单向固结试验操作规程OEDOMETER-TEST-HANDOUT

TheOedometerTestObjectives1.Todeterminetheone-dimensionalcompressionresponseofaclayeysoil.2.Toinvestigatethetime-dependentbehaviourofthesoilduringone-dimensionalconsolidation.IntroductionTheoedometertestiswidelyusedinpracticetoprovidetheparametersthatarerequiredtoestimatethesettlement,andrateofsettlement,offoundationsonsoftsoils.Intheoedometerlateralmovementofthesoilispreventedbycontainingthesoilinastiffmetalringsothatonlyvertical(one-dimensional)soilmovementsarepossible.Drainageisonlypermittedfromthehorizontalfacessothatthewaterintheporesisalsoconstrainedtoflowvertically.Whenanincrementofstressisappliedtotheoedometertestspecimenthereisaninstantaneousincreaseinporepressureequaltothestressincrement.Asdrainageoccurstheporepressuresdecreaseandtheeffectivestressesincrease,untilwhendrainageiscompletethechangeineffectivestressiseverywhereequaltotheappliedtotalstressincrement.Duringthestagewhenwaterisdrainingfromthesoilthevariationofporepressurewithtime,andheight(z)isgivenbytheconsolidationequation:22zuczuvwherecvisthecoefficientofconsolidation.Solutionstotheconsolidationequationforthecaseof1-waydrainageinanoedometertestareshowninFigure1.Thecurvesarereferredtoasisochrones,andgivethevariationofporepressure(andhenceeffectivestress)withtimeatdifferentdepths.ThecurvesaregivenintermsofadimensionlesstimefactorTvwhere:2HtcTvvandHisthemaximumdrainagepathforthesoillayer.Therelationbetweenthesettlement(afterconsolidationiscomplete)andtheeffectivestressdefinesthecompressionresponseofthesoil.Itisconventionaltopresentthecompressionresponseinaplotofvoidratioagainstthelogarithmofthever1icaleffectivestress(seeFigure2).Thesoilcompressibilityisdescribedbythestiffnessparametermvwhichisgivenbytheratiooftheverticalstrain(settlement/originalsamplethickness)totheeffectivestresschange.hhmvFigure1:IsochronesshowingthevariationofporepressurewithtimefactorTvProcedure1.SmalloedometerAlightlyover-consolidatedsampleofclayhasbeenplacedinasmalloedometerandallowedtoreachequilibriumunderasmallhangerload.Aseriesofloadincrementshavebeenappliedandthedialgaugereadinghasbeenrecordedatthestartandafterequilibriumhasbeenreachedforeachincrement.Applythenextloadincrementandrecordthechangingdialgaugereadingwithtime.Takereadingsattimesof10s,20s,30s,1,1.5,2,3,4,6,9,12,16and25minutes.Afterbothgroupshaveaddedaloadincrement,theloadwillthenberemovedintwodecrementsforwhichitwillbenecessarytorecordonlythefinaldialgaugereadings.Afterthefinaldecrementremovetheloadandhangerandmeasurethefinalheightandweightofthesample.Thenplacethesampleintheoventodeterminethefinalmoisturecontent.2.HydraulicoedometerTheprocedureissimilartothesmalloedometer.However,beforeaddingthenextload(pressure)incrementyoushouldrecordtheappliedverticalstress,theporepressureinthesoil,andthevolumegaugereading.Thenclosethetaptopreventdrainageandapplya50kPatotalstressincrease.Observetheporepressurechange.Thenopenthedrainagetapandtakereadingsfromthevolumegauge,dialgauge,andporepressureatthesametimesasforthesmalloedometergivenabove.LaboratoryRequirementTocompletethelaboratoryexerciseallstudentsnotrequiredtowritethereportmustproducethegraphsforpoints1and3belowandshowthesetothelaboratorydemonstratoratthenextlaboratorysession.ReportYourreportshouldinclude:1.Aplotofvoidratio(e)versuslog'forthetestperformedinthesmalloedometer,andthevaluesofCcandCrforthissoil.Thefinalvoidratiocanbedeterminedfromthefinalmoisturecontent,assumingthesoilissaturated,using.ef=wfGs,whereGsisthespecificgravity(2.6forthissoil),andwfisthefinalmoisturecontent.Alternativelythefinalvoidratiocanbecalculatedfromthedrymassandthefinalvolume(height×area).TocalculatethevoidratioatotherstagesofthetestwemakeuseoftherelationbetweenchangeofvolumeandchangeofvoidratioeeVV1andbecausethecross-sectionalareaisconstanth/h=V/Vwecanwrite)1(ehhe2.Estimatethepre-consolidation’vcfromthee,log’plotusingCassagrande'smethod.Theprocedure,illustratedinFigure2,istodeterminethepoint(A)ofmaximumcurvatureonthee,log'plot,drawahorizontalline(AB)andtangenttothecurve(AD)throughA,anddrawalinebisectingtheselinesAC.ThevalueoftheeffectivestresswherethebisectorlineACmeetstheextendedportionofthestraightlinepartofthee,log'plot(P)givesthepre-consolidationpressure.Figure2:Procedurefordeterminingthepre-consolidationpressure3.Determinethecoefficientofconsolidationcvbythesquareroottimemethod.PlotthedialgaugereadingsagainstthesquarerootoftimeasiIIustratedinFigure3.Todeterminecvproceedasfollows:(i)Obtainthetruezeroreadingbyextrapolatingtheinitialstraightlinetozerotime(Thetheorypredictsaninitiallinearresponse).Thedifferencebetweenthiszeroandtheinitialzeroreadingrepresentscompressioninpartsoftheapparatus,andshouldbeallowedforwhencalculatingtheheightchangesofthesample.(ii)Constructalinefromthecorrectedzeropointwheretheabscissaeofthelineare1.15timestheabscissaeofthestraightlineapproximationoftheexperimentalcurve.Theintersectionofthislinewiththeconsolidationcurvegivesthetimecorrespondingto90%primaryconsolidation.VerticalStressVoidRatioe(logscale)PAEvmBDFigure