STRESS PATHS IN RELATION TO DEEP EXCAVATIONS

JOURNALOFGEOTECHNICALANDGEOENVIRONMENTALENGINEERING/MAY1999/357STRESSPATHSINRELATIONTODEEPEXCAVATIONSByCharlesW.W.Ng,1Member,ASCEABSTRACT:Themechanicalbehaviorofmanysoilssuchasstiffclaysdependsontheircurrenteffective-stressstatesandstresshistory.Forimprovingdesignandanalysisofsoil-structureinteractionassociatedwithdeepexcavationsinthesesoils,itisimportanttounderstandeffective-stresschangesaroundexcavationscausedbybothhorizontalandverticalstressrelief.Inthispaper,totalandeffective-stressvariationsadjacenttoadiaphragmwallduringconstructionofa10-m-deepexcavationinstifffissuredclayarereportedanddiscussed.Interpretedfieldstresspathsarecomparedwithsomerelevantlaboratorytriaxialstresspathtests,whichsimulatethehorizontalandverticalstressreliefinthefieldatanappropriatestresslevel.Theinterpretedfieldeffective-stresspathsinfrontofthewallarefoundtobesimilartolaboratorystresspathsdeterminedinundrainedextensiontests.Fieldstresspathsbehindthewalldonotcorrespondparticularlywellwiththosefromlaboratoryundrainedcompressiontests,exceptwhenthestressstateapproachesactivefailure.Theconventionalundrainedassumptiondoesnotseemtoholdforthesoillocatedimmediatelybehindthewallduringarelativelyrapidexcavationinthestiffclay.INTRODUCTIONDeepexcavationsareusedincreasinglyincongestedcitiestoprovideundergroundspace.Excavationalterstheinitialstressstatesintheground.Understandingtheeffective-stresschangescausedbytheexcavationsisimportantfordesigningsafe,economical,andserviceableexcavationsupportsforfoundationsandundergroundstructures.Unlikeotherman-madematerials,thebehaviorofsoilsisdependentnotonlyonthecurrenteffective-stressstates,butalsoonstresshistory.ThestresspathmethodpresentedbyLambe(1967)providesarationalapproachtothestudyoffieldandlaboratorysoilbehavior.Becausethestabilityanddeformationcharacteristicsofanexcavationareinfluencedbystresshistoryandstressstate,anticipatedfieldbehaviorshouldbesimulatedinthelaboratoryforappropriatedeterminationofshearstrengthandstiffnessparameters.Anunderstandingofthefieldstresspathisnecessarytoidentifythecriticalele-mentsthatarelikelytoaffecttheshearstrength.Oncethepotentialchangesinthefieldhavebeenreviewed,appropriatelaboratorytestscanbeassignedtodeterminetheinsitustrengthanddeformationcharacteristicsrequiredfordesignandanalysis.Thispapercomparesandexaminessomefieldstresspathsandrelevantlaboratorytriaxialstresspathtests,whichsimulatethehorizontalandverticalstressreliefofamultiproppedexcavationinstiffgaultclayatLionYard,Cam-bridge,U.K.Theinterpretedstresspathsarecomparedwithinanidealizedconceptualframework.IDEALIZEDSTRESSPATHSADJACENTTODEEPEXCAVATIONSRetainingwallconstructionandexcavationcausesverticalandhorizontalstressrelief.Themagnitudeofstressreliefde-pendsonmanyfactors,suchasinitialstressintheground,constructionmethod,typeofretainingwall,andthedepthofexcavation.Thestresschangesassociatedwithverticalandhorizontalstressreliefareverycomplex.Nevertheless,thestatesofstressatcertainlocationsaroundanexcavationmaybestudiedqualitativelyifsimplifyingassumptionsaremade.Ifanexcavationisconsideredtobelongwithrespecttoits1Asst.Prof.,HongKongUniv.ofSci.andTechnol.,ClearWaterBay,Kowloon,HongKong.Note.DiscussionopenuntilOctober1,1999.Toextendtheclosingdateonemonth,awrittenrequestmustbefiledwiththeASCEManagerofJournals.ThemanuscriptforthispaperwassubmittedforreviewandpossiblepublicationonJanuary21,1997.ThispaperispartoftheJour-nalofGeotechnicalandGeoenvironmentalEngineering,Vol.125,No.5,May,1999.᭧ASCE,ISSN1090-0241/99/0005-0357–0363/$8.00ϩ$.50perpage.PaperNo.14980.width(Fig.1)andifthesoilisassumedtobeisotropic,thenthestresschangesfortwosoilelements(AandP)canbeidealizedasundergoingplanestraindeformations.ThesoilelementAislocatedataboutmidheightbetweentheretainingwall,andtheelementPislocatedinthecenterofthesitebelowthefinalexcavationlevel.Thestresspathsareexpressedintermsofmeantotalstress[s=(␴vϩ␴h)/2]ormeaneffec-tivestress[sЈ=ϩandthemaximumdeviatoric(␴Ј␴Ј)/2],vhshearstresst=(␴vϪ␴h)/2=Ϫintheplaneof(␴Ј␴Ј)/2vhshearing.Axessandtareorientedat45Њtothe(␴h,␴v)axes,witha͌2scalingfactor.Thetwosetsofstresspathplotsarecompletelyinterchangeable.Beforeconstruction,thetwosoilelementsareassumedtohavebeensubjectedtoslightlydifferentstresshistories,whichincludedone-dimensionalloading,unloading,andreloadingassociatedwithnaturaldeposition,erosion,andredepositionprocesses,respectively.Astheresultofthesenaturalloadingandunloadingprocesses,theinitialstressconditionsforele-mentsAandPmaybe,respectively,representedasA1ЈandP1Јintheeffective-stressspace,andA1andP1inthetotalstressspaceasshowninFigs.1(aandb),respectively.Thedifferencebetweenthetwocorrespondingstresspathsinthesetwofiguresisthepore-waterpressure.Whenatrenchisexcavatedforconstructionofadiaphragmwallinanoverconsolidatedstiffclay,asignificantreductioninlateralstresstakesplaceintheclay.Ifchangesinverticalstressareignoredandtheconstructionofthediaphragmwallisrapidsuchthatanundrainedassumptionisvalid,theeffec-tive-stressstatesforAandPwilltravelverticallytoA2ЈandP2Ј,respectively[Fig.1(a)].Ontheotherhand,thetotalstresspathsA1A2andP1P2willmovewithagradientof45Њtothehorizontalaxis,i.e.,⌬t/⌬s=Ϫ1[Fig.1(b)].Duringsubsequentexcavationstages,thestresschangesforelementPconsistofareductioninverticalstress,accompa-niedbyarelativelysmallreductioninhorizontalstress.ThetotalstresspathP2P3[Fig.1(b)]willhaveaslopeof45Њtothehorizontal(⌬t/⌬s=1)ifthereductioninhorizontalstresscanbeignored.Thecorrespondingeffective-stresspathP2ЈP3Ј[Fig.1(a)]willtravelverticallydownwardassumingthatthesoildeformsunderundrainedconditionsandpassivefailurehasnotbeenreached.Theexcessporepressuregenerateddur-ingexcavationisnegative.Inthelong-term,thetotalstressstateofP3willnotsignificantlyalteraftertheconstruction.Ontheotherhand,theeffectivestresswillreduceasaresultofdissipationofnegativeporepressure,sothattheeffective-stresspathwillmovetowardtheextensionKfline.ThefinalpositionofP4Јmayormaynotreachthefailureenvelopedependingonsoilshearstrengthandthemagnitudeoftheeffective-stresschanges.J. Geotech. Geoenviron. Eng. 1999.125:357-363.Downloaded from ascelibrary.org by Tongji University on 12/04/12. Copyright ASCE. For personal use only; all rights reserved.