BearingCapacityofSoil(WithDiagram)byZ.KhanSoil

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Duringrainyseason,immediatelyafterrain,whenwesteppedintosoilsoftenbytherain,ourshoepenetratesintothesoil.Thesoilsqueezesoutfromunderourshoeandemergesaroundthesidesofourshoe.Duringthisprocessweloseourbalancetillthesoilbelowthesurfacesupportsourweightandprovidesusstability.

Thesoilbelowourshoejustgaveway.ThisgivingwayorrunningawayofsoiliscalledbearingcapacityfailurebyGeotechnicalengineers.Bearingcapacityfailureisafailurebyshear.Soknowledgeofbearingcapacityofanysoilbeforeconstructionofstructureisveryessential.

Definitions:

(i)Foundation:

Itisthelowestpartofthestructurewhichsupportsastructure.

(ii)Foundationbed:

Thematerialonwhichafoundationrestsiscalledfoundationbed.

(iii)Shallowfoundation:

Whenthedepthofthefoundationislessthanorequaltothewidthofthefoundation,itiscalledshallowfoundation.

(iv)Deepfoundation:

Whenthedepthofafoundationisgreaterthanthewidth,itiscalleddeepfoundation.

(v)Bearingcapacity:

Itistheloadcarryingcapacityofsoil.

(vi)Ultimatebearingcapacity(qu):

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Itistheminimumgrosspressureatthebaseofthefoundationatwhichsoilfailsinshear.

(vii)Grosspressure(q):

Thegrosspressureisthetotalpressureatthebaseofthefootingduetotheweightofthesuperstructure,selfweightofthefootingandweightoftheearthfill.

(viii)Netpressureintensity(qn):

Itisthedifferenceinintensitiesofthegrosspressureandtheoriginaloverburdenpressure.IfDisdepthoffoundation,thenqn=qyD

(ix)Netultimatebearingcapacity(qnu):

Itistheminimumnetpressureintensitywhichcausesshearfailureofsoil.

Qu=qnu+yD

qnu=qu+yD

(x)Safebearingcapacity(qs):

Itisthemaximumpressureintensitywhichthesoilcancarrysafelywithouttheriskofshearfailure.

qs=qns+yD

=qnu/F+yD

whereFisthefactorofsafety

(xi)Netsafebearingcapacity(qns):

Itisthenetultimatebearingcapacitydividedbyafactorofsafety.

Qns=qns/F

(xii)Allowablebearingcapacity(qa):

Itisthenetloadingintensityatwhichneitherthesoilfailsinshearnorthereisexcessivesettlementdetrimentaltothestructure.

ConceptofBearingCapacity:

Allcivilengineeringstructureswhethertheyarebuildings,dams,bridgesetc.arebuiltonsoils.Afoundationisrequiredtotransmittheloadofthestructureonalargeareaofsoil.Thefoundationofthestructureshouldbesodesignedthatthesoilbelowdoesnotfailinshearnorthereisexcessivesettlementofthestructure.Theconventionalmethodoffoundationdesignisbasedontheconceptofbearingcapacity.

Thebearingcapacityoffoundationisthemaximumloadperunitareawhichthesoilcansupportwithoutfailure.Itdependsupontheshearstrengthofsoilaswellasshape,size,depthandtypeoffoundation.Figure9.1showsatypicalloadvs.settlementcurveofafooting.Fromthefigureitisclearthatasthefootingloadisincreased,thesettlementalsoincreases.

Thesettlementincreaseslinearlywithloadattheinitialstage.Onfurtherincreaseinload,thesettlementincreasesmorerapidlyandthencontinuestoincreasewithoutanyappreciableincreaseinload.Thisstageiscalledfailureoffoundationi.e.,thesoilhasreacheditscapacitytobearload.

Toavoidbearingcapacityfailureoffoundationitisessentialtotakeintoconsideration,beforedesignoffoundation,twotypesofactionbythesoilwhensubjectedtoload:

(i)Thebearingcapacityshouldbelowenoughtoensurethatthesettlementcausedisnotexcessive.

(ii)Thebearingcapacityshouldbesuchthatexcessiveshearstrainisnotcaused.

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BearingCapacityofShallowFoundations(TerzaghiAnalysis):

AssumptionsinTerzaghisAnalysis:

1.Thefootingisstriponeatshallowdepthandhasroughbase(L>5B,D>B,whereL=length,B=widthandD=depthofthefooting).

2.Thesoilishomogeneous,isotropic#andrelativelyincompressible.

3.Thefailurezonesdonotextendabovethehorizontalplanethroughthebaseofthefooting.

4.Theelasticzonehasstraightboundariesinclinedat=tothehorizontalandtheplasticzonesfullydeveloped.

Alsocalledgeneralbearingcapacityequationforstripfooting

qu=CNc+0.5BN+qNq

wherequ=ultimatebearingcapacity

q=overburdenpressureatthebase

=yD(useD,ifsubmerged)Ccohesionofsoil

=unitweightofsoilatbaseleveloffoundation

(useofsubmerged)

B=Widthoffoundation

D=Depthoffoundation

NC,NgandNqarebearingcapacityfactorswhichdependon(angleofinternal

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friction).

BearingCapacityfromBuildingCodes:

Forpreliminarydesignofanystructureandfordesignoffoundationoflightlyloadedstructures,thepresumptivebearingcapacitymaybeused.Table9.1givespresumptivesafebearingcapacitiesforvarioustypesofsoilrecommendedbyNationalbuildingcodeofIndia.

Note1:

Valuesofbearingcapacitylistedarefromshearconsiderationonly.

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Note2:

Valueslistedinthetableareverymuchroughforthefollowingreasons:

(i)Effectofdepth,width,shapeandroughnessoffoundationhavenotbeenconsidered.

(ii)EffectOfangleoffriction,cohesion,watertable,densityetc.,havenotbeenconsidered.

(iii)Effectofeccentricityandindicationofloadshasnotbeenconsidered.

Note3:

Drymeansthatthegroundwaterlevelisatadepthnotlessthanthewidthoffoundationbelowthebaseofthefoundation.

Note4:

Forcohesionlesssoils,thevalueslistedinthetableshallbereducedby50%ifthewatertableisaboveornearthebaseoffooting.

Note5:

Compactnessofcohesionlesssoilsmaybedeterminedbydrivingaconeof65mmdiaand60apexanglebyahammerof65kgfallingfrom75cmIfthecorrectedNvaluefor30cmpenetrationislessthan10,thesoiliscalledloose,ifNliesbetween10and30,itismediumandifmorethan30,thesoiliscalleddense.

FactorsAffectingBearingCapacityofSoils

Thefollowingfactorsaffectthebearingcapacityofsoils:

(i)Typeofsoil:

(ii)Physicalcharacteristicsoffoundation

(iii)Soilproperties

(iv)Typeoffoundation

(v)Watertable

(vi)Amountofsettlement

(vii)Eccentricityofloading.

(i)Typeofsoil:

Thebearingcapacityofsoilsdependsuponthetypeofsoil.Dependinguponthetypeofsoil,thebearingcapacityofsoilisdifferentwhichisclearfromTerzaghibearingcapacityequation.

qu=CNC+0.5yBNy+qNq

Forpurelycohesionlesssoil

C=0

Equation(9.1)reduceto

qu=0.5yBNy,+qNq

Forpurelycohesivesoil

=0,

thevaluesofbearingcapacityfactorsare

Nc=5.7

Nq=1andN=0

Equation(9.1)isthen

qu=5.7C+q

(ii)Physicalcharacteristicsoffoundation:

Physicalcharacteristicslikewidth,shapeanddepthoffoundationaffectthebearingcapacityofsoils.Eq.9.1showsthatthebearingcapacityofsoilsdependsuponthewidthBanddepth(D)offoundation.SoanychangeinthevalueofBandDoffoundationwillaffectthebaringcapacity.

Theshapeoffoundationalsoaffectsthebearingcapacitywhichisas

follows:

Forsquarefootings:

qu=1.2CNc+0.4BN+DNq(9.2)

Forcircularfootings:

qu=1.2CNC+0.3BN+DNq(9.3)

whereBisthediameterofcircularfooting.

(iii)Soilproperties:

Soilpropertieslikeshearstrength,density,permeabilityetc.,affectthebearingcapacityofsoil.Densesandwillhavemorebearingcapacitythanloosesandasunitweightofdensesandismorethanloosesand.

(iv)Typeoffoundation:

Thetypeoffoundationselectedalsoaffectsthebearingcapacityofsoils.Raftormatfoundationadoptedsupportstheloadofstructuresafelybyspreadingtheloadtoawiderarea,evenifthesoilishavinglowbearingcapacity.

(v)Watertable:

Whenthewaterisabovethebaseofthefooting,thesubmergedunitweightofsoilisusedtocalculatetheoverburdenpressureandthebearingcapacityofthesoilreducesby50%.

Foranypositionofthewatertablethegeneralbearingcapacitymaybemodifiedasunder:

(vi)Amountofsettlement:

Theamountofsettlementofthestructurealsoaffectsthebearingcapacityofsoil.Ifthesettlementexceedsthepossiblesettlement,thebearingcapacityofsoilisreduced.

(vii)Eccentricityofloading:

IftheloadactseccentricallyinafootingthewidthBandlengthLshouldbereducedasunder

B=B2e

L=L2eand

A=BXL

Theultimatebearingcapacity(qu)ofsuchfootingsaredeterminedbyusingBandLinsteadof8andL.Hencequislessthanthatcorrespondstoactualsizeoffootingas

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showninfigure9.4.

ConceptofVerticalStressDistributioninSoilsDuetoFoundationLoads:

Whenasoilmassisloaded,verticalstressesaredevelopedinthesoil.Theestimationofverticalstressesatanypointinasoilmassduetoexternalloadingareofgreatsignificanceinthepredictionofsettlementofbuildings,bridges,embankmentsandotherstructures.Thestressesduetoexternalloadingaregreatestatshallowdepths,closetothepointloadapplicationandtheybecomesmallerastheverticaldistancebelowtheloadorthehorizontaldistancefromtheloadincreases.

Theverticalstressdistributioninasoilmassdependsupon:

(i)Thenatureofloadingi.e.,themannerofloadplacement,loaddistributionandshapeoftheloadedarea

(ii)PhysicalpropertiesofsoillikePoissonsratio,modulusofelasticity,compressibilityetc.

Indeterminingthestressesbelowafoundation,itisgenerallyassumedthatthesoilbehavesasanelasticmediumwithidenticalpropertiesatallpointsandinalldirections.Manyformulaebasedontheoryofelasticityhavebeenusedtocomputestressesinsoils.OnesuchformulawasfirstdevelopedbyBoussinesq(1885)forthestressesanddeformationintheinteriorofasoilmassduetoverticalpointload.ABritishscientistWestergaardin1938alsoproposedaformulaforcomputationofverticalstressinthesoilmassduetoverticalpointload.

PointLoad:

Businesssformula:

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Businesssformulaisbasedonthefollowingassumptions:

(i)Thesoilmassislinearlyelastic,homogeneous,isotropicandsemiinfinite.

(ii)Theloadactsasaverticalconcentratedload.

(iii)Thesoilisweightless.

Theequationforverticalstressatapointasshowninfigure9.5

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LineLoad:

TheequationforverticalstressduetoalineloadP1perunitlengthonthesurfaceatapointlocatedatadepthzanddistancexlaterallyasshowninfigure9.6is

Z=2p1/

z3/(x2+z2)2

UniformlyLoadedStrip:

TheequationforverticalstressduetoauniformloadqonastripareaofwidthBandinfinitelengthintermsofandasshowninthefigure9.7is

z=q/(+SinCos2)

Belowthecentreofthestrip,verticalstressoatadepthzisgivenby

Z=q/(a+sin)(iszeroandcos2=1)

or2=qloz

Thevaluesoftheinfluencefactoraregivenintable9.3,

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SoilPropertiesGoverningChoiceofFoundationType:

Thefollowingpropertiesofsoilgovernthechoiceoffoundationtype:

(i)Bearingcapacityofsoil

(ii)Settlementofsoil

Theknowledgeofbearingcapacityandsettlementofthesoilisveryessentialfordesignoffoundationofanystructure.Thefoundationofanystructureshouldbesoselectedthatthesoilbelowdoesnotfailinshearandsettlementiswithinthepermissiblelimits.

Ifthebearingcapacityofsoilatshallowdepthissufficienttosafelytaketheloadofthestructure,ashallowfoundationisprovided.Isolatedfooting,combinedfootingorstripfootingaretheoptionforshallowfoundation.Deepfoundationsareprovidedwhensoilimmediatelybelowthestructuredoesnothaveadequatebearingcapacity.Pile,piersorwellaretheoptionsfordeepfoundations.Matorraftfoundationsareusefulforsoilwhicharesubjectedtodifferentialsettlementorwherethereisawidevariationinloadingbetweenadjacentcolumns.Table9.4givessuitabilityoffoundationfor

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buildingsbasedonsoiltype.

InSituTestsforDeterminationofUltimateBearingCapacity

Thefollowinginsitutestsmaybeusedtodeterminetheultimatebearingcapacityorallowablebearingcapacityofsoil:

(a)Plateloadtests

(b)Standardpenetrationtest

(c)Dynamicconepenetrationtest

(d)Staticconepenetrationtest

(e)Pressuremetertest

PlateLoadTest:

Plateloadtestessentiallyconsistsofloadingarigidplateatthefoundationlevelandrecordingthesettlementscorrespondingtoeachloadincrement.Theultimatebearingcapacityisthentakenastheloadatwhichtheplatestartssinkingatarapidrate.Theminimumandmaximumrecommendedsizestotestplateare30cmsquareand75cmsquarerespectively.Thethicknessofthesteelplateshouldnotbelessthan25mm.AlamSinghhasrecommendedthesizeofthetestplatetobe32cmsquare.

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Thetestiscarriedinapithavingwidthequalto5timesthewidthofthetestplate.Atthecentreofthepit,asmallsquareholeisdugwhosesizeisequaltothesizeoftheplateandthebottomlevelofthepitcorrespondtotheleveloftheactualfoundation.

Theloadingtothetestplatemaybeappliedbythefollowingtwomethods:

(a)GravityLoadingPlatformmethod

(b)ReactionTrussMethod

Reactiontrussloadingisfoundconvenientandlesstimeconsuming,hencegenerallyused.Forthispurpose,asteeltrussisanchoredtogroundacrossthepit.Ahydraulicjackwithattachedpressuregaugeisplacedbetweentheundersideofthetrussandthetestplate.Atleasttwodialgauge,havingaccuracyof0.2mm,isusedtomeasurethesettlementofthetestplate.Thedialgaugesaremountedonindependentdatumbarandarejusttouchingthetestplate.

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