Small-strain stiffness of saprolites

2023-10-03 来源：星星旅游

Small-strainstiffnessofgraniticandvolcanicsaprolitesinHongKong

C.W.W.Ng

HongKongUniversityofScienceandTechnology,HKSAR

E.H.Y.Leung

OveArupandPartners(HK)Ltd.FormerlyHongKongUniversityofScienceandTechnology,HKSAR

ABSTRACT:Whilethestudyofthesmall-strainbehaviourofsedimentarysoilsandsandshaswit-nessedsignificantprogress,thestudyofthesmall-straincharacteristicsofgeomaterialsderivedfromthedecompositionofrockshasattractedrelativelylessattention.Inthispaper,variousaspectsofthesmall-strainstiffnessofthreesaproliticsoilsinHongKongarestudied.Theresultsoffieldtests(geo-physicaltestsandself-boringpressuremetertests)andlaboratorytests(triaxialtestswithmultidirectionalshearwavevelocitymeasurements,localstrainmeasurementsandsuctioncontrol)arepresentedanddiscussed.Stiffnessanisotropyisstudiedandtheeffectsofsuction,cyclicloading,creepandsampledisturbanceonsmall-strainstiffnessarediscussed.Someuniquefeaturesandcharacteristicsofsaproliticsoilsatsmallstrainsarehighlighted.

1INTRODUCTION

Itisgenerallybelievedthatthebehaviourofsaproliticsoils(soilsthatpreservetheoriginalstructuresinheritedfromtheparentrock(GCO1988))differsconsiderablyfromthatoftransportedsoilsundersimilarconditionsduetotheirspecialmineralogicalandmicrostructuralcharacteristicsdevelopedduringweatheringprocesses.PriortotheestablishmentoftheGeotechnicalControlOffice(GCO)oftheHongKongGovernmentin1977,somestudiesontheshearstrengthofgraniticsaproliteshavebeenreportedbyLumb(1962,1965).Sincethelate1970s,theGCO(nowGeotechnicalEngineeringOffice(GEO)),hascarriedoutsignificantamountofresearchbymeasuringtheshearstrengthofsaturatedandunsaturatedgraniticsaprolitesthroughtriaxialcompressionanddirectsheartestsinthelaboratoryandinthefield(Massey1983,Brandetal.1983,Shen1985).Basedonalargedatabaseofmeasuredresultsfromtriaxialtests,Pun&Ho(1996)reportedtheshearstrengthofintactgraniticsaprolitesandprovidedasetofgeneralizedshearstrengthparametersofcompletelydecomposedgranite(CDG).CommissionedbytheGEO,Gan&Fredlund(1996)alsoinvestigatedtheshearstrengthofsaturatedandunsaturatedCDGusingdirectshearboxandtriaxialapparatus.TheeffectsofmatricsuctiononshearstrengthwerequantifiedusingasimplifiedextendedMohr-Coulombfailurecriterion.

ThemechanicalbehaviouroflooselycompactedCDGandcompletelydecomposedvolcanic(CDV)soilshasbeeninvestigatedextensivelyinanattempttoimprovetheanalysisanddesignoftheuseofsoilnailsinloosefillslopes(Ng&Chiu2001,2003,Ngetal.2004a).Theeffectsofstressstate,stresspathandsoilsuctiononthestress-strainrelationship,shearstrength,andvolumetricbehaviourofthematerialswereassessedthroughaseriesofcomputer-controlledtriaxialstresspathtests.Specificstresspathwascarriedouttomimicrainfallinfiltrationintoaninitiallyunsaturatedsoilslopebyreducingsoilsuctionbutkeepingotherstresscomponentsconstant.Alltestresultswereinterpretedandmodelledunderastate-dependentextendedcriticalstateframework(Chiu&Ng2003).

Withtheadvancementinfieldmeasurementtechniques(Baldietal.1988,Clarke1995)andtheimprovementintheaccuracyofstrainmeasurementsinthelaboratory(Clayton&Khatrush1986,Tatsuoka1988),ourunderstandingofthesmall-strainstiffnessofsoilshasbeenimprovingcontinuouslyandhasbeenutilizedinsomeroutinegeotechnicalengineeringanalysesanddesignsoverthelast15years

2507

(Burland1989,Ng&Lings1995,Atkinson2000).Variousstudiesrevealedthatthestiffnessofsoilsatverysmallstrains(ε<0.001%)isinfluencedbyanumberoffactorsincludingconfiningstress,voidratioandsoilstructure.Thestress-strainbehaviourofnaturalsoilsisnonlinear,stress-pathandstress-historydependentatsmallstrains(0.001%<ε<1%).Soilsaregenerallyanisotropicandbehavedifferentlyundersaturatedandunsaturatedconditions.Despitethesignificantprogressinunderstandingthesmall-strainstiffnessofmanygeomaterials,suchassedimentarysoils,softrocks,andcleansands,relativelylittleattentionhasbeenpaidtothesmall-strainbehaviourofgeomaterialsresultingfromthedecompositionofrocks.

Inthispaper,variousaspectsofthesmall-strainstiffnessofthreetypesofsaproliticsoilsinHongKongareconsidered.Thesesoilsincludecompletelydecomposedgranite(CDG),completelydecomposedtuff(CDT)andcompletelydecomposedrhyolite(CDR).ThegeologicalsettingofHongKongisbrieflyintroducedinSection2.TheclassificationofdecomposedmaterialsandthecommonmethodsofsoilsamplinginHongKongarepresentedinSections3and4,respectively.Section5describesthegroundconditionsofthetestsitesandthephysicalpropertiesofthetestedmaterials.Section6reportsthefindingsoffieldmeasurementsonthesmall-strainstiffnessofdecomposedgranite.InSection7,developmentsinlaboratorytestingtechniquesaredescribed.Section7alsoreportssomerecentfindingsonthestiffnessofsaproliticsoilsatverysmalltosmallstrainlevelsandstiffnessanisotropyatverysmallstrains.Theeffectsofsuction,cyclicloading,creepandsampledisturbanceonthesmall-strainstiffnessofthesaproliticsoilsareassessedanddiscussed.Finally,conclusionsaredrawninSection8.Inordertofocusonsoilstiffnessonly,furtherdiscussiononshearstrengthofsaprolitesisnotincludedinthispaper.2GRANITICANDVOLCANICROCKSINHONGKONG

Figure1showsageologicalmapofHongKongthatillustratesthedistributionsofrocktypesinHongKong.ThepredominantrocktypesinHongKongareMesozoicvolcanicrockandgranite,whichconstituteabout85%ofrockoutcropontheland(Sewelletal.2000).MostoftheigneousrockswereformedduringtheLateJurassicandEarlyCretaceousperiods.Volcanicrocksoccupyabout50%ofHongKong’ssurfacearea,formingmostofthemountainousgroundinHongKong.Theyincludetuffs,tuffites,lavasandsedimentaryrocks.IntrusiveigneousrocksinHongKongaccountforabout35%ofHongKong’ssurfacearea,whichcomprisesmainlygraniteandgranodiorite.DetailedclassificationanddescriptionsofthevolcanicandgraniticrocksinHongKongwereprovidedbySewelletal.(2000).UndertheinfluenceofthewarmandhumidsubtropicalclimateinHongKong,rockweatheringisdominatedbychemicalalterationprocesses(Irfan1996).Weatheringleadstophysicalandchemical

113¢X 50'E114¢X 00'E114¢X 30'EGuangdong Sheng( Shenzhen Special Economic Zone )Sheung Shui22¢X 30'NDeep BayLuen Wo HuiMirs Bay22¢ X30'NSite A Yuen LongTai PoSai KungTuenMunTsuen WanSha TinChek Lap KokYen Chow Street Kowloon BaKowloonLegendReclaimed Land22¢ X20'NLantau IslandQuaternary DepositsCretaceous - TertiarySedimentary RocksJurassic / CretaceousGranitoidsJurassic / CretaceousVolcanic RocksDevonian - PermianSedimentary RocksHong Kong Island22¢X 10'N113¢X 50'E024681012km114¢X 20'EFigure1.GeologicalmapofHongKongshowingthelocationofthetestsites.

2508

changesinrockmaterial.Theprocessstartsattherock’ssurfaceandprogressesinwardsalongrockjoints.Ingressofwateralongrockjointspromotesoxidation,hydrolysisandsolutionoftheconstituentminerals.Theigneousrock,suchasgranite,iscomposedmainlyofquartz,feldsparandmica.Quartzisresistanttochemicaldecomposition,whilefeldsparandmicaaretransformedmainlytoclaymineralsduringtheweatheringprocess.Asweatheringproceeds,thestressreleaseasaresultoftheremovaloftheover-lyingmaterialacceleratestherateofexfoliation(stressreleasejointing)andthewettinganddryingprocessesintheunderlyingfreshrock.Theseprocessesincreasethesurfaceareaoftherockonwhichchemicalweatheringproceeds,whichleadstoweatheringprofilesofuptotensofmetresthick(Irfan1996).WeatheredrockprofilesaregenerallyoverlaidwithQuaternarysuperficialdepositsofvaryingthickness.WeatheringeffectsonthemineralogicalandtexturalaspectsofweatheredgraniteshavebeencarriedoutbyIran(1996,1999)andShaw(1997).WeatheringmechanismsofvolcanicandgraniticrocksinHongKonghavebeenstudiedbychemicalanalysis,opticalmicroscopyonthinsectionsandmagneticsusceptibilitymeasurements(Ngetal.2001).Themobilityofelementsofcalcium,sodium,iron,potassium,magnesium,silicon,aluminiumandconstitutionalwaterhasbeeninvestigatedindetail.ChemicalweatheringprocessesinHongKongaredominatedbydecompositionoffeldsparsandbiotite,leachingofalkaliandalkalineearthmetals,andenrichmentofferricoxideunderprolongedsubtropicalclimateconditions.Basedonthemicropetrographicstudies,itisevidentthattheprocessofweatheringgraduallyreducesthecontentsoffeldsparinbothvolcanicandgraniticrocks.Onthecontrary,theamountofclayminerals,microfracturesandvoidsincreaseswiththedegreeofweathering.Asexpected,thequartzremainsfairlyconstantthroughouttheweatheringprocesses.Themicropetrographicindex(Ip),whichexaminesquantitativechangesofminerals,microcracksandvoidsinasample,isshowntobesuitableforgraniticrockbutnotparticularlygoodforvolcanicrockduetoitsfine-grainedmineralsandsmallporesize(Ngetal.2001).

3CLASSIFICATIONOFDECOMPOSEDMATERIALS

Asix-foldmaterialdecompositiongradeschemeiscommonlyusedtoclassifythestateofdecompo-sitionofigneousrocksinHongKong(GCO1988).Theterm‘decomposed’insteadof‘weathered’isusedinthematerialclassificationschemebecausethedominantweatheringprocessinHongKongischemicaldecomposition.Thegeneralcharacteristicsofeachmaterialdecompositiongradearesum-marisedinTable1.Forthepurposeofengineeringdesign,materialsofGradeItoGradeIII,which

Table1.

Classificationofrockmaterialdecompositiongrades(GCO1988).

RockMaterialDescriptionResidualSoil

GradesVI

GeneralCharacteristics•••••••••••••••••••••

Originalrocktexturecompletelydestroyed

CanbecrumbledbyhandandfingerpressureintoconstituentgrainsOriginalrocktexturepreserved

CanbecrumbledbyhandandfingerpressureintoconstituentgrainsEasilyindentedbythepointofageologicalpickSlakeswhenimmersedinwater

CompletelydiscolouredcomparedwithfreshrockCanbebrokenbyhandintosmallerpieces

MakesadullsoundwhenstruckbyageologicalhammerNoteasilyindentedbythepointofageologicalpickDoesnotslakewhenimmersedinwater

Completelydiscolouredcomparedwithfreshrock

Cannotusuallybebrokenbyhand;easilybrokenbyageologicalhammerMakesadullorslightlyringingsoundwhenstruckbyageologicalhammerCompletelystainedthroughout

Cannotbebrokeneasilybyageologicalhammer

Makesaringingsoundwhenstruckbyageologicalhammer

FreshrockcoloursgenerallyretainedbutstainednearjointsurfacesCannotbebrokeneasilybyageologicalhammer

MakesaringingsoundwhenstruckbyageologicalhammerNovisiblesignsofdecomposition(i.e.nodiscolouration)

CompletelyDecomposedV

HighlyDecomposedIV

ModeratelyDecomposedSlightlyDecomposedFresh

IIIIII

2509

generallycannotbebrokendownbyhand,areconsideredasrocks.MaterialsofGradeIVtoVI,whichgenerallycanbebrokendownbyhandintotheirconstituentgrains,areconsideredassoils.SoilsofGradeIVandGradeVaretermedsaprolites.Saproliteisazoneinaweatheringprofilethathasnotundergonemarkedvolumechangesandpreservesoriginalstructuresinheritedfromtheparentrock(Fyfeetal.2000).Aweatheringprofileisconsideredtobemainlycomposedofcompletelytohighlydecomposedinsitumaterials(gradesVandIVrespectively)withsomeresidualsoil(gradeVI)(GCO1988).

DescriptionsofthepetrologicalchangesineachmaterialdecompositiongradeofgraniticrocksandvolcanicrocksaregivenbyIrfan(1996,1999).Basedonresultsfromchemicalanalysesanddrydensitytestsonrhyolitictuffandgraniticsamples,anewtheoreticalweatheringmodelforcalculatingamobilityindexofmajorelementsinrockandsoilhasbeenreported(Guanetal.2001).Itisfoundthatthisnewindexisusefulfordescribingchangesofrockandsoilduringweatheringandclassifyingthedegreesofweathering.Basedonthetheoreticalmodel,anewparametercalledthevolumeindex,whichcombineschemicalandphysicaldata,issuggested.ThisindexappearstobeagoodindicatorforclassifyingthedegreeofweatheringinHongKong.AttemptshavebeenmadebyMasseyetal.(1989)andIrfan(1996)tocorrelaterelationshipsbetweenthedegreesofweatheringandengineeringpropertiesofweatheredrockswithreasonablesuccess.

Themicrostructureofsaproliticsoilinheritedfromtheparentrockandtheweatheringprocessleadstoconsiderablydifferentbehaviourinsaproliticsoilscomparedwithtransportedsoils.Therelictprimaryinterparticlebondingandthesecondarybondingresultingfromcementationinsaproliticsoilsaffecttheengineeringproperties,suchasstrengthandstiffness,ofsaproliticsoils(Irfan1996,1999).Interparticlebondingissensitivetodisturbancesarisingfromsamplingandsamplepreparation.

Figure2.Retractabletriple-tubecore-barrel(Mazier)(GCO1987).

2510

4SAMPLING

Themethodofsamplingaffectsthedegreeofsampledisturbanceinsoilspecimens.Indeterminingpropertiessuchasshearstrengthandstiffnessofanaturalsoil,intactsoilsamplesarepreferred.Rotarysamplingandblocksamplingareconsideredassuitablesamplingtechniquestoobtainsufficientlyintactsamplesfordeterminingshearstrengthandstiffnessofsoilsderivedfrominsiturockdecomposition(GCO1987).InHongKong,theMaziercore-barreliscommonlyusedforsoilsampling(Fig.2;GCO1987).Itisatriple-tubecore-barrelfittedwitharetractableshoe.Thecuttingshoeandconnectedinnerbarrelprojectsaheadofthebitwhendrillinginsoilandretractswhenthedrillingpressureincreasesinhardermaterials.Thisgreatlyreducesthepossibilityofanydrillingfluidcomingintocontactwiththecoreatorjustabovethepointofcuttingandhenceminimizessampledisturbance.Moreover,theMaziercore-barrelhasaninnerplasticlinerthatprotectsacoredsampleduringtransportationtoalaboratory.Thecorediameteris74mm,whichiscompatiblewiththelaboratorytriaxialtestingapparatus.LocalexperiencecommonlyregardstheMaziersamplingtechniqueasthemostsuitablesamplingmethodavailableforweatheredgranularmaterialsatdepths.

Whenasoilsamplewiththeleastpossibledisturbanceisrequired,theblocksamplingtechniquecanbeappliedtoretrievethesoilsample.Blocksamplesareobtainedbycuttingexposedsoilintrialpitsorexcavations.Speciallyorientedsamplescanbeobtainedbyblocksamplingfortestssuchasmeasurementofshearstrengthonspecificdiscontinuitiesandanisotropicsoilstiffness(Ng&Leung2006).

ThecomparisonofthedegreesofsampledisturbanceassociatedwiththeMazierandblocksamplingtechniquesandtheeffectsofsampledisturbanceonthesmall-strainstiffnessofsoilsarepresentedinSection7.7

5TESTSITESANDTESTEDMATERIALS

Inthispaper,foursitesinHongKongareselectedforstudyingthesmall-strainstiffnessofsaprolites.ThelocationsofthetestsitesareshowninFigure1.Twoofthesites,KowloonBayandYenChowStreet,arelocatedontheKowloonPeninsulawheregranitepredominates.ThesiteinthenorthernNewTerritories,LuenWoHui,isunderlainbycoarshashcrystaltuff.Theremainingsite(SiteA),whererhyolitewasencountered,islocatedinthenorthwesternNewTerritories.Fieldand/orlaboratorytestswerecarriedoutonthedecomposedmaterialsderivedfromtheaforementionedigneousrocks.

Figures3aand3bshowthesoilprofileandtheresultsofstandardpenetrationtests(SPT‘N’profile)ofthesitesatKowloonBay(Ngetal.2000)andYenChowStreet(Ng&Wang2001).ThegraniteatbothsitesisclassifiedasKowloonGraniteintheLionRockSuite(Sewelletal.2000).KowloonGranitehasbeendatedto138±1millionyearsagoandrepresentsthefinalpulseofplutonicactivityrecordedinHongKong.ItformsasubcircularbiotitemonzograniteplutoncentredonKowloonandHongKongIsland.Thegraniteisuniformintextureandcomposition.Itcontainsquartzandplagioclasemegacrystssetinamatrixofquartz,alkalifeldsparandplagioclasewithabundantbiotite.Atbothsites,graniteatvaryingdegreeofdecompositionwasidentifiedalongthedepth.Thedegreeofdecompositiondecreaseswithdepth,wherecompletelydecomposedgranite(CDG;GradeV)overlieshighlydecomposedgranite(HDG;GradeIV)andmoderatelydecomposedgranite(MDG;GradeIII).TheCDGatthetwositescanbedescribedasclayeysandysilt.ThephysicalpropertiesandtheparticlesizedistributionsofthematerialareshowninTable2andFigure4,respectively.

Figure3cshowsthesoilprofileandtheSPT‘N’profileofthesiteatLuenWoHui.TheMesozoicvolcanicrockintheareabelongstotheTaiMoShanformationoftheUpperJurassicTsuenWanVolcanicGroup(Sewelletal.2000).TheformationiswidespreadandvoluminousinthenorthernpartoftheNewTerritories.Thelithology(physicalcharacteristicsofrocksbasedongrainsize,mineralcontentandcolour)oftheformationisuniform,withpalegreytodarkgreylapilli-ashorcoarseashcrystaltuff.Therockismainlymadeupofquartz,plagioclase,alkalifeldsparandsomebiotiteandlithiclapilliofpalesandstone.Atthedepthofsampleretrieval,theoriginalrockiscompletelydecomposedanddiscolouredtoreddishbrownclayeysiltwithsomecrystalfragments.Thepatternoffoliation,whichisaresultofmetamorphismoftheparentrock,isvisibleinhandspecimensofCDT.TheCDTsamplescanbedescribedasclayeysilt(MI).ThephysicalpropertiesandtheparticlesizedistributionsofthematerialareshowninTable2andFigure4,respectively.Thecompletelydecomposedrhyolite(CDR)retrievedfromSiteAcanbedescribedasafirm,moist,lightgreymottledwithorangebrown,slightlysandysilt/clay.

2511

SPT 'N'100150

SPT 'N'

200

100

150

200

BP110

BH2BH620Depth (m)Fill10

Alluvium YCS1AYCS1BFill20Depth (m)Alluvium CDG30

HDG40

CDG40

(a) 60

MDG50

(b) 60SPT 'N'

HDGMDG0

050100150200

FillAlluvium 10

CDT20Depth (m)HDT3040

Figure3.

SPT‘N’profilesatthetestsites:(a)KowloonBay;(b)YenChowStreet;(c)LuenWoHui.

Table2.Physicalpropertiesofthetestedmaterials.

CDG(KowloonBay)

CDG(YenChowStreet)Siltysand(SM)

///2.6415–341410–1890

CDT(LuenWoHui)Clayeysilt(MI)4329142.73171730

Classification

Liquidlimit(LL)(%)Plasticlimit(PL)(%)Plasticityindex(PI)(%)Specificgravity,GsMoisturecontent(%)Drydensity(kg/m3)Siltysand(SM)///2.6319–351210–1580

2512

Figure4.Particlesizedistributionofthetestingmaterials–CDGandCDT.

6FIELDMEASUREMENTS6.1Fieldtestingtechniques

AlthoughinsitumeasurementofsoilandweakrockstiffnessusinggeophysicalmethodsisnotverycommoninHongKong,severalstudieswerecarriedoutinHongKongusingmethodsincludingsus-pensionP-S(compressionandshearwave)velocitylogging(Kwong1998,Ngetal.2000),crosshole(Ng&Wang2001)anddownhole(Wongetal.1998)seismicmeasurements.InsuspensionP-Svelocitylogging,P-waveandS-wavevelocitiesaremeasuredat1-mintervalsinasingleuncasedboreholeusingadownholeprobecontainingasourceandtworeceivers.Bymeasuringthewavetraveltimebetweenthetworeceivers,whichareseparatedby1m,theaveragewavevelocityintheregionbetweenthereceiverscanbedetermined.Theinducedstrainisbelievedtobeintheorderof0.001%orsmaller,whichissufficientlysmallforthedeterminationoftheelasticmoduliofsoils(Imai&Tonouchi1982).Forthemeasurementofshearwavevelocity,shearwavesaretransmittedverticallywithhorizontalpolarizationandthusthevelocityoftheshearwaveintheverticalplane(vs(vh))ismeasured.Incrossholeseismicmeasurements,ahammerprovidesasourceofhorizontallypropagating,verticallypolarizedshearwaves.Thewavesaredetectedbytwoborehole-pick,three-componentgeophoneslocatedatthesamehorizontallevelasthesourceintwoadjacentin-lineboreholes.Thevelocityofthehorizontallypropagatingshearwavewithverticalpolarization(vs(hv))iscalculatedbythemeasureddifferenceintraveltimesoftheshearwavefromthesourcetothetwogeophonesandthemeasuredpathlengthofwavepropagationalongthetwoboreholeswithgeophones.

Withthemeasuredshearwavevelocity(vs(vh)orvs(hv)),theshearmodulusintheverticalplane(GvhorGhv)atverysmallstrainscanbecalculatedbythefollowingequation:

whereρisthebulkdensityofthesoil.

Insitumeasurementofthevariationintheshearmoduluswiththeshearstraincanbeachievedbyperformingself-boringpressuremeter(SBPM)testsusing,forexample,theCambridge-typeSBPM(Clarke1995).Theself-boringmechanismofthepressuremeterminimizesdisturbancetothesurroundingsoilduringinstallation.TheCambridge-typeSBPMprovidesmeasurementsofhorizontalstrainand

2513

Table3.Summaryoftestsperformedatthetestsites.

TestSiteKowloonBay(Ngetal.2000)

YenChowStreet(Ng&Wang)2001)

LuenWoHui(Ng&Leung)2006)DecomposedtuffN/AN/AN/A󰀁

SiteADecomposedrhyoliteN/AN/AN/A󰀁

SoilTypeFieldTest

SuspensionP-SvelocityloggingCrossholeseismicmeasurementSelf-boring

pressuremetertest

LaboratoryTest

ShearwavevelocitymeasurementintheverticalplaneusingbenderelementsShearwavevelocitymeasurementinthehorizontalplaneusingbenderelementsSmallstraintriaxialstresspathtestwithlocalstrainmeasurements

Decomposedgranite󰀁N/A󰀁N/A

N/A󰀁󰀁󰀁

N/AN/A󰀁N/A

󰀁󰀁󰀁󰀁

󰀁:Testwasperformed

N/A:Notestwasperformed

porepressure,thusallowingacompletestress-straincurvetobedrawn.Severalunload-reloadloopsaregenerallyrecommendedandconductedforthedeterminationoftheshearmodulusofsoilfromthestress-straincurve.

6.2Shearstiffnessatverysmallstrains

FieldmeasurementsofshearwavevelocityofdecomposedgranitealongdepthweretakenattheKowloonBaysite(Ngetal.2000)usingsuspensionP-SvelocityloggingandtheYenChowStreetsiteusingcrossholeseismicmeasurements(Ng&Wang2001).AsummaryofthefieldandlaboratorytestsperformedatthetestsitesisgiveninTable3.

AtKowloonBay,shearwavevelocitymeasurementsweretakenacrossadepthfrom10mto43mwherealluviumanddecomposedgraniteoccupiedthesoilprofile(Fig.3a).AtYenChowStreet,shearwavevelocitymeasurementsweretakenongraniteatvariousdegreesofdecompositionacrossadepthfrom29mto59m.Figures5aand5bshowtheshearwavevelocityprofilesatKowloonBayandYenChowStreet,respectively.TheshearwavevelocityprofilesaregenerallyconsistentwiththeSPT‘N’profilesoftherespectivesiteasshowninFigures3aand3b.Atthesamedepth,theshearwavevelocitiesmeasuredatthetwositeswithsimilarsoilprofilesareconsistentwitheachother.TheshearwavevelocityofthesoilprofileatKowloonBaygraduallyincreaseswithdepth.Inthealluviumlayer,theshearwavevelocityincreasesfrom130m/sto200m/sacrossadepthfrom10mto22.5m.Theshearwavevelocityofdecomposedgraniteincreasesfrom200m/satadepthof23mto350m/satadepthof43m.AtYenChowStreet,theshearwavevelocityofCDGgraduallyincreasesfromabout250m/sto350m/soveradepthof29mto42m.Therateofincreaseintheshearwavevelocityincreasesbelowadepthof42m.Althoughthematerialatadepthbetween42mand48misalsoclassifiedasCDG,thedegreeofdecompositionislikelytodecreaseasthedepthincreasesandresultsinahighershearwavevelocity.TheresultsoftheshearwavevelocitymeasurementsareconsistentwiththeresultsofthestandardpenetrationtestswhereasignificantincreaseintheSPT‘N’valuebelowadepthof42misobserved(Fig.3b).Theincreaseintheshearwavevelocitybetween42mand48misaresultoftheincreaseinthesoilstiffness.

2514

0510

Shear wave velocity, vs(vh) (m/s)100200300400500(a)Shear wave velocity, vs(hv) (m/s)

05100

(b)FillMDGHDGCDGAlluvium500100015002000

FillAlluvium1520Depth (m)25

1520Depth (m)25303540

Crosshole measurementacross boreholesYCS1A and YCS1B354045505560

P-S velocity logging at BF-1 P-S velocity logging at BF-2 P-S velocity logging at BF-6 Ordinary least square 95% Confidence interval95% Prediction intervalCDG30

HDG 505560

Figure5.Shearwavevelocityprofilesat:(a)KowloonBay;(b)YenChowStreet.

Thearrivaloftherefractedwavetravellinginthestrongerunderlyingstratummayalsoleadtoanincreaseinthemeasuredshearwavevelocity.ThemeasuredshearwavevelocityofthelimitedthicknessofHDGisbelievedtobeaffectedbythearrivaloftherefractedwaveandthereforeitislikelytobeoverestimated.TheshearwavevelocityofMDGincreasesfrom1500m/sto1700m/soveradepthof52mto60m.Theshearmodulus(GvhorGhv)atverysmallstrainscanbecalculatedfromthemeasuredshearwavevelocityandthebulkdensityofthesoilusingEquation1.ThebulkdensityoftheCDGatthetwositesisabout2000kg/m3.ThebulkdensitiesofHDGandMDGatYenChowStreetare2400kg/m3and2550kg/m3,respectively.Figures6aand6bshowthevariationsinshearmoduluswithdepthatthetwosites.ThevalueofGvh(orGhv)increasesgraduallywithdepthacrosstheCDGlayeratbothsites.AtKowloonBay,thevalueofGvhincreasesfromabout45MPaatadepthof23mtoabout200MPaatadepthof39m.AtYenChowStreet,thevalueofGhvoftheCDGlayerincreasesfrom50MPato280MPaacrossadepthfrom29mto42m.ThevaluesofGhvatdepthsabovethestrongunderlyingstratum(CDG/HDGboundary,HDG/MDGboundary)maybeoverestimatedduetothearrivaloftherefractedwave.AlongtheMDGlayer,thevalueofGhvincreasesfrom5500MPato7000MPaacrossadepthfrom52mto58m.

Theshearmoduliatverysmallstrains(GvhorGhv)ofdecomposedgraniteattheKowloonBayandYenChowStreetsitesarecorrelatedwiththeSPT‘N’valuesinFigure7.ThevaluesofGhvofgraniticsaproliteinPortugal(VianadaFonsecaetal.1997)determinedbycrossholeseismicmeasurementsarealsoshowninthefigureforcomparison.ThevalueofGhvofthegraniticsaproliteinPortugalvariesfromabout85MPato120MPaacrossadepthfrom1mto5m,wheretheSPT‘N’valueincreasesfromabout10to35acrossthelayer.Abest-fitlinethroughallthedataforCDGinHongKongisshowninthefigure.ThevalueofGvh(orGhv)ofCDGcanbeempiricallycorrelatedwiththeSPT‘N’valuebyusingthefollowingequation:

TheempiricalcorrelationbetweenGvhandtheSPT‘N’valueforCDGissimilartothatproposedbyImai&Tonouchi(1982)forsands(Gvh=14.4×N0.68).ThedataforCDGinHongKongandPortugal(VianadaFonsecaetal.1997)aregenerallyconsistenteventhoughthemeasurementsonCDGinPortugalweretakenatashallowersoilprofile(Depth=1–5m).

2515

005101520Depth (m)2530354045

Shear modulus, Gvh (MPa)200400600(a) Shear modulus, Ghv (MPa)

800

Fill 510

Alluvium 1520Depth (m)2530354045

HDG CDG 0

(b) 2000400060008000

Crosshole measurementacross boreholesYCS1A and YCS1BCDG 505560Figure6.

P-S velocity logging:BF-1BF-2BF-6505560

Shearmodulusprofilesat:(a)KowloonBay;(b)YenChowStreet.

1000

Gvh (or Ghv) = 14.3 x N0.64R2 = 0.60Gvh or Ghv (MPa)100

Kowloon Bay (Ng et al. 2000):BF-1BF-2BF-6Yen Chow Street (Ng & Wang 2001)Portugal (Viana da Fonseca 1997)1010

100SPT 'N'

1000

Figure7.

Gvhvs.SPT‘N’forcompletelydecomposedgranite(CDG).

2516

MDGHDG Alluvium Fill 1000

(a)800

Depth = 30 - 39 mtest1test2test3test4test5test6test7test8test9test10test11test12test13test14test15Gsec/√p'600

400

From P-S velocity logging 200

00.001

0.01

Shear strain, εs (%)

0.11

1000

(b)800

Depth = 39.4 mDepth = 42.4 mDepth = 45.0 mDepth = 48.5 mGsec/√p'600

From crosshole seismicmeasurements 400

200

00.001

0.01

Shear strain, εs (%)

0.11

Figure8.Stiffness-strainrelationshipsofdecomposedgranitefromself-boringpressuremetertests:(a)KowloonBay;(b)YenChowStreet.

6.3Variationsinshearstiffnesswithshearstrain

Self-boringpressuremeter(SBPM)testswerecarriedoutattheKowloonBayandYenChowStreetsitestostudythevariationsinthesecantshearmoduluswithshearstrainofdecomposedgranite(Ngetal.2000,Ng&Wang2001).SBPMtestswereperformedusingtheCambridge-typeSBPMatbothsites.Fifteentestswereconductedalongadepthof30mto39matKowloonBay,whilefourtestswereperformedacrossadepthof39mto49matChowStreet.Figure8ashowstherelationshipbetween√Yen󰀁

thenormalizedsecantshearmodulus(Gsec/p,wherep󰀁isthemeaneffectivestress)andtheshearstrain(εs)interpretedfromthethirdcycleoftheunload-reloadloopofthetestsatKowloonBay.Thevariations√󰀁

inGsec/pwithεsatYenChowStreetareshowninFigure8b,wheretheresultswerederivedfromthelastcompleteunloadingloop.Itwaspreviouslyfoundthatthesecantshearmodulusderivedfrom

2517

thelastcompleteunloadingloopisconsistentwiththatderivedfromeachunload-reloadloop(Ng&Wang2001),thusitisbelievedthattheshowninFigures8aand8barecomparable.Atthetwo√results󰀁

sites,therelationshipsbetweenGsec/pandεsfordecomposedgranitearegenerallyconsistent.The√

stiffness-strainrelationshipishighlynon-linear.ThevalueofGsec/p󰀁decreasessignificantlyfromabout300toabout50asεsincreasesfrom0.02%to1%.Therangeoftheshearmodulusatverysmallstrains(GvhorGhv)determinedfromseismicmeasurementsisalsoshowninthefigures.ThemeasuredvalueofGvhorGhvcorrespondstothemaximumshearmodulusofthedecomposedgranite.

7LABORATORYMEASUREMENTS7.1Laboratorytestingtechniques

Shirley&Hampton(1978)developedashearwavetransducerutilizingpiezoceramicbenderelementstomeasuretheshearwavevelocitiesandtherebytheshearmoduliofsoilsatverysmallstrains.Benderelementscanbeincorporatedintovariouslaboratorytestingdevicestodeterminetheshearwavevelocityandshearmodulusofsoils.Thesedevicesincludeoedometercells,resonantcolumndevices,triaxialcellsandmanyothers(Dyvik&Madshus1985,Thomann&Hryciw1990).Inanattempttoinvestigatethestiffnessanisotropyofsoilsatverysmallstrains,multidirectionalshearwavevelocitymeasure-menttechniqueshavebeendeveloped.Insuchtechniques,benderelementsareinstalledindifferentdirectionsinlaboratorytestingequipment,suchasoedometercellsandtriaxialtestingapparatus.Inanoedometercell,benderelementscanbeinstalledinthetopcapandbasepedestaltodeterminethevelocityofverticallypropagatedshearwavewithhorizontalpolarization(vs(vh))(Thomann&Hryciw1990,Shibuyaetal.1997).Benderelementscanalsobeinstalledacrosstheoppositefacesoftheringofanoedometercelltodeterminethevelocityofhorizontallypropagatedshearwaveswithhorizontalpolarization(vs(hh))(Jamiolkowskietal.1995).Inatriaxialtest,benderelementscanbeinstalledinthetopcapandbasepedestaltodeterminethevelocityofverticallypropagatedshearwaveswithhorizontalpolarization(vs(vh)).Thistechniquehasbeenwidelyadoptedtodeterminetheshearwavevelocityofsoilsintriaxialtests,exceptnaturallydecomposedgeomaterials(Dyvik&Madshus1985,Viggiani&Atkinson1995,Joviˇci´c&Coop1998).Recentadvancesinlaboratorytestingtechniqueshaveenabledthemeasurementofthevelocityofhorizontallypropagatedshearwaveswithverticalandhorizontalpolar-ization(vs(hv)andvs(hh))intriaxialtests(Fioravante2000,Kuwanoetal.2000,Penningtonetal.2001,Ngetal.2004b).

Assessmentofsoilstiffnessinlaboratorytestsrequiresaccuratemeasurementofstrainsatlowlevels.Externalmeasurementsofstrainsintriaxialtestshaveseveralsourcesoferrors.Theseincludebeddingerrorsattheendsofthespecimen,alignmenterrors,seatingerrorsandsystemcompliance(Scholeyetal.1995).Assessmentofsoilstiffnessinarangeofstrainsatlowlevelsisaccomplishedthroughtheuseoflocalstraintransducers.Theuseoflocalstraintransducerscaneliminatetheerrorsandimprovetheaccuracyandresolutionofstrainmeasurements.ThetypesoflocalstraintransducersincludesubmersibleLVDT(Brown&Snaith1974),proximitytransducers(Hird&Yung1989),inclinometergauges(Burland&Symes1982),Halleffectlocalstraintransducers(Clayton&Khatrush1986,Claytonetal.1989,Ngetal.1998)andlocaldeformationtransducers(LDT)(Tatsuoka1988,Ngetal.1995).Scholeyetal.(1995)andYimsiri&Soga(2002)provideddetailedreviewsofthesetransducers.

Figure9ashowsacomputer-controlledtriaxialstresspathapparatusequippedwithlocalstraintrans-ducersandbenderelementsusedinastudyoftheanisotropicsmallstrainstiffnessofacompletelydecomposedtuff(CDT)attheHongKongUniversityofScienceandTechnology(Ngetal.2004b,Ng&Leung2006,Leung2005).Localaxialandradialstrainsatthemid-heightofthespecimensweremeas-uredbyHallEffecttransducers.Amid-planeporepressureprobewasusedtomeasuretheporepressureatthemid-heightofthespecimens.Threepairsofbenderelementswereusedtomeasuretheshearwavevelocityinthreeorthogonalplanesofasoilspecimen.Apairofbenderelementswasinsertedintothetopandbottomendsofaspecimentodeterminetheshearwavevelocityofverticallytransmittedshearwaveswithhorizontalpolarization,vs(vh).Twopairsofbenderelementswereinsertedatthemid-heightofaspecimentodeterminetheshearwavevelocityofthehorizontallytransmittedshearwaveswithhorizontalandverticalpolarization,vs(hh)andvs(hv).

Ng&Yung(2005)modifiedthetriaxialapparatusshowninFigure9afortestingunsaturatedsoilsusingtheaxistranslationtechnique(Hilf1956).Figure9bshowsaschematicdiagramofthemodifiedtriaxialapparatus.Airpressureiscontrolledthroughacoarse,lowair-entryvaluecorundumdiskplacedonthetopofthesoilspecimen,whereaswaterpressureiscontrolledthroughasaturated,highair-entryvalue(3bars)ceramicdisksealedtothepedestaloftheapparatus.Abasepedestalwithspiral-shaped

2518

Figure9.Triaxialapparatuswithmultidirectionalshearwavevelocitymeasurementsandlocalstrainmeasurementsfortesting:(a)saturatedsoils;(b)unsaturatedsoils.

drainageductswasdesignedtofacilitatetheremovalofanydiffusedairthroughthehighair-entryvalueceramicdisk.A16mm-diameterrecessinthecenterwasdesignedtohouseabenderelementforshearwavevelocity(vs(vh))measurements.Apre-drilled,highair-entryvalueceramicdiskwasplacedinthepedestalandsealedontheinnerandoutercircumferencesofthediskwithepoxyresin.Shearwavevelocity(vs(hh)andvs(hv))measurementsacrossthemid-heightofasoilspecimenandlocalstrainmeasurementsusingHallEffecttransducerswerealsoinstalledasshowninFigure9a.

2519

400000350000300000250000Gvh/pr20000015000010000050000

Measured shear modulus – Depth = 23.0 - 24.0 mDepth = 27.5 - 28.5 mDepth = 36.5 - 37.5 mDepth = 44.0 - 45.0 mDepth = 53.0 - 54.0 mCrosshole seismic measurements(Ng & Wang 2001) Computed shear modulus (Gvh/pr = 7641e−0.641(p'/pr)0.448) –Depth = 23.0 - 24.0 mDepth = 27.5 - 28.5 mDepth = 36.5 - 37.5 mDepth = 44.0 - 45.0 mDepth = 53.0 - 54.0 m050100150

200p'/pr

250300350400

Figure10.

Normalizedshearmodulusatverysmallstrains(Gvh/pr)ofCDGatYenChowStreet.Table4.Soiltype

CDG(32%silt,18%clay)

Ottawasand(Salgadoetal.2000)0%silt5%silt10%silt15%silt

RegressionparametersC,a,andnforCDGandOttawasand.

C7641547410135101

a−0.641−1.051−1.044−2.376−2.069

n0.4480.4430.4580.5570.715

r20.900.970.950.960.94

7.2Small-strainstiffnessofdecomposedgranite

7.2.1Shearmodulusatverysmallstrains

Theshearmodulusatverysmallstrains(Gvh)ofCDGwasdeterminedfromshearwavevelocitymeasure-mentsusingbenderelementsinatriaxialapparatus(Wang&Ng2005).MazierspecimenstakenfromthesiteatYenChowStreetatdepthsbetween23mand54mweretested.Thespecimenswereisotropicallyconsolidatedtotheestimatedinsitumeaneffectivestressforshearwavevelocitymeasurement.Thespecimenswerethensubjectedto50kPaincrementsofmeaneffectivestressforthedeterminationoftheshearmodulusatthosestresslevels.Figure10showsthenormalizedshearmodulusatverysmallstrains(Gvh/pr)plottedagainstthenormalizedmeaneffectivestress(p󰀁/pr),wherepristhereferencepressuretakenas1kPa.ThetestdatawereinterpretedusinganempiricalnonlinearequationsuggestedbySalgadoetal.(2000)asshownbelow:

whereC,a,andnareregressionconstantsandeisthevoidratio.ThevaluesoftheregressionconstantsaresummarizedinTable4.TheresultsoftheregressionanalysisonthetestdataofOttawasand(Salgadoetal.2000)arealsoshowninthetableforcomparison.

TheCvalueforCDGissignificantlylargerthanthatfortheOttawasandmixtures.ThisseemstoimplythattheintactCDGhasahigherGvhthanhastheartificialOttawasandmixturesforagivenvoid

2520

ratio(e)andmeaneffectivestress(p󰀁).TheavalueofCDGislessthanthatofOttawasand,indicatingthatthevoidratio(e)haslesssignificantinfluenceonGvhinintactCDGthanthatinOttawasandmixtureswithhighfinecontents.ThisdifferencemaybeattributedtothepresenceofsoilfabricandbondingintheintactCDGinheritedfromtheparentrock.Onedistinctivefeatureforsaproliticsoilsisthestructureandbondingresultingfromweatheringprocesses(Blight1997).Ithasbeenshownthattheshearwavevelocity,andthustheshearmodulus,islesssensitivetovoidratiochangesincementedsoil(Yun&Santamarina2005).

Theshearmodulusdeterminedincrossholeseismicmeasurements(Ghv)(Ng&Wang2001)conductedatthesamedepthsandatthesamesitewheretheMazierspecimenswereextractedforlaboratorytestsareshowninFigure10.Theshearmodulusdeterminedinthelaboratorytests(Gvh)isabout50%to80%ofthatmeasuredinthefield(Ghv).Theinsituhorizontalstressesmaybehigherthantheestimatedhorizontalstresses(assumingK0=0.4),whichresultinahigherstiffnessdeterminedinthefield.Thediscrepanciesinthemeasuredshearmodulusmayalsobeattributedtothelossofsoilstructureandbondinginthelaboratoryspecimensasaresultofsampledisturbance.

7.2.2Variationsinshearmoduluswithshearstrain

Wang&Ng(2005)studiedtheinfluenceofstresspathsandrecentstresshistoryonthesmall-strainstiffnessofCDGfromaseriesofconstantp󰀁triaxialcompressionandextensiontests.Therecentstresshistoryisdefinedastherotationofthecurrentstresspathfromthepenultimatestresspath(Atkinsonetal.1990a).Inadditiontotherecentstresshistory,thestiffnessofsoilsisinfluencedbythestateofthesoil,forexample,thestressstateandthevoidratio.Inthestudyofstiffnessdegradationofsoils,normalizationisrequiredinordertocomparetheresultsfromdifferenttestsandmaterials.Toaccountfortheeffectsofthestressstateandvoidratio,theshearmodulusdeterminedinthetriaxialstresspathtestspresentedinthispaperisnormalizedbytheshearmodulusatverysmallstrains,Gvh,obtainedfromtheshearwavevelocitymeasurements.Thiswayofnormalizationisoftenadoptedinpresentingtheshearmodulusofsoilsdeterminedinmonotonicandcyclictests(Stokoeetal.1995,Tatsuoka2001).

Constantp󰀁triaxialcompressiontestswerecarriedoutontheMazierspecimensofCDGtakenfromvariousdepthsatYenChowStreet(Wang&Ng2005).Thetestswereperformedinatriaxialapparatusequippedwithlocalstraintransducers(Hall-effecttransducers).Figure11ashowsthevariationsinthenormalizedsecantshearmodulus(Gsec/Gvh)withdeviatoricstrain(εq)oftheconstantp󰀁compressiontests.ThevalueofGvhateachmeaneffectivestresslevel(p󰀁)wasdeterminedfromtheshearwavevelocitymeasurementinanotherseriesoftestsusingbenderelementsembeddedintheendplatensofthetriaxialapparatus.Gsec/Gvh=1thusrepresentstheelasticshearmodulus.Asexpected,theshearmodulusdecreasesasthedeviatoricstrainincreases.Theresultsamongthefivetestsarescattered,whichmaybetheresultofthesamplevariabilityoftheCDGspecimensalongthedepthfrom23mto54m.Theaverageshearmodulusdegradationcurveisshowninthefigure.TheaveragecurveshowsthattheshearmodulusofCDGinthecompressiontestsdropstoabout30%oftheinitialvalue(Gvh)atadeviatoricstrainof0.01%andfurtherreducesto15%oftheinitialvalue(Gvh)atadeviatoricstrainof0.1%.

Aseriesofconstantp󰀁triaxialextensiontestswerealsoconductedonMazierspecimensofCDGtakenfromthesamesite(Wang&Ng2005).Figure11bshowstheshearmodulusdegradationcurvesoftheconstantp󰀁extensiontests.ThevalueofGvhfornormalizationateachmeaneffectivestresslevel(p󰀁)wasthesameasthatinthecompressiontests.Anaverageshearmodulusdegradationcurveisshowninthefigure.ItcanbeseenthattheshearmodulusofCDGalongtheextensionpathreducestoabout50%and20%oftheinitialvalue(Gvh)atdeviatoricstrainsof0.01%and0.1%,respectively.Thematerialshowsastifferresponsealongtheextensionpath,whereGsec/Gvhisabout60%higherthanthatalongthecompressionpathatadeviatoricstrainof0.01%.ThedifferenceinGsec/Gvhbetweentheextensionandcompressionpathsdiminishestoabout20%atadeviatoricstrainof0.1%andvanishesatadeviatoricstrainof1%.Thehighershearmodulusalongtheextensionpathmaybeattributedtostresspathreversal.Duringtheweatheringprocess,someoftheoriginalmineralsinthegraniteweredecomposedandleached.ThisweatheringprocessisencouragedbysufficientprecipitationinHongKongandleadstoasignificantvolumelossandgroundsettlement(Goodman1993).Thisleachingprocessisanalogoustoapplyingcompressionloadstosoilspecimensinsuchawaythatthesoilsaresubjectedtosimilardownwarddeformation.Whentheintactsoilspecimenswereloadedalongtheextensionpath,stresspathreversaltookplaceandresultedinahigherstiffnessresponse(Atkinsonetal.1990a).Onthecontrary,thecompressiontestsfollowedthecontinuingstresspathofthenaturalweatheringprocess.Therewasnostresspathreversal,whichresultedinalowerstiffnessresponse.

2521

1.0

(a)0.90.80.70.6Gsec/Gvh0.50.40.30.20.10.00.001

0.010.1Deviatoric strain, εq (%)

p' = 100 kPap' = 150 kPap' = 200 kPap' = 250 kPap' = 300 kPaAverage1.00.90.80.70.6Gsec/Gvh0.50.40.30.20.10.00.001

0.01

Deviatoric strain, εq (%)

0.1

(b)p' = 100 kPap' = 150 kPap' = 200 kPap' = 250 kPap' = 300 kPaAverageFigure11.

Gsec/Gvhvs.εqofCDG:(a)Compressiontests;(b)Extensiontests.

7.2.3Comparisonoftheshearmodulus-shearstrainrelationshipdeterminedintriaxialtestsand

in-situSBPMtests

Figure12comparesthevariationsintheshearmodulusofCDGwiththeshearstraindeterminedintriaxialtestsandinSBPMtests.Theresultsoftheconstantp󰀁(p󰀁=200kPa)triaxialcompressiontestsontheMazierspecimensofCDGatKowloonBay(Depth=36m)(Ngetal.2000)areincludedforcomparison.Theupperandlowerboundsofthemeasurementsareshown.TheCDGatYenChowStreetshowsahighershearmodulusinthefieldteststhaninthelaboratorytests.Thereasonsforthediscrepancyincludesampledisturbanceanddifferencesinstresshistory.SomedegreeofdamagetothebondingandstructureofnaturalCDGisinevitableduringsampling,transportation,storageandspecimenpreparation.Thelossofbondingandstructureinsoilsresultsinasignificantreductioninsoilstiffness(Atkinsonetal.1990b,Cuccovillo&Coop1997).Theshearmodulusdeterminedinthe

2522

800700

Laboratory measurements -Constant p' compression (Ng et al. 2000)Constant p' compression (Ng & Wang 2001)Constant p' extension (Ng & Wang 2001)Field measurements -SBPM tests (Ng et al. 2000)SBPM tests (Ng & Wang 2001)600500Gsec/p'400300

20010000.001

0.01

Shear strain, εs (%)

0.11

Figure12.ComparisonofthefieldandlaboratorymeasurementsonthevariationsintheshearmodulusofCDGwithshearstrain.

SBPMtestsisderivedfromtheunloadingstress-straincurve.Unloadinginvolvesstresspathreversalandthusresultsinahighershearmodulus.WhiletheupperboundsoftheshearmodulusdeterminedintheSBPMtestsconductedatYenChowStreetandKowloonBayaresimilar,theshearmodulusdeterminedinthelaboratorytests(constantp󰀁triaxialcompression)ontheCDGatKowloonBayishigherthanthatdeterminedinsimilartestsontheCDGatYenChowStreet.ItshouldbenotedthatthevoidratiosoftheCDGspecimensfromthetwositesaresimilaratthesamemeaneffectivestresslevel(p󰀁=200kPa).ThehighershearmodulusdeterminedinthelaboratorytestsontheCDGatKowloonBaymightbeduetoalowerdegreeofdecompositionoftheCDGsampledinKowloonBay.

7.2.4StudyofstiffnessanisotropyofnaturalCDGusingMazierspecimens

Itiswelldocumentedthattheshearwavevelocityofsoildependsonthestressesinthedirectionofwavepropagationandparticlemotionandisindependentofthestressnormaltotheplaneofshear(Roesler1979).Assumingthattheeffectsofthetwostressesintheplaneofshearontheshearwavevelocityaresimilar(Bellottietal.1996),asemi-empiricalrelationshipbetweentheshearmodulusandthestateofthesoilcanbeexpressedasfollows(Ng&Leung2006):

wherei:asubscriptindicatingthedirectionofwavepropagation,whichisdenotedbyh(horizontal

direction)orbyv(verticaldirection);j:asubscriptindicatingthedirectionofparticlemotion,whichisdenotedbyh(horizontal

direction)orbyv(verticaldirection);Gij:theelasticshearmodulusinthei–jplane;vs(ij):theshearwavevelocityinthei–jplane;ρ:thebulkdensityofthematerial;

Cij:aconstantreflectingthesoilfabricinthei–jplane,withdimensionm/s;F(e):avoidratiofunctionrelatingshearmodulustothevoidratio;

2523

Table5.Theshearmoduli,GhvandGhh,ofCDG(Mazierspecimen).p󰀁(kPa)80160400

q(kPa)80160400

󰀁(kPa)σv

󰀁(kPa)σh

Ghv(MPa)82

126275

Ghh(MPa)65110220

13326766753107267

1000

GhhGhv Gij (MPa)10010100010000σi'xσj' (kPa2)

1000001000000Figure13.VariationsinGijwithσi󰀁×σj󰀁ofCDG(Mazierspecimen).

ρwprσi󰀁σj󰀁n

:::::thedensityofwater;

thereferencepressure,takenas1kPainthisstudy;

theeffectiveprincipalstressinthedirectionofwavepropagation;theeffectiveprincipalstressinthedirectionofparticlemotion;anempiricalstressexponent.

ByusingtheequipmentshowninFigure9a,multidirectionalshearwavevelocitymeasurementsweretakenonaMazierspecimenofCDGtakenfromYenChowStreetatadepthof27m.Measurementsweretakenatanisotropicstressstatesofp󰀁=q=80,160and400kPa.Themeasuredvaluesoftheshearmoduliinthevertical(Ghv)andthehorizontalplanes(Ghh)aresummarizedinTable5.Undertheinfluenceofahigherverticaleffectivestress,GhvisonaveragehigherthanGhhbyabout22%ateachofthestressstates(p󰀁=q=80,160and400kPa),showingstress-inducedanisotropy.Figure13showsthevariationsinGhvandGhhwiththeproductofeffectivestressesintheplaneofshear(σi󰀁×σj󰀁).ByplottingGhvandGhhagainst(σi󰀁×σj󰀁),theeffectofhorizontalandverticaleffectivestressesontheshearmodulicanbeaccountedfor.Best-fitlinesthroughthedataofGhvandGhhareshowninthefigure.Thebest-fitlinesthroughthedataofGhvandGhhareclosetoeachother.Thissuggeststhatunder

󰀁󰀁󰀁󰀁󰀁󰀁

isotropicstresscondition,i.e.σh=σvandσh×σv=σh×σh,thevaluesofGhvandGhharesimilar.Theinherentstiffnessanisotropy(degreeofanisotropyunderisotropicstressstate)ofCDGthusappearsnottobeobviousfromthislimitedsetoftestresults.IthasbeendemonstratedthatthemeasureddegreeofinherentstiffnessanisotropyofCDTislowerinMazierspecimensthaninblockspecimensduetoahigherdegreeofsampledisturbance(Ng&Leung2006).Theeffectsofsampledisturbanceonthesmall-strainstiffnessandstiffnessanisotropyofsoilarefurtherdiscussedinSection7.7.AstheCDGspecimenwasobtainedbyMaziersampling,theanisotropicstructureofCDG,ifany,maybedamaged

2524

1000

Gij (MPa)100

Block Mazier specimen specimen GhhGhhIsotropic GhvGhvGvhGvhAnisotropic 10

1000

GhhGhvGvhGhhGhvGvh1000000

10000

σi'xσj' (kPa2)

100000

Figure14.VariationsinGijwithσi󰀁×σj󰀁ofCDT(blockandMazierspecimens)(Ng&Leung2006).

bythesampledisturbance.FurthertestsarerequiredtostudythestiffnessanisotropyofCDGusingspecimenswiththeleastdegreeofsampledisturbance,i.e.blockspecimens.7.3Stiffnessanisotropyofcompletelydecomposedtuff(CDT)

7.3.1Inherentstiffnessanisotropyatverysmallstrains

BlockandMaziersamplesofCDTweretakenfromanexcavationsiteinLuenWoHui(Fig.1)fortheinvestigationofstiffnessanisotropyatverysmallstrainsinthelaboratory(Ng&Leung2006).Thetestswereconductedinacomputer-controlledtriaxialapparatusequippedwithbenderelementsandlocalstraintransducersasshowninFigure9a.

BlockandMazierspecimensofCDTweresubjectedtoisotropiceffectivestressesintherangesof80kPato400kPaand100kPato400kPa,respectively.Shearwavevelocitiesinthreeorthogonalplanes,vs(hh),vs(hv)andvs(vh),weremeasuredtodeterminetheinherentanisotropyofthematerial.Figure14showstheshearmoduli(Ghh,GhvandGvh)oftheblockandMazierspecimensofCDTunderisotropicstressstates.TheshearmoduliofCDTmeasuredunderanisotropicstressstatesarealsoshowninthefigureandtheresultsarediscussedinthenextsection.CDTshowsstiffnessanisotropy,withtheshearmodulusinthehorizontalplane(Ghh)higherthantheshearmodulusintheverticalplane(GhvorGvh).ThevaluesofGvh,GhvandGhhandthedegreeofstiffnessanisotropyexpressedasGhh/GhvoftheblockandMazierspecimensaresummarizedinTable6.TheaveragevaluesofGhh/GhvoftheblockandMazierspecimenswere1.48and1.36,respectively.Theshearmoduli,Ghh,GhvandGvh,oftheblockspecimenareconsistentlyhigherthanthoseoftheMazierspecimen.TheMazierspecimenshowsalowerdegreeofanisotropyandlowershearmoduliduetosampledisturbance,whichisfurtherdiscussedinSection7.7.Itshouldbenotedthatinanidealelasticcontinuum,GvhequalsGhv.Inbothspecimens,GvhdoesnotequalGhv.Asimilarobservationismadeinthestudiesofstiffnessanisotropyofsandsandclays(Kuwanoetal.1999,Pennington1999).Thediscrepancymaybeduetothedeviationfromtheassumptionoftheidealcontinuumofsoilspecimens.

Table7summarizessomerecentfindingsonthedegreeofinherentstiffnessanisotropyofdifferentgeomaterialsbyshearwavevelocitymeasurements.Whileithasbeenshownthatthedegreeofinherentstiffnessanisotropyofreconstitutedorair-pluviatedmaterialsishigherinfinermaterials(i.e.Ghh/Ghv

2525

Table6.Typeofspecimen

Laboratorymeasurementsoftheshearmoduli,Gvh,GhvandGhh,ofCDT(Ng&Leung2006).

Stressstatep󰀁(kPa)

q(kPa)

Voidratio

ShearmodulusGvh(MPa)7177109139178426277927579117154176426384109

Ghv(MPa)827013116319455781081248686126157194567797130

Ghh(MPa)127127194254277761071491651061091582032396590119136

DegreeofanisotropyGhh/Ghv1.541.401.471.561.431.361.381.381.321.241.271.261.301.231.161.161.221.04

Block

Mazier

Block

Mazier

Isotropic–80010002000300040001000200030004000Anisotropic–8080100100200200300300400400100100200200300300400400

0.56

0.550.540.530.520.760.740.720.700.580.580.560.550.540.830.800.780.76

ishigherinclaythaninsiltandsand;Kuwanoetal.1999),thedegreeofinherentstiffnessanisotropyoftheblockspecimenofnaturalCDT(Ghh/Ghv=1.48)isofasimilarordertosomenaturalclayssuchasLondonclay.TherelativelyhighdegreeofanisotropyofCDTispossiblyattributedtosomegeologicprocessessuchasmetamorphismoftheparentrockmassofCDT.Thetuffintheareaisslightlyschistoseasaresultofmetamorphismandpossessesametamorphicmatrixdominatedbyfinelydividedquartzandsericitewithaweakorientation(Ho&Langford1987,Langfordetal.1989).Theanisotropicfabricisretainedinthesaproliteofthetuffandresultsinstiffnessanisotropy.ShearwavevelocitymeasurementsonsaturatedreconstitutedCDTshowedthattheshearmodulusatverysmallstrainsinthehorizontalplaneissimilartothatintheverticalplane(Yung2004).ThisshowsthattheanisotropicsoilstructureofnaturalCDTresultingfromgeologicprocessesisresponsibleforthestiffnessanisotropy.AhigherdegreeofinherentstiffnessanisotropyinnaturalsoilisalsoexhibitedinPentresilt(Kuwanoetal.1999),anaturalglacio-lacustrineclayey-siltpossessingalaminatedmacro-structure,wherethedegreeofinherentanisotropy(Ghh/Ghv=1.7)ishigherthaninsomenaturalclays,suchasLondonclayandPisaclay(Ghh/Ghv=1.4–1.5).Inair-pluviatedsandandreconstitutedclay,theinherentanisotropyisduetothedepositionalfabricandgraincharacteristicsofsoilparticles(Arthur&Menzies1972,Kuwanoetal.1999).Thedegreeofinherentanisotropyofair-pluviatedsandrangesfrom1.1to1.2,whilethatofreconstitutedclayisintherangeof1.2to2.0.

7.3.2Stress-inducedstiffnessanisotropyofCDTatverysmallstrains

ThedegreeofstiffnessanisotropyintheblockandMazierspecimensofCDTunderanisotropicstressstateshasbeenreportedbyNg&Leung(2006).Specimensweresubjectedtoanisotropiceffectivestressesrangingfromp󰀁=q=80kPatop󰀁=q=400kPa.Thestressratio(η=p󰀁/q)waskeptconstantat

󰀁󰀁

η=1.0,correspondingtoK=σh/σv=0.4.Shearwavevelocitiesinthreeorthogonalplanes,vs(hh),vs(hv)andvs(vh),weremeasuredandthecorrespondingshearmoduli,Ghh,GhvandGvh,werecalculated(Gij=ρvs(ij)).Figure14showstheshearmoduli(Ghh,GhvandGvh)oftheblockandMazierspecimensofCDTunderanisotropicstressstates.Theshearmodulideterminedintheanisotropicstressstatesaregenerallyconsistentwiththedataobtainedintheisotropicstressstateafterconsideringtheeffectiveprincipalstressesintheshearplanes.ThisimpliesthattheshearwavevelocitiesdependonthetwoeffectivestressesintheshearplanesassuggestedinEquation4.ThehighervalueofGhhinbothspecimensisduetothehorizontallayeringstructureofCDTasdiscussedinSection7.3.1.ThevalueofGhvisgenerallyhigherthanGvhinbothspecimens,butthedifferencedecreaseswithincreasingstresslevel.Theincreasingverticaleffectivestressenhancesthedevelopmentoftheforcechainsthroughthesoilparticlesintheverticaldirection,resultinginarelativeincreaseinGvh(Jardineetal.1999).

2526

Table7.Inherentstiffnessanisotropyofdifferentgeomaterials(Ng&Leung2006).

Samplepreparationmethod/natureReconstitutedReconstitutedReconstitutedReconstitutedAirpluviationAirpluviationAirpluviationAirpluviationNaturalNaturalNaturalNaturalNaturalNaturalNatural

MaterialSpeswhitekaolinSpeswhitekaolinLondonclayHPF4siltToyourasandTicinosandTicinosandKenyasandPisaclay

PanigagliaclayLondonclay

CambridgeGaultclayPentresilt

EdogawaPleistocenedeposit(sand)

Completelydecomposedtuff(CDT)

∗†

p󰀁(kPa)250–60030040040010025–400100–30010–700N/AN/A120–4009050N/A80–400

eN/AN/AN/AN/AN/A0.81

0.80–0.811.3N/AN/AN/A0.81N/A

0.80–0.950.52–0.58

Ghh/Ghv1.7∗2.0∗1.24∗1.5∗1.2∗1.1∗1.21†1.2∗1.4‡1.6‡1.5∗2.01∗1.7∗1.1§1.48∗

Reference

Jovˇci´c&Coop(1998)

Kuwanoetal.(1999)Joviˇci´c&Coop(1998)Kuwanoetal.(1999)Kuwanoetal.(1999)Fioravante(2000)Bellottietal.(1996)Fioravante(2000)

Jamiolkowskietal.(1995)Jamiolkowskietal.(1995)Joviˇci´c&Coop(1998)Pennington(1999)Kuwanoetal.(1999)Nishio&Katsura(1995)Ng&Leung(2006)

BenderelementsintriaxialapparatusGeophonesincalibrationchamber‡Benderelementsinoedometer

§BenderelementsmountedinbearingplatesplacedatoppositeendsofsoilblockN/AInformationisnotavailable

ThemeasuredshearmoduliaresummarizedinTable6.Asinthetestsunderisotropicstresses,GhhishigherthanGhvandGvhinbothspecimensunderanisotropicstresses.Thisisnotexpectedasitwouldbeintuitivetothinkthattheshearmodulusintheverticalplanewouldbehigherthanthatinthehorizontalplaneunderhigherverticaleffectivestressconditions(refertoEquation4)iftherewerenopriorknowledgeofanyanisotropicsoilfabric.Theeffectofastrongerlayeringstructureinthehorizontalplaneontheshearmodulus(i.e.Ghh>Ghv)prevailsevenunderahigherverticaleffectivestresscondition(i.e.K=0.4).Theaveragevaluesofthedegreeofstiffnessanisotropy(Ghh/Ghv)are1.26and1.15forblockandMazierspecimens,respectively.Thereduceddegreeofstiffnessanisotropyintheanisotropicstressstatecomparedwiththeisotropicstressstateisduetotheeffectofahigherverticaleffectivestressintheanisotropicstressstate.AsshowninEquation4,ahigherverticaleffectivestressincreasestheshearmodulusintheverticalplane,Ghv,andthusreducesthedegreeofstiffnessanisotropy(Ghh/Ghv).Theshearmoduli(Ghh,GhvandGvh)oftheblockspecimensareagainconsistentlyhigherthanthoseoftheMazierspecimenduetothelowerdegreeofsampledisturbanceoftheblockspecimens.TheeffectsofsampledisturbanceareaddressedinSection7.7.7.4Effectsofsuctiononanisotropicstiffness

UsingtheequipmentshowninFigure9b,Ng&Yung(2005)investigatedtheanisotropicstiffnessofareconstitutedCDTretrievedfromLuenWoHuisite(Fig.1)atvariousmatricsuctions(ua–uw=0,50,100and200kPa),whereuaanduwaretheporeairpressureandtheporewaterpressure,respectively.Itshouldbenotedthatua–uw=0representsthesaturatedcase.Theshearwavevelocitiesinthreeorthogonalplanes,vs(vh),vs(hv)andvs(hh),weremeasuredatdifferentmatricsuctions.DetailsofthetestingproceduresaredescribedbyNg&Yung(2005).Figure15showsthevariationsinshearwavevelocitywithmatricsuction(ua–uw)fordifferentvaluesofnetmeanstress(p–ua),wherepisthemeantotalstress.Theshearwavevelocitiesincreasewiththetwostressstatevariables,matricsuctionandnetmeanstress.Theshearwavevelocitiesincreasenon-linearlywithincreasingmatricsuction.Withintheair-entryvalue,i.e.ua–uw=50kPa,ofthereconstitutedCDT,theshearwavevelocitiesincreasesignificantly.Thesoilspecimensbehaveaswereessentiallysaturatedatamatricsuctionbelowtheair-entryvalue.Anincreaseinthematricsuctioncanbeconsideredasequivalenttoanincreasein

2527

400(a)Shear wave velocity, vs(vh) (m/s)3503002502001500p-ua = 110 kPa p-ua = 200 kPa p-ua = 300 kPa p-ua = 400 kPa p-ua = 500 kPa 50100150200Matric suction, ua-uw (kPa)400(b)Shear wave velocity, vs(hv) (m/s)3503002502001500

p-ua = 110 kPap-ua = 200 kPa p-ua = 300 kPa p-ua = 400 kPa p-ua = 500 kPa 50

100

150

200

Matric suction, ua-uw (kPa)

400Shear wave velocity, vs(hh) (m/s)(c)3503002502001500

p-ua = 110 kPa p-ua = 200 kPa p-ua = 300 kPa p-ua = 400 kPa p-ua = 500 kPa 50

100150

Matric suction, ua-uw (kPa)

200

Figure15.Theshearwavevelocity,(a)vs(vh),(b)vs(hv)and(c)vs(hh),plottedagainstmatricsuction(Ng&Yung2005).

themeaneffectivestress,andhencetheshearwavevelocitiesincrease(Mancusoetal.2000).Therateofincreaseoftheshearwavevelocitiesreducesasthematricsuctionexceedstheair-entryvalueofthematerial(ua–uw=50kPa).Air-watermenisci(orcontractileskins)formattheinter-particlecontactsasthesoilspecimensstarttodesaturateatmatricsuctionsexceedingtheair-entryvalue.Thisresultsinanincreaseinthenormalforceactingontheparticlesandthustheshearwavevelocitiesincrease.Upon

2528

1.81.71.61.5Ghh/Ghv1.41.31.21.11.0

Reconstituted specimens:ua – uw = 0 kPaua – uw = 50 kPa ua – uw = 100 kPaua – uw = 200 kPaBlock specimens:ua – uw = 0 kPa(Ng et al 2004b)ua – uw = 0 kPa ua – uw = 100 kPa 050100150200250300Net mean stress, p - ua (kPa)

350400450500Figure16.DegreeofstiffnessanisotropyofintactandreconstitutedspecimensofCDTunderisotropicnetmeanstress(Ng&Yung2005).

furtherincreaseofthematricsuction,themeniscusradiusreduces,whichlimitstheincreaseoftheshearwavevelocities(Mancusoetal.2000).

Figure16showsthedegreeofstiffnessanisotropy,expressedasGhh/Ghv,ofreconstitutedspecimensofCDTunderisotropicnetmeanstressesatvariousvaluesofmatricsuctions.ThemeasureddegreesofstiffnessanisotropyoftheintactspecimensofCDTatmatricsuctionsof0and100kPaarealsoshowninthefigureforcomparison.Thedegreeofstiffnessanisotropyofthereconstitutedandintactspecimensappearsnottobeaffectedbythematricsuction.Overtherangeoftheappliednetmeanstress,thedegreeofstiffnessanisotropyisessentiallyconstantinbothtypesofspecimen.Theanisotropyofthesoilfabricisprobablynotalteredbythenetmeanstressovertherangeofthenetmeanstressconsidered.Therecon-stitutedspecimensconsistentlyshowalowerdegreeofstiffnessanisotropy(Ghh/Ghv=1.0−1.1)thandotheintactspecimens(Ghh/Ghv=1.4−1.5).TheanisotropicsoilfabricofnaturalCDTisdestroyedinthereconstitutedspecimens,resultinginsimilarshearmoduliinthehorizontalandverticalplane.7.5Effectsofcyclicloadingandcreeponthesmall-strainstiffnessofdecomposedtuffandrhyoliteUndrainedcyclictriaxialcompressiontestswithlocalstrainmeasurementswereconductedonMazierspecimensofcompletelydecomposedtuff(CDT)fromLuenWoHuitakenatadepthof29mandcompletelydecomposedrhyolite(CDR)fromSiteAtakenatadepthof27m(Fig.1).Figure17showsthestresspathsthroughoutthetests.Back-pressuresaturationandisotropicconsolidationwerecar-riedoutunderanisotropiceffectivestress,whichisequivalenttotheinsitulateraleffectivestress(p󰀁=118kPa;q=0kPa).Thespecimenswerethenanisotropicallyconsolidatedtotheinsitueffectivestress(p󰀁=170kPa;q=156kPa).Aftertheanisotropicconsolidationwascompleted,thespecimenswereanisotropicallyconsolidatedtoaneffectivestressstateofp󰀁=548kPaandq=504kPa,whichwasafoundationstressstate.Thestressstatewasmaintainedfor10hourssuchthattheaxialstrainratewasmaintainedatlessthan0.002%perhour.

Anundrainedcyclictriaxialtestof100cyclesstartingwithcompressionwascarriedoutaftercon-solidation,withacyclicloadingamplitude(󰀈σv)of±153kPaandaloadingfrequencyofonecycleperminute(0.0167Hz).TheundrainedsecantYoung’smodulus(Eu)ofthematerialsduringcyclicloadingwasdeterminedandthevaluesofEuduringthefirstcompressioninthecyclicloading(AtoBinFig.17)andthefirstrecompressioninthecyclicloading(CtoDinFig.17)arepresentedinthispaper.Aftercyclicloading,aperiodof11hourswasallowedforthespecimenstocreepuntilthestressstatereturned

2529

1000900800700600q (kPa)5004003002001000

CAAnisotropic consolidation to in situ effective stressAnisotropic consolidation to foundation loading stress stateUndrained cyclic loading (100 cycles)CreepUndrained compressionEDB0100200300p' (kPa)

400500600Figure17.StresspathsofthetestsonCDTandCDR(Mazierspecimen).

toAinFigure17andtheaxialstrainratewasmaintainedlessthan0.002%perhour.Thespecimenswerethenshearedunderundrainedcompressionatarateof0.2%perhour.TheundrainedsecantYoung’smodulusduringundrainedcompressionispresented(AtoE)inFigure17.

Figure18showstheundrainedsecantYoung’smodulus(Eu)ofthematerialsatvariousstagesnormal-izedbytheinitialmeaneffectivestress(p󰀁0).ThenormalizedundrainedsecantYoung’smoduli(Eu/p󰀁0)atthreestagesarepresented:(a)firstcompressioninthecyclicloading(AtoBinFig.17);(b)firstrecompressioninthecyclicloading(CtoDinFig.17);and(c)compressionaftercyclicloadingandcreep(AtoEinFig.17).ThenormalizedundrainedsecantYoung’smoduli(Eu/p󰀁0)decreaseasaxialstrainsincreaseovertherangeofthemeasuredaxialstrainsinallthreestages.TheundrainedsecantYoung’smodulusofCDRisslightlyhigherthanthatofCDTateachrespectivestage.TherateofYoung’smodulusdegradationisdifferentatdifferentstagesofundrainedcompressionforbothmaterials.Bothmaterialsshowtheloweststiffnessinthefirstcompressionofthecyclicloading(AtoB).Aftersub-sequentunloading,thematerialsshowthehigheststiffnessalongtherecompressionloadingpath(CtoD),whereEu/p󰀁0increasesbyabout100%comparedwiththefirstcompressionloading(AtoB)forbothmaterials.A180◦stresspathrotationisinvolvedintherecompression,resultinginthestiffestresponseofthematerials,whichdemonstratestheeffectoftherecentstresshistoryondecomposedmaterials(Wang&Ng2005).TheundrainedYoung’smodulusofCDRandCDTatanaxialstrainof0.01%duringtheundrainedcompressionaftercyclicloadingandcreep(AtoE)increasedbyabout30%and15%,respectively,comparedwiththeundrainedYoung’smodulusofthematerialsdeterminedinthefirstcompressionofthecyclicloading(AtoB).Astimewasallowedforthespecimenstocreep,deformationofthespecimenswasreducedtoaratethatwasnotsignificantinaffectingsubsequentstiffnessmeasurements.TheincreaseintheundrainedYoung’smodulusinthematerialsaftercyclicloadingandcreep(AtoE)maybeduetotheexpansionofyieldsurfaceofthematerialsduringthefirstcompressionofthecyclicloading(AtoB).

7.6Comparisonofthesmall-straincharacteristicsofnaturallydecomposedgraniteandtuff7.6.1Shearmodulusatverysmallstrains

Figure19showstheshearmodulusofCDGandCDTintheverticalplane,Gvh,determinedfromshearwavevelocitymeasurementsusingbenderelements.TheshearmodulusoftheMazierspecimensofCDG

2530

1400

CDT – 1st loading (A-B); p’0 = 548 kPa Reloading (C-D); p’0 = 488 kPa After cyclic loading and creep (A-E); p′0 = 548 kPaCDR – 1st loading (A-B); p’0 = 548 kPa Reloading (C-D); p’0= 474 kPa After cyclic loading and creep (A-E); p’0 = 548 kPa1200

1000

800Eu/p′06004002000

0.001

0.01

Axial strain, εa (%)

0.11

Figure18.

NormalizedundrainedsecantYoung’smodulus(Eu/p󰀁0)ofCDTandCDR.

1000

CDG -Depth = 23 mDepth = 27 mDepth = 28 mDepth = 37 mDepth = 45 mDepth = 54 mCDT -Depth = 14 mDepth = 29 mGvh (MPa)100

101000

10000

σv'x σh' (kPa2)

1000001000000

Figure19.

ComparisonofGvhofCDGandCDT(Mazierspecimens).

(fromYenChowStreet)atdepthsfrom23mto54misshown.ThereisanincreaseinGvhinspecimenstakenatgreaterdepths(Depth=45mand54m),whichisrelatedtothelowerdegreeofdecompositionatdeeperlevels.ThevaluesofGvhoftheMazierspecimensofCDGtakenatdepthsof45mand54mare22%and50%,respectively,higherthanthattakenatadepthof28.5munderthesameeffectivestress

2531

1000

Gij (MPa)100

CDG -GhhGhv CDT -GhhGhv 101000

10000

100000

σi'x σj' (kPa2)

1000000

Figure20.ComparisonofthestiffnessanisotropyofCDGandCDT(Mazierspecimens).

level.ThefigurealsoshowsthevaluesofGvhoftheMazierspecimensofCDTtakenatdepthsof14mand29m.Atasimilareffectivestresslevel,thevalueofGvhoftheMazierspecimenofCDTatadepthof29mis64%higherthanthatatadepthof14m,whichreflectsthelowerdegreeofdecompositionatagreaterdepth.Atasimilardepth,thevaluesofGvhoftheMazierspecimensofCDT(Depth=29m)andCDG(Depth=28.5m)aresimilar.ThereappearstobenoobviousdifferenceinthemeasuredshearmodulusatverysmallstrainsbetweenCDGandCDTusingMazierspecimens.

ThestiffnessanisotropiesoftheMazierspecimensofCDGandCDTarecomparedinFigure20.Thefigureshowstheshearmoduliinthehorizontalandverticalplanes,GhhandGhv,determinedfrommulti-directionalshearwavevelocitymeasurementsonMazierspecimensusingbenderelements.TheMazierspecimenofCDTclearlyshowstiffnessanisotropy,withtheshearmodulusinthehorizontalplanehigherthanthatintheverticalplane(Ghh>Ghv).ThestiffnessanisotropyisattributedtothehorizontallayeringstructureofCDTasaresultofmetamorphismintheparentrock,whichisdiscussedinSection7.3.1.StiffnessanisotropyisapparentlynotobviousintheMazierspecimenofCDGasrevealedinthesinglesetoflaboratorytestdata.DisturbanceduringMaziersamplingmaydamagetheanisotropicstructureofCDGifsuchasoilstructureexists.FurthertestsarerequiredtostudytheshearmodulusofCDGindifferentplanesusingspecimenswiththeleastdegreeofsampledisturbance(e.g.blockspecimens).7.6.2Variationsinshearmoduluswithshearstrain

Constantp󰀁triaxialcompressionandextensiontestswereperformedonMazierspecimensofCDTinatriaxialapparatusequippedwithbenderelementsandlocalstraintransducers(Fig.9a).Shearwavevelocitymeasurementsweretakenbeforeconstantp󰀁shearingtodeterminetheshearmodulusatverysmallstrains.Thecompressiontestsandtheextensiontestsinvolvedifferentanglesofstresspathrotation(theanglemeasuredanti-clockwisefromtheapproachingstresspath).Theangleofstresspathrotationinthecompressiontestswas90◦,whilethatintheextensiontestswas135◦.Figures21aand21bshowthenormalizedshearmodulusdegradationcurves(Gsec/Gvhvs.εq)ofCDTalongthecompressionandextensionpaths,respectively.ThetestdataforCDGarealsoshowninthefiguresforcomparison.

ThevariationsintheshearmodulusofCDTwithdeviatoricstrainaregenerallyconsistentwiththoseofCDGalongbothcompressionandextensionpaths.ThedatafromthetestsonCDTlieclosetotheupperboundofthetestdatafromCDGalongbothcompressionandextensionpaths.Finematerials(e.g.,clays)exhibitlessshearmodulusdegradationatagivenstrainlevelcomparedwithcoarsematerials(e.g.,sands)(Stokoeetal.1995).CDTisslightlyfinerthanCDG(Fig.4),butthedifferenceinparticlesizeisnotsosignificanttoresultinanobviouschangeintheshearmodulusdegradationcurve.

Alongthecompressionpath(θ=90◦;Fig.21a),thenormalizedshearmodulusofCDTreducesby60%atadeviatoricstrainof0.01%.TheshearmodulusofCDTishigheralongtheextensionpath(θ=135◦;Fig.21b),wherethenormalizedshearmodulusreducesby30%atadeviatoricstrainof

2532

1.00.90.80.7Gsec/Gvh0.60.50.40.30.20.10.00.001

0.01

0.1

(a) CDT -p′ = 400 kPa; θ = 90°CDG -p′ = 100 kPa; θ = 90°p′ = 150 kPa; θ = 90°p′ = 200 kPa; θ = 90°p′ = 250 kPa; θ = 90°p′ = 300 kPa; θ = 90°Deviatoric strain, εq (%)

1.00.90.80.7Gsec/Gvh0.60.50.40.30.20.10.00.001

0.010.1Deviatoric strain, εq (%)

(b) CDT -p′ = 400kPa; θ = 135°CDG -p′ = 100kPa; θ ≈180° p′ = 150kPa; θ ≈180° p′ = 200kPa; θ ≈180° p′ = 250kPa; θ ≈180° p′ = 300kPa; θ ≈180° Figure21.

Gsec/Gvhvs.εqofCDGandCDT(Mazierspecimen):(a)Compressiontests;(b)Extensiontests.

0.01%.Thedifferenceinthemeasurednormalizedshearmodulusisclearlyshownuntilthedeviatoricstrainreachesabout1%.Atkinsonetal.(1990a)suggestedthatthechangeofsubsequentstiffnessasaresultofeitherasuddenchangeinthedirectionofthestresspathoraperiodoftimeataconstantstressstateistheeffectofrecentstresshistory.TheincreaseinsoilstiffnesswithincreasinganglesofstresspathrotationshowninFigures21aand21bistheeffectoftherecentstresshistoryasaresultofchangesinthedirectionofthestresspath.Similarbehaviourisalsoobservedinothertypesofsoil.Atkinsonetal.(1990a)showedthattheshearmodulusofreconstitutedLondonClayincreasesuptooneorderofmagnitude(atadeviatoricstrainofabout0.01%)withincreasingmagnitudeofθandthattheeffectofstresspathrotationvanisheswhendeviatoricstrainreachesabout0.5%.Hird&Pierpoint(1997)andPowrieetal.(1998)showedthattheshearmodulusofclayishigheraftera180◦stresspathrotation.Amorosietal.(1999)andClayton&Heymann(2001)demonstratedthedifferenceinsoilstiffnessderivedfromcompressionandextensiontestsduetotherecentconsolidationstresshistory.

2533

Table8.VoidratiochangesofCDTresultingfromreconsolidationtoinsitustresses.SpecimenB_anisoM_aniso

p󰀁(kPa)80100

q(kPa)80100

󰀈e/e0−0.01−0.01

εv(%)−0.2−0.4

εa(%)+1.1+3.0

εr(%)−0.6−1.7

εv:volumeticstrain.εa:axialstrain.εr:radialstrain.

7.7Sampledisturbance–comparisonofMazierandblocksampling

Samplinginducesirrecoverablestrainsinnaturalsoilsamples.Thesoilstructuremaybedamagedsignificantlyduringthesamplingprocess.Soilstructurehasbeenshowntohavesignificanteffectsonthemechanicalpropertiesofsoilssuchascompressioncharacteristics(Burland1990),strength(Burland1990,Claytonetal.1992),stiffness(Claytonetal.1992,Cuccovillo&Coop1997),yieldingandlarge-strainbehaviour(Cuccovillo&Coop1999).Ithasbeenshownthatdifferentsamplingtechniquesresultindifferentdegreesofsampledisturbanceintermsofthemagnitudeofinducedstrainsandthereductionofthemeaneffectivestressandshearwavevelocity(Balighetal.1987,Hight1993,LoPrestietal.1999,Lohanietal.1999).Itisthereforeimportanttoassesstheeffectofsamplingwhendeterminingthemechanicalpropertiesofnaturalsoils.

Thedegreeofsampledisturbancecanbeassessedbytheamountofvolumechangeresultingfromreconsolidationofasoilspecimentotheestimatedinsitueffectivestresses(Hight1993,Tanakaetal.1996,Lunneetal.1997,LoPrestietal.1999).Table8showsthemeasuredvoidratiochangeandstrainsresultingfromthereconsolidationoftheblock(B_aniso)andMazier(M_aniso)specimensofCDTtotheestimatedinsitueffectivestresses.Thevoidratiochangeoftheblockspecimenresultingfromreconsolidationissmall(󰀈e/e0=0.01).AccordingtothecriterionproposedbyLunneetal.(1997)forevaluatingsampledisturbance,thequalityofasoilsampleisconsideredverygoodtoexcellentif󰀈e/e0islessthan0.04forsoilswithoverconsolidationratiosbetween1and2.AlthoughthevoidratiochangeoftheMazierspecimenisalsosmall(󰀈e/e0=0.01),theassociatedaxialandradialstrainsarerelativelylarge.SampledisturbanceintheMazierspecimenisbelievedtobelargerthanthatintheblockspecimen.

AhigherdegreeofsampledisturbanceisresponsibleforthelowershearmodulianddegreeofstiffnessanisotropyoftheMazierspecimens.ItisshowninSection7.3.1thattheMazierspecimenshowsalowerdegreeofanisotropy(Ghh/Ghv=1.36)comparedwiththeblockspecimen(Ghh/Ghv=1.48).TheanisotropicstructureofCDTmaybedamagedbythetubesamplingprocess,resultinginalowerdegreeofanisotropyintheMazierspecimen.TheshearmodulioftheMazierspecimenarealsolowerthanthatoftheblockspecimenasshowninSections7.3.1and7.3.2.Theshearmoduli,Ghh,GhvandGvh,oftheMazierspecimenare72%,58%and81%lowerthanthoseoftheblockspecimen,respectively(Fig.14).Thesedifferencescorrespondtothedecreaseinshearwavevelocities,vs(hh),vs(hv)andvs(vh),by27%,21%and29%,respectively,andadecreaseofbulkmodulusby7%intheMazierspecimen.

8SUMMARYANDCONCLUSIONS

Thesmall-strainstiffnessofcompletelydecomposedgranite(CDG),tuff(CDT)andrhyolite(CDR)atfourtestsitesinHongKongwasstudiedthroughfieldandlaboratorymeasurements.FieldtestingtechniquesincludingsuspensionP-Svelocitylogging,crossholeseismicmeasurementsandself-boringpressuremeter(SBPM)testswereconductedondecomposedgranitetostudytheshearmoduliofthematerialatsmallstrains.Laboratorytestingtechniquesweredeveloped,whichenabledmultidirectionalshearwavevelocitymeasurementsusingbenderelementsandcontrolofmatricsuctioninatriaxialapparatuswithlocalstrainmeasurements.Variousaspectsofsoilstiffnessatsmallstrainswerestudied,includingthevariationsinshearmoduluswithincreasingshearstrains(CDGandCDT),thestiffnessanisotropyatverysmallstrains(CDGandCDT),theeffectsofmatricsuctiononstiffnessanisotropy(CDT),theeffectsofcyclicloadingandcreeponsmall-strainstiffness(CDTandCDR),andtheeffectsofsampledisturbanceonshearmodulusandstiffnessanisotropy(CDGandCDT).

Inthefield,similarshearwavevelocitiesofdecomposedgraniteweremeasuredbythesuspensionP-Svelocityloggingandcrossholeseismicmeasurementsconductedattwositeswithsimilarsoilprofiles.

2534

Themeasuredshearwavevelocitiesareconsistentwiththesoilprofiles,whereboththeshearwavevelocityandtheSPT‘N’valueincreasewithdepthandthemeasuredshearwavevelocityincreasesasthematerialissubjectedtolessweatheringatadeeperlevel.Theshearwavevelocity(vs(vh)orvs(hv))andshearmodulus(GvhorGhv)ofcompletelydecomposedgranite(CDG)variesfrom200m/sto350m/sand45MPato280MPa,respectively,accrossadepthofabout20mto40m.AnempiricalcorrelationbetweentheshearmodulusandtheSPT‘N’valueofCDG(Gvh=14.3×N0.68)wasestablishedandwasfoundtobesimilartothatproposedbyImai&Tonouchi(1982)forsands.TheSBPMtestsconductedattwositesshowconsistentresults,wherethestiffness-strainrelationshipishighlynon-linearwithGsec/p󰀁decreasingfromabout300toabout50asshearstrainincreasesfrom0.02%to1%.

Laboratorymeasurementsoftheshearmodulus(Gvh)oftheMazierspecimensofCDGshowa20%to50%reductioncomparedwiththatmeasuredinthefield.Thelowershearmodulusmeasuredinthelaboratorymaybeattributedtosampledisturbance.Atthesameeffectivestresslevel,themeasuredGvhinMazierspecimenstakenatagreaterdepthishigherthanthatatashallowerdepth.Forinstance,thevalueofGvhoftheMazierspecimensofCDGtakenatadepthof54mis50%higherthanthattakenatadepthof28.5mwhentestedatthesameeffectivestresslevel,whilea64%increaseinGvhisobservedintheMazierspecimenofCDTtakenatadepthof29mcomparedwiththatatadepthof14m.ThehigherGvhinspecimenstakenatadeeperlevelreflectsalowerdegreeofdecompositionofthematerialsatagreaterdepth.Themeasuredshearmodulus(Gvh)oftheMazierspecimensofCDGatverysmallstrainsissimilartothatofCDTforMazierspecimenstakenatsimilardepthsandtestedundersimilareffectiveconfiningstresses.

Constantp󰀁triaxialcompressionandextensiontestsonMazierspecimensofCDGandCDTshowedthatthematerialsarehighlynon-linearandthenormalizedshearmodulus(Gsec/Gvh)ishigheralongtheextensionpath(angleofstresspathrotation,θ>90◦)thanalongthecompressionpath(θ≤90◦).ForCDG,thevalueofGsec/Gvhalongtheextensionpathisabout60%higherthanthatalongthecompressionpathatadeviatoricstrainof0.01%andthedifferencediminishesasdeviatoricstrainincreases.ForCDT,thevalueofGsec/Gvhishigheralongtheextensionpaththanalongthecompressionpathbyabout75%atadeviatoricstrainof0.01%.TheshearmodulusofCDTishigheralongtheextensionpath(θ=135◦),wherethenormalizedshearmodulusreducesby30%atadeviatoricstrainof0.01%.Thedifferenceinthestiffnessofthematerialsalongthedifferentpathsisrelatedtotheeffectoftherecentstresshistorythroughwhichsoilsshowastifferresponseastheangleofthestresspathrotationincreases.Thevariationsinthenormalizedshearmodulus(Gsec/Gvh)ofCDGandCDTwithdeviatoricstrain(εq)aregenerallyconsistentalongbothcompressionandextensionpaths,withthestiffness-strainrelationshipofCDTlyingclosetotheupperboundofthatofCDG.

Theeffectoftherecentstresshistoryonthesmall-strainstiffnessofsaprolitesisalsoexhibitedduringtheundrainedcyclictriaxialcompressiontests.TheMazierspecimensofCDTandCDRshowthehigheststiffnessalongtherecompressionloadingpath,whereEu/p󰀁0increasesbyabout100%comparedwiththefirstcompressionloading.A180◦stresspathrotationisinvolvedintherecompression,resultinginthestiffestresponseofthematerials,whichdemonstratestheeffectoftherecentstresshistoryonthesmall-strainstiffness.

MultidirectionalshearwavevelocitymeasurementsrevealedtheanisotropiccharacteristicofCDT.Thedegreesofinherentanisotropy(Ghh/Ghv)determinedattheisotropicstressstate(K=1.0)are1.48and1.36fortheblockandMazierspecimens,respectively.Ahighershearmodulusinthehorizontalplaneisattributedtothelayeringstructureinthehorizontalplaneresultingfrommetamorphismoftheparentrock.ThelowerdegreeofinherentstiffnessanisotropyoftheMazierspecimenisduetoahigherdegreeofsampledisturbanceintheMazierspecimen.Attheanisotropicstressstate(K=0.4),thedegreesofanisotropy(Ghh/Ghv)reducedto1.26and1.15intheblockandMazierspecimens,respectively,whichisa15%reductionfromthemeasuredinherentanisotropyduetothestress-inducedeffect.Theeffectofastrongerstructureinthehorizontalplaneontheshearmoduli(i.e.Ghh>Ghv)prevailsevenunderahigherverticaleffectivestresscondition(K=0.4),whichisunexpectedwithnopriorknowledgeoftheanisotropicstructureofthematerial.Theshearmoduli(Ghh,GhvandGvh)oftheMazierspecimenofCDTareonaverage70%lowerthanthoseoftheblockspecimenduetoahigherdegreeofsampledisturbance.PreliminarymeasurementsofGhhandGhvoftheMazierspecimenofCDGsuggestedthatthedegreeofanisotropyofCDG,ifany,islowercomparedwiththeMazierspecimensofCDT.FurthertestsarerequiredtostudythestiffnessanisotropyofCDGusingspecimenswiththeleastsampledisturbance(e.g.,blockspecimens).

Theshearwavevelocities,vs(vh),vs(hv)andvs(hh),ofunsaturatedreconstitutedCDTdependonthetwostressstatevariables,matricsuction(ua–uw)andnetmeanstress(p–ua).Theshearwavevelocitiesincreasenon-linearlywithanincreaseinmatricsuctionataconstantnetmeanstress.Therateofincreaseintheshearwavevelocitiesreduceswhentheappliedmatricsuctionexceedstheair-entryvalueofthe

2535

soil.Thedegreeofstiffnessanisotropyofthereconstitutedandintactspecimensisnotsensitivetothematricsuctionfortherangeofmatricsuctionconsidered(ua–uw=0to200kPa).

REFERENCES

Amorosi,A.,Callisto,L.&Rampello,S.1999.Observedbehaviorofreconstitutedclayalongstresspathstypicalofexcavations.InM.Jamiolkowski,R.Lancellotta&D.C.F.LoPresti(eds),Proc.Int.Symp.onPre-failureDeformationofGeomaterials1:35–42.Rotterdam:Balkema.

Arthur,J.R.F.&Menzies,B.K.1972.Inherentanisotropyinasand.Géotechnique22(1):115–128.

Atkinson,J.H.,Richardson,D.&Stallebrass,S.E.1990a.Effectofrecentstresshistoryonthestiffnessofoverconsolidatedsoil.Géotechnique40(4):531–540.

Atkinson,J.H.,Coop,M.R.,Stallebrass,S.E.&Viggiani,G.1990b.Measurementofstiffnessofsilsandweakrocksinlaboratorytests.Proc.of25thAnnualcon.of.Engrg.Geo.Group,Leeds,BritishGeologySociety,21–27.Atkinson,J.H.2000.Non-linearsoilstiffnessinroutinedesign.Géotechnique50(5):487–508.

Baldi,G.,Bruzzi,D.,Superbo,S.,Battaglio,M.&Jamiolkowski,M.1988.SeismicconeinPoRiversand.Proceed-ingsofInternationalSymposiumonPentrationTesting,ISOPT-1,Orlando2:643–650.Rotterdam:Balkema.Baligh,M.M.,Azzouz,A.S.&Chin,C.T.1987.Disturbanceduetoidealtubesampling.JournalofGeotechnicalEngineering113(7):739–757.

Bellotti,R.,Jamiolkowski,M.,LoPresti,D.C.F.&O’Neill,D.A.1996.AnisotropyofsmallstrainstiffnessinTicinosand.Géotechnique46(1):115–131.

Blight,G.E.1997.Mechanicsofresidualsoils.A.A.Balkema,Rotterdam,theNetherlands.

Brand,E.W.,Phillipson,H.B.,Borrie,G.W.&Clover,A.W.1983.In-situdirectsheartestsonHongKongresidualsoils.Proc.,Int.Symp.onSoilandRockInvestigationsbyIn-SituTesing2:13–17.

Brown,S.F.&Snaith,M.S.1974.Themeasurementofrecoverableandirrecoverabledeformationsintherepeatedloadtriaxialtest.Géotechnique24(2):255–259.

Burland,J.B.&Symes,M.1982.Asimpleaxialdisplacementgaugeforuseinthetriaxialapparatus.Géotechnique32(1):62–65.

Burland,J.B.1989.Smallisbeautiful–thestiffnessofsoilsatsmallstrains.CanadianGeotechnicalJournal26(4):499–516.

Burland,J.B.1990.Onthecompressibilityandshearstrengthofnaturalclays.Géotechnique40(3):329–378.

Chiu,C.F.&Ng,C.W.W.2003.Astate-dependentelasto-plasticmodelforsaturatedandunsaturatedsoils.Géotechnique53(9):809–829.

Clarke,B.G.1995.Pressuremetersingeotechnicaldesign.BlackieAcademicandProfessional.

Clayton,C.R.I.&Khatrush,S.A.1986.Anewdeviceformeasuringlocalaxialstrainsontriaxialspecimen.Géotechnique36(4):593–597.

Clayton,C.R.I.,Khatrush,S.A.,Bica,A.V.D.&Siddique,A.1989.TheuseofHallEffectSemiconductorsinGeotechnicalEngineering.GeotechnicalTestingJournal12(1):69–76.

Clayton,C.R.I.,Hight,D.W.&Hopper,R.J.1992.ProgressivedestructuringofBothkennarclay:implicationsforsamplingandreconsolidationprocedures.Géotechnique42(2):219–239.

Clayton,C.R.I.&Heymann,G.2001.Stiffnessofgeomaterialsatverysmallstrains.Géotechnique51(3):245–255.

Cuccovillo,T.&Coop,M.R.1997.Yieldingandpre-failuredeformationofstructuredsands.Géotechnique47(3):491–508.

Cuccovillo,T.&Coop,M.R.1999.Onthemechanicsofstructuredsands.Géotechnique,49(6):741–760.

Dyvik,R.&Madshus,C.1985.LaboratorymeasurementofGmaxusingbenderelements.ProceedingsofASCEAnnualconvention:AdvancesintheArtofTestingSoilsunderCyclicCondition,Detroit:186–196.Fioravante,V.2000.AnisotropyofsmallstrainstiffnessofTicinoandKenyasandsfromseismicwavepropagationmeasuredintriaxialtesting.SoilsandFoundations40(4):129–142.

Fyfe,J.A.,Shaw,R.,Campbell,S.D.G.,Lai,K.W.&Kirk,P.A.2000.TheQuaternaryGeologyofHongKong.GeotechnicalEngineeringOffice,CivilEngineeringandDevelopmentDepartment,theGovernmentoftheHongKongSAR.

Gan,J.K.M.&Fredlund,D.G.1996.DirectshearandtriaxialtestingofaHongKongsoilundersaturatedandunsaturatedconditions.GEOReportNo.46,GeotechnicalEngineeringOffice,HongKongGovernment.GeotechnicalControlOffice(GCO)1987.Guidetositeinvestigation,Geoguide2.HongKongGovernment.

GeotechnicalControlOffice(GCO)1988.Guidetorockandsoildescriptions,Geoguide3.HongKongGovernment.Goodman,R.E.1993.Engineeringgeology:rockinengineeringconstruction.JohnWiley&Sons,NewYork.

Guan,P.,Ng,C.W.W.,Sun,M.&Tang,W.H.2001.WeatheringindicesforrhyolitictuffandgraniteinHongKong.EngineeringGeology59(1–2):147–159.

Hilf,J.W.1956.Aninvestigationofpore-waterpressureincompactedcohesivesoils.PhDDissertation,Tech.Memo.No.654,U.S.Dep.oftheInterior,BureauofReclamation,DesignandConstructionDiv.,Denver,CO.

Hight,D.W.1993.Areviewofsamplingeffectsinclaysandsands.OffshoreSiteInvestigationandFoundationBehaviour28:115–146.

Hird,C.C.&Yung,P.C.Y.1989.Theuseofproximitytransducersforlocalstrainmeasurementsintriaxialtests.GeotechnicalTestingJournal12(4):292-296.

2536

Hird,C.C.&Pierpoint,N.D.1997.StiffnessdeterminationanddeformationanalysisforatrialexcavationinOxfordClay.Géotechnique47(3):665–691.

Ho,J.L.P.&Langford,R.L.1987.PetrographicstudyofmetamorphismintuffsoftheTaiMoShanformation,northwesternNewTerritories,HongKong.GeologicalSocietyofHongKongNewsletter5(4):9–13.

Imai,T.&Tonouchi,K.1982.CorrelationofNvaluewithS-wavevelocityandshearmodulus.Proc.,2ndEur.Symp.onPenetrationTesting1:67–72.

Irfan,T.Y.1996.MineralogyandFabricCharacterizationandClassificationofWeatheredGraniticRocksinHongKong(GEOReportNo.41).GeotechnicalEngineeringOffice,CivilEngineeringandDevelopmentDepartment,theGovernmentoftheHongKongSAR.

Irfan,T.Y.1999.CharacterizationofweatheredvolcanicrocksinHongKong.QuarterlyJournalofEngineeringGeology32:317–348.

Jamiolkowski,M.,Lancellotta,R.&LoPresti,D.C.F.1995.RemarksonthestiffnessatsmallstrainsofsixItalianclays.InS.Shibuya,T.Mitachi&S.Muira(eds),Proc.Int.Symp.onPre-failureDeformationofGeomaterials2:817–836.

Jardine,R.J.,Kuwano,R.,Zdravkovic,L.&Thornton,C.1999.Somefundamentalaspectsofthepre-failurebehaviourofgranularsoils.InM.Jamiolkowski,R.Lancellotta&D.C.F.LoPresti(eds),Proc.Int.Symp.onPre-failureDeformationofGeomaterials2:1077–1111.Joviˇci´c,V.&Coop,M.R.1998.Themeasurementofstiffnessanisotropyinclayswithbenderelementtestsinthetriaxialapparatus.GeotechnicalTestingJournal21(1):3–10.

Kuwano,R.,Connolly,T.M.,&Kuwano,J.1999.Shearstiffnessanisotropymeasuredbymulti-directionalbenderelementtransducers.InM.Jamiolkowski,R.Lancellotta&D.C.F.LoPresti(eds),Proc.Int.Symp.onPre-failureDeformationofGeomaterials1:205–212.

Kuwano,R.,Connolly,T.M.&Jardine,R.J.2000.Anisotropicstiffnessmeasurementsinastress-pathtriaxialcell.GeotechnicalTestingJournal23(2):141–157.

Kwong,J.S.M.1998.PilotstudyoftheuseofsuspensionPSlogging.TechnicalNoteNo.TN2/98,GeotechnicalEngineeringOffice,CivilEngineeringDepartment,TheGovernmentoftheHongKongSpecialAdministrativeRegion.

Langford,R.L.,Lai,K.W.,Arthurton,R.S.&Shaw,R.1989.GeologyoftheWesternNewTerritories.HongKongGeologicalSurveyMemoirNo.3,GeotechnicalControlOffice,CivilEngineeringDepartment,HongKongGovernment,HongKong.

Leung,H.Y.2005.Theanisotropicsmallstrainstiffnessofcompletelydecomposedtuffanditseffectsondeformationsassociatedwithexcavations.PhDthesis,TheHongKongUniversityofScienceandTechnology,HongKong.Lohani,T.N.,Imai,G.&Shibuya,S.1999.EffectofsampledisturbanceonGmaxofHoloceneclaydeposits.InM.Jamiolkowski,R.Lancellotta&D.C.F.LoPresti(eds),Proc.Int.Symp.onPre-failureDeformationofGeomaterials1:115–122.

LoPresti,D.C.F.,Pallara,O.,Cavallaro,A.&Jamiolkowski,M.1999.Influenceofreconsolidationtechniquesandstrainrateonthestiffnessofundisturbedclaysfromtriaxialtests.GeotechnicalTestingJournal22(3):211–225.

Lumb,P.1962.Thepropertiesofdecomposedgranite.Géotechnique12,226–243.Lumb,P.1965.TheresidualsoilsofHongKong.Géotechnique15,180–194.

Lunne,T.,Berre,T.,&Strandvik,S.1997.SampledisturbanceinsoftlowplasticNorwegianclay.InM.Almeida(ed.),Proc.RecentDevelopmentsinSoilandPavementMechanics:81–102.Rotterdam:Balkema.

Mancuso,C.,Vassallo,R.&d’Onofrio,A.2000.Soilbehaviourinsuctioncontrolledcyclicanddynamictorsionalsheartests.Proc.of1stAsianRegionalConferenceonUnsaturatedSoils,Singapore:539–544.

Massey,J.B.1983.ShearstrengthofHongKongresidualsoil:AreviewofworkcarriedoutbytheGeotechnicalControlOffice.ReportoftheGeotechnicalControlOffice25/83,55p.

Massey,J.B.,Irfan,T.Y.&Cipullo,A.1989.Thecharacterisationofgraniticsaproliticsoils.Proceedingsofthe12thInternationalConferenceonSoilMechanics&FoundationEngineering,RiodeJaneiro6:533–542.

Ng,C.W.W.,Bolton,M.D.&Dasari,G.R.1995.Thesmallstrainstiffnessofacarbonatestiffclay.SoilsandFoundations35(4):109–114.

Ng,C.W.W.,Sun,Y.F.&Lee,K.M.1998.Laboratorymeasurementsofsmallstrainstiffnessofgraniticsaprolites.GeotechnicalEngineering,SoutheastAsianGeotechnicalSociety,29(2),233–248.

Ng,C.W.W.,Pun,W.K.&Pang,R.P.L.2000.SmallstrainstiffnessofnaturalgraniticsaproliteinHongKong.JournalofGeotechnicalandGeoenvironmentalEngineering126(9):819–833.

Ng,C.W.W.&Chiu,C.F.2001.Behaviorofalooselycompactedunsaturatedvolcanicsoil.JournalofGeotechnicalandGeoenvironmentalEngineering127(12):1027–1036.

Ng,C.W.W.,Guan,P.&Shang,Y.J.2001.WeatheringmechanismsandindicesofigneousrocksofHongKong.QuarterlyJournalofEngineeringGeologyandHydrology34(2):133–151.

Ng,C.W.W.&Wang,Y.2001.Fieldandlaboratorymeasurementsofsmallstrainstiffnessofdecomposedgranites.SoilsandFoundations41(3):57–71.

Ng,C.W.W.&Chiu,C.F.2003.Laboratorystudyofloosesaturatedandunsaturateddecomposedgraniticsoil.JournalofGeotechnicalandGeoenvironmentalEngineering129(6):550–559.

Ng,C.W.W.,Fung,W.T.,Cheuk,C.Y.&Zhang,L.2004a.Influenceofstressratioandstresspathonbehaviourofloosedecomposedgranite.JournalofGeotechnicalandGeoenvironmentalEngineering130(1):36–44.

Ng,C.W.W.,Leung,E.H.Y.&Lau,C.K.2004b.Inherentanisotropicstiffnessofweatheredgeomaterialanditsinfluenceongrounddeformationsarounddeepexcavations.CanadianGeotechnicalJournal41(1),12–24.

2537

Ng,C.W.W.&Yung,S.Y.2005.Determinationoftheanisotropicshearstiffnessofanunsaturateddecomposedsoil.SubmittedtoGéotechnique.

Ng,C.W.W.&Leung,E.H.Y.2006.Determinationofshearwavevelocitiesandshearmoduliofcompletelydecomposedtuff.JournalofGeotechnicalandGeoenvironmentalEngineering(Accepted).

Nishio,S.&Katsura,Y.1995.ShearwaveanisotropyinEdogawaPleistocenedeposit.InS.Shibuya,T.Mitachi&S.Muira(eds),Proc.Int.Symp.onPre-failureDeformationofGeomaterials1:169–174.Rotterdam:Balkema.Pennington,D.S.1999.TheanisotropicsmallstrainstiffnessofCambridgeGaultclay.PhDthesis,UniversityofBristol,Bristol,UK.

Pennington,D.S.,Nash,D.F.T.&Lings,M.L.2001.Horizontallymountedbenderelementsformeasuringanisotropicshearmoduliintriaxialclayspecimens.GeotechnicalTestingJournal24(2):133–144.

Powrie,W.,Pantelidou,H.&Stallebrass,S.E.1998.Soilstiffnessinstresspathsrelevanttodiaphragmwallsinclay.Géotechnique48(4):483–494.

Pun,W.K.&Ho,K.K.S.1996.AnalysisoftriaxialtestsongraniticsaproliteperformedatPublicWorksCentralLaboratory.DiscussionnoteDN4/96,GeotechnicalEngineeringOffice,HongKongGovernment.

Roesler,S.K.1979.Anisotropicshearmodulusduetostressanisotropy.JournaloftheGeotechnicalEngineeringDivision105(7):871–880.

Salgado,R.,Bandini,P.&Karim,A.2000.Shearstrengthandstiffnessofsiltysand.JournalofGeotechnicalandGeoenvironmentalEngineering126(5):451–462.

Scholey,G.K.,Frost,J.D.,LoPresti,D.C.F.&Jamiolkowski,M.1995.Areviewofinstrumentationformeasuringsmallstrainsduringtriaxialtestingofsoilspecimens.GeotechnicalTestingJournal18(2):137–156.

Sewell,R.J.,Campbell,S.D.G.,Fletcher,C.J.N.,Lai,K.W.&Kirk,P.A.2000.ThePre-QuaternaryGeologyofHongKong.GeotechnicalEngineeringOffice,CivilEngineeringandDevelopmentDepartment,theGovernmentoftheHongKongSAR.

Shaw,R.1997.Variationsinsub-tropicaldeepweatheringprofilesovertheKowloonGranite,HongKong.JournaloftheGeologicalSociety,London,154:1077–1085.

Shen,J.M.1985.GCOresearchintounsaturatedshearstrength1978–1982.GeotechnicalControlOffice,ResearchReportRR1/85,183p.

Shibuya,S.,Hwang,S.C.&Mitachi,T.1997.Elasticshearmodulisofsoftclaysfromshearwavevelocitymeasurement.Géotechnique47(3):593–601.

Shirley,D.J.&Hampton,L.D.1978.Shear-wavemeasurementsinlaboratorysediments.JournalofAcousticalSocietyofAmerica63(2):607–613.

Stokoe,K.H.II,Hwang,S.K.,Lee,J.N.K.&Andrus,R.D.1995.Effectsofvariousparametersonthestiffnessanddampingofsoilsatsmalltomediumstrains.InS.Shibuya,T.Mitachi&S.Muira(eds),Proc.Int.Symp.onPre-failureDeformationofGeomaterials2:785–816.Rotterdam:Balkema.

Tanaka,H.,Sharma,P.,Tsuchida,T.&Tanaka,M.1996.Comparativestudyonsamplequalityusingseveraltypesofsamplers.SoilsandFoundation36(2),57–68.

Tatsuoka,F.1988.Somerecentdevelopmentsintriaxialtestingsystems.Advancedtriaxialtestingofsoilandrock,ASTMSTP977:7–67.

Tatsuoka,F.2001.Impactsongeotechnicalengineeringofseveralrecentfindingsfromlaboratorystress-straintestsongeomaterials,2000BurmisterLectureatColumbiaUniversity.InCorreia&Brandle(eds),GeotechnicsforRoads,RailTracksandEarthStructures:69–140.Rotterdam:Balkema.

Thomann,T.G.&Hryciw,R.D.1990.LaboratorymeasurementofsmallstrainshearmodulusunderK0conditions.GeotechnicalTestingJournal13(2):97–105.

VianadaFonseca,A.,MatosFermandes,M.&SilvaCardoso,A.1997.Interpretationofafootingloadtestonasaproliticsoilfromgranite.Géotechnique47(3):633–651.

Viggiani,G.&Atkinson,J.H.1995.Stiffnessoffine-grainedsoilatverysmallstrains.Géotechnique45(2):249–265.

Wang,Y.&Ng,C.W.W.2005.Effectsofstresspathsonthesmall-strainstiffnessofcompletelydecomposedgranite.CanadianGeotechnicalJournal42(4):1200–1211.

Wong,Y.L.,Lam,E.S.S.,Zhao,J.X.&Chau,K.T.1998.AssessingSeismicResponseofSoftSoilSitesinHongKongUsingMicrotremorRecords,TheHKIETransaction5(3):70–78.

Yimsiri,S.&Soga,K.2002.Areviewoflocalstrainmeasurementsystemsfortriaxialtestingofsoils.JournaloftheSoutheastAsianGeotechnicalSociety,April2002:41–52.

Yun,T.S.&Santamarina,J.C.2005.Decementation,softening,andcollapse:changesinsmall-strainshearstiffnessink0loading.JournalofGeotechnicalandGeoenvironmentalEngineering131(3):350–358.

Yung,S.Y.2004.Determinationofshearwavevelocityandanisotropicshearmodulusofanunsaturatedsoil.MPhilthesis,TheHongKongUniversityofScienceandTechnology,HongKong.

2538

因篇幅问题不能全部显示，请点此查看更多更全内容

查看全文

全部栏目

Small-strain stiffness of saprolites