FP-CH328

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HarmonisingRockEngineeringandtheEnvironment–Qian&Zhou(eds)©2012Taylor&FrancisGroup,London,ISBN978-0-415-80444-8

3DECmodellingofp-ybehaviouroflaterallyloadedpilesinjointedrockW.L.Chong,A.Haque&P.G.RanjithDepartmentofCivilEngineering,MonashUniversity,Victoria,Australia

A.ShahinuzzamanPowerlink,Queensland,Australia

ABSTRACT:Rockmassisahighlydiscontinuousmediumwheresecondarystructuressuchasjointsareofcommonexistence.Forlaterallyloadedrocksocketedpiles,jointcharacteristicssuchasjointset,spacinganddipanglemayeachplayanimportantpartindeterminingthep-ybehaviourandthecorrespondingload-deflectionresponseofthepile-rocksystem.Inthisstudy,extensivenumericalmodelingwasundertakenforpilessocketedintojointedmudstoneusingathree-dimensionaldistinctelementcomputerpackage,3DEC.Thenumericalmodelwasinitiallycalibratedagainstlaboratorymodelpiletests.Thecalibratedmodelwassubsequentlyextendedtospecificallystudytheeffectofdifferentjointdipangles(30◦,45◦and60◦)onthep-yandthepileheadload-deflectionbehaviourofa1-jointsetmodel.Inthisstudy,itwasfoundthatjointdipanglehaslittleimpactonthep-yandthepileheadload-deflectionbehaviourfora1-jointsetmodel.Thelinearresponseofthepileremainedsimilarforthedipanglessimulateduptoapiledeflectionof10%ofpilediameter.Beyondthis,asteeperdipangledemonstratedhigherloadcarryingcapacity.

Subject:ModellingandnumericalmethodsKeywords:Numericalmodelling,rockjoints,rockslopesandfoundations,weakrock

1INTRODUCTIONPilefoundationsareoftenemployedtoprovidesupportforcivilengineeringinfrastructuressuchasbridges,offshoreplat-forms,windturbinesandtransmissiontowerstoresistlateralloads.Fortheanalysisoflaterallyloadedpiles,thep-ymethodalsoknownassubgradereactionapproachiswidelyusedwherepisthenetsoil/rockresistanceperunitpilelengthandyisthecorrespondingpiledeflection.Atypicalp-ycurveisshowninFigure1wheretheinitiallinearslopeisrep-resentedbyKi,initialsubgradereactionmodulus,followed

byanon-linearsegmentleadingtotheultimateresistanceofrock,pu.Thep-ycurvederivedforaspecificrockconditionisusuallyusedinconjunctionwithnumericalsoftwaresuchasPYGMY(Stewart,2010)togeneratethecorrespondingpileheadload-deflectionresponse.Extensiveresearchhasbeenconductedandwellestablishedp-ymethodsbasedoncontinuumapproachhavebeendevel-opedforsoils(Matlock,1970;Reeseetal.,1975).Forlaterallyloadedrocksocketedpiles,theexistingp-ymethodshaveextendedthecontinuumtheorybyadoptingrockclassificationsystemssuchasRockMassRating(RMR)(Bieniawski,1976)orGeologicalStrengthIndex(GSI)(Hoeketal.,1988)tosimplifytherockmasstoahomogeneouscondition(Carteretal.,1992;Zhangetal.,2000;Gabretal.,2002;Yang,2006).However,rockinrealityisahighlydiscontinuousmediumwheresecondarystructuressuchasfractures,jointsandthepresenceofinfillmaterialarecommonoccurrence.Theassumptionofhomogenousandcontinuumrockconditionhasledtosignificantinaccuracyobtainedwhentheanalyticalmethodswereappliedonotherrocktypesorgroundconditions(Choetal,2001;Vu,2006).Chongetal.(2008)carriedoutabroadreviewoftheexist-ingmethodsfordesignoflaterallyloadedrocksocketedpiles,

Figure1.Typicalrepresentationofap-ycurve.andconcludedthatthesemethodsdonotconsiderthephysi-caleffectofsecondarystructuresinrockmasssuchasjointsetordipangleonthep-yandthepileheadload-deflectionresponses.Withsuchlimitedknowledgeinthisarea,currentdesignmethodscaneitherproduceconservativeandexpensivesolutionsorsometimesunsafedesigns.Therefore,itisevidentthatfurtherresearchisrequiredtoprovidebetterunderstand-ingofthecomplexp-yandthepileheadload-deflectionbehaviouroflaterallyloadedpilessocketedintojointedrockmass.Inthisstudy,athree-dimensionaldistinctelementcode,3DECisemployedtoinvestigatenumericallythecomplexinteractionbetweenthejointdipangleandthep-yresponse,aswellasthepileheadload-deflectionbehaviouroflaterallyloadedrocksocketedpile.Theinitialnumericalmodelwasfirstcalibratedagainsttwolaboratorymodelpiletestscon-ductedbyFrancis(2003)onlateralpilessocketedintojointedmudstone.Thecalibratedmodelissubsequentlyextendedto

1881Figure2.Dimensionsandspecificationsof3DECmodel.incorporatea1-jointsetmodelhavingadipangle45◦andthep-ycurveforthismodelisthenderived.Thenormalisedp-ycurvederivedfromthelaboratoryscalemodelshowsgoodsimilaritywhencomparedwiththep-ycurveobtainedfromfieldloadtests(Chongetal.,2010).Withthisvalidation,dif-ferentjointdipangles(30◦and60◦)aresimulatedandits

effectsonthep-yandthepileheadload-deflectionresponsesarecarriedout.

2NUMERICALMODELLING3DEC(2008)wasemployedinthisstudytosimulatethelaboratorybehaviouroflaterallyloadedpilesocketedintojointedmudstone.Figure2showsthedimensionsandspecificationsofthelaboratoryexperiments(Francis,2003)simulatedinthenumericalmodel.Ablocksizeof575mm×575mm×415mm,asperrecommendationbyWallace(2002),wasmodeledwithapilediameterof25mmandasocketlengthof110mm.Table1showstheinputmate-rialpropertiesforthenumericalsimulation.ThepilewassimulatedasanelasticmaterialwhileMohr-Coulombconsti-tutivemodelwasassignedtotherock.Thepile-rockinterfaceandjointsweresimulatedusingCoulombslipmodel.Chongetal.(2009)presentedadetaileddescriptionofthenumericalmodelanditsabilitytocapturethepileheadload-deflectionbehaviourofthetwolaboratorymodelpiletestsquiteclosely.Withthis,themodelwasextendedtostudytheeffectofjointdipanglesina1-jointsetmodel.