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Vane

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Vane

    Vane

TransactionsofTianjinUniversity

    ;Vo1.10No.3Sep.2004

    ;VaneShearStrengthBasedabilAnalysisofSlopes

    ;LIURun刘涓,YAN8hu-wang闫澍旺,Z/-/ANGL/an-flJ张莲福2

    ;(1.SchoolofCivilEngineering,TianjinUniversity,Tianjin300072,China; ;2.TianjinHarborCokePortCompany,Tianjin3130452,China) ;Abstract:/nsituvanesheartestis~equenflyperformedtodetermineshearstrengthforslopestabilityan

    alysis

    ;inTianjinNewHarbor.However,thesoilshearstrengthvarieswiththeshearplaneorientation.Apossible

    means

    ;toreducetheeffectofdirectionaldependencyofshearstrengthistoconverttheinsituvaneshearstrengthinto

    ;undrainedshearstrengthparameters.Amethodofconvertinginsituvaneshearstrengthintoundrainedshear

    ;strengthparametersispresented.Theshearstrengthparametersdeterminedforalloftheinsituvanesh

    ear

    ;strengthsaresubjectedtostatisticalregressionanalysistotakeintoconsiderationthepossibleeffectofno

    n’?horn’?

    ;ogeneityinthesoftclaydeposit.Usingtheregressedshearstrengthparameters,slopestabilityanalysesar

    eper-

    ;formedforfiveexistingsoilslopes.Theresultsofstabilityanalysesindicatethatthesafetyfactorsobtaine

    dfrom

    ;theconvertedparametersreflectthestateoftheslopesanalyzedmuchbetterthanthoseobtainedfromin

    situ

    ;vaneshearstrengthandlaboratoryconsolidatedundrainedandunconsolidatedundrainedstrength

    parameters.It

    ;isconcludedthatthepresentedmethsodofdeterminingundrainedshearstrengthparametersforinsitu

    vane

    ;shearstrengthiseffective.

    ;Keywords:softclay;vaneshear;undrainedstrength;slopestability ;ArticleID:1006.4982(2004)03-0195.06

    ;InTianjinNewHarborarea,theuppersoillayerof

    ;20mconsistsofsoftclay.Thesoftlayerisasedimentary ;depositofrecentgeologicalagesandconsistsofveryfine ;clayparticles.Thissoillayerisunconsolidated;therefore, ;itishighlycompressiblewithlargevoidratioandlowshear ;strength.Meanwhile,thesoilishighlysensitivewitha ;sensitivityof3—4….SinceTianjinNewHarboriscon—

    ;structedonsuchasoil.thestabilityofslopesinportcon

;structionhasbeenofagreatconcern.

    ;Thehighlysensitivenatureoftheunc0ns0lidatedsoft ;depositintheTianjinNewHarborareamakesitdicultto

    ;obtainundisturbedsoilsamplesforlaboratorytesting.Soil ;sampling,handling,transportation,andtestingwillinevi

    ;tablycausedisturbanceandreducetheshearstrengthprop

    ;ertyofthesoil.Usingthesestrengthdataintheanalysis ;anddesignofsoilslopesoftenresultsinasafetyfactorun

    ;realisticallylowerthanunityevenforstableslopes. ;Toavoidpossiblesampledisturbance,theinsituvane

    ;sheartestmethod(seeFig.1)hasbeencommonlyadopted ;todeterminetheshearstrengthpropertyfordesignanda

    ;nalysisofsoilslopesinTianjinNewHarbor.However,e

    ;venwiththeuseofinsituvaneshearstrength,thesafety ;factorsofstableslopesareoftenunderestimated,sugges

    ;ringthattheinsituvaneshearstrengthmaynotrepresent ;theshearstrengthmobilizedalongtheanalyzedslidingsur

    ;faces.Thisispossible,becausethesoilshearstrengthva

    ;deswiththeshearplaneorientation;namely,theshear ;strengthisdirectiondependent.Anotherfactordeserving ;(b)Soilcolumnto

    ;resistthetorque

    ;Fig.1Keyschemeforvanesheartest

    ;Accepteddate:2003ll-28.

    ;LIURun.bornin1974,female,Dr,associateProf. ;E-mail:liurun74@yahoo.coll1.cn

    ;

    ;TransactionsofTianjinUniversityVo1.10No.32004 ;attentionisthatthevaneshearstrengthsaremeasuredfromnessoftheapproachisdiscussedindetails.

    ;verticaldirectionofslopeslidingsurface.Asaconse

    ;quence,thesafetyfactorsdeterminedfromvaneshear

    ;ii

    ;strengthsmayn.trealisticallyreflectthestability.ftheae一专

    ;tualslope.

    ;Ithasbeenrealizedthatthesafetyfactorscalculated ;withthevaneshearstrengthgreatlydependontheplastici

    ;tyindex,,P.Leroueiletal(1990)compileddataon ;cutfailureswhichoccurredduringexcavationorshortlyaf- ;terexcavationandplottedthefactorofsafetycalculatedon ;thebasisofundrainedshearstrengthmeasuredwiththe ;uncorrectedfieldvanedataortheundrainedtriaxialtests, ;asafunctionoftheplasticityindex,,P(seeFig.2).For ;soilinTianjinNewHarbor,theplasticityindex,,P=22. ;FromFig.2itcanbefoundthatthefactorsofsafetyof

    ;failedslopes,whencalculatedwithvaneshearstrength, ;shouldbecloseto1.0.

    ;Apossiblemeanstoreducetheeffectofdirectional ;dependencyofshearstrengthonsafetyfactoragainstslope ;failureistoconvertthein?-situvaneshearstrengthintoun-- ;drainedshearstrengthparameters(cand)withduecon

    ;solidationsoftheeffeetofdirectionaswellasstressde

    ;pendencyonshearresistance.Suchanapproachispresen

    ;tedinthispaper.Meanwhile,theresultsofslopestability ;analysisforsomeexistingslopesbyusingtheconvertedun

    ;drainedshearstrengthparametersarecomparedwiththose ;obtainedbyusingthevaneshearstrength.Theeffective

    ;3.2

    ;2.8

    ;2.4

    ;2.0

    ;1.6

    ;1.2

    ;l,0

    ;0.8

    ;0.4

    ;-Triaxialtest>0.4,L<0.2

    ;?Vanetest,L>0.4

    ;failre

    ;u

    ;s

    ;t:一一

    ;020406080l00l20l40

    ;Plasticityindex,P

    ;Fig.2Factorsofsafetyobtainedbytotalstressanalysis ;ofshort-termfailuresversusplasticityindex ;1Soilprofileandproperty

    ;1.1Soilprofile

    ;ThesoilprofileatthesiteofTianjinNewHarboris ;describedinTab.1.Itisseenthatthegroundcontainsa1 ;mthickhardcrustontop,followedsuccessivelybysoft ;clay,siltyclay,clayeysilt,anddensesand.Thesoftclay ;stratum,nearly18mthick,consistsofvaryingthinlayers ;oforganicmatters,siltysandandsandysoilpockets.This ;stratum,formedbynaturalsedimentationtogetherwithhy

    ;draulicfillingapproximately2yearsbeforeportconstruc

    ;Tab.1Softstratumdescriptions

    ;tion,isstillunderconsolidation,i.e.inune0ns0lidated ;state,accordingtovariousinvestigations.Therefore,the

;soilinthisstratumisinliquidtoplasticstateofconsisten

    ;cy,andishighlysensitiveandcompressiblewithlow ;

    ;l96

    ;strength..

    ;Thephysicalandmechanicalpropertiesofthesoilsin ;differentlayersarepresentedinTab.2andTab.3,respec

    ;tively.ThemechanicalpropertiesinTab.3weredeter- ;Bs’10J0l.n.u

    ;

    ;LIURunetal:VaneShearStrengthBasedStabilityAnalysisofSlopesinUnconsolidatedSoftClay

    ;minedinthelaboratoryonundisturbedsamplesusingdif-compressiontest,unconsolidatedundrained(UU)triaxial

    ;ferenttestmethodsincludingconsolidatedundrained(CU)compressiontest,andunconsolidatedu

    nconfinedcompres

    ;directsheartest,consolidatedundrained(CU)triaxialsiontest. ;Tab.2Physicalpropertiesofsoils

    ;Tab.3Mechanicalpropertiesofsoils

    ;Inadditiontothelaboratorytest,thousandsofinsituSh=ztan+C(2)

    ;vanesheartestswereperformedatvariouslocationsofdif-where ;ferentdepths.Fig.3presentssomeoftheinsituvane

    ;shearstrengthvariationwithdepthtogetherwiththedatafaces ;obtainedfromthelaboratoryCUandUUtriaxialcompres

    ;siontests.Itisseenthatinsituvaneshearstrengthsare

    ;bracketedbetweentheCUandUUtriaxialshearstrengths ;ThegreaterCUstrengththanUUandvaneshearstrength ;isasexpected.AnobviousreasonforlowertriaxialUU ;strengththanvaneshearstrengthistheeffectofsample ;disturbanceasmentionedearlier.

    ;1.2Undrainedshearstrengthparameters

    ;Althoughthesoftclaystratumisstillunderconsolida

    ;tion,independentstudieshaveshownthatthedegreeof ;consolidationhasreachedatleast85%.Underthiscondi

    ;tion,theinternalfrictionshouldbeasignificantcomponent ;ofshearingresistancewhencomparedwiththecohesion ;component(seeFig.4).Thefrictionalresistancedepends ;ontheeffectivenormalstressactingontheshearplane. ;Sincetheeffectivenormalstressvarieswithplanedirection ;aswellasdepth,theshearingresistanceonthecylindrical ;verticalsurfaceandhorizontalshearsurfaceinthevane ;sheartestwillbedifferentasshownbelow:

    ;S=UKoTztan+C(1)

    ;S,Shshearstrengthsonverticalandhorizontalsur- ;,

;respectively;

    ;U--degreeofconsolidation;

    ;--

    ;coefficientofearthpressureatrest; ;--

    ;submergedunitweightofsoil;

    ;z——depthbelowgroundsurface;

    ;~--intemalfrictionangleofsoil;

    ;c--cohesionofsoil. ;Atfailure,thelistingequilibriumequationcanbeex ;pressedasfollows,assumingauniformshearstressdistil

    ;but

    ;\

    ;

    ;

    ;

    ;E

    ;\

    ;

    ;

    ;

    ;Shearstrength/kPa ;(a)BoreholeNo:C+832

    ;O204060

    ;Shearstrength/kPa ;(e)BoreholeNo:C+838

    ;80

    ;E

    ;\

    ;

    ;

    ;

    ;E

    ;\

    ;

    ;

    ;

    ;E

    ;\

    ;

    ;

    ;

    ;Shearstrength/kPa ;(b)BoreholeNo:C+837

;O20406O8O

    ;Shearstrength/kPa

    ;(d)BoreholeNo:C+8317

    ;Shearstrength/kPaShearstrength/kPa

    ;(e)BoreholeNo:C+8318(f)B0eholeNo:C+8319

    ;Fig.3TypicalvanesheartestresultsinTianjinNewHarbor ;Fig.4Mohr-Coulombfailurecriterion

    ;--——

    ;198--——

    ;Or

    ;丢盯d+d1T口凡+1Td

    ;盯譬+,+zUtan?+C

    ;(5)

    ;ItiSinterestingtonotethatthetermontheleftsideof ;Eq.(5)isequaltotheconventionalvaneshearstrength ;(S).AlsonotethatEq.(5)canbeexpressedbythe ;followingsimpleform:

    ;Y=X+b(6)

    ;

    ;LIURunetal:VaneShearStrengthBasedStabilityAnalysisofSlopesinUnconsolidatedSoftClay

    ;where

    ;Y=

    ;(

    ;=

    ;S

    ;ItappearsthatbothXandYaredependentonthedi

    ;mensionofthevane.TodetermineXrequiresthevalueof ;.Theresuhsoflaboratorytestsonundisturbedsoilsam

    ;plesindicatethatthevalueofrangesbetween0.65and ;0.70.TheparametersaandbinEq.(6)canbedeter

    ;minedbyperformingstatisticallinearregressionforall ;vanesheartestdataobtainedfromthissoftclaystratum ;(seeFig.5).

    ;Theresultofregressionlinearequationis

    ;Y=0.2414X+0.06(7)

    ;/kPa

    ;Fig.5Regressionresultsfromtheobserveddata ;ofvanesheartests

    ;Thevalueofcorrelationcoemcientr=0.742.Con

    ;sideringthenonhomogeneousnatureofthesoftclayinthis ;stratum,sucharelativelylowrvalueshouldbetolerable. ;Accordingtothiscorrelation,thecorrespondingundrained ;shearstrengthparametersareapproximatelyc=6kPa,and ;(D15.8.for?85V00.Thesestrengthparametersareused

    ;intheslopestabilityanalysis.whichisdescribedbelow. ;2Slopestabilityanalysis

    ;TheFelleniusmethodofsliceisadoptedintheslope ;stabilityanalysis.Themethodassumescircularfailuresur

    ;facesandparallelsideforcestothebaseofeachslice.The ;safetyfactoragainstslopefailureisexpressedintermsof ;effectivestressas[‘]

    ;F:(8)

    ;‘?sin,

    ;where

    ;.

    ;

    ;baselengthofslidingsurface;

    ;

    ;weightofeachslice;

    ;

    ;baseanglefromhorizontalofeachslice:

    ;Fstyfactor.

    ;Otherfactorshavebeendefinedearlier.

    ;ItisseenfromEq.(8)thatUandtan’Darealways

    ;togetherandareassociatedwithnoFnlalforceforcalculat

    ;ingtheantislidingfrictionalresistance.Foreachpairofc ;andUtan,thesafetyfactorforeachassumedsliding ;surfacecanbecomputed.

    ;Fiveslopeswereanalyzed.theyaredescribedinTab. ;4.0fthesefiveslopes.fourarestableandoneisa ;failedslope.Theanalysesofthesefiveslopeswerealso ;madeforshearstrengthparametersobtainedfromCUtriax

    ;ia1.UUtriaxialandnsituvanesheartests.Theanalyzed ;safetyfactorsaresummarizedinTab.5.

    ;Tab.4DetailsofdesigningtheslopesinTian]inNewHarborarea ;ThedatainTab.5show,asexpected,thatthesafety ;factorsobtainedfromCUstrengthsarethehighestdueto ;consolidationeffect,andthoseobtainedfromUUstrength ;arethelowestduetosampledistributioneffect.Also,the ;safetyfactorsdeterminedfromvaneshearstrengthsarebe

    ;tweentheCUandUUvalues.Examiningthesethreesets ;

    ;199

    ;\LIcls?LI?

    ;

    ;TransactionsofTianjinUniversityVo1.10No.32004 ;ofsafetyfactorvaluesagainstthestateoftheanalyzed ;slopes,itisseenthattheCUresultsoverestimatethesafe-

    ;tyfactorofthefailedslope,andtheUUresultsunderesti- ;matethreestableslopes.Thesafetyfactorsobtainedfrom ;regressedstrengthparametersappeartomatchthestateof ;slopeverywel1.Thisindicatesthatthemethodofdetermi- ;ningunconsolidatedundrainedshearstrengthparameters ;forslopestabilityanalysisisvalidandaccurate. ;Tab.5Safetyfactorsandstatusoftheanalyzedslopes ;3Summaryandconclusions

    ;Unconsolidatedclaystypicallyaresoft,sensitive,and ;compressiblewithlowstrength.Itisdifficulttoobtainun- ;disturbedsoilsamplesfordeterminingshearstrengthpa- ;rametersforstabilityanalysisofslopesinsuchsoils.Thus, ;in-?situvanesheartestsarefrequentlyperformedtodeter-? ;minetheundrainedshearstrengthforanalysis.Whenvane ;shearstrengthisadopted,thefrictionalcomponentof ;shearingresistanceisneglected.Theomissionoffrictional ;componentmayresuhinsignificanterrorintheanalysises- ;peciallyforsoilswithconsiderableinternalfriction.Mean- ;while,insoils,thevaneshearstrength,whichisdeter- ;minedbybothverticalandhorizontalplanes,maynotre- ;presenttheshearstrengthonthepotentialslidingsurface. ;Apossiblemeanstominimizetheseeffectsistoconvertthe ;vaneshearstrengthintoundrainedshearstrengthparame- ;terswithdueconsiderationoftheeffectsofplanedirection ;andfrictionalresistance.

    ;Inthisstudyamethodofdeterminingtheundrained ;shearstrengthparametersfromin-?situvaneshearstrengths ;ispresented.Theshearstrengthparametersdeterminedfor ;allofthein-situvaneshearstrengthsweresubjectedtosta- ;tisticalregressionanalysistotakeintoconsiderationthe ;possibleeffectofnon-homogeneityinthesoftclaydeposit. ;Usingtheregressedshearstrengthparameters,slopesta- ;bilityanalyseswereperformedforfiveexistingsoilslopes. ;Thesameslopeswerealsoanalyzedusingin-situvane ;shearstrengths,aswellasconsolidated-undrainedandun- ;consolidated-undrainedshearstrengthparametersobtained ;20o

    ;fromlaboratorytestonundisturbedsamples.,I’hesafety

    ;factorsdeterminedfromthevariousstrengthdatawereeva1. ;uatedagainstthestateoftheslopesanalyzed.,I’heresults

    ;indicatethat,ofthesestrengthinputdata,onlytheshear ;strengthparametersconvertedfromin-situvaneshear ;strengthareabletoprovidereasonablesafetyfactors. ;Basedontheresultsofthestudy.itcanbeconcludedthat

    ;thepresentedmethodisavalidandeffectivewaytoconvert ;in-situvaneshearstrengthintoundrainedshearstrength ;parametersforslopestabilityanalysis.

    ;References

    ;[1]ChenH.Analysisofthemechanismofvacuumpreloadingmethod ;A].In:The5SoilFoundationofTianfinM].TianjinScienceand

    ;TechnologyPublishingHouse.1987:7383.

    ;[2]BishopAW.Thest~nsthofsoils8sengineeringmaterials[J]. ;Geotechnique.1966.6:1l28.

    ;[3]BjerrumL.Problemsofsoilmechanicsandconstructiononsoftclays ;[A].In:P,oc8thInternationalConferenceSMFE[C].Moscow. ;1973.3:111l59.

    ;4]CraigRF.SoilMechanicsM].5thed.London:Chapman&Hal1. ;1992.

    ;Mo~enstemNR.PriceVE.Anumericalmethodforsolvingthee? ;quationsofstabilityofgeneralslipsurfaces[J].ComputerJouma/, ;l967.9:388393.

    ;LeroueilS,VaughanPR.Thegeneralandcongruenteffectsofstruc? ;tureinnaturalsoilsandweakrocks[J].Geotechnique.1990.40(3): ;467488.

    ;MitchellJamesK.FoundamentalsofSoilBehavior[M].2nded.New ;York:JohnWiley&SonsInc.1993.

    ;YanS.LiS,SunWH.Casestudyandbackanalysisofthefailedslope ;ofacoaltransportingportonTanggusoftclay[A].In:2ndInternation? ;alConferenceonLandslidesSlopeStability&theSafetyofInfia?Struc? ;tllJ’e[C].Singapore.1999,6:2l7—222.

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