement on the alternative assump-tions of a rigid raft (heavy beam and slab construction) or a fullyflexible raft (thin flat slab construction). It is then a matter forthe structural designer's judgement to assess the degree offlexibility of the raft and its interaction with the superstructurefor the particular design under consideration. The complexitiesof this assessment have already been discussed in section17.2.6.2. Particular points to be taken into consideration withbasement floor designs are noted below.

Basements constructed in water-bearing strata may becomebuoyant if the groundwater level in the excavation around thecompleted (or partly completed) structure is allowed to rise toits normal rest level. At this stage there may not be sufficientloading from the superstructure to prevent uplift occurring.Therefore care should be taken to keep the excavation pumpeddown until the structural loads have reached the stage whenuplift cannot occur.

Figure 17.13 Basement floor founded on compressible stratum

When basements are supported on piles and settlements areexpected in the pile group, i.e. where the piles terminate oncompressible soils, some loading will be transferred to theunderside of the floor slab. The magnitude of the pressure whichdevelops will depend on the amount of settlement of the piles,the amount of heave of the base of the excavation due to relief ofoverburden pressure, the amount of heave and reconsolidationof the soil due to the installation of the piles and the timeinterval between completion of the excavation (including finaltrimming and removal of heaved soil) and the time whenyielding of the piles commences due to superstructure loading.In all cases where there is potential transfer to the underside ofthe floor slab, or where hydrostatic pressure has to be resisted,the piled raft (Figure 17.14(a)) is the appropriate form ofconstruction. The problems of load sharing between the pilesand basement slab of a piled raft have been reviewed by Padfieldand Sharrock' and by Hooper.15

Where the piles are terminated on rock or other relativelyincompressible material and there is no hydrostatic pressure,there will be no load transfer to the floor slab, the latter beingonly of nominal thickness (Figure 17.14(b)). This assumes thatground heave causing uplift on the underside of the slab hasceased and that the heaved soil has been stripped off beforeplacing the floor concrete.

17.3.2.2 Design of basement floors

Basement floors founded on rock or other relatively incompres-sible soils will not undergo appreciable downward movementdue to elastic or consolidation settlement of the subgradematerial. Then differential settlements will be negligible and itwill be necessary only to design the floor to resist upward waterpressure. If no water table exists or cannot develop in the futurethen columns and walls can be designed with independentfoundations, the floor slab being only of nominal thickness(Figure 17.12).

Figure 17.12 Basement floor founded on relativelyincompressible stratum

Where appreciable total and differential settlements of thesubstructure can occur the basement floor should be designed asa stiff raft, either in slab and beam construction (Figure17.13(a)) or as a flat slab (Figure 17.13(b)). Design practices aresimilar to those described in section 17.2.6.2 for surface rafts.

Joint

Wall foundation Column bases

Floor slab

Relatively incompressiblesoil or rock

Compressible soil

Columns

Column bases

Columns

FillingFloating

floorslab

Figure 17.14 Piled basement floors, (a) With load transfer tofloor slab; (b) with no load transfer to floor slab

17.3.2.3 Design of basement walls

Although the exterior walls of basements are supported by theground-floor slab of the main structure and any intermediatesubfloors in deep basements, they should be designed as free-standing cantilever retaining walls (Figure 17.15). This isbecause the supporting floors are not usually constructed untilthe final stage of the work (a special method of supporting theexternal walls of deep basements is shown in Figure 17.21, page17/13). Similarly, the foundation slab of the retaining wallshould not be dependent on its connection to the basement floorslab for stability.

The structural form of the retaining wall is governed to someextent by the ground conditions and by the need or otherwisefor waterproofing treatment (see below). Thus, the sloping backand projecting heel shown in Figure 17.15(a) require additionalwidth of excavation, the cost of which may outweigh theincrease in concrete volume required by a wall of uniformthickness (Figure 17.15(b)). In stable ground it may be possibleto undercut the excavated face to form the heel enlargement.The wider excavation required for the sloping back wall (Figure17.15(a)) may be needed in any case to allow room for applyinga waterproof asphalt layer, whereas the vertical back requireseither an enlarged excavation or the construction of a separatevertical backing wall on which to apply asphalt.

Figure 17.16 Diaphragm wall construction

17.3.2.4 Waterproofing basements

Watertightness of a basement can be obtained either by relyingon impervious concrete and leaktight joints, or by providing animpermeable membrane in the form of trowelled-on asphalttanking or preformed sheathing material. Neither method isentirely satisfactory.

If complete watertightness is required for functional reasonsin a basement it is probable that the asphalt tanking method hasa slight advantage compared with relying on the concrete alone,as tanking is a distinct operation carried out by skilled opera-tives, and the work can be restricted to favourable weatherconditions and subjected to intensive supervision; whereas if theconcrete alone is to be relied upon for watertightness, theconcreting operations proceed in stages over a long constructionperiod, in all weathers, with comparatively unskilled labour,and in congested situations, thus making close supervisiondifficult at all times.

Asphalt tanking or self-adhesive plastics sheathing is laid onblinding concrete beneath the basement floor and may beapplied either to the exterior of the retaining walls if space isavailable around the excavations or, in restricted space condi-tions, it can be applied to a vertical backing wall beforeconstructing the main wall (Figure 17.17). It is useless to applytanking to the interior of the structural wall as the waterpressure will merely force it off. Tanking applied to the exteriorof the retaining wall should be protected by a 100-mm thickbacking wall (in a manner similar to that shown in Figure 17.17)to prevent damage by sharp objects in the backfill materials.

manner. Diaphragm walls are designed as retaining walls usingconventional methods for calculating earth pressure (Chapter9). However, they cannot usually be designed to act as cantileverwalls at the final stage of excavation, and they require to bepropped by shores (or held at the top or intermediate levels byground anchors) as described in section 17.3.2.5.

Contiguous bored pile walls faced with reinforced concretecan also be used for basements (see Figure 17.43(0, page 17/24).

Guide wall concrete left inplaceGuide wall concrete removed

Ground-floor slabacting as prop

Basement slab(to be cast)

Diaphragm wall in3 — 6 m panels

Indent

Figure 17.15 Basement floors, (a) With sloping back and heel;(b) with vertical back and no heel

The basement walls can be constructed as diaphragm walls byexcavating a narrow trench by a mechanical grab using bento-nite to support the excavation (Figure 17.16). The excavation istaken out in alternate panels 3 to 6 m long between guide walls.The level of the guide walls should be such that there is at least a1-m head of bentonite slurry above the highest groundwaterlevel. A preassembled reinforcing cage is lowered into thebentonite-filled trench and then concrete is placed by tremiepipe. The intermediate panels are then constructed in a similar

100mm brickbacking wall

Fillet

Structuralretaining wall3-coat asphalt appliedto backing wall

Base concrete

Figure 17.17 Asphalt tanking to basement

Asphalt tanking is covered by BS 988 and BS 1162 for lime-stone aggregate and natural rock asphalt aggregate respectively.The tanking should be applied in three coats to a total thicknessof not less than 27 mm for horizontal work and 20 mm forvertical work. Other points of workmanship are covered in CP102. An alternative to asphalt tanking is the use of Volclaypanels. These consist of fluted cardboard slabs. The flutes arefilled with bentonite which swells when wetted to form apermanent flexible gel.

Pumps keeping down the groundwater level around theexcavation should not be shut down until the structural concretewalls have been concreted and have attained their designstrength.

17.3.2.5 Construction of basements

If space around the substructure permits, the most economicalmethod of constructing a basement is to form the excavationwith sloping sides, followed by concreting the floor slab andthen the retaining walls. If the space is restricted it will benecessary to support the vertical face of the excavation with steelsheet piling (Figure 17.18) or by horizontal timber sheeting inconjunction with vertical soldier piles (Figure 17.19). The sheetpiling method is suitable for soft or water-bearing ground wherecontinuous support is necessary and where it is desired tomaintain the surrounding groundwater table at its normal levelto safeguard existing structures. Horizontal sheeting can be usedin 'dry' ground conditions, or where drainage towards theexcavation can be permitted. In the latter case, hydrostaticpressures do not develop with correspondingly reduced loads tobe carried by the bracing system.

Figure 17.19 Excavation supported by soldier piles and sheetingWhere sheet piling is supported by berms of soft clay sloping

not steeper than 2 horizontal: 1 vertical, observations haveshown a maximum inward deflection of about 2% of theexcavation depth.2

If there are existing structures within a distance of 3 times theexcavation depth from the excavation line then considerationwill have to be given to the need for underpinning them beforeexcavation commences. For reasonably good ground condi-tions, underpinning is unlikely to be needed if the existingstructures are not nearer than a distance equal to the excavationdepth. For example, Figure 17.20 shows the order of settlementsof the ground around a 10-m deep basement. A building in the

bearing blocks at the toe. These may give difficulties withmaintaining waterproofing in thin basement slabs.

Inward movement of the sheeted sides of an excavation willtake place inevitably owing to relief of lateral pressure onremoval of the excavation, the compression of the supportingstruts (or stretch and creep of ground anchors) and the thermalmovements of the support system if the work is properlydesigned and carefully executed. The inward movement isproportional to the depth of the excavation and appears to beindependent of the type of soil and the particular supportsystem.

The inward movements of strutted or anchored diaphragmwalls in a wide range of soil types have been shown byobservation to be in the general range of 0.05 to 0.6% of theexcavation depth.2 The inward movement is accompanied by avertical settlement of the same magnitude of the ground surfaceclose to the perimeter of the excavation. The settlement is abouthalf this maximum value at half the excavation depth from theface and falls to a negligible amount at a distance of 3 or 4 timesthe excavation depth from the face.

Timbersheeting

CleatRaking shore

Thrust block(later removed)

Basementfloor

Unreinforcedconcretesoldier piles

Waterlevel

Double channelwalings

Figure 17.18 Excavation supported by tied-back sheet piling

The bracing system required to support sheeting to excava-tions of moderate width (say up to 30 m) can be in the form ofhorizontal struts and walings restrained against buckling byking piles and vertical cross-bracing (Figure 17.20). The strutscan be preloaded by jacking to minimize inward movement ofthe sides. Where wide excavations have to be supported it ispreferable to use a system of ground anchors (shown in variousstages of construction in conjunction with sheet piling in Figure17.18) or raking shores (shown in conjunction with horizontalsheeting in Figure 17.19).

Ground anchors have the advantage of providing a clearworking space within the excavation and they can convenientlyprovide a preloading force to minimize inward movement, butthere may be problems with existing sewers or other obstruc-tions preventing their installation; also, it may be impossible toobtain wayleaves from surrounding property owners. Rakingshores obstruct the working space and require substantial

Sand

ClayGroundanchors

Figure 17.20 Bracing to wide excavation (also showing inwardmovement)

Maximum settlementA = 30 mm

Maximum inward yielding= 30 mm = 0-3%

Kingpiles

Cross bracing,

Firm clay

Figure 17.21 Construction of deep basement, (a) Excavation tolevel A and ground anchors installed; (b) excavation to level B andfloor slab cast; (c) excavation to level C and further floor slab cast;(d) completed excavation with all basement floor slabs cast

17.3.3 Buoyancy rafts (hollow box foundations)

The substructure should be as light as possible consistent withthe requirement of stiffness. A cellular ('egg box') constructionis suitable. This structural form does not normally allow thesubstructure to be used for any purpose other than its functionas a foundation element.

A cellular buoyancy raft may be designed as a caisson (Figure17.22) which is an economical method of sinking for soft groundconditions, but ground disturbance during sinking can result insome settlement. A buoyancy raft should preferably be con-structed within an open excavation. If necessary, the cells maybe constructed in individual small areas or strips which aresubsequently bonded together. By limiting the area of theexcavation in this way, the heave and subsequent reconsolida-tion of a soft clay can be minimized to a marked degree.

Although considerable gain in uplift can be obtained ifbuoyancy rafts are designed as watertight structures, there arepractical difficulties in achieving this. The space within the cellsof a buoyancy raft is normally unoccupied and, if leaks occur,either through the substructure or from fracture of water pipes

Figure 17.22 Caisson-type cellular buoyancy raft

17.3.4 Caisson foundations

17.3.4.1 General

The types of caisson foundation are:

(1) A box caisson, which is closed at the bottom but open toatmosphere at the top.

(2) An open caisson, which is open both at the top and bottom.(3) A compressed air or pneumatic caisson, which has a working

chamber in which air is maintained above atmosphericpressure to prevent the entry of water and soil into theexcavation.

(4) A monolith, which is an open caisson of heavy mass concreteor masonry construction containing one or more wells forexcavation.

The allowable bearing pressures beneath caissons are calculatedby the methods described in Chapter 9. However, allowancemust be made for the disturbance which may occur during theinstallation of the foundation. These factors are noted in thefollowing subsections which describe the design and construc-tion methods for the various types.

Caissons are often required to carry horizontal or inclinedloads in addition to the vertical loading. As examples, caissonpiers to river bridges have to carry lateral loading from windforces on the superstructure, traction of vehicles on the bridge,river currents, wave forces and sometimes floating ice or debris.Caissons in berthing structures have to be designed to withstandimpact forces from ships, mooring-rope pull, and wave forces.Methods of calculating the bearing pressures beneath eccentri-cally loaded foundations are described in section 17.2.5. A

position indicated would not need to be underpinned. Consider-ation should be given to the comparative cost of repairs to makegood cracking caused by small settlements and that of underpin-ning, bearing in mind that underpinning operations are them-selves usually accompanied by some small settlement.

The various stages of excavation of a four-level deep base-ment using ground anchors to support the upper two levels andthe basement floors to support the lower levels of a diaphragmwall are shown in Figure 17.21. Excavation is undertakenbeneath the completed floors and openings are left for removalof spoil. The permanent columns supporting the basementfloors are set in drilled holes before commencing the excavation.The inherent stiffness of a diaphragm wall combined withpreloading of ground anchors, say to 50% higher than thecalculated working load, reduces to a minimum (but does noteliminate) inward yielding of the wall.

within the structure, the flooding of the cells may remainundetected. While the cells can be interconnected and providedwith a drainage sump and automatic pumping arrangementsthere can be no certainty that these arrangements will bemaintained in a sound working condition throughout the life ofthe supported structure. Therefore, unless drainage by gravityto an existing piped system is possible, the net bearing pressuresbeneath the buoyancy raft should be calculated on the assump-tion that the cells will become flooded to the level at whichgravity drainage can be assured. As noted in section 17.3.2.4, thetanking of a buoyancy raft with asphalt does not give anyguarantee of lasting watertightness.

Pipes carrying potentially explosive gases should not berouted through the cells of a buoyancy raft. Leakage of gas intothe unventilated cells could remain undetected with a conse-quent risk of an explosion from accidental ignition.

Grab

Sealing concrete

Founding level Plugging concrete

Cell

Groundsanchors

Indents castinto wall

Diaphragmwall

Column,set indrilledholeBackfill

Bored pile

Floorsupportedbycolumn}

Excavation

Openings leftfor removal of,spoil

Openingsfilled in

caisson will be safe against overturning provided that thebearing pressure beneath its edge does not exceed the safebearing capacity of the foundation material, but it is alsonecessary to ensure that tilting due to elastic compression andconsolidation of the foundation soil or rock does not exceedtolerable limits.

The walls of caissons are frequently subjected to severestresses during construction. These stresses may arise fromlaunching operations (when caissons are constructed on aslipway and allowed to slide into the water), from: (1) waveforces when floating under tow or during sinking; (2) rackingdue to uneven support whilst excavating individual cells; (3)superimposed kentledge; and (4) the drag effects of skin friction.

Lateral pressures on the external walls of caissons initiallymay be relatively low, corresponding to active pressure of soilloosened by the sinking process. However, with time the loos-ened soil will reconsolidate and, because the walls may be rigidand unyielding the conditions of earth pressure 'at rest* maydevelop (the coefficients appropriate to 'active' or 'at rest' earthpressure conditions are stated in Chapter 9). Where caissons aresunk through stiff over-consolidated clays or shales it may benecessary to cut the excavation larger than the plan dimensionsof the foundation. With time the soil will swell to fill the gap andsubstantial swelling pressures may develop on the external walls.

173.4.2 Box caissons

Box caissons are designed to be floated in water and sunk on toa prepared foundation bed. The stages of sinking are shown inFigure 17.23. The foundation bed is prepared under water bydivers, and the caisson is lowered by opening flood valves toallow the unit to sink at a controlled rate. Box caissons aresuitable for site conditions where the bed can be prepared withlittle or no excavation below the sea- or river-bed. Thus, they areunsuitable for conditions where scour can undermine a shallowfoundation. They are also unsuitable for conditions where scourcan occur during the final stages of sinking by the action ofeddies and currents in the gap between the base of the caissonand the bed material as the gap diminishes. For founding on softclay or in scouring conditions, box caissons can be sunk on to apiled raft constructed underwater, but this method is normallymore expensive than adopting an open-well caisson.

Box caissons can be of relatively light reinforced-concreteconstruction, since they are not subjected to severe stressesduring sinking. Light construction is desirable to give therequired freeboard whilst floating. After sinking they can befilled with mass concrete or sand if dead weight is required forthe purpose of increasing the resistance to overturning or lateralforces.

17.3.4.3 Open caissons

Open caissons are designed to be sunk by excavating whileremoving soil beneath them through the open cells. They aredesigned in such a manner that the dead weight of the caissontogether with any kentledge which may be placed upon itexceeds the skin friction of the soil around the walls and theresistance of the soil beneath the bottom (cutting) edges of thewalls. To aid sinking, the soil may be excavated from beneaththe cutting edges, or kentledge may be placed on the top of thewalls to increase the dead weight. The skin friction around theexternal walls can be reduced considerably by injecting abentonite slurry above the cutting edge between the walls andthe soil. On reaching founding level, mass concrete is placed toplug each cell after which any water in the cells can be pumpedout and further concrete placed to form the final seal. Theportions of the cells above the sealing plugs can be left empty, orthey can be filled with mass concrete, sand, or fresh waterdepending on the function of the unit and the allowable netbearing pressure. The stages of sinking are shown in Figure17.24.

The lower part of an open caisson is known as the shoe. Thisis usually of thin mild steel plating stiffened at the edges withsteel tees or angles and provided with internal bracing members.Concrete is placed in the space between the skin plates of theshoe to provide ballast for sinking through water and thereaftermore concrete and further strakes of skin plating are added toobtain the required downward forces to overcome skin frictionand the bearing resistance of the soil beneath the cutting edges.While the top of the shoe is still above water level, formwork isassembled and the walls extended above the shoe in reinforcedconcrete. The formwork is usually arranged in lifts of about1.5 m and a 24-h cycle of operations comprises grabbing to sink1.5m, erecting steel skin plating or formwork in the walls,placing the concrete and striking the formwork. Sinking pro-ceeds steadily throughout this cycle. Thick walls are needed forrigidity and to provide dead weight. As well as being reinforcedto withstand external earth and hydrostatic pressures, they mustresist racking stresses and vertical tension stresses. The lattermay occur when the upper part of the caisson is held by skinfriction and the lower part tends to fall into the undercut andloosened zone beneath the shoe.

The form of construction, incorporating a shoe fabricated insteel plating, is the traditional method of design, which providesoptimum conditions for control of sinking at all stages. How-ever, the introduction of bentonite injection techniques to aidsinking has improved the control conditions making it possibleto design caissons entirely in reinforced concrete and enablingthem to be sunk to great depths. Circular caissons were sunk todepths of as much as 105m below the bed of the JamunaRiver.16

Handling rope

Floodingvalve

Waterballast Sand fill

Crushed rock blanketlevelled by diver v

Figure 17.23 Stages in sinking a buoyancy raft, (a) Floodingvalve opened to admit water ballast; (b)caisson sunk in finalposition

Figure 17.24 Stages in sinking an open caisson, (a) Grabbingfrom cells and concreting in walls; (b) plugging and sealingconcrete in place with caisson at final level

Some typical values used to give a rough guide to skin frictionare shown in Table 17.2.17

Table 17.2 (After Terzaghi and Peck (1967) Soil mechanics inengineering practice. Wiley)

Skin frictionType of soil (kN/m2)

Silt and soft clay 7-30Very stiff clay 50-200Loose sand 10-35Dense sand 30-70Dense gravel 50-100

The soil is excavated from within the cells and, wherenecessary, from below cutting edge level by mechanical grab. Inuncemented granular soils, the spoil can be removed by an airliftpump. On reaching founding level any kentledge placed on thewalls is removed to arrest sinking and mass concrete is quicklyplaced at and below cutting edge level in the corner cells toprovide a bearing on which the caisson comes to rest. Theremaining outer cells are then plugged with concrete followed bycompletion of excavating and plugging of the inner cells. Theconcrete plugs are placed under water and after the concrete hashardened the cells are pumped out and further sealing concreteis placed.

Accuracy in the positioning of caissons and control of verti-cality while sinking are necessary. Various methods of achievingthese are:

(1) Sinking between moored pontoons (Figure 17.25).(2) Sinking within a piled enclosure (Figure 17.26).(3) Sinking through a sand island (Figure 17.27).

The choice of method depends on the site conditions, i.e. thedepth of water, degree of exposure, and velocity of sea or rivercurrents. It also depends on the number of caissons to be sunkon any particular project. The cost of an elaborate floatingsinking set as shown in Figure 17.25 is justified if spread over anumber of sinking sites. Lowering during sinking can beachieved by using suspension links and jacks (Figure 17.26) bylowering from block and tackle (Figure 17.25) by free sinkingwith the use of guides (Figure 17.27) or by the controlledexpulsion of air from the cells in conjunction with air domes(Figure 17.28).

Open-well caissons are best suited to sinking in soft or loosesoils to reach a founding level on stiff or compact material, i.e.

Figure 17.27 Sinking caisson through a sand island

Figure 17.26 Lowering caisson from piled stagingDerrick cranes

Suspensionrope

Winch Mooringropes

PontoonBallastconcrete

CaissonRiver-bed.

Figure 17.25 Lowering caisson from pontoons

Guide pilestaging

Scour protectionmattress

CaissonSand fill

Steel sheetpilingH.W.

River-bed

Suspension link

Hydraulic jack

Girder

Enclosurepiles

Derrick crane

Staging

Formwork

Dredgingwells -

Concrete,filling

(stein ing)Cutting edge

Skin platesBracing Haunch plates Plugging concreteDredging wells

Sealingconcrete

Cofferdam(later removed)

PierH.W.

through materials which can be dredged readily and are free ofobstruction such as boulders, tree trunks or sunken vessels.They are unsuitable for ground containing obstructions whichcannot be broken out from beneath the cutting edge, and arealso unsuitable for sinking on to an irregular rock surface.Problems also arise when founding on weak rocks. Grabbingthrough water causes softening and breakdown of the rock,making it difficult to judge when a satisfactory bearing stratumhas been reached and to clean the rock surface to receive theconcrete plug.

Removal of soil from within or below the cells of an opencaisson causes quite appreciable loss of ground, i.e. the totalvolume of soil excavated exceeds the volume displaced by thecaisson. Open caissons are therefore unsuitable for sinking closeto existing structures.

Some of the difficulties mentioned above can be overcome byproviding an open caisson with air domes. These are providedwith airlocks and are designed to be placed over individual cellsas required. Having placed a dome on top of a cell, compressedair is introduced to expel water, after which workmen can enterthrough an airlock to remove obstructions or to prepare thebottom to receive the sealing concrete. There are limits to the airpressure under which operatives can work in this manner (seesection 17.3.4.4). Air domes provided on all cells can be used asthe means of floating an open caisson to the sinking site and forcontrolling its vertical aspect during sinking by varying the rateof expulsion of air from individual cells. Caissons designed inthis way are known as flotation caissons. A design used for theTagus River bridge18 is shown in Figure 17.28. The cutting edgeof this caisson was 'tailored' to suit the profile of the rocksurface on which the caisson was landed. The domes of flotation

caissons are not normally provided with an airlock. After theyhave been removed, grabbing proceeds in the normal way foropen well caissons.

17.3.4.4 Pneumatic caissons

Pneumatic caissons are designed to be sunk with the assistanceof compressed air to obtain a 'dry' working chamber. Thegeneral arrangement is shown in Figure 17.29. The caissonconsists of a single working chamber surrounded by the shoewith its cutting edge, and a heavy roof. Walls are extendedabove the shoe in the form of double steel skin plating with massconcrete infilling. The height of the walls depends on the weightrequired to provide sinking effort and the need to providefreeboard when sinking through water. The airshaft extendsfrom the working chamber to the full height of the caisson and itis surmounted by a combined manlock and mucklock. As thenames imply, the former is used for access and egress byoperatives and the latter for removal of spoil in crane buckets.The manlocks must at all times be above the highest tide or riverflood levels, with due allowance being made for rapid sinking insoft or loose soils.19

Work in pneumatic caissons is regulated by the statutoryregulations governing working conditions in compressed air.The regulations require 0.3 m3 of fresh air per minute per personin the working chamber at the pressure in the chamber. The air issupplied from stationary compressors powered by diesel orelectric motors. Standby power must be available if the siteconditions are such as to endanger life or property if the mainsupply fails. To improve working conditions and to reduce theincidence of caisson-sickness the air supply should be treated to

Figure 17.28 Flotation caisson for the Tagus River bridge. (AfterRiggs (1965) Tagus River Bridge - tower piers', Civ. Engng (USA)(Feb.) 41-45)

Basalt

Tuff -35mMud and rocks

MudBasalt

40-79 mCut t ing edge

Fill concreteAnchorsystem

Concretewall

Water

Water surface elevationin pressurized dredging

wells

^BridgeAir domes21 Dredging wells

4-71 mID

Figure 17.29 Compressed-air caisson. (After Wilson and Sully(1949) Compressed air caisson foundations. Works ConstructionPaper Number 13, Institution Civil Engineers)warm it for working in cold weather and to cool it for hot-weather working. In tropical climates the air should be dehumi-dified to keep the wet bulb temperature at less than 25° C. Invery permeable ground the escape of air into the soil beneath theworking chamber may cause too great a demand on the airsupply. This can be reduced by pregrouting the ground withclay, cement or chemicals.

If the dead weight of the caisson, together with any addedkentledge, is insufficient to overcome the skin friction, theeffective sinking weight can be increased temporarily by 'blow-ing down' the caisson. This involves removing the operativesfrom the working chamber, then reducing the air pressure byabout one-quarter of the gauge pressure.

On nearing founding level, concrete blocks are placed on thefloor of the working chamber and the roof is allowed to come torest on them. The working chamber is then filled with concreteand the airshaft and airlocks removed.

The pneumatic caisson is suitable for sinking close to existingstructures since the excavation is not accompanied by loss ofground. It is also suitable for sinking in ground containingobstructions, and for founding on an irregular rock bed. Pneu-matic caissons have the severe limitation that the depth ofsinking cannot exceed a level at which the required air pressureto exclude water from the working chamber exceeds the limit atwhich operatives can work without danger to their health. Apressure of 345 kN/m2 is considered generally to be a safemaximum but stringent medical precautions and supervision are

Figure 17.30 Concrete monolith

173.4.6 Shaft foundations

Where deep foundations are required for the heavily loadedcolumns of a structure it may be desirable to sink the foundationin the form of a lined shaft excavated by hand or by mechanicalgrab. This type of foundation is similar to the large bored pile asdescribed in section 17.4.3.1 but its distinguishing characteristicis the construction of the lining in place, taken down stage-by-stage as the shaft is deepened. The shaft foundation would beselected in cases where the required diameter was larger than thecapacity of the large-bored-pile drilling machine, in groundcontaining boulders or other obstructions which could preventmachine drilling or caisson sinking, and in localities wherespecialist pile-drilling plant is not available but where labour forhand excavation can be provided from local resources.

Shaft foundations can be of any desired shape but thecylindrical form is the most convenient since internal bracing isnot required. The lining can consist of mass concrete placed insitu behind formwork (Figure 17.31 (a)) or bolted precast con-crete, steel or cast-iron segments (Figure 17.31(b)). The in situconcrete lining is suitable for relatively dry ground which canstand without support for a height of about 1.5m. Segmentallining can be used in water-bearing ground which can standunsupported for the height of a segment. Cement grout must be

Cut t ing edge

Working chamber

required at all stages of the work.20 The high cost of compressed-air sinking generally precludes pneumatic caissons for all butspecial foundations where no alternatives are feasible or eco-nomically possible.

17.3.4.5 Monoliths and cylinders

Monoliths are open caissons of reinforced concrete or massconcrete construction (Figure 17.30) and are mainly used forquay walls where their heavy weight and massive constructionare favourable for resisting the thrust of the filling behind thewall and for withstanding the impact forces from berthing ships.Because of their weight they are unsuitable for sinking throughdeep soft deposits. Their design and method of constructiongenerally follow the same principles as those for open caissonsin section 17.3.4.3.

Open caissons of cylindrical form and having a single cell aresometimes referred to as cylinder foundations.

Air shaft

Bracing

Muck bucket

Man lock Man lockAir supply

Muck lock

Hoisting rope

Skin

plat

ing

Caiss

on

shoe

Figure 17.31 Shaft foundations, (a) With mass concrete liningconstructed below caisson; (b) with precast concrete segmentallining constructed below a sheet-piled cofferdam

17.4 Piled foundations

17.4.1 General descriptions of pile typesThere is a large variety of types of pile used for foundationwork.21 The choice depends on the environmental and groundconditions, the presence or absence of groundwater, the func-tion of the pile, i.e. whether compression, uplift or lateral loadsare to be carried, the desired speed of construction and consider-ation of relative cost. The ability of the pile to resist aggressivesubstances or organisms in the ground or in surrounding watermust also be considered.

In BS 8004, piles are grouped into three categories:

(1) Large displacement piles: these include all solid piles, includ-ing timber and precast concrete and steel or concrete tubesclosed at the lower end by a shoe or plug, which may beeither left in place or extruded to form an enlarged foot.

(2) Small displacement piles: these include rolled-steel sections,open-ended tubes and hollow sections if the ground entersfreely during driving.

(3) Replacement piles: these are formed by boring or othermethods of excavation; the borehole may be lined with acasing or tube that is either left in place or extracted as thehole is filled.

Large or small displacement piles In preformed sections theseare suitable for open sites where large numbers of piles arerequired. They can be precast or fabricated by mass-productionmethods and driven at a fast rate by mobile rigs. They aresuitable for soft and aggressive soil conditions when the wholematerial of the pile can be checked for soundness before beingdriven. Preformed piles are not damaged by the driving ofadjacent piles, nor is their installation affected by groundwater.

They are normally selected for river and marine works wherethey can be driven through water and in sections suitable forresisting lateral and uplift loads. They can also be driven in verylong lengths.

Displacement piles in preformed sections cannot be variedreadily in length to suit the varying level of the bearing stratum,but certain types of precast concrete piles can be assembled fromshort sections jointed to form assemblies of variable length. Inhard driving conditions preformed piles may break causingdelays when the broken units are withdrawn or replacementpiles driven. A worse feature is unseen damage particularlywhen driving slender units in long lengths which may bedeflected from the correct alignment to the extent that thebending stresses cause fracture of the pile.

When solid pile sections are driven in large groups theresulting displacement of the ground may lift piles alreadydriven from their seating on the bearing stratum, or maydamage existing underground structures or services. Problemsof ground heave can be overcome or partially overcome in somecircumstances by redriving risen piles, or by inserting the piles inprebored holes. Small-displacement piles are advantageous forsoil conditions giving rise to ground heave.

Displacement piles suffer a major disadvantage when used inurban areas where the noise and vibration caused by drivingthem can cause a nuisance to the public and damage to existingstructures. Other disadvantages are the inability to drive them invery large diameters, and they cannot be used where theavailable headroom is insufficient to accommodate the drivingrig-

Driven and cast-in-place piles These are widely used in thedisplacement pile group. A tube closed at its lower end by adetachable shoe or by a plug of gravel or dry concrete is drivento the desired penetration. Steel reinforcement is lowered downthe tube and the latter is then withdrawn during or after placingthe concrete. These types have the advantages that: (1) thelength can be varied readily to suit variation in the level of thebearing stratum; (2) the closed end excludes groundwater; (3) anenlarged base can be formed by hammering out the concreteplaced at the toe; (4) the reinforcement is required only for thefunction of the pile as a foundation element, i.e. not fromconsiderations of lifting and driving as for the precast concretepile; and (5) the noise and vibration are not severe when the pilesare driven by a drop hammer operating within the drive tube.

Driven and cast-in-place piles may not be suitable for verysoft soil conditions where the newly placed concrete can besqueezed inwards as the drive tube is withdrawn causing 'neck-ing' of the pile shaft, nor is the uncased shaft suitable for groundwhere water is encountered under artesian head which washesout the cement from the unset concrete. These problems can beovercome by providing a permanent casing. Ground heave candamage adjacent piles before the concrete has hard-ened, and heaved piles cannot easily be redriven. However, thisproblem can be overcome either by preboring or by driving anumber of tubes in a group in advance of placing the concrete.The latter is delayed until pile driving has proceeded to adistance of at least 6.5 pile diameters from the one beingconcreted if small (up to 3mm) uplift is permitted, or 8diameters away if negligible (less than 3 mm) uplift must beachieved.22 The lengths of driven and cast-in-place piles arelimited by the ability of the driving rigs to extract the drive tubeand they cannot be installed in very large diameters. They areunsuitable for river or marine works unless specially adapted forextending them through water and cannot be driven in situa-tions of low headroom.

Replacement piles or bored piles These are formed by drilling aborehole to the desired depth, followed by placing a cage of steel

injected at intervals into the space between the back of thesegments and the soil. This is necessary to prevent excessive flowof water down the back of the lining, and also to support thesegments from dropping under their own weight augmented bydowndrag forces from the loosened soil. The collar at the top ofthe shaft is also required to support the lining.

Shaft foundations may be constructed as a second stage afterfirst sinking through soft or loose ground as a caisson (Figure17.3l(a)) or at the base of a sheet piled cofferdam (Figure17.31(b)).

Reinforcedconcretering waling

.Steel sheetpilingMass concretecollar

Pre-castconcretesegments

Foundationlevel

Form work

Concretepouredin situ

Soft orloosesoil

Circular reinforcedconcrete caisson

Sealingconcrete