Geology Applied to Civil Engineering 土木工程地质 英文课件 第十章 Geologic problems in foundation engineering

10GeologicproblemsinfoundationengineeringGeologyAppliedtoCivilEngineering123contentsBearingcapacityoffoundationGroundimprovementTypesoffailureinsoil4Casestudies231Typesoffailureinsoil(1)GeneralshearfailureThefoundationisloadedbyastripfootingwiththewidthofB.Thedimensionperpendiculartotheplaneofthepaperwillbetakenasunit,butallmotionissupposedintheplane.Now,iftheaveragepressureqisgraduallyappliedtothefoundation,settlementwillincrease.Atacertainpoint,whentheloadperunitareaequalsquthefoundationwillundergosuddenshearfailure.Inaddition,thefailuresurfaceinthesoilwillextendtothegroundsurface.Thisloadperunitareaisusuallyreferredtotheultimatebearingcapacity.Whensuchsuddenfailureinsoiltakesplace,itiscalledgeneralshearfailure.Natureofbearingcapacityfailureinsoil:(a)generalshearfailure1Typesoffailureinsoil(2)LocalshearfailureIfthesamefoundationisnowsupportedbyamediumcompressibilitymaterial,theplotofqvs.settlementwillbedifferent.Notethatthemagnitudeofqincreaseswithsettlementuptoq=qu(1),whichisusuallyreferredtothefirstfailureload.However,iftheloadonthefoundationisfurtherincreased,theload-settlementcurveproceedsinalmostalinearmanneralongwiththegradualprogressofthefailuresurfaceinsoil.Whenqincreasestoqu,thefailuresurfaceextendsuptogroundsurface.Beyondthat,theplotofqvs.settlementtakesalinearshapeandapeakloadisneverobserved.Natureofbearingcapacityfailureinsoil:(b)localshearfailure41Typesoffailureinsoil(3)PunchingshearfailureIfthefoundationissupportedbyafairlyloosesoil,theload–settlementplotwillbeasthefollowingfigure.Inthiscase,thefailuresurfaceinsoilwillnotextendtothegroundsurface.Beyondtheultimatefailureload,theload–settlementplotwillbesteepandpracticallylinear.Thistypeoffailureinsoiliscalledthepunchingshearfailure.Natureofbearingcapacityfailureinsoil:(c)punchingshearfailure52BearingcapacityoffoundationPrandtl’ssolutiononultimatebearingcapacityBasicassumptionofPrandtl’stheoryThegroundsoilunderthefoundationisconsideredtobeweightless,namelyγ1=0;Thebaseofthefootingissmooth.Namely,thefrictionangleonthebaseoffootingiszero,sothatthefoundationpressureshouldbeimposedonthegroundinverticaldirection;Theembedmentdepthisnotgreaterthanthewidthofthefooting.Hence,thiskindoffoundationisdefinedasshallowfoundation.Inaddition,theleveloffoundationbottomisassumedtobethevirtualplane,onwhichthefailuresurfaceextendsto.Thesoilbelowthevirtualplaneissubjectedtosurcharge.Sliplinenetworkofweightlesssoil62BearingcapacityoffoundationPrandtl’ssolutiononultimatebearingcapacityb)FailuresurfaceZoneⅠofRankineactivestate.Thetriangularwedgesoil(ΔABC)ispusheddownwards,withinwhichthemajorprincipalstressisverticalandtheminorprincipalstressishorizontal.ThistriangularΔABCbehaveslikearigidbodyandwouldmovedownwardwithnodeformation.TheanglebetweenABandACis45°+φ/2.ZoneⅡofradialshearstate.ThesectorACEandBCE’areforcedsidewaystorotateabouttheedgeofthefooting.ThecurveCEandCE’aresectionsoflogarithmicspirals,andtheanglebetweenACandAEis90°.ZoneⅢofRankinpassivestate.IntheouterwedgeAEFandBE’F’,theprincipalstressarealignedinaccordancewithRankinpassivestatebecauseoftheprincipalstressrotationinZoneⅡ.TheanglebetweenAFandAEis45°-φ/2.Sliplinenetworkofweightlesssoil72BearingcapacityoffoundationPrandtl’ssolutiononultimatebearingcapacityc)TheultimatebearingcapacitypuTheaboveequationcanbeexpressedas:82Bearingcapacityoffoundation(2)Terzaghi’stheoryontheultimatebearingcapacityBasicassumptionofTerzaghi’stheoryThesoilisasemi-infinite,homogeneous,isotropic,rigid-plasticmaterial；Thebaseofthefoundationisrough；Theembedmentdepthisnotgreaterthanthewidthofthefooting(d<B)；Generalshearfailureoccurs；ThelinesACandBCformanangleψwiththehorizontaldrawnthroughthepointAandB.ψisrelevanttothetoughdegreeofthebase,andisinarangeofφ<ψ<45°+φ/2；TheShearresistanceofthesoilabovethelevelofthebaseofthefootingisneglected.Theoverburdenpressureduetotheembedmentdepthofdisconsideredasq=γ2d.Terzaghi’smechanismsforbearingcapacityanalysis92Bearingcapacityoffoundation(2)Terzaghi’stheoryontheultimatebearingcapacityb)Terzaghi’ssolutionontheultimatebearingcapacityTheultimatebearingcapacitycanbeexpressedaswhere:NqandNccanbecalculatedusingPrandtl-Reissner’ssolution:N

isderivedusingNqandNc:(TerzaghiandPeck)(Meyerhof)(Hansen)Terzaghi’smechanismsforbearingcapacityanalysis10112Bearingcapacityoffoundation(2)Terzaghi’stheoryontheultimatebearingcapacityInTerzaghi’ssolutionontheultimatebearingcapacityforlocalshearfailure,thelowbearingcapacityisDuetothereductionofthestrengthparameters,thefactorsNr',Nc',Nq'arelessthanNr,Nc,Nq,respectively.ThevalueofthefactorNr',Nc',Nq'versusthevalueofφcanbefoundinTerzaghi’sbook.122Bearingcapacityoffoundation(3)ThegeneralbearingcapacityequationRectangularfoundation:inwhichlisthelengthoffoundation.b)Squarefoundation:c)Circularfoundation:whereRistheradiusofthefoundation.132Bearingcapacityoffoundation(4)AllowablebearingpressureThedesignofshallowfoundationsneedstosatisfythefollowingtwopoints:(i)maintainanadequatefactorofsafety(FS)relativetoshearfailureofthebearingsoil;(ii)limitthesettlementofthefoundationtomeetserviceabilityrequirements.Thelesserofcalculatedpressurecorrespondingtotheabovetwopointsisdefinedastheallowablebearingcapacity.In-situbearingtestThefieldplateloadtestTheplateloadtestisasemi-directmethodtoestimatetheallowablebearingpressureofsoiltoinduceagivenamountofsettlement.Plates,roundorsquare,varyinginsize,from30to60cmandthicknessofabout2.5cmareemployedforthetest.Schematicsketchofplateloadingtestapparatus.1—bearingplate；2—hydraulicjack；3—jackingplate；4—bar；5—inclinedjackingplate；6、7—pin；8—pressuregauge；9—dialgauge；10—referencebeam；11—spreaderbeam142Bearingcapacityoffoundation(4)AllowablebearingpressureIn-situbearingtestII.VanesheartestVanetestsareusedprimarilytodeterminetheundrainedshearstrengthofsaturatedclaydepositswithstrengthgenerallyupto200kPa.Theycanalsobeusedinfine-grainedsoilssuchassilts,organicpeats,tailingsandothergeomaterialswhereapredictionoftheundrainedshearresistanceisrequired.Vanebladesfortestinginboreholes152Bearingcapacityoffoundation(4)AllowablebearingpressureIn-situbearingtestIII.Standardpenetrationtest(SPT)Ofthevariousmethodsofgroundinvestigationcurrentlypracticedworldwide,thestandardpenetrationtest(SPT)remainsthemostpopularandiswidelyusedinsitutestingtechnique.Primarilyrecommendedforgranularsoilsandothergroundconditionsinwhichitisdifficulttosampleandtestinthelaboratoryortoperformotherinsitutests,theSPTisbeingusedforpreliminaryexplorationinvirtuallyanygroundcondition.Additionally,itprovidesassessmentofsoilproperties,foundationdesignparametersandliquefactionpotential.SchematicviewofSTPusingdonuthammer,ropeandcatheadmethod162Bearingcapacityoffoundation(5)CorrectionfordepthandwidthInChina,thecorrectionequationprovidedin“CodeforDesignofBuildingFoundation”(GB50007-2011)isshownasTypeofsoil

b

dsludgeandsludgysoil01.0artificialfill,redclay(e≥0.85orIL≥0.85)01.0RedclayWatercontentratioαw＞0.8;Watercontentratioαw≤0.800.151.21.4AgreatdealofcompactedfillSiltysoil(compactingfactor>0.95andclaycontent(

c)≥10%);Gradingsandwithmaximumdrydensitygreaterthan2.lt/m3001.52.0SiltysoilSiltysoil(claycontent(

c)≥10%);Siltysoil(claycontent(

c)≤10%)0.30.51.52.0Claysoil(e≥0.85&rIL≥0.85);Siltysand,finesand(excludingtheextremelywetorsaturatedsoil);Mediumsand,coarsesand,gravelandgravelsand0.32.03.01.63.04.4Correctioncoefficientforwidthandembedmentdepth173GroundimprovementGroundimprovementinitsbroadestsenseisthealterationofanypropertyofsoilorrocktoimproveitsengineeringperformance.Next,thegroundimprovementtechniquesforsoftgroundandproblemsoilgroundarediscussed.SoftgroundSoftgroundmeansagroundconsistingmainlyofsoftsoil,hydraulicfills,miscellaneousfillsorotherhighcompressibilitysoils.SoftsoilThecharacteristicsofasoftsoilarehighwatercontent,largeporeratio,lowpermeabilitycoefficient,highcompressibilityandlowshearstrength.Thebearingcapacityofthesoftsoilgroundislow,andthedeformationislarge.183Groundimprovement(1)Softgroundb)HydraulicfillsThesoilneededforthisisusuallyobtainedbydevelopingitfromtheadjacentriver,lake,oroceantoplaceitatthedesiredlocation.Generallyawellgradedsoilispreferredforhydraulicfill.Sanddepositedbyhydraulicmethodswillberelativelyinlooseconditionandhastobedensified.Siltandclayhydraulicfillsaredifficulttocompactafterplacement.Whentimepermits,suchfillislefttoconsolidateandstabilizenaturally.c)MiscellaneousfillMiscellaneousfillisakindofirregularfillingmaterialsuchasconstructionwaste,industrialwasteandlivinggarbageformedbyhumanactivities.Themainproblemsofmiscellaneousfillarelowstrength,highcompressibilityandpooruniformity.193Groundimprovement(2)ProblemsoilgroundThe“collapsibleloess”region,thedifferentialsettlementusuallyoccurwhilethegroundsoilisimmersedinwater.Thevolumeofswellingsoilwillchangealotindifferentwatercontent.Thefrozensoil,especiallytheseasonalfrozensoilwillmeltinsummer,whichinducesthedecreaseofthebearingcapacity.Hence,theproblematicsoilsneedtobereinforced.203Groundimprovement(3)ThepurposeofgroundimprovementToenhancethebearingcapacity.Toensurethegeneralshearorlocalshearnotoccurringintheground.Toreducethecompressibility.Thesettlementanddifferentialsettlementmustbekeptwithintolerablebounds.Toimprovethedynamiccharacteristicsoftheground.Theseismiccapacityofthegroundneedstobeimprovedtopreventtheliquefaction.Toimprovethepermeability.Thepermeabilityofthegroundneedstobeimprovedtopreventthesandboilandpiping.Toensurethestabilityofthegroundinproblemsoils.213Groundimprovement(3)ThepurposeofgroundimprovementToenhancethebearingcapacity.Toensurethegeneralshearorlocalshearnotoccurringintheground.Toreducethecompressibility.Thesettlementanddifferentialsettlementmustbekeptwithintolerablebounds.Toimprovethedynamiccharacteristicsoftheground.Theseismiccapacityofthegroundneedstobeimprovedtopreventtheliquefaction.Toimprovethepermeability.Thepermeabilityofthegroundneedstobeimprovedtopreventthesandboilandpiping.Toensurethestabilityofthegroundinproblemsoils.223Groundimprovement(4)TheapproachofgroundimprovementDrainageConsolidationPreloadingmethod,surchargepreloadingmethod,vacuumpreloadingmethod,pumpingmethod.b)GroutingmethodDeepmixingmethod,jetgroutingmethod,permeationgrouting,compactiongroutingmethod,hydraulicfracturinggroutingmethod.c)GeosyntheticmethodReinforcementmethod,geotextile,rootpile,anchormethod.d)CompositefoundationVibro-replacementstonecolumnmethod,CFGcolumn,limepile,flyash&limecolumn,dynamicreplacement.e)CompactionmethodRollingcompaction,heavytampingmethod,dynamiccompactionmethod.234CasestudiesCase1softsubgradeimprovementTongliao-Ranghulu(Daqing)RailwayislocatedinthewesternpartofJilinprovinceandsouthernHeilongjiangProvince.Thetotaltracklengthis421km.Alongtherailwayline,thereareatleastfiveareasthatimmersedinwaterforalongtime.Intheseareas,thegroundiscomposedofsoftsoil.Generally,thesoilalongtherailwaylineismainlyQuaternaryalluvialmaterial.Muckyclay,muckysand,andsiltsandaredistributedintheseareas.(a)(b)SanddrainssetupinErdaojiang244CasestudiesCase1softsubgradeimprovement：improvementmethodThereareintotal1271sanddrainsinthissectionwithplumblossom-likearrangement.Thediameterofthesanddrainis200mm,theintervalsare2.2(alongtherailway)and2.5(cross-sectiondirection),respectively.Thelengthofthesanddrainis7-11m.Thesandgrooveinsteadofthesandcushionisusedfordrainage.Thespeedofconstructionisstrictlycontrolled,inordertoimprovethestrengthofthesoftlayerbydrainageinducedbytheloadingduringtheconstruction.Untilnow,thereisnolargedeformationduringtheoperationperiod.(a)(b)SanddrainssetupinErdaojiang254CasestudiesCase2softgroundimprovementwithdynamicreplacementmethodThefourthsinteringplantofWuhanIronandSteelCompanyisloca