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INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING This paper was downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. 655 EquivalentpiertheoryforpiledraftdesignÉquivalencedelathéoriedelajetéepourlaconceptionderadeauempiléBalakumarSimplexInfrastructuresLimited,Chennai,TamilNadu,India.HuangM. ArupGeotechnics,Brisbane,Australia. E.,BalasubramaniamA.S.GriffithUniversityGoldCoastCampus,Brisbane,Australia.ABSTRACT Theobjectiveofgeneratinganeconomicalandsafefoundationsystemreducingthesettlementratherthaneliminating hasledtothechangeinthedesignphilosophy recognisingthefactthatmoststructurescantolerateacertainamountofsettlement Thepiledraftanalysesanddesignisthreedimensionalinteractionproblems,wherein,theappliedloadistransferredbya complicatedinteractionprocessbetweenthepi lesandtheraft..Thenecessitytohavearelativelysimpledesignproceduresothatthe preliminarydesigncangiveadequatebutreasonablyaccuratedataforthefinalanalysesisexplained.Thepaperpresentssucha simpledesignprocessintheformof equivalentpierapproachbyestablishingitsapplicabilitybyapplyingittotwocases.Alsothe studyhasbroughtouttheeffectofdeepcompressibledepositsandwichedbetweentwodenselayers.RÉSUMÉ / ¶ R E M H F W L I G H F U p H U X Q V \ V W q P H G H I R Q G D W L R Q p F R Q R P L queetfiable,parlaréductiondesaffaissementsplutôtquedeleur éradication,aentraîné unchangementdanslaphilosophiedeconception,admettantlefaitquelaplupartdesstructuressontcapables G H W R O p U H U X Q F H U W D L Q Q R P E U H G ¶ D I I D L V V H P H Q W V / H V D nalysesetlaconceptionduradiersurpieuxcomportentdesproblèmes G ¶ L Q W H U D F W L R Q W U L G L P H Q V L R Q Q H O V R O D F K D U J H D S S O L T X p H H V W W U D Q V I p U p H S D U X Q S U R F H V V X V G ¶ L Q W H U D F W L R Q F R P S O H [ H H Q W U H O H V S L H X [ H W O H S L O L H U , O H V W H [ S O L T X p O D Q p F H V V L W p G H G L V S R V H U G ¶ X Q eprocéduredeconceptionrelativementsimple,demanièreàcequelaconception S U p O L P L Q D L U H S X L V V H D S S R U W H U G H V G R Q Q p H V D S S U R S U L p H V V X I I L V D P P H Q W S U p F L V H V S R X U O H V D Q D O \ V H V I L Q D O H V / ¶ D U W L F O H S U p V H Q W H X Q processusdeconceptionaussisimple,sousformed ¶ D S S U R F K H G H S L O L H U p T X L Y D O H Q W H Q S U R X Y D Q W V D I D L V D E L O L W p S D U V R Q D S S O L F D W L R Q G D Q V G H X [ F D V / ¶ p W X G H D p J D O H P H Q W U p Y p O p O H V H I I H W V G X G p S { W I R U W H P H Q W F R P S U H V V L E O H F R L Q F p H Q W U H G H X [ F R X F K H V p S D L V V H V EYWORDSPiledraft,Pier,PeatINTRODUCTIONThedesignoffoundationsystemforstructuresthatcannottoleratesettlements,theaspectofbalancingtheperformanceandcost,hadalwaysbeenachallengeforthefoundationdesigners.Duetothecomplexityinvolvedinthesoilstructureinteractionanalyses,requiredforanoptimumdesign,designershavesofarbeenresortingtothetraditionallydesignedpilefoundationssystempermittingverysmalllimitingsettlements.Eventhoughthisapproachproducesasafedesign,theeconomicsofthedesignbecomesquestionable.Theobjectiveofgeneratinganeconomicalandsafefoundat
ionsystemreducingthesettlementratherthaneliminatinghasledtothechangeinthedesignphilosophyKeepingtheaboveobjectiveinmindresearcherslikeBurland(1995)andsubsequentlyPolous(2001)hadbroughtouttheuseofpileswiththerafttoreducethesettlementoftheraft.Thishadledtheadventofthecombinedpiledraftfoundationsystem,whichprovidesaskilfulgeotechnicalconcepttodesignthefoundationforstructureswhicharesensitivetolargesettlements.Thepiledraftanalysesthreedimensionalinteractionproblems,wherein,theloadtransfermechanismisacomplicatedinteractionprocessbywhichtheloadissharedTheinteractiveprocessbetweentVariousproceduresbasedonobservationalstudy(Katzenbachetal.,2000a)smallscalemodelstudiessuchascentrifugemodelsHorikoshi1995)1gmodelstudies(Balakumar,2008)andtheresultinginteractiveprocesswiththenumericalmodeling(Clancy1993;Russo1998;supportedbythedevelopmentofnewgeotechnicalcomputationalfacilities(PolousandSmall2007hasledtothethepiledraftfoundationsystembeingextensivelyusedtosupporttallandheavilyloadedstructuresinasuccessfulmannerpermittinglargersettlementsclosetothepermissiblevalue(Polous,2008;.,Yamashitaetal.,2010).SIGNPROCESSThesatisfactoryperformancesofpiledraftlargelydependupontheperformanceofthepilegroupofpiledraftinprovidingtheinitialstiffnessandthenallowtherafttohaveahighercapacitybyfunctioningassettlementreducer.Henceafterascertainingthefeasibilityofthepiledrafttosupportthestructure,apreliminaryanalyseshastobedonetofinalizethecomputationaldetailoftheconstituentelements.rimarilythenumber,lengthofthepiles,theloadsharedbythepilegrouparetheessentialparametersinadditiontothepropertiesofthesupportingsoillayers.thecaseofthepiledraftthepilegroupcapacityandtheoverallcapacityofthepiledraftplayanimportantrole.Thesecondstageofanalyseshastoproducethesedatainareliablemannersuchthatwhenusedinthefinalanalyses,theanalyseswillproduceadesignwhichneednotbesubjectedtoanyiterationprocess.hisrequirementmakestheproceduretobemorerealisticandsimpleenoughsuchthatthecomputationaleffortsareminimumandeconomicalEventhoughtheexistingmethodscanprovideadesignapproach,theseinvolveaverydetailedcomputationalefforts,notreallywarrantedforthesecondstageofdesign,fromthecommercialdesignorganisationpointofview..Thereforeitisessentialtohavearelativelysimpledesignproceduresothatthesecondstageofworkcangiveadequatebutreasonablyaccuratedataforthefinalanalyses. 656 Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013 SELECTIONOFDESIGNPROCESSAmongthevariousmethodsstudieditwasconsideredthattheequivalentpierconceptwasfoundtobemoresuitable.TheapplicabilityoftheequivalentpiertheorytopledraftanalyseshasbeenestablishedbyHorikosh(1995)Butthestudywasrestrictedonlytoasmallpilegroupplacedinthecenteroftheraft,placedonaoverconsolidatedclaylayer.Althoughthestudyhasproducedveryimportantandusefuldata,theapplicabilityneedstobevalidatedwithotheravailableresultsfromageneralsoilprofile.Inthisparticularstudytheresultsoftwosuchcasesonefromtheobservationalstudyconductedonaninstrumentedpiledraftsupportinga12storeyedbuildingandtheotherfromtheparametricstudyconductedindependentlyarereanalyzedusingequivalentpierconcept.InthisparticularcasetheratioLe/Lnamelytheratioofthepierlengthtothepileworksouttounityandhencetheequivalentlengthofthepieristakentobethesameasthatofthepile.Oncethepilesarereplacedbyapierthenthesolutionforthesinglepilecanbeappliedtoestimatetheloadsettlementcharacteristics,andtheloadsharingresponse;theloadsharedbythepierbecomestheloadsharedbythepilegroup.WiththisidealisationitispossibletoruntheanalysessanaxisymmetrictwodimensionalproblemVALIDATIONInordertoestablishtheapplicabilityoftheequivalentpiertheorytwo
caseswereconsideredforwhichpublishedresultsareavailable.Themodelswereselected,onefromaparametricstudycarriedoutanalyticallyandtheothermodelwasfromanobservationalstudycarriedoutonthebehaviourofpiledraftsupportinga12storiedstructure.4.1ValidationbasedonnumericalstudyExtensiveparametricstudieshavebeencarriedoutinGriffithuniversityGoldCoastcampusandtheresultshadbeenpublishedbyOhetal.,().Thesestudieshadbeenbasedonthegeneralsoilprofilecompiledfromthenumberofgeotechnicalinvestigationdatacollected.A9pilegroup(3x3)with5dspacinghasbeenconsideredThespacingofthepilesconsideredis5d(ddiameterofthepile).Thed/tratioistakenasunityandaccordinglytheraftthicknessandthepilediameterhavebeentakenas800mm.Thegeneralsoilprofilecomprisesof13mthickmediumdensetodensesandlayer,followedby3mthickhighlycompressibleorganiclayertermedaspeat.Thislayerisfollowedbydensesandandhardclay.heEsvaluesofvariouslayershavebeentakenbasedontheNvaluesfromthestandardcorrelations.Theequivalentpiermodulusistakenfromtheexpression,+(E(1)WhereEeqistheequivalentpiermodulus,Esistheelasticmodulusofthesoil,Epistheelasticmodulusofthepile,Atistotalcrosssectionalareaofthepile,andAgistheplanareaofthepilegroup.ThepierconsideredalongwiththeparametersispresentedinFigure1.4.2TheobservationalstudyAsapartofanextensiveresearchprogramme,a12storeyedcommercialcumresidentialapartmentwasdesignedandsupportedonpiledraft(BalakumarandIlamparuthy)wasinstrumentedandmonitored.Thepiledraftsystemcomprisedof93pilesof600mmdiameterand14Mdeepfromthebottomoftheraft.Theraftthicknesswas600mmsothatthed/tratiowasmaintainedasunity.Thelayoutofpilesandotherpertinentdataaregiveninearlierpublications.Atwopilegroupswithatributaryraftdiameterof6mwasconvertedintoanequivalentpierandwasloadedinsmallincrementstillthesettlementreached100mm.Thepierwasrestinginamediumdensetodensesand.Thedetailsofthepier,andthegeotechnicalparameterstogetherarepresentedinFigure2.TheanalysesinboththecaseswerecarriedoutwithPlaxis2DthemodelandthemesharegivenFigure3.Figure1.Pier&geotechnicaldata(numericalstudy)Eforraft=2.74x10 4 MN/m 2 16.00m Peat V N 1 P=17kN/mEs=8MN/mDenseSand I =36º;=20kN/mEs=35MN/mStifftoHardclay V N 1 P=19kN/mEs=20MN/m22.00m 22.00m d/2 D/2=5.65DenseSand I=36º=20kN/mEs=30MN/m C/LofPIER 0.0 13.00m 0.00m 4.00m 11.00m 14.00m 650mm I PIER Sandysiltyclayc=0.2kg/cm=25º J =1.6t/m 3 ;E=50N/mm Clayeysiltysandc=0.1kg/cm=27º=1.7t/m;E=50N/mm=34º;=1.8t/m 3 E=60N/mm VerydensestrataE=70N/mm 600mm 3000mmFigure2.Observationalstudy Figure3.TypicalmeshPLAXIS2D 657 Technical Committee 103 / RESULTS,ANALYSES,ANDDISCUSSION.Theresultsobtainedfromthemodel1areplottedintheformofloadsettlementresponsecurvesandpresentedinFigure4.Theloadsettlementresponseof12mpierispresentedseparatelyinfigure5.orthepierlengthsof12m,16.8m,and18mmAtanystageofsettlement,itwasfoundthattheloadtakenbythepiledraftwasfarhigherthantheloadtakenbytheunpiledraftforthecorrespondingsettlement.Theresultsarestudiedindependentlyforthethreecasesanalysed,andthentheyarecompared.Fromtheloadsettlementresponseoftheunpiledraftandthepiledraft,theloadsharedbythepier(pilegroup)iscomputedatdifferentsettlementlevelsnamely12mm,25mm,50mm,80mmand100mmandhasbeenpresentedinTable1.Table1LoadSharingRatioAtVariousSettlementLevelsSettlement Pierlength20mm50mm100mm 12.0m0.540.530.370.300.25 16.8m0.640.650.600.580.56 18.0m0.610.560.640.590.59 Inthecaseofloadsettlementresponseofallthethreecases,theinitialstagesuptoasettlementlevelof25mm,thepiledraftexhibitsahigherstiffness,withtheverysmallrateofchangeinthestiffness.Asseenfromthetable1,theloadss
haredbythepierintheinitialstagesarehigherandthengraduallyreduceswithsettlement.Thisindicatesthatthemajorpartoftheappliedloadistakenbythepilegrouporthepier.Beyondthisleveltherateoffallinthestiffnessincreasesrapidlyindicatingthatthefullfrictionhasbeenmobilisedandtheraftstartstakingahigherload.Thisstageexistsuptoasettlementlevelof75mm.Beyondthisleveltherateoffallofstiffnessfurtherincreasesrapidlyevenforasmallincrementintheload.Inthecaseofpiledraftwith16.8mdeeppiertheloadcorrespondingto25mmsettlementishigherthanthepreviouscaseby100%indicatingthatthepiermobilisesahigherfrictioninthelinearelasticstage.At75mmsettlementleveltheincreaseintheloadtakenbythe16.8mdeeppierishigherby60%,indicatingthattheloadsharedbythepierreducesgradually.andinthecaseof18mdeeppierthisincreaseisonly15%whencomparedto16.8mdeeppier.EFFECTOFPEATLAYERThestudyoftheTable1andtheFigurewhichpresentstheshaftstressdistributionwiththedepthindicatesthattheloadsharingratioandtheshaftstressindicateanincreaseandthenafall.Theshaftstressincreasecommencesatalevelof13mandextendsupto16mlevel;andthenitreduces.Inthecaseofloadsharingratiotheincreasetakesplaceatasettlementlevelof20mminthecaseof16.8mdeeppierand50mmlevelinthecaseof18mdeeppier.Thistrendisabsentinthecaseof12mdeeppierwhichisabovethepeatlayer.Themostprobablereasonforthisbehavioristhatatahigherloadthepeatlayergeneratesanegativeskinfrictioncausingahigherloadonthepilegroup.Thisresultsinthesuddenincreaseintheshaftstressandtheloadsharingratiovalue.Figurepresentsthemobilisationofshaftfrictionwithdepth.Itisseenthattheshaftfrictionincreasesandthenfallsdownrapidlywithdepthconfirmingtheductilebehaviourinthesensethatmajorpartoftheloadistransferredbyfriction.HYPERBOLICBEHAVIOURThecurverelatingtothe12mdeeppierexhibitsinadistinctmannerathreephasebehaviour;namelyOA,whichisalinearelasticstageABaviscoplasticstageandBCtheplasticstage.Intheothertwocasesthethirdstageishasnotreachedmainlybecausethepiledwasstillcapableoftakinghigherload.Typicallythepiledraftwith12mpierdepthhadexhibitedahyperbolicbehaviourandithasbeenloadedclosetofailure Figure4Loadsettlementresponse Figureoadsettlementresponse12mpierFigure6Shaftstressmobilisation(18mdeeppile) Figure 7 & K L Q ¶ V J U D S K P S L H U 658 Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013 hasexhibitedahyperbolicbehaviour.IthasbeenestablishedthatthehyperbolicbehaviourcanbeexpressedintermsofChin-Kondtnertypefunctions,whentheinverseofthestiffnessisplottedagainstsettlement,alinearplotcanbeobtained.Inthatcasetheinverseoftheslopegivestheasymptoticultimatecapacityofthepier.Accordinglyasatypicalcasetheloadsettlementresponseofthepiledraftwith12mpierwasplotted D V & K L Q ¶ V J U D S K FigureTheasymptoticultimatecapacitywasfoundtobeoftheorderof750kN,indicatingthattheasymptoticultimatecapacityisthreetimesthecapacityattheelasticlimitand1.5timestheloadcorrespondingtotheelastoplasticstage.Thiswouldmeanthatthecapacityhastobelimitedtotheloadcorrespondingtotheelastoplasticstage.Thereforethelimitingcapacityofthepiledraftcanbethecapacityatasettlementlevelof10%ofthepilediameterwhenthepilecanbeseatedinthenoncompressiblelayerandwhenthepilehastopassthroughacompressiblelayer,andthenthenegativefrictionhastobeaccountedfor.OBSERVATIONALSTUDYMODELFigurepresentstheloadsettlementresponseofthepierrepresentingthepilegroupandtheraftwhichformsapartofthepiledraftsupportingthestructure.Hereitisseenthattheelasticstageisseenuptoaloadlevelof200kNpersq.m.Howeverintheanalysesthepressurewasappliedcontinuouslywhereasduringconstructiontheloadwasappliedingradualmanneroveraperiodoftime.Howeverthesettlementshownbt
heequivalentpieranalysesis12mmasagainsttheobservedvalueof14mmreportedinthereferredpublication,Theloadsharingbehaviourandtheshaftstressmobilisationexhibitedasimilartrendasinthecaseofearliermodelandasobservedintheobservationalstudy.Inthiscasetheloadlevelwasfoundtobewellwithintheelasticlimits.Thesettlementobservedfromtheobservationalstudywas14mmandfromthepieranalysesthesettlementobtainedwas12mmindicatingaverycloseagreementindicatingthattheequivalentpierconceptcanfilltheneedofasimpledesignprocedure.CONCLUSIONTheextensivestudycarriedoutonthetwoindependentcasesadoptingtheequivalentpiertheoryhasestablishedthattheequivalentpiertheory,althoughinvolvesnumericalapproximation,theperformanceofthepiledraftinboththecasespredictedbyequivalentpiertheoryisinconformitywiththeearliarworksbyOhetal.,(2008)andBalakumar(2008)Thestudyhasfurtherpointedoutthatinthecaseofpiledraftswiththepilegrouppassingthroughseamsofcompressiblelayerthebehaviourisaffectedbythemobilisationofnegativeskinfrictionandtheequivalentpierconceptisabletopredictthiseffectively.Thecompressiblelayergeneratesnegativeskinfrictionandincreasestheloadonthepileasshownbytheincreaseintheloadsharingratioandthenallowsthepilegroupbehaviourtobeductile.Inshorttheequivalentpiertheoryisanidealtheoryforthepiledraftanalyses.REFERENCESClancyP.(1993),NumericalAnalysisofPiledRaftFoundations,UniversityofWesternAustralia,PhDThesis. µ . D W ] H Q E D F K 5 $ U V O D Q 9 D Q G 0 R R U P D Q F K D µ 1 X P H U L F D O StimulationsofCombinedPiledRaftFoundationsfortheNewHighRiseBuilding,Max L Q ) U D Q N I X U W D P P D L Q ¶ 3 U R F Q G , Q W Conf.onSoilStructureInteractioninUrbanCivilEngineering. 3 R X O R V + * µ 3 L O H G 5 D I W ) R X Q G D W L R Q ' H V L J Q D Q G $ S S O L F D W L R Q ¶ * H R W H F K Q L T X H 9 R O 1 R S S 113 + R U L N R V K L . µ 2 S W L P X P ' H V L J Q R I 3 L O H G 5 D I W ) R X Q G D W L R Q V ¶ DissertationsubmittedforthedegreeofDoctorofPhilosophy,UniversityofWesternAustralia.E.Y.NOH,M.Huang,C.Surarak,R.Adamecand $ 6 % D O D V X E U D P D Q L D P µ ) L Q L W H ( O H P H Q W 0 R G H O L Q J I R U 3 L O H G RaftFoundationi Q 6 D Q G ¶ ( O H Y H Q W K ( D V W $ V L D PacificConferenceonStructuralEngineering&Construction(EASEC11). 3 R O R X V + * ³ 7 K H 3 L O H G 5 D I W ) R X Q G D W L R Q I R U W K H % X U M ' X E D L ' H V L J Q 3 H U I R U P D Q F H ´ , * 6 FerrocoTerzaghi,Oration2008.Balakumar.V(2008), ³ ( [ S H U L P H Q W D O 6 W X G L H V R I 0 R G H O 3 L O H G 5 D I W R Q 6 D Q G D Q G ) L H O G 6 W X G \ R I 3 U R W R W \ S H % H K D Y L R U ´ 3 K ' 7 K H V L V $ Q Q D University,Chennai. % X U O D Q G - % µ 3 L O H V D V 6 H W W O H P H Q W 5 H G X F H U ¶ W K , W D O L D Q & R Q J U H V V onSoilMech.,Pavia. 5 X V V R * µ 1 X P H U L F D O $ Q D O \ V H V R I 3 L O H G 5 D I W V ¶ , Q W O - Q O 1 X P andAnl.MethodsinGeomech,Vol.22,pp.477493. 6 P D O O - D Q G 3 R X O R V + * ³ $ 0 H W K R G R I $ Q D O \ V L V R I 3 L O H G 5 D I W ´ 10thAustraliaNewzelandConferenceonGeoMechanics,PP.555.Yamashita.K,Hamada.J,Yamada.T.(2010),FieldMeasurementsOnPiledRaftswithGridFormDeepMixingWallsonSoftGround,GeaotechnicalEngineeringSEAGSVol.42No2June2011 Figureoadsettlementresponse