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:/n—situvanesheartestis~equenflyperformedtodetermineshearstrengthforslopestabilityan
alysis
;inTianjinNewHarbor.However,thesoilshearstrengthvarieswiththeshearplaneorientation.Apossible
means
;toreducetheeffectofdirectionaldependencyofshearstrengthistoconvertthein—situvaneshearstrengthinto
;undrainedshearstrengthparameters.Amethodofconvertingin—situvaneshearstrengthintoundrainedshear
;strengthparametersispresented.Theshearstrengthparametersdeterminedforallofthein—situvanesh
ear
;strengthsaresubjectedtostatisticalregressionanalysistotakeintoconsiderationthepossibleeffectofno
n’?horn’?
;ogeneityinthesoftclaydeposit.Usingtheregressedshearstrengthparameters,slopestabilityanalysesar
eper-
;formedforfiveexistingsoilslopes.Theresultsofstabilityanalysesindicatethatthesafetyfactorsobtaine
dfrom
;theconvertedparametersreflectthestateoftheslopesanalyzedmuchbetterthanthoseobtainedfromin—
situ
;vaneshearstrengthandlaboratoryconsolidated—undrainedandunconsolidated—undrainedstrength
parameters.It
;isconcludedthatthepresentedmethsodofdeterminingundrainedshearstrengthparametersforin—situ
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 ;depositintheTianjinNewHarborareamakesitdi伍cultto
;obtainundisturbedsoilsamplesforlaboratorytesting.Soil ;sampling,handling,transportation,andtestingwillinevi—
;tablycausedisturbanceandreducetheshearstrengthprop—
;ertyofthesoil.Usingthesestrengthdataintheanalysis ;anddesignofsoilslopesoftenresultsinasafetyfactorun—
;realisticallylowerthanunityevenforstableslopes. ;Toavoidpossiblesampledisturbance,thein—situvane
;sheartestmethod(seeFig.1)hasbeencommonlyadopted ;todeterminetheshearstrengthpropertyfordesignanda—
;nalysisofsoilslopesinTianjinNewHarbor.However,e—
;venwiththeuseofin—situvaneshearstrength,thesafety ;factorsofstableslopesareoftenunderestimated,sugges—
;ringthatthein—situvaneshearstrengthmaynotrepresent ;theshearstrengthmobilizedalongtheanalyzedslidingsur—
;faces.Thisispossible,becausethesoilshearstrengthva—
;deswiththeshearplaneorientation;namely,theshear ;strengthisdirectiondependent.Anotherfactordeserving ;(b)Soilcolumnto
;resistthetorque
;Fig.1Keyschemeforvanesheartest
;Accepteddate:2003一ll-28.
;LIURun.bornin1974,female,Dr,associateProf. ;E-mail:liurun74@yahoo.coll1.cn
;
;TransactionsofTianjinUniversityVo1.10No.32004 ;attentionisthatthevaneshearstrengthsaremeasuredfromnessoftheapproachisdiscussedindetails.
;verticaldirectionofslopeslidingsurface.Asaconse—
;quence,thesafetyfactorsdeterminedfromvaneshear占
;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—
;solidationsoftheeffeetofdirectionaswellasstress—de—
;pendencyonshearresistance.Suchanapproachispresen—
;tedinthispaper.Meanwhile,theresultsofslopestability ;analysisforsomeexistingslopesbyusingtheconvertedun—
;drainedshearstrengthparametersarecomparedwiththose ;obtainedbyusingthevaneshearstrength.Theeffective一
;3.2
;2.8
;2.4
;2.0
;1.6
;1.2
;l,0
;0.8
;0.4
;-Triaxialtest>0.4口,L<0.2
;一?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 ;—
;l96一
;strength..
;Thephysicalandmechanicalpropertiesofthesoilsin ;differentlayersarepresentedinTab.2andTab.3,respec—
;tively.ThemechanicalpropertiesinTab.3weredeter- ;Bs’10J0l.一n.u
;
;LIURunetal:VaneShearStrengthBasedStabilityAnalysisofSlopesinUnconsolidatedSoftClay
;minedinthelaboratoryonundisturbedsamplesusingdif-compressiontest,unconsolidated—undrained(UU)triaxial
;ferenttestmethodsincludingconsolidated—undrained(CU)compressiontest,andunconsolidated—u
nconfinedcompres—
;directsheartest,consolidated—undrained(CU)triaxialsiontest. ;Tab.2Physicalpropertiesofsoils
;Tab.3Mechanicalpropertiesofsoils
;Inadditiontothelaboratorytest,thousandsofin—situSh=ztan+C(2)
;vanesheartestswereperformedatvariouslocationsofdif-where ;ferentdepths.Fig.3presentssomeofthein—situvane
;shearstrengthvariationwithdepthtogetherwiththedatafaces ;obtainedfromthelaboratoryCUandUUtriaxialcompres—
;siontests.Itisseenthatin—situvaneshearstrengthsare
;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,Sh—shearstrengthsonverticalandhorizontalsur- ;,
;respectively;
;U--degreeofconsolidation;
;--
;coefficientofearthpressureat—rest; ;--
;submergedunitweightofsoil;
;z——depthbelowgroundsurface;
;~--intemalfrictionangleofsoil;
;c--cohesionofsoil. ;Atfailure,thelistingequilibriumequationcanbeex— ;pressedasfollows,assumingauniformshearstressdistil—
;but
;\
;？
;厶
;凸
;E
;\
;？
;厶
;凸
;Shearstrength/kPa ;(a)BoreholeNo:C+83—2
;O204060
;Shearstrength/kPa ;(e)BoreholeNo:C+83—8
;80
;E
;\
;？
;厶
;凸
;E
;\
;？
;厶
;凸
;E
;\
;？
;厶
;凸
;Shearstrength/kPa ;(b)BoreholeNo:C+83—7
;O20406O8O
;Shearstrength/kPa
;(d)BoreholeNo:C+83—17
;Shearstrength/kPaShearstrength/kPa
;(e)BoreholeNo:C+83—18(f)B0『eholeNo:C+83—19
;Fig.3TypicalvanesheartestresultsinTianjinNewHarbor ;Fig.4Mohr-Coulombfailurecriterion
;--——
;198--——
;Or
;丢盯d+盯d1T口凡+1Td
;盯譬+譬,一+zUtan?+C
;(5)
;ItiSinterestingtonotethatthetermontheleftsideof ;Eq.(5)isequaltotheconventionalvaneshearstrength ;(S).AlsonotethatEq.(5)canbeexpressedbythe ;followingsimpleform:
;Y=口X+b(6)
;
;LIURunetal:VaneShearStrengthBasedStabilityAnalysisofSlopesinUnconsolidatedSoftClay
;where
;Y=
;耵(
;=
;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—
;sideringthenon—homogeneousnatureofthesoftclayinthis ;stratum,sucharelativelylowrvalueshouldbetolerable. ;Accordingtothiscorrelation,thecorrespondingundrained ;shearstrengthparametersareapproximatelyc=6kPa,and ;(D15.8.for?85V00.Thesestrengthparametersareused
;intheslopestabilityanalysis.whichisdescribedbelow. ;2Slopestabilityanalysis
;TheFelleniusmethodofsliceisadoptedintheslope ;stabilityanalysis.Themethodassumescircularfailuresur—
;facesandparallelsideforcestothebaseofeachslice.The ;safetyfactoragainstslopefailureisexpressedintermsof ;effectivestressas[‘]
;F:生(8)
;‘?sin,
;where
;.
;一
;baselengthofslidingsurface;
;一
;weightofeachslice;
;—
;baseanglefromhorizontalofeachslice:
;F—styfactor.
;Otherfactorshavebeendefinedearlier.
;ItisseenfromEq.(8)thatUandtan’Darealways
;togetherandareassociatedwithnoFnlalforceforcalculat—
;ingtheanti—slidingfrictionalresistance.Foreachpairofc ;andUtan,thesafetyfactorforeachassumedsliding ;surfacecanbecomputed.
;Fiveslopeswereanalyzed.theyaredescribedinTab. ;4.0fthesefiveslopes.fourarestableandoneisa ;failedslope.Theanalysesofthesefiveslopeswerealso ;madeforshearstrengthparametersobtainedfromCUtriax—
;ia1.UUtriaxialandn—situvanesheartests.Theanalyzed ;safetyfactorsaresummarizedinTab.5.
;Tab.4DetailsofdesigningtheslopesinTian]inNewHarborarea ;ThedatainTab.5show,asexpected,thatthesafety ;factorsobtainedfromCUstrengthsarethehighestdueto ;consolidationeffect,andthoseobtainedfromUUstrength ;arethelowestduetosampledistributioneffect.Also,the ;safetyfactorsdeterminedfromvaneshearstrengthsarebe—
;tweentheCUandUUvalues.Examiningthesethreesets ;一
;199一
;日\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.
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