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pile groups-bridge structure interaction in liquefiable ground

By Nicholas Graham,2014-02-18 10:33
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pile groups-bridge structure interaction in liquefiable groundin,pile,Pile

    pile groups-bridge structure interaction in

    liquefiable ground

    Vl01.9.N0.1EARTHQUAKEENGINEERINGANDENGINEERINGVIBRATIONMar

    ch.20l0

    EanhqEng&EngVib(201019:3950

    Shaketabletestof?…l?1sOil-pilegroups--bridge

    inliquefiableground

    DoI:10.1007/s11803009.8131.7

    structureinteraction

    TangLiang",LingXianzhang",XuPengju,GaoXiaandWangDongsheng{

    1.SchoolofCivilEngineering,HarbininstituteofTechnology,Harbin150090,China 2.ResearchCentreofRoadandBridgeEngineering,DalianMaritimeUniversity,Dalian116026,China

    Abstract:ThispaperdescribesashaketableteststudyODtheseismicresponseoflow

    cappilegroupsandabridge

    structureinliquefiableground.Thesoilprofile.containedina1arge

    scalelaminarshearbox.consistedofahorizontally

    saturatedsandlayeroverlaidwithasiltyclaylayer,withthesimulatedlow

    cappilegroupsembedded.Thecontainerwas

    excitedinthreeE1Centroearthquakeeventsofdifferent1evels.Testresultsindicatethatexcessiveporepressure(EPP)

    duringslightshakingonlyslightlyaccumulated.andtheaccumulationmainlyoccurredduringstrongshaking.TheEPPwas

    graduallyenhancedastheamplitudeanddurationoftheinputaccelerationincreased.Theaccelerationresponseofthesand

    wasremarkablyinfluencedbysoi1liquefaction.Assoil1iquefactionoccurred.thepeaksanddisplacementgraduallylagged

    behindtheinputacceleration;meanwhile.thesanddisplacementexhibitedanincreasingeffectonthebendingmomentofthe

    pile.andaccelerationresponsesofthepileandthesand1ayergraduallychangedfromdecreasingtoincreasinginthevertica1

    directionfromthebottomtothetop.Aiumpvariationofthebendingmomentonthepilewasobservednearthesoi?nterface

    inalIthreeinputearthquakeevents.Itisthoughtthattheshaketabletestscouldprovidethegroundworkforfurtherseismic

    performancestudiesof1owcappilegroupsusedinbridgeslocatedonliquefiablegroun. Keywords:liquefiableground;seismicsoil--pile--structureinteraction;pilegroupsofbridge;shaketabletest

    1Introduction

    Itiswellknownthatsoilliquefactionisoneofthe

    maincausesofearthquakeinduceddamagetobridge

    piles,andthesignificanceofthisdamagehasbeen

    clearlydemonstratedinmanyearthquakes.mostnotablv

    the1995Hyogo.kenNambuEa~hquakeinJapan.and

    the1976TangshanEarthquakeinChina.Although

    soilliquefactionmightreducetheinertialforceofthe

    superstructure,damagewasoftenattributedtofailure

    ofthepiles(Fuiiieta1..1998;FinnandFujita.2002).

    Therefore.itisimportanttoconsiderseismicsoilpile

    structureinteractioninthedesignofpilesparticularly

    inliquefiablegroundconditions.However.sinceless

    attentionhasbeenpaidtothebehaviorofthistypeof

    pilegroups.thereiscurrentlyonlylimitedinformation

    availableregardingtheseismicresponseoflowcap

    pilegroupsandbridgestructuresinliquefiableground

    (AbdounandDobry,2002;Boulangeretal1999).More

    C0rresp0ndencetO:TangLiang,SchoolofCivilEngineering,

    HarbinInstituteofTechnology,Harbin150090C'hina Tel:8645186l27799

    Email:hittl163.corn

    rPhDStudent;*Professor;?AssociateProfessor

    Supportedby:MaiorResearchPlanofNationalNatural ScienceFoundationofChinaUnderGrantNo.90815009; NationalNaturalScienceFoundationofChinaUnderGrant No.5037803land50l78027;WestemTransportConstruction TechnologyProiectsUnderGrantNo.2009318000100 ReceivedNovember9,2008;AcceptedMarch29,2009 recently,thesetypesofpileshavebeenwidelyadopted inbridgeengineering.especiallyforbridgesbuiltinsoil susceptibletoliquefaction.Thereisnotmuchphysica1 modelingdataavailabletounderstandthemechanisms ofsoillowcappilegroup.bridgestructureinteractionin 1iquefiedsoil.norarethereproceduresavailableforthe designofthistypeofpilegroup(Ohtsukieta1.,1998; Imamuraela1.,2004;LiandYuan,2004;Lingeta1.,2004). Ashaketabletestwasconductedin2006togain insightintothemechanismsofseismicsoil?-pile-- structureinteractionforlowcappilegroupsand

    understandthedynamicbehaviorofpilegroupsandthe bridgestructureinliquefiableground.Meanwhile,one ofthekeytasksofthistestwastogeneratereliabledata, whichcouldbeusedtoimproveanalysistechniquesand designguidelinesforlowcappilegroups.Inthispaper,

    theshaketabletestresultsarepresentedandanalyzed. 2Desciptionoftestfacilitiesandconditions 2.1Testfacility

    ThesketabletestwasperformedattheStateKey

    LaboratoryforDisasterReductioninCivilEngineering. TongiiUniversity,Shanghai,China,usingtheMTSshake tablefacility.Thetableisthreedimensionalandhassix

    degrees.of-freedommotion.Thedimensionofthetable is4mx41TI.andthemaximumpayloadis25,000kg. Theshaketablevibrateswithtwomaximumhorizontal EARTHQUAKEENGINEERINGANDENGINEERINGVIBRATIONVb1.9 accelerationsof1.2gand0.8g.andamaximum accelerationof0.7gvertically.Itsfrequencyranges from0.1to50Hzandthereare96channelsavailablefor dataacquisitionduringthetestingprogress. 2.2Laminarshearbox

    Alargescalelaminarshearboxwasdesigned

    tostudytheseismicpilegroup..soil..bridgestructure interaction(Wueta1..2002).Theboxis2mhigh.1.5m wideand2mlong(inthemajorshakingdirection);its movingpartsweighapproximately0.7t.Itistypically 5%oftheweightoftheentiretestmode1includingthe shearbox.Theshearboxwasdesignedtobesufficiently lightwhencomparedtothemode1.Therefore.theshear boxcanproduceanapproximateonedimensionalwave

    propagationfield.

    2.3Testsoil

    Thesoilprofilesusedinthetestconsistedof twohorizontalsolIIayers.Thetestsand.withanon

    uniformitycoefficientof3.0.meanparticlediameterof 0_32mm.densityof2.72maximumvoidratioof0.96, minimumvoidratioof0.57andmaximumdiameterof 2mm.wasobtainedinShanghai.China.Thelaminar shearboxwaspartiallyfilledwithwaterinadvance.

    andafterthat,theunwashedsandwasdirectlyplaced jntoit.usingafleefallmethodforthesaturatedsand layer(Lingeta1..2005)iethesandwas1iftedtoa

    pre.specifiedheightanddroppedintotheboxthrougha cone.Thedroppingheightwasdeterminedaccordingto therequiredrelativedensity.Specialattentionwasgiven sothatthesandlayerwasashomogeneousandlooseas possible.Theheightofthelowersandlayerwas1.6m withanaveragevoidratioof0.8.relativedensityof60% andpermeabilitycoemcientofabout0.0035cm/s.This methodforthesaturatedsandhasbeenusedsuccessfully inpreviousmodelstudies(Lingeta1..2004).The0.3 mthickupper1ayerwasanormallyconsolidated reconstitutedsiltyclayforthe"modelclay".Themodel clayhadliquidandplasticlimits0fLL45andPL28.and aplasticityindexofPI17.Inordertoensurethatthe sandlayerwascompletelysubmerged,thewatertable wasapproximatelyattheinterfacebetweentheupper claylayerandlowersaturatedsandlayer.

    2.4Modelstructure

    Alow?-capfour.pilegroupwithasinglepierwas

    usedinthetest.Thereinforcedconcretepiles,capand columnpierweremodeledwithfineaggregateconcrete

    andgalvanizedfineironwires.Allthebarsembedded inthemode1structurewerereplacedbygalvanized ironwirewithdifierentdiametersandhadthesamebar mark"".Theverticalreinforcements.whichconsisted Of92barsevenlydistributedaroundacirclewitha diameterOf70mm.wereembeddedineachpilewitha diameterof80mm.Theconfinementtotheverticalstee1

    wasprovidedby1barspiralswithapitchof20mm. especiallywithapitchof10mmwithintherangeof600 mmofthepilesnearthecap.A5mmconcretecoverof thepileswasmaintainedtoprotectthereinforcement bars.TheverticaIreinforcementoftheI60mmdiameter pierconsisted0f322barsevenlydistributedarounda circlewithadiameterofl50mm.andtheconfinement totheverticalsteelwasprovidedby1barspiralswith apitchof20mm.especiallywithapitch0f10mill withintherangeOf150mmofthepierbottom.A5mm concretecoverofthepierwasmaintained.Inaddition, thepilegroupswere1inedup2by2withaspacingof 3.75timesdiameterofthepile.Basedontheconcrete materialspecimentests,thefineaggregateconcrete

    hadanaverage28.daycompressivestrengthOf8.5 MPa.Consideringthefactthatsimplysupposedgirder bridgeshavebeenwidelyusedinbridgeengineering, thesuperstructureinthetestwasrepresentedbyan individualmassof360kgonthetopofthepierto characterizetheinertialefrectofthebridgestructure. 2.5Testprocedure

    Toensureaneffectivetransmissionofthetable motiontothebaseofthetestground.thesoilcontainer wasfirmlymountedontheshaketableusingbolt connections.Detailedconstructionoftheshaketable testincludingthelayoutofthetransducers.thegeometry andthepilegroupsystemareshowninFig.1.ThemodeI Sec

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Fig.1Schematicfigureofthetestsystem

    No.1TangLiangeta1.:Shaketabletestofsoilpilegroups

    bridgestructureinteractioninliquefiableground41 structurewasinstalledintothelaminarshearboxbefore theboxwasfilledwithtestsand.Thepiletipwaslocated inthesandatadepthof1700mm.Thearrangementof theinstrumentationwhichbasicallyconsistedof8pairs ofstraingaugesfY1toY8andY1+toY8+1attachedto thepileandatthepierbottom.2laccelerometersinthe soilandonthepile.and6porepressuretransducersinthe sandlayer,isshowninFig.1.Apairofstraingaugeswas fixedonthepilesatthesamedepth.Theaccelerometers inthesoillayerwereinstalledintheperspexboxes. wereabletofollowthemovementofthesoillayer simultaneouslyduringshaking.Alltransducersinthe soiweresealedforwaterproofing.Theeffectivenessof thisinstallationmethodwasconfirmedinpreliminary shaketableteststudiesconductedin2002.

    Itwassuggestedthatexcessiveearthquakeshaking conductedbeforeasoiIIiquefactiontestdetracts fromitssuccessfu1execution.Inordertoavoidthese disadvanta2esinthetests.thecontainerbasewasexcited bymicroamplitudewhitenoiseandthreeElCentro earthquakeevents.summarizedinTable1.Thesuite ofshakingeventsbeganwithvery1owleve1shaking

    tocharacterizethe1owstrainresponseofsoilandthe

    soilstructuresystem,andthensuccessivelyprogressed throughverystrongmotionstobringthepilesintothe nonlinearstageandtocausesoilliquefaction.Inorder toinvestigatetheeffectofthefrequencycomponent

    oftheinputaccelerationonsoilliquefaction.Event Bwasscaledbythetimescalingfactorof1/?10

    relativetoEventC.Thedominantfrequencyofthe inputaccelerationbetweenEventBandEventCwas apparentlydifferent.EventsCandDhadapeakamplitude of0.15gand0.5g.respectively,occurringatthesame time.Theinterva1betweentwoshakingeventswaslong enoughtodissipatetheEPPgeneratedinlastevent. Table1Suiteofshakingevents

    3Testresults

    3.1Naturalcharacteristics

    Figure2showssomepicturesofthetestmodeltaken afterthetestsetup.Thefundamentalfrequencyandthe dampingratioofthetestsystemareabcIut12.5Hzand 15.6%.respectively,obtainedfromthemicroamplitude

    whitenoisescanningtestperformedbeforetheseismic shakingruns.Thefrequencyoftheemptyboxwas1.4 Hzundersmallamplitudeinputacceleration.Theempty boxwasgivenaninitialhorizontaldisplacementof5cm andreleased,andthenthefrequencyandthedamping ratiowereabout1.2Hzand3.53%.respectively.Itis obviousthatthefundamentalfrequencyandthedamping ratiooftheemptyboxwerenotsimilartothetestsystem, sothatthedynamiceffectofthesoilboxontheseismic responseofthetestsystemwouldbeunremarkable. 3.2Macroscopicphenomena

    Thefollowingphenomenawereobservedfrom

    thistest.InEventB,wherethevibrationamplitudeof thepierwassmallwhilethevibrationfrequencywas relativelyhigh,waterspoutsonthegroundsurfacedid

    notoccur.InEventC,thevibrationamplitudeofthe superstructurewasstillvervsmallbutthevibration frequencydecreasedandthetopofthesandlayerwas partlyliquefied.InEventD.with0.5ginputacceleration significantliquefactionoccurredinthesandlayer,and sandboilsandwaterspoutsappearedduringshaking, asshowninFig.2.Manysandhillocksformedby sandboilsweredistributedonthegroundsurface.thesoiI subsidencerangedfrom3to5cm,andthesettlementof thepilegroupsrangedfrom10to17cm.althoughthe pilesdidnotfail.Anumberoftransversecracksonthe piles.mainlydistributedwithinthepilenearthecap wereobserved.andthepierwasstil1ingoodcondition. Thefirstcrackonthepilesappearednearthepileheads andsomecrackswereobservedatasolldepthofabout 120cm.whichindicatedthattheinertialforceofthe superstructureandseismicpile--soil--structureinteraction hadasignificanteffectontheseismicperformanceof thepilesinliquefiableground.itwasconcludedthat theshaketabletestadequatelyreproducedthesoil liquefactionandtheseismicresponseofthepilegroups andthebridgestructureinliquefiableground. 3.3Liquefactioncharacteristicsoftheground Timehistoriesofexcessporepressure(EPP)and theEPPratioarepresentedinFig.3toFig.5fora varietyofdepthsofthesandprofileineventsB.Cand D.ItappearsthattheEPPgraduallyincreasedwithsoi1 depthineventsB.CandD.Thisisbecausethevertica1 staticeffectivestressintheoverlaidsoilwasgreater, regardlessofthesizeofthedynamicshearstressof

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