Chapter 3 Geotechnical Conditions - OCDI 一般財団 ... · the investigation should continue until a stratum of engineering rock with a shear wave ... Bearing capacity ... CHAPTER

PART II ACTIONS AND MATERIAL STRENGTH REQUIREMENTS, CHAPTER 3 GEOTECHNICAL CONDITIONS

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Chapter 3 Geotechnical Conditions

Public NoticeGeotechnical Conditions

Article 13Geotechnicalconditionsshallbeappropriatelysetintermsofthephysicalandmechanicalpropertiesofthesoilbasedontheresultsofgroundinvestigationsandsoiltests.

[Commentary]

(1)GeotechnicalConditionsThegeotechnicalconditionsarethevariousconditionsthatrepresent thegeotechnicalcharacteristicstaken into consideration in the verification of the performance of the facility concerned against thetechnicalstandards.Insettingthegeotechnicalconditions,thereliabilityisdeterminedbasedontheresultsofagroundinvestigationandsoiltestscarriedoutbyappropriatemethods.

(2)GroundinvestigationThegroundinvestigationforsettingthegeotechnicalconditionstakesintoconsiderationthestructure,scale,andimportanceofthefacilitythatissubjecttothetechnicalstandards,aswellasthenatureofthegroundclosetothelocationofthefacility.

(3)SoiltestsThesoiltestsforsettingthegeotechnicalconditionsusesmethodsthatenablethegeotechnicalconditionstakenintoconsiderationintheperformanceverificationofthefacilitythatissubjecttothetechnicalstandardstobeappropriatelyset.

[Technical Note]

1 Ground Investigation1.1 Methods of Determining Geotechnical ConditionsThegeotechnicalconditionsnecessaryfortheperformanceverificationandtheconstructionplanningincludedepthofthebearingstrata,depthoftheengineeringfoundationstrata,thicknessofweakstrata,andotherstratigraphicalconditionsoftheground,waterlevels(residualwaterlevel),thedensity(degreeofcompaction),physicalcharacteristics,shearcharacteristics,consolidationcharacteristics,hydraulicconductivity,liquefactioncharacteristics,etc.Soilisamaterialthatisstronglystress-dependent,anditscharacteristicscanchangegreatlyduetoconsolidationwithtime,orchangesinoverburden,etc.Thereforewhennecessaryanewgroundinvestigationshouldbecarriedout.However,the size of ground investigations is limited, so past ground information (including databases, etc.) obtained fromdocumentsurveysshouldbepositivelyutilized.Inthiscase,itisimportanttoconfirmthatthegeotechnicalconditionshavenotchangedduetochangesinoverburdenorconsolidation,ortotakeintoconsiderationthatthegeotechnicalconditionshavechanged.

1.2 Position, Spacing, and Depth of Ground Investigation Locations

(1)Thelocationofagroundinvestigation,andthespacingandthedepthshouldbedeterminedinaccordancewiththesizeofthefacility,thestressdistributioninthegroundcausedbytheweightofthefacility,andtheuniformityofthestratigraphyoftheground.However,thereisalsotheproblemoftheconstructioncostandimportanceofthefacility,soitisnotpossibletocategoricallyregulatethenumberofsurveypointsandtheirdepth.Indeterminingthe number of survey points the uniformity or non-uniformity of the ground is themost important aspect. Itiseffective tochecktheuniformityornon-uniformityof thegroundfromtheresultsofpast investigations, thetopographyof the land,andgeophysicalexplorationmethodssuchassonicwaveandsurfacewaveexplorationmethods. Automaticallydetermining thespacingofground investigationpoints shouldbeavoidedasmuchaspossible,butforreferenceTable 1.2.1 showsthespacingofgroundinvestigationpointsforboringandsoundingsurveys. Thedepthofthegroundinvestigationshallbesufficienttoconfirmstratawithsufficientbearingcapacity.Whetherastratumhassufficientbearingcapacityornotvariesdependingontheshapeandscaleofthefacility,soitcannotbecategoricallydetermined.However,asaguide,forcomparativelysmallscalefacilitiesorwhenthefoundationsarenotendbearingpiles,theinvestigationmaybeterminatedifstratumofafewmetersthicknessisconfirmedwith theN-valueobtainedfromthestandardpenetration test is30orhigher,or fora largescale

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TECHNICAL STANDARDS AND COMMENTARIES FOR PORT AND HARBOUR FACILITIES IN JAPAN

facilitywhereendbearingpilesareanticipatedtheinvestigationmaybeterminatedifstratumofafewmetersthicknessisconfirmedwiththeNvalueis50orhigher.Also,forperformanceverificationofseismic-resistance,theinvestigationshouldcontinueuntilastratumofengineeringrockwithashearwavevelocityof300m/sormoreisconfirmed.

Table 1.2.1 Guideline for Investigation Location and Spacing for Boring and Sounding Investigations

① Inthecasewherethestratigraphicalconditionsarecomparativelyuniformbothhorizontallyandvertically(Units:m)Facelinedirection Perpendiculartofacelinedirection

Spacinglayout Spacinglayout Distancefromfaceline(maximum)

Boring Sounding Boring Sounding Boring Sounding

Preliminarysurvey

Widearea 300–500 100–30050 25

50–100Smallarea 50–100 20–50Detailedsurvey 50–100 20–50 20–30 10–15

② Whenthestratigraphicalconditionsarecomplex(Units:m)Facelinedirection Perpendiculartofacelinedirection

Spacinglayout Spacinglayout Distancefromfaceline(maximum)

Boring Sounding Boring Sounding Boring SoundingPreliminarysurvey 50orless 15–20 20–30 10–15

50–100Detailedsurvey 10–30 5–10 10–20 5–10

Note)Asoundingsurveymayormaynotrequireaborehole Thesoundingsurveysinthetableareonlythoseforwhichaboreholeisnotnecessary. Forsoundingsurveysthatrequireaborehole,“theboringcolumn”isapplicable.

1.3 Selection of Investigation Methods

(1) Investigationmethodsthatarethemostsuitableforthesurveyobjectivesareselectedtakingintoconsiderationtheextentofthesurvey,theimportanceofthefacilityandeconomics.

(2)Table 1.2.2 shows the surveymethods for each survey objective, and the ground information obtained fromthem.


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Table 1.2.2Survey Methods according to Survey Objectives

Classification Surveyobjective Surveymethod SurveydetailsStratigraphicalconditions

Confirmationofstratigraphicalconditions

BoringSoundingGeophysicalexploration

FoundationdepthThicknessofweakstrataSequenceofstrata

Physicalcharacteristics Classificationof

soilpropertiesUndisturbedsampling(structuraldisturbanceispossibleforallexceptγt)

UnitweightWatercontentSoilparticledensityParticlesizedistributionConsistency

γtwρs

wL, wP, IP

(Hydraulicconductivity)

Hydraulicconductivity

UndisturbedsamplingIn-situtests Hydraulicconductivity k

Mechanicalproperties

BearingcapacityUndisturbedsamplingSoundingInsitutests

UnconfinedcompressivestrengthShearstrengthAngleofshearresistanceRelativedensity

quτf

Dr

Slopestability

Earthpressure

Consolidationcharacteristics

UndisturbedsamplingCompressionindexCompressioncurveCoefficientofconsolidationCoefficientofvolumecompressibility

Cce-logpcvmv

Compactioncharacteristics

DisturbedsamplingalsoapplicableIn-situtests

MaximumdrydensityOptimumwatercontentCBR

γdmaxwopt

Dynamiccharacteristics

UndisturbedsamplingIn-situtests

ShearmodulusAttenuationcoefficientLiquefactioncharacteristics

GhP

References

1) PortandHarbourBureau,MinistryofTransport:GuidelineforPortSurveys.JapanPortAssociation,19872) TheJapanGeotechnicalSociety:MethodologyandCommentaryofSoilSurvey,2004

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2 Ground Constants2.1 Estimation of Ground Constants 1)

(1)GeneralThe ground constants/parameters used in the performance verification are generally estimated in accordancewiththeflowshowninFig. 2.1.1.However,ifthereisarationalreasonbasedonthecharacteristicsofthegroundinvestigationandthesoiltests,derivedvaluesmaybeusedasthecharacteristicvalues.Forexample,asamethodofestimatingthederivedvaluesfrommeasuredvaluesoftheN-valueobtainedfromstandardpenetrationtests,empirical equations or correlation equations have been proposed that take into consideration the variation inthemeasuredvalues,sothederivedvaluescanbeusedascharacteristicvalueastheyare.Also,fortheshearwavevelocitymeasuredbygeophysicalexploration, themeasuredvaluesevaluate thecomplexconditionsandcharacteristicsofthein-situground,andthesubjectbeingevaluateddifferswitheachmeasurementlocation,andtherearecaseswheretheuseofstatisticalprocessingofmanymeasurementresultsisnotappropriate.Inthiscasethederivedvaluesmayalsobeusedastheyareascharacteristicvalues. Partialfactorsthataremultipliedbythecharacteristicvaluestocalculatedesignvaluesmaybesetbasedonthevariabilityofthegroundparametersandthesensitivitytotheverificationresultoftheparameter.Therefore,partial factors are set for each performance verification method for each facility. Also, for each individualperformance verification, it is difficult to separately take into account the extent of variation of the groundparametersthatdependsongroundinvestigationsorthesoiltestmethods.Therefore,thecharacteristicvaluesarecalculatedbyapplyingacorrectioncorrespondingtothereliabilityofthesoiltestmethod.Thisapproachisadeviceforsimplifyingtheperformanceverificationmethodbymakingthepartialfactorssetforeachperformanceverificationmethod for each facility independent of the ground investigationmethods and soil testmethods.However,itisslightlydifferentfromtheconceptsofJGS4001thatmakes“thecharacteristicvalueistheaveragevalueofthederivedvalues”asaprinciple.

Representative values of ground parameters (characteristic values)

Estimated ground parameters (derived values)

Direct results of various surveys, tests, measurements,and observations/ monitoring (measurement values)

Ground parameter used in foundation and ground model (design values)

Application of theory, experience, and correlation,including primary processing

Statistical processing taking account of limit stateand variation

Apply partial factor

Modeling of ground (estimated values)

Classification of strata

Fig. 2.1.1 Example of Procedure for setting the Design Values of Ground Parameters 1)

(2)MethodsofEstimatingtheDerivedValuesAsshownbelow,methodsofestimatingderivedvaluesincludethemethodofusingthemeasuredvaluesastheyareasthederivedvalues,themethodofapplyingprimaryprocessingonlytoobtainthederivedvalues,andthemethodofobtainingthederivedvaluesbyconvertingmeasuredvaluesintodifferentengineeringquantities.

① Themethodofusingthemeasurementvaluesastheyareasderivedvaluesis,literally,directmeasurementofthegroundparameters.

②Withinthemethodofapplyingprimaryprocessingonlytoobtainthederivedvalues,theprimarycorrectionsareanareacorrectionforsheartests,acorrectionfortheeffectofstrainrateontheshearstrength,andthesimplecorrectioncorrespondstojustmultiplyingbythecoefficients.Also,applyingsimpleprocessingtotestresults,suchasapplyingtheprimaryprocessingtocalculatethewatercontentw,thewetdensityρt,thesoilparticledensityρs,grainsizedistribution,obtainingthedeformationmodulusEfromthestress-strainrelationship,andobtainingtheconsolidationyieldstresspcfromthee-logprelationship,correspondstothismethod.


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③ Themethodofobtainingderivedvaluesbyconvertingthemeasuredvaluesintodifferentengineeringquantitiesisthemethodofconvertingthemeasuredresultsintoengineeringquantitiesbasedontheoreticalorempiricalequations,or,obtainingfittingparametersinaccordancewiththeory.ConvertingN-valuesintotheangleofshearresistance usingempiricalequations,andfittingtheoreticalcurvesofconsolidationtosettlement-timecurvestoobtainthecoefficientofconsolidation cv,correspondtothismethod.

(3)MethodsofsettingtheCharacteristicValues

① GeneralThecharacteristicvaluesaresetgenerallyinaccordancewiththeflowshowninFig. 2.1.2.Ifthereisasufficientnumberofthederivedvaluedatatocarryoutstatisticalprocessing,andifthevariationinthederivedvaluesissmall,asarule,thecharacteristicvaluemaybecalculatedastheaveragevalue,expectedvalue,ofthederivedvalues.Here,ifthenumberofdataentriesnofthederivedvaluesis10ormore,andtheamountofvariationisnotlarge,andifthecoefficientofvariationCV islessthan0.1,itisconsideredthatacertain reliabilitycanbeguaranteed for thestatistical results,and theaveragevalue,expectedvalue,of thederivedvaluesmaybetakentobethecharacteristicvalue.However,ifthereareaninsufficientnumberofdataentriesofthederivedvaluestocarryoutstatisticalprocessingandifthevariationinthederivedvaluesislarge,it isnecessarytoset thecharacteristicvaluebycorrectingtheaveragevalue,expectedvalue,of thederivedvaluesbasedonthemethodshownbelow.

Ground parameters estimated from measured values (derived values)

Modeling of ground (estimated values)

Variation in estimated valueswith respect to derived values

0.1 ≤ coefficient of variation CV < 0.6

Review of measured values

Is No. of data entries sufficient?No. of data entries ≥ 10

CV < 0.10.6 ≤ CV

Yes

No

Yes

Representative values of ground parameters (characteristic values) ak =b1×b2×a*)

b1 = 1

b2 = 1

Correction for variation in the data(0.75 < b1 ≤ 1.0, or 1.0 ≤ b1 < 1.25)

Correction for number of data entries(0.5 < b2 ≤ 1.0, or 1.0 ≤ b2 < 1.5)

Fig. 2.1.2Example of Procedure for setting Characteristic Values of Ground Parameters

② Correctionoftheaveragevalue,expectedvalue,ofthederivedvaluesWhenthenumberofderivedvaluedataentriesislimited,orthevariationinthederivedvaluesislarge,thecharacteristic values cannot simply and automatically be taken to be the average value, expected value, ofthederivedvalues,butit isnecessarytoappropriatelysetthecharacteristicvaluestakingintoconsiderationthe estimated error of the statistical average values. In this case the followingmethodmaybe used. Theuncertaintyfactorsinthecharacteristicvaluesincludeerrorsinthegroundinvestigationorsoiltests,estimationerrorsinthederivedvalues,andinhomogeneityinthegrounditself.Therefore,itisdesirablethatthegroundinvestigationconditionssuchastypesofsurveyequipment,soiltestconditionssuchastypesoftestequipment,testmethodsandconditionoftestspecimen,thesoilstratigraphy,andothersoilinformationneedtobecarefullyexamined.Themethodofcorrectionoftheaveragevalue,expectedvalueofthederivedvaluesdescribedhereisnotlimitedtothevaluesforstabilityverificationofthefacility,butit issupposedthatitcanbegenerallyappliedtogroundconstants,includingvaluesusedforsettlementpredictions.InJGS4001amethodofsettingthecharacteristicvaluesinaccordancewithconfidencelevelsisdescribed,inwhichanormaldistributionis

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assumedifthestandarddeviationofthepopulationisknown,andat-distributionisassumedifthestandarddeviationisnotknown.However,whendealingwithgroundparameters,thedistributionandvariationinthederivedvaluesareduetoerrorsinthegroundinvestigationorsoiltests,estimationerrorsinthederivedvalues,andinhomogeneityinthegrounditself,andhence,thisisdifferentfromdealingwithqualityindicesoffactoryproducts,andsimplestatisticalprocessingishardlyapplicable. Toobtainthegroundparameters,whichareobtainedbyadjustingtheaveragevaluesforthestatisticalerrors,andcorrespondtothecharacteristicvalues,forreliability-baseddesign, it isnecessarytoobtainasufficientnumberoftestresultsforstatisticalprocessing.Also,inordertoreflectthesoilinvestigationandsoiltestresultsintheperformanceverification,itisnecessarytomodelthedistributioninthedepthdirectionoftheestimatedvaluesa*ofthegroundparameteraasconstantwithdepth(a*=c1),linearlyincreasingwithdepth(a*=c1z+c2),orashavingaquadraticdistributionwithdepth(a*=c1z2+c2z+c3).Here,c1,c2,andc3areconstants.Ifacertainrangeofdepthistobemodeled,asufficientnumberoftestsare10ormoredataentriesinordertocarryoutstatisticalprocessingonthegroundmodel.Thereliabilityofgroundparametersobtainedfromdifferentsoiltestmethodssuchastheundrainedshearstrengthofcohesivesoilsobtainedfromtriaxialtestsandunconfinedcompressiontestsdiffers,sodifferentpartialfactorsshouldbesetaccordingly,butitisnotknowntowhatextentthefactorsshoulddiffer.However,itiswellknownthatthecoefficientsofvariationofthetwotestresultsaresignificantlydifferent.Basedonthisthecharacteristicvaluesarecalculatednotsimplyasthearithmeticmean,butbymultiplicationbyacorrectioncoefficientthattakesintoaccountthevariationofthederivedvaluestotheestimatedvalues.However,thisisbasedontheassumptionthatthereisasufficientnumberofdataentriestocarryoutstatisticalprocessing,soifthenumberofdataentriesisinsufficient,itisnecessarytofurthersetthecharacteristicvaluesonthesafetyside,bymultiplyingbyacorrectioncoefficientforthenumberofdatapoints.Inotherwords,thecharacteristicvaluesarecalculatedfromthefollowingequation (2.1.1)orequation (2.1.2).Here,ifitisreasonabletoconsiderthevariationonlogarithmicaxes,equation (2.1.2)isused.

(2.1.1)

(2.1.2)

where ak :representativevalueofgroundparameter(characteristicvalue) b1 :correctioncoefficientforvariationinthederivedvalues b2 :correctioncoefficientfornumberofdatapointsofderivedvalues a* :modelvalueofthegroundparameter(estimatedvalue)

Aspecificmethodofsettingthecorrectioncoefficient isdescribedbelow.However,whendealingwiththeunitweightofthein-situgroundinstabilityanalysis,fordeterminingthevaluesatwhichtheactionsideandtheresistancesidearesubstantiallyinbalance,thecorrectioncoefficientsmaybetakentobeb1=1,b2=1.

③MethodofsettingthecorrectioncoefficientforvariationinthederivedvaluesIftheestimatedparameterformodelingthedistributionoftestresultsisrepresentedbya*,whenconsideringthevariationintestresultsa,itisconvenienttousethestandarddeviationof(a/a*)whichreferstothecoefficientofvariation.Hereitisassumedthata*isestimatedastheaveragevalueofauniformdistributionwithinastratumthatismodeled,oradistributioninwhicherrorsareminimizedbytheleastsquaresmethodorsimilar.Itisknownthatforauniformground,thecoefficientofvariationofthegroundparametersobtainedasaresultoftakingundisturbedclaytestsamplesusingafixedpistontypethin-walledtubesampler,andcarefullycarryingouteachtypeofsoiltest,is0.1orless.Inotherwords,eventhoughitisauniformground,thereisacertainamountofnon-uniformity,andthereareerrorscausedbythesoiltestmethods,sothisextentofvariationintheresultsisinevitable.However,ifthevariationisgreater,ifthenon-uniformityinthegroundislarge,ifthedisturbanceduringsamplingislarge,ifthesoiltestmethodsareinappropriate,orifthemodelingwithrespecttodepthisinappropriate,theestimatedvaluesa*cannotbetakentobecharacteristicvaluesastheyare,butitisnecessarytosetthecharacteristicvaluesonthesafetyside,takingtheuncertaintyfactorsintoaccount. Therefore,thecorrectioncoefficientb1forvariationofthederivedvaluesissetcorrespondingtothecoefficientofvariationCVdefinedasthestandarddeviationSDof(a/a*).Whentheparameteraiscontributingtotheresistancesidesuchasshearstrength,inaperformanceverification,thecorrectioncoefficientb1=1–(CV/2),andwhencontributingtotheactionsidesuchasunitweightofanembankment,andcompressionindex,b1=1+(CV/2)are set, and thevalues shown inTable 2.1.1 shouldbeused in theperformanceverification. Thiscorrectsthevaluetoavaluecorrespondingtoabout70%probabilityofnon-exceedance,foruseasthecharacteristicvalue.Ifthecoefficientofvariationis0.6orhigher,thereliabilityispoor,soperformanceverificationcannotbecarriedout,interpretationofthetestresultsmustbecarriedoutagain,andifnecessarythemodelingofthegroundmustbere-investigated.Incertaincasesitmaybenecessarytocarryoutthesoilinvestigationagain.


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Table 2.1.1 Values of Correction Coefficients

CoefficientofvariationCV

Correctioncoefficientb1Whenitisnecessarytocorrectthe

characteristicvaluetoavaluesmallerthanthederivedvalues

Whenitisnecessarytocorrectthecharacteristicvaluetoavaluelargerthan

thederivedvalues≥0,<0.1 1.0 1.0≥0.1,<0.15 0.95 1.05≥0.15,<0.25 0.9 1.1≥0.25,<0.4 0.85 1.15≥0.4,<0.6 0.75 1.25≥0.6 Re-investigatetheinterpretationoftheresultsorthemodeling,orre-dothesurvey

Thegroundparametersincludeparameterswhoseresultsareevaluatedaslogarithmicdistributions,suchastheconsolidationyieldstresspc,thecoefficientofconsolidationcv,andthecoefficientofvolumecompressibilitymv. Inorder toobtain thecharacteristicvaluesof theseparameters,several testsarecarriedout,and if thegroundistobetreatedasuniform,theseparametersaredistributedaslog-normal,soitisreasonabletoconsiderthevariationonthelogarithmicaxis.Inotherwords,fortheparametera,ifthestandarddeviationof(loga)/(loga*)isSD,andthisbecomesthecoefficientofvariationCV,thevaluesinTable 2.1.1canbeusedastheyareasthecorrectioncoefficientb1onthelogarithmicaxis.Ontheotherhand,fortheangleofshearresistance ,thevariationof itselfisnotconsidered,butthevariationoftan isconsidered.Inthecaseoftheangleofshearresistanceofamoundmaterial,ifthevalueusedintheperformanceverificationisspecifiedbasedonexperience,thespecifiedvaluealreadyhastheeffectofvariationtakenintoconsideration,soitisnotnecessarytoconsideracorrectioncoefficient.Thecorrectioncoefficientsshownhereareusedaftercarryingoutstatisticalprocessinginordertoobtainthecharacteristicvaluesfromthereportedsoiltestresults.Therefore,itisnecessarytobeawarethatthecoefficientsofvariationinTable 2.1.1donotindicatethelevelofvariationobtainedfromsoilinvestigationsorsoiltestresults.

④MethodofsettingthecorrectioncoefficientforthenumberofdataentriesofthederivedvaluesForthe Method of setting the correction coefficient for variation in the derived valuesin ③above,itwasassumedthatthenumberofdatapointsissufficienttocarryoutstatisticalprocessing.However,inthecasewherethenumberofdatapointsisinsufficientforcarryingoutstatisticalprocessing,thecorrectioncoefficientb2forthenumberofdataentriesofderivedvaluesissetasfollows.Inotherwords,ifthenumberofdataentriesnis10ormore,therewillbeacertainreliabilityinthestatisticalresults,butifthenumberisinsufficientthecorrectioncoefficientshouldbesettob2={1±(0.5/n)}.Herethenegativesignisusedwhenitisnecessarytocorrectthecharacteristicvalueofagroundparameterusedinperformanceverificationtowardsmallervaluesthanthederivedvalues,andthepositivesignisusedwhenitisnecessarytocorrectthevaluetowardlargervaluesthanthederivedvalues.Fortheperformanceverificationtheremustbeatleasttwoormoredataentries.However,eveninthecasewherethereisonlyonedataentries,ifotherparametersforexampleN-valueorgrainsizedistributionhavebeenobtained,andifthedistributioninthedepthdirectionismodeledfromacorrelationwiththeseprovidedonlycommonlyknowncorrelationsareused,thenthatonedataentrymaybeusedintheperformanceverification.Inthiscase,b1=1,andb2=1±0.5areassumed.

⑤MethodofsettingthecharacteristicvaluestakingthemodeoftheperformanceverificationintoaccountThegroundconstantsforconsolidationandthegroundconstantsforsheararenotmutuallyindependent.Intheperformanceverification,iftheseconstantsareconsideredtobeindependent,thecharacteristicvaluescanbeobtainedtakingintoconsiderationthereliabilityoftherespectiveparameters.However,ifastrengthincreasedue to consolidation is expected for stability evaluation, theparameters in respect of consolidation and theparametersinrespectofshearingmustbecloselylinked.Inthesecircumstances,intheprocessofobtainingcharacteristicvaluesfromderivedvalues,theparametersaremodeledasmutuallylinkedwhenmodelingthedistributionofsoiltestresultstoderiveestimatedvalues.Forexample,thecharacteristicvaluesmustbesetbystatisticalprocessingforthevariation,toestimatecompatiblegroundparameters,takingintoconsiderationtherelationshipcu=m×OCR×σ'v0betweentheeffectivesoiloverburdenpressureσ'v0,theconsolidationyieldstresspc,andtheundrainedshearstrengthcu,usingthestrengthincreaseratiom=cu/pc,andtheoverconsolidationratioOCR=pc/σ'v0.

(4)MethodofCalculatingDesignValuesInthevariouscalculationswhenthegroundparametersareusedinperformanceverification,designvaluesareobtainedbymultiplyingthecharacteristicvaluesbyapartialfactorγ.Avalueofthepartialfactorγmaybesetforeachperformanceverificationmethodforeachfacility,butifnotspecifiedotherwise,γmaybetakentobe1.0.

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2.2 Physical Properties of Soils2.2.1 Unit Weight of Soil

(1)Theunitweightmustbeobtainedbycollectingundisturbedsamplesonsite,ordirectlyobtainingitonsite.

(2)Theunitweightisnormallytheweightperunitvolumeinair,andincludesthewetunitweightanddryunitweight.Also,theunitweightinwater(weightperunitvolumefromwhichbuoyancyisdeducted)isreferredtoastheimmersedunitweight.Forthemeasurementoftheunitweight,methodsofcollectingundisturbedsamplesofclaysoilshavebeenestablished,anditispossibletoobtaintestsamplesthatarerepresentativeofthesoilin-situ.Thereforetheunitweightofclaysoilscanbeobtainedfromlaboratorytests.However,theunitweightofsandysoilsorsandmustbeobtaineddirectlyin-situ. Thewetunitweight isoneof the indices indicating the fundamentalpropertiesof a soil, and isused forrecognizingthesoilstiffness,anddegreeoflooseness,andforcalculating,theweightofasoilmassandthevoidratio.

①WetunitweightThewetunitweightisgenerallyexpressedasshowninequation (2.2.1),bycombiningboththeweightofsoilparticlesperunitvolumeandtheweightofwaterwithinthevoid.

(2.2.1)

where γt :wetunitweight(kN/m3) ρt :bulkdensity(t/m3) ρs :soilparticledensity(t/m3) e :voidratio Sr :degreeofsaturation(%) w :watercontent(%) ρw :densityofseawater(t/m3) g :gravitationalacceleration(m/s2)

Theapproximatevaluesof theunitweightof soilsnormallyencountered inharborareas inJapanareasshowninTable 2.2.1.

Table 2.2.1Unit Weight and Water Content of Representative Soils

Holoceneclays Holoceneclays SandysoilsWetunitweightγt(kN/m3)Dryunitweightγd(kN/m3)

Watercontentw(%)

12–165–14

150–30

16–2011–1460–20

16–2012–1830–10

② DryunitweightOnlysoilparticlesareconsideredintheunitweight,sobyputtingw=0orSr=0,thedryweightperunitvolumeisexpressedbyequation (2.2.2).

(2.2.2)

where γd :dryunitweight(kN/m3) ρd :drydensity(t/m3)

Also, therelationshipbetweenthewetunitweightγtandthedryunitweightγd isgivenbythefollowingequation.

(2.2.3)


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③ ImmersedunitweightIfthevoidiscompletelysaturatedwithwater,theimmersedunitweightisexpressedbyequation (2.2.4)takingbuoyancyintoaccount.

(2.2.4)

whereγ' :immersedunitweight(kN/m3)γsat :saturatedunitweight(kN/m3)

Although the unitweight ofwater γw is somewhat dependent on salt concentration and temperature, itscorrectvalueisknown.Therefore,whenobtainingthecharacteristicvaluesofasaturatedfoundationtakingthevariationinunitweightintoaccount,thevariationinγ'shouldbeconsidered,notγsat.Inotherwords,whenmultiplyingthecharacteristicvaluesbyapartialfactortoobtainthedesignvalues,thereisnonecessitytoapplyapartialfactortotheunitweightofwaterγw,sotheimmersedunitweightγ'ismultipliedbythepartialfactor,notthesaturatedunitweightγsat.

(3)MeasurementofUnitWeightIn-situMethodsfordirectlyobtainingtheunitweightin-situincludemethodsinwhichmeasurementisonlypossiblenearthegroundsurface,andmethodsofmeasurementinfirmground.Theformerincludesforexampletheso-calledsandreplacementmethod,asimpleandeasymethodasprescribedbyJISA1214Method of Soil Density TestbySand Replacement Method.Also,thelatterincludesexamplemethodsofmeasurementusingradioisotopes(RI).

①MethodsusingthesandreplacementmethodThesandreplacementmethodismainlyappliedtomeasurementonlandnearthegroundsurfaceforcontrolofearthworks,butitcanbeuseddowntoacertaindepthwherepitscanbeexcavated.ThismeasurementmethodisdescribedinJISA1214.

② Radioisotopes(RI)InrecentyearstheuseofRIhasbecomecomparativelyeasy,andalthoughtherearestrictlawsandregulationssuchasthe Law to Prevent Radiological Hazards Caused by Radioactive Isotopes.(LawNo.167,1957)anditsassociatedregulations,therehavebeenmanycasesofmeasurementusingaγ-raydensitometerasanin-situtestwhereitisdifficulttoobtainundisturbedsamplesofsandandsandysoil.Incidentally,theselegalrestrictionsdonotapplyinthecaseofsealedradioactivesourceswhoseradiationsourcestrengthis3.7MBq(megabequerel)orless. Therearetwotypesofγ-raydensitometerthatuseRI:asurfacetypeandaninsertedtype,andthesearedescribedinSoil Density TestMethods using RI Equipment,JGS1614,theStandardofGeotechnicalSocietyofJapan.Thesurfacetypeisappliedtomeasurementnearthegroundsurface,asimpliedbyitsname,andisusedforcontrolofearthworkssameasthesandreplacementmethod. Thesurfacetypeisfurtherclassifiedintobackscattering typesandtransmissiontypes. Measuringequipmentusingthe initiallydevelopedbackscatteringmethodisfrequentlyused,butinrecentyearsequipmentusingthetransmissionmethodhasbecomepopularbecauseof its accuracy. On theotherhand, the insertion type is applied tomeasuring thedensitydistributionintheverticaldirection,inotherwordsforsurveysinthedepthdirection.Forexample,itisusedfor investigating thedensitydistribution in thedepthdirection forgroundsurveys, fordetermining thesoilimprovementeffectbydensitymeasurementofreplacedsand,andmeasurementofthedensityoffilledsandincaissons. TheRImethodhas theadvantage that it isanon-destructive test fromwhich the in- situdensitycanbedirectlymeasured.Also,althoughthemeasurementoperationitselfissimplesoithasahighusabilityvalue,ontheotherhandbecausethereisdangerassociatedwiththeradioactivematerialtherearemanyregulationsregardingitshandling,soitcannotbesimplybroughtoutandusedin-situ.Inaddition,insurveysassociatedwithportconstruction,theinsertedtypeismainlyused,sothereisanoperationofinsertingtheequipmentintotheaccesspipe.Themeasurementaccuracyisgovernedbythematerialandqualityofthepipe,ortheinsertionaccuracy,inotherwords,themeasurementaccuracyisgovernedbythedisturbanceofthesurroundingswhenthepipeisinserted,andhowgoodthecontactbetweenthepipeandthesoilis,socautionisnecessary.RecentlytheRIconepenetrometer,whichincorporatesRIinaconeprobe,isbeingdevelopedasadevicecapableofdirectlypenetratingintothegroundforsurveys.

(4)RelativeDensityThedegreeofcompactionofsandmaybeexpressedbytherelativedensityusingequation(2.2.5).

(2.2.5)

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where,Dr :relativedensity

emax :voidratiointhelooseststateemin :voidratiointhedenseststatee :voidratiointhepresentstateofthetestsample

ρdmin :drydensityinthelooseststate(g/cm3)ρdmax :drydensityinthedenseststate(g/cm3)

ρd :drydensityinthepresentstateofthetestsample(g/cm3)

Thedensityofsandisgreatlyaffectedbytheshapeoftheparticlesandbythegrainsizecomposition.Sofromtheunitweightsandthevoidratioscalculatedfromit,thedensityofsandcannotbecorrectlyevaluated.Therefore,therelativedensityisusedtoindicatetherelativevaluewithintherangeofvoidratiosthatcanbetakenwiththissoil.Measurementofemax,emin,(ρdmin,ρdmax)forobtainingDrcanbecarriedoutinaccordancewithJapaneseIndustrialStandardJISA1224Method of Measurement of the Minimum and Maximum Density of Sand. Itisdifficulttotakeundisturbedsamplesofsand,sotherelativedensityisfrequentlymeasuredindirectlybysounding.(see2.3.4(4) Angle of shear resistance of sandy ground).

2.2.2 Classification of Soils

(1)Soilclassificationisperformedbythegradingforcoarsesoilsandbytheconsistencyforfinesoils.

(2)Mechanicalpropertiesofsoilsuchasstrengthordeformationhaveacloserelationshipwiththegradingforcoarsesoils,andwiththeconsistencyforfinesoils.

(3)EngineeringClassificationMethodforSubsoilMaterials(JapaneseUnifiedSoilClassificationSystem)Theclassifyingmethodofsoilandrock,andtheirnomenclatureshouldbeinaccordancewiththeengineeringclassificationmethod for subsoil material prescribed by the JGS 0051 Japanese Unified Soil Classification SystemoftheGeotechnicalSocietyofJapan.ThegrainsizeclassificationsandtheirnamesareshowninFig. 2.2.1.Thecoarse-grainedsoilreferstosoilcomposedmainlyofcoarsefractionwithagrainsizerangingfrom75µmto75mm.Soilconsistingofcomponentswithagrainsizelessthan75µmiscalledthefine-grainedsoil.Fig. 2.2.1 and Fig. 2.2.2 showtheengineeringclassificationsystemforsoil,andFig. 2.2.3 showstheplasticitydiagramusedinclassifyingfine-grainedsoil.

ParticleDiameter

5µm 75µm 250µm 425µm 850µm 2mm 4.75mm 19mm 75mm 300mm

Clay SiltFinesand Coarse

sandFinegravel

Mediumgravel

Coarsegravel Cobble BoulderMediumsand

Sand Gravel StoneFinegrainfraction Coarsegrainfraction Stonefraction

(Note) The word "particle" is affixed when referring to a constituent particle belonging to a particular category; andtheword"fraction"isaffixedwhenreferringtoacomponentbelongingtoaparticularcategory.

Fig. 2.2.1 The Grain Size Classifications and their Names (JGS 0051)

(4)ClassificationbyGrainSizeThe uniformity coefficient is an index showing the grain size characteristics of sandy soil and is defined byequation(2.2.6).

(2.2.6)

where Uc :uniformitycoefficientD60 :grainsizecorrespondingto60%passingbymassingrainsizedistributioncurve(mm)D10 :grainsizecorrespondingto10%passingbymassingrainsizedistributioncurve(mm)

A large uniformity coefficientmeans that the grain size is broadly distributed, and such a soil is called


–217–

“wellgraded”.Incontrast,asmallvalueofUcmeansthatthegrainsizedistributionisnarroworthegrainsizeisuniform.Suchasoiliscalled“poorlygraded”. IntheJapaneseUnifiedSoilClassificationSystem,coarsesoilwherefinecontentsarelessthan5%ofthetotalmassisfurtherdividedinto“broadly-distributedsoil”and“uniformedsoil”.

Broadly-distributedsoil :10 ≤ UcUniformedsoil :Uc<10

2.2.3 Hydraulic Conductivity of Soil

(1)Whentheseepageflowinacompletelysaturatedgroundisasteadylaminarflow,thehydraulicconductivityshallbeestimatedbyusingDarcy’slaw.

(2)Thehydraulicconductivitykiscalculatedbyequation(2.2.7),takingintoaccountofthemeasurementofcross-sectionalareaofsoilA,hydraulicgradientiandvolumeofseepageflowinunittime.

(2.2.7)where

k :coefficientofpermeability(cm/s) q :volumeofwaterflowinsoilinunittime(cm3/s)

i :hydraulicgradient,

h :headloss(cm) L :lengthoftheseepagepath(cm) A :cross-sectionalarea(cm2)

Themeasurement fordeterminingcoefficientofpermeabilityk includes a laboratorypermeability testofundisturbedsoilsamplestakenin-situ,orain-situpermeabilitytest.

(3)ApproximatevaluesofthecoefficientofpermeabilityHazenshowedthattheeffectivegrainsizeD10andthepermeabilityofsandkarerelated,andgaveequation(2.2.8)tocalculatekofrelativelyuniformsandwiththeuniformitycoefficientofUclessthan5,andtheeffectivegrainsizeD10from0.1mmto0.3mm.5)

(2.2.8)where

k :coefficientofpermeability(cm/s) C :constant(C=100(1/cm·s)) D10 :grainsizecalledastheeffectivegrainsizecorrespondingto10percentagepassingofmass

ingrainsizedistributioncurve(cm)

Terzaghi has pointed out that equation (2.2.8) can also be applied to cohesive soils by usingC≒2. TheapproximatevaluesofthecoefficientofpermeabilityarelistedinTable 2.2.2).5)

Table 2.2.2 Approximate Values of Coefficient of Permeability 5)

Soil Sand Silt ClayHydraulicconductivity 10-2cm/s 10-5cm/s 10-7cm/s

–218–


2.3 Mechanical Properties of Soil2.3.1 Elastic Constants

(1)Whenanalyzingsoilbehaviorasanelasticbody,theelasticconstantsaredeterminedwithdueconsiderationforthenonlinearityofstress-strainrelationofsoils.

(2)Whenanalyzingsoilbehaviorasanelasticbody,thedeformationmodulusandPoisson’sratioarenormallyusedastheelasticconstants.Becauseofthestrongnonlinearityofstress-strainrelationshipofsoil,theelasticconstantsinanalysismustbedeterminedbyconsideringthestrainlevelofthegroundtobeanalyzed.

(3)StrainDependencyofDeformationModulusThestress-strainrelationofsoilusuallyshowsastrongnonlinearity.Whenthestrainleveliswithinarangeof10-5orlessnamely0.001%orless,thedeformationmodulusislargestandnearlyconstant.ThismaximumvalueEmaxiscorrespondingtothemeasuredvalueinthedynamictestingmethodssuchastheelasticwaveexploration,andiscalledthedynamicelasticitymodulus.Asthestrainlevelincreases,theelasticitymodulusdecreases.ThesecantmodulusE50,determinedfromaconventionalunconfinedcompressiontestoratriaxialcompressiontest,isconsideredasthedeformationmoduluswhenthestrainisoftheorderof10-3(0.1%).Whenconductinganelasticanalysisofsoil,itisnecessarytodeterminetheelasticconstantbyconsideringthestrainlevelofthesoil.

(4)RelationshipbetweenUndrainedShearStrengthandDeformationModulusForcohesivesoils,theapproximatevaluesfortheinitialtangentelasticmodulusEi,andthesecantelasticmodulusE50canbedeterminedbyusingequation(2.3.1)andequation(2.3.2).7)

(2.3.1)

(2.3.2)

where Ei :initialtangentelasticmodulus(kN/m2) E50 :secantelasticmodulus(kN/m2) cu :undrainedshearstrength(kN/m2)

Theequation(2.3.1)isapplicableonlyforhighlystructuredmarinecohesivesoilwithhighplasticity.

(5)Poisson’sRatioFordeterminingPoisson’sratioofsoil,thereisnoestablishedmethodcurrently,althoughanumberofmethodshavebeenproposed.Practically,v =1/2isusedforundrainedconditionsofsaturatedsoil,andv =1/3–1/2isusedformanyothersituations.

2.3.2 Compression and Consolidation Characteristics

(1)CompressioncharacteristicsofsoilandthecoefficientsforestimatingsettlementoffoundationsduetoconsolidationcanbecalculatedfromthevaluesobtainedbasedonJISA1217Test Method forConsolidation Testof Soils Using Incremental Loading.

(2)When soil is loaded one-dimensionally, compression of the structure with the soil particles which causessettlementisreferredtoascompression.Ifthevoidsofthesoilaresaturatedwithwater,itisnecessaryfortheporewatertobedrainedinordertocontactthestructurewiththesoilparticles.Forsandysoilswithhighhydraulicconductivity,drainageisfast,socontractionoccursimmediatelyafterloadingandissooncompleted.However,forcohesivesoilgroundthehydraulicconductivityisverylow,soalongperiodoftimeisneededfordrainage,andcompressionsettlementoccursslowly.Thisphenomenoninwhichcompressionsettlementincohesivesoilgroundoccursoveralongperiodoftimeisreferredtoasconsolidation. Theconsolidationcharacteristicsofsoilsareusednotonlyforcalculatingthesettlementduetoloading,butalsoforestimatingtheincreaseinshearstrengthofsoilsinsoilimprovementwork.

(3)CalculationofthefinalsettlementduetoconsolidationWhentheconsolidationpressureand thevoidratiowhenconsolidation iscompletedat thatpressure(after24hours)inaconsolidationtestareplottedonsemi-logarithmicgraph,theso-callede–logpcurveorcompressioncurveisobtained,asshownin Fig. 2.3.1.The“abc”portionofthee–logpcurveindicatestheloadingprocess,andisvirtuallylinear.Theconsolidationstateindicatedbythe“abc”portionisreferredtoasthenormalconsolidationstate.Ontheotherhand,ifthesoilisunloadedfromthestateatpoint“b”,therelationshipbetweenthevoidratioandthepressurewhentheequilibriumstateisreachedunderthereducedpressuredescribesthepath“bd”.Ifthepressureisincreasedagain,thepath“db”isdescribed.Thestaterepresentedby“bd”and“db”isreferred


–219–

toastheoverconsolidation.Whenaconsolidationtestiscarriedout,thepath“d→b→c”isdescribed,thepoint“b” isobtainedat theboundaryof“d→b”indicating theelasticdeformationand“b→c”indicating theplasticdeformation,andthepressurecorrespondingtothisboundaryisreferredtoastheconsolidationyieldstress.

a

b

c

d

log p

1

2

1 2

e

e

e

p p

Fig. 2.3.1 e–log p Relationship during Consolidation

The relationshipbetween thevoid ratioe and thepressurep for the segment“abc”,normalconsolidationdomaininFig. 2.3.1 isexpressedbyequation(2.3.3).

(2.3.3)

whereCc isanon-dimensionalnumbershowingthedegreeofinclinationofsegment“abc”andiscalledthecompressionindex.

Thefinalsettlementresultingfromtheconsolidationloadcanbecalculatedusingthreemethods:thee-logp curvemethod,theCc method,andthecoefficientofvolumecompressibilitymvmethod.Thedecreaseinvoidratio ewhenthepressureincreasesfromtheoverburdenpressurein-situp0to(p0+ p)canbedeterminedbydirectlyreadingthee–logprelationshipcurveobtainedfromconsolidationtests.Otherwise,ifthesettlementisexpectedtobeoverestimatedtothesafeside,itcanalsobeevaluatedbyequation (2.3.4)usingequation (2.3.3).

(2.3.4)

In thee-logpcurvemethod, thesettlementS iscalculatedbythefollowingequationusing eeitherreaddirectlyordeterminedfromequation (2.3.4):

(2.3.5)

where h :thicknessoflayer

IntheCc method,thesettlementS iscalculatedbythefollowingequation(2.3.6):

(2.3.6)

Thisequationcorrespondstothatwherebyequation (2.3.4)issubstitutedinequation (2.3.5).Thecoefficientofvolumecompressibilitymv isusedforestimatingsettlementandtheamountofcompressionbyaloadisproportionaltomv.However,thisiseffectiveonlywithsmallincreasesinconsolidationpressuresuchaswheremvcanbeassumedtobeconstant,becauseitwouldlinearizethesoilwithstrongnonlinearity.Equation (2.3.7)isusedtocalculatethesettlementSusingmv.

(2.3.7)

where mv :coefficientofvolumecompressibilitywhentheconsolidationpressureis )( 00 ppp ∆+×

–220–


Generally,thevalueofmv duringconsolidationdecreaseswiththeincreaseofeffectiveoverburdenpressure.Undernormallyconsolidatedstate,therelationshipbetweenpandmvplottedonadoublelogarithmicgraphwouldalmostbeastraightline.Themv usedinequation(2.3.7)forcalculatingsettlementisthemeanvalueduringthechangeineffectiveoverburdenpressureofthegroundfromp0to(p0+ p).Usually,thiswouldbethemvforthegeometricmeanoftheeffectiveoverburdenpressure .

(4)SettlementRateInTerzaghi’stheorywhichisaclassicaltheoryofconsolidation,themethodofanalyzingthesettlementrateisasfollows:Whenapressureincrementpisaddedtoasaturatedcohesivesoilunderundrainedconditions,anexcessporewaterpressureequaltothemagnitudeofpisgenerated.Asconsolidationprogresses,thisexcessporewaterpressuregraduallydissipates,andatthesametimethestressσ'actingbetweensoilparticlesincreases.Thisstressisreferredtoasthe“effectivestress”.However,thesumoftheexcessporewaterpressureuandtheincrementofstressσ'betweensoilparticlesisalwaysequaltotheincrementofloadingpressurep,soequation(2.3.8)isestablished.

(2.3.8)

Considerthecasewherehighlypermeablesandlayersexistaboveandbeneathaclaylayerofthickness2H.Whenaconsolidationpressureincrementpisapplied,thedistributionwithdepthofσ'anduareasshowninFig. 2.3.2.Inotherwords,atthetimeofstartofconsolidation(t=0),thestateisindicatedbythelineDCwithu=p,σ'=0,andwhenconsolidationiscompletedthestateisasindicatedbythelineAB,withu=0,σ'=p.ThecurveAEBistheporewaterpressuredistributionatthetimet1afterstartofconsolidation.Thiscurveiscalled“isochrone”.Asshowninthefigure,thepartsofsoildistantfromthedrainagelayershaverelativelyslowrateofconsolidation. Theratiooftheeffectivestressincrementtotheconsolidationpressureincrement(σ'/p)atacertaindepthzisreferredtoasthedegreeofconsolidationUzatthatdepth.ThedegreeofconsolidationateachdepthaveragedoverthewholelayerisreferredtoastheaveragedegreeofconsolidationU.TheaverageconsolidationistheratiooftheareaofAEBCDtotheareaABCDinFig. 2.3.2.

A

BC

D

(　=

0 )

(　=　

)t

E

＝

＝1

1

vv

∞

u

u

H

H

p p

z

h

h

t

tt T

T

Drainage layer

Drainage layer

Clay layer

'

'

Fig. 2.3.2Distribution of Pore Water Pressure with Depth

The consolidation is the time-dependent settlement phenomenon. The rate of consolidation for an entirecohesivesoillayerisrepresentedwiththeparameterU fortheaveragedegreeofconsolidation.TherelationshipbetweenU andthenon-dimensionaltimefactorTv isobtainedbythetheoryofconsolidation.Therelationshipbetweenthenon-dimensionaltimefactorTv andtheactualtimet isshownbythefollowingequation:

(2.3.9)

where Tv :timefactor cv :coefficientofconsolidation t :timeaftertheconsolidationstarts H* :maximumdrainagedistance

Whenthepermeablelayerexistatbothsidesofthecohesivesoillayer,themaximumdrainagedistanceH*isthesameasH.However,whenthepermeablelayeronlyexistsononeside,H*isequalto2H.ThedegreeofconsolidationateachdepthisshownbytheconsolidationisochronesinFig. 2.3.3.Furthermore,Fig. 2.3.4 shows


–221–

thetheoreticalrelationshipbetweentheaveragedegreeofconsolidationandthetimefactor.

0

0.5

1

1.5

2.00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

H―Z

Degree of consolidation Uz

=0

T v

=0.05

0.10 0.15 0.200.30

0.400.50

0.60

0.70

0.80 0.848

0.90

Tv

Fig. 2.3.3 Consolidation Isochrones

0

20

40

60

80

100 0 0.1 0.2 0.5 0.6 0.7 0.8 0.9

u

u u u

u u uuii = = +

0

0

1

1

2

2

2H

H z-

0.3 0.4

H

Ave

rage

deg

ree

of c

onso

lidat

ion U

(%)

Time factor Tv

Fig. 2.3.4 Theoretical Relationship between Average Degree of Consolidation and Time Factor

(5)PrimaryConsolidationandSecondaryConsolidationIftherelationshipbetweenamountofsettlementandtimemeasuredinaconsolidationtestisshownasdegreeofconsolidationagainsttime,Fig. 2.3.5 isobtained.However,asshowninthefigure,atthefinalstageofconsolidation,thetestcurvedoesnotcoincidewiththetheoreticalcurve.ConsolidationuntilU=100%asdeterminedbythetimesettlementrelationshipvirtuallyagreeingwithconsolidationtheoryisreferredtoas“primaryconsolidation”,andthepartinwhichU>100%andconsolidationisnotinaccordancewithconsolidationtheoryisreferredtoas“secondaryconsolidation”.Secondaryconsolidationisconsideredtobeacreepphenomenon,andinthiscasethesettlementtendstooccurlinearlywithrespecttothelogarithmoftime. Intheperformanceverificationofportfacilities,normallytheconsolidationpressureduetoloadingreachesseveraltimestheconsolidationyieldstressoftheground.Undertheseconditions,theamountofsettlementduetoprimaryconsolidationislarge,andtheamountofsettlementduetosecondaryconsolidationiscomparativelysmall,soinmostcasessecondaryconsolidationisnotconsideredwhencarryingouttheperformanceverification.Also,if thesettlementislarge,theeffectoftheincreaseinbuoyancywithsettlementcancelsouttheeffectofsecondaryconsolidation,soapparentlysecondaryconsolidationisnotseen.Inthefollowingcases,secondaryconsolidationmustbetakenintoconsiderationattheperformanceverification.

① Theadvancementingroundsettlementwithelapseoftimesubsequenttoconstructionishavingseriouseffectsonthefacility.

② As in the case of deep Pleistocene clayey ground, when the consolidation pressure does not exceed theconsolidationyieldstressofthesoillayersignificantly,thecontributionofsecondaryconsolidationinanentiresettlementcannotbeneglected.

–222–


0

2040

6080

100120

t100

Theoretical curve

Experimental curveU = 100%

Secondaryconsolidation

Ave

rage

deg

ree

of c

onso

lidat

ion U

(%)

Primaryconsolidation

Fig. 2.3.5 Primary Consolidation and Secondary Consolidation

(6)ConsolidationSettlementofVerySoftCohesiveSoilsWhen the landfill iscarriedoutwithdredgingordisposedsludge, it isnecessary topredict theconsolidationsettlementofextremelysoftdeposits.Mikasa’s8)consolidationtheorywhichtakesintoaccounttheeffectofselfweightofclaylayerandthechangesinlayerthicknessduringconsolidationcanbeappliedtoanalyzethisproblem..Inthiscase,theamountandspeedofsettlementcannotbedeterminedanalytically;itmustbecalculatedwiththefinitedifferentialmethod. Whenthereductioninthicknessofalayerduetosettlementcomparedwiththeoriginalthicknessissolargethat it cannotbe ignored, the errors in thenormal consolidation settlement calculationmethodbecome large.Forexample,ifthereductioninthelayerthicknessis10to50%,thedifferencebetweenthenormalcalculationmethodandacalculationthattakesintoconsiderationtheeffectofthechangeinlayerthicknessisintheregion3to30%.Also,theeffectofdeadweightislargestafterallowingtostandafterdredgingandfilling,andastheloadincreases,theeffectisreducedrelatively.Forloadingthatistwiceormoretheaverageownweightofaweaklayer,theeffectofselfweightbecomesverysmall,andcanbevirtuallyignored. Inordertoestimatetheconsolidationparametersofverysoftcohesivesoil,thereisaconstantrateofstrainconsolidationtestinwhichdisplacementiscontinuouslyappliedasstipulatedinJISA1227Test Method for One-dimensional Consolidation Properties of Soils using Constant Rate of Strain Loading. Forcohesivesoilswithalargeagingeffect,orforcohesivesoilsofwhichsettlementcanbesuddenlyseenaftertheconsolidationyieldstress,theconstantrateofstrainconsolidationtestfromwhichacontinuouse–logpcurvecanbeobtainedisaveryusefulmethodforobtaining theconsolidationyieldstress9) However, thee–logpcurve isstronglyaffectedbythestrainrate,andthee–logpcurveobtainedfromthistestisnormallygreatlyshiftedtothelargeconsolidationpressuresidecomparedwiththee–logpcurveobtainedfromanicrementalloadingconsolidationtestasstipulatedinJISA1217.Thereforeitisnecessarytobeawarethattheconsolidationyieldstressbecomeslarger.

(7)CorrelationbetweentheCompressionandConsolidationCoefficientsandthePhysicalPropertiesOfallthesoiltests,theconsolidationtestrequiresthelongestamountoftime.Iftheresultsofconsolidationtestcanbeestimatedfromphysical test results,whichrequireonlydisturbedtestsamples,andisacomparativelysimpletestmethod,andmoreoverwhoseresultscanbequicklyobtained,thiswouldbeveryuseful.Skemptonhasproposedthecorrelationequation (2.3.10)astherelationshipbetweenthecompressionindexCcandtheliquidlimitwL.

(2.3.10)

Equation(2.3.10)isapplicabletoclaythatisre-moldedandre-consolidatedinthelaboratory,oryoungclaygroundformedbyartificialfilling,butittendstoeitheroverorunderestimatethecompressioncharacteristicsofnaturallydepositedclays. The reason why natural cohesive soil grounds have larger compression index values than young clay isbecauseintheprocessofsedimentationwhichoccursovermanyyears,astructureisformedduetoagingeffectssuchascementation. Whenthisstructure isdestroyedasaresultof theconsolidationpressureexceedingtheconsolidationyieldstress,highcompressibilityisdemonstrated.


–223–

2.3.3 Shear Characteristics

(1)Theshearstrengthparametersofsoilaredeterminedbyclassifyingsoilintosandysoilandcohesivesoil.Theshearstrengthofsandysoilisdeterminedunderdrainedconditions,whiletheshearstrengthforcohesivesoilisdeterminedunderundrainedconditions.

(2)Ingeneral,thehydraulicconductivityofsandysoilis103–105timesthatofcohesivesoil.Forsandysoillayer,theexcesswaterinporesisconsideredtobecompletelydrainedduringconstruction.Forcohesivesoillayer,ontheotherhand,almostnodrainageisexpectedduringconstructionbecausethehydraulicconductivityissignificantlylow.Thusinmanycasesforsandysoillayertheshearstrengthisevaluatedusingtheangleofshearresistanceindrainedcondition D andthecohesionindrainedconditioncD.BecausethevalueofcD isusuallyverysmall,practicallycD isignoredandonly D isusedasthestrengthparameter. Inthecaseofsaturatedcohesivesoillayer,theshearstrengthofthelayerundergoesalmostnochangebetweenbeforeandafterconstruction,asthedrainagecannottakeplaceduringconstruction.Theundrainedshearstrengthbeforeconstructionisthereforeusedasthestrengthparameter.Forintermediatesoilthathasthepermeabilitysomewherebetweenthoseofsandysoilandcohesivesoil,thesoilshouldbeviewedassandysoilorcohesivesoilbasedonthecoefficientofpermeabilityandconstructionconditions.

(3)ConsiderationsonShearStrengthTheshearstrengthτ fofasoilisgenerallyexpressedbythefollowingequation.

(2.3.11)where

τf :shearstrength c :cohesionorapparentcohesion :angleofshearresistance(°) σ :normalstressontheshearsurface

Whenastressisappliedtoasoil,thestressactingontheskeletalstructureofthesoilparticles,referredtoastheeffectivestress,andtheporewaterpressure,)bothchange.Ifthetotalstressappliedtothesoildenotesσ,theeffectivestressdenotesσ',andtheporewaterpressuredenotesu,thefollowingrelationshipcanbeestablished.

(2.3.12)

(2.3.13)

Inequation (2.3.11), thestrengthconstantssuchascand ,varydependingon theconditionsduring thesheartests,buttheconditionthathasthegreatesteffectisthedrainageconditionofthesoil.Becausesoilhasthetendencyofchangingvolumeswhichisknownas“dilatancy”whilebeingsheared,shearstrengthofsoilisgreatlydependentuponwhetheravolumechangetakesplaceduringtheshearornot.Thedrainageconditionisclassifiedintothefollowingthreecategoriesanddifferentstrengthparametersareusedforeachcase:

① Unconsolidated,Undrainedcondition(UUcondition)

② Consolidated,Undrainedcondition(CUcondition)

③ Consolidated,Drainedcondition(CDcondition)

InFig. 2.3.6,patterndiagramsareshownfortheshearstrengthwhendirectsheartestsarecarriedoutunderthedrainagecondition①,②,③.10)Inthefigure,thechangeinshearstrengthunderincreasedorreducednormalstressσ isshownonthesoilsamplesconsolidatedinadvancetothepressurep0.Asshowninthefigure,undertheunconsolidatedundrainedcondition①,thestrengthisconstantanddoesnotdependonσ.Inthecaseoftheconsolidatedundrainedcondition②,withintherangep0<σthestrengthincreaseslinearlyasσincreases.Undertheconsolidateddrainedcondition③, thestrength isoverallgreater than①,②,and this isbecause thevoidratioisreducedbyconsolidationorshearinginthecaseofweakcohesivesoilorloosesand.However,whenσissignificantlysmallerthanp0(inthefigurethislimitofthenormalstressisindicatedasσ*),thestrengthundertheconsolidateddrainedconditionissmallerthanthestrengthundertheconsolidatedundrainedconditionduetotheeffectofswellingduringshearing.Summarizingthisrelationshipfortherangeofσthefollowingisobtained.

Intherangep0<σ , namelytheappliedloadingislargerthanthepre-consolidationpressure;①<②<③

In the rangeσ*<σ <p0 , namely the applied loading is somewhat smaller than thepreceding consolidation

–224–


pressure;②<①<③or②<③<①Intherangeσ<σ*,namelytheappliedloadingissignificantlysmallerthantheprecedingconsolidationpressure;③<②<①

Fig. 2.3.6 Relationship between Drainage Conditions and Shear Strength

Theshearstrengthusedfortheperformanceverificationofgroundshouldbetheshearstrengthforthemostdangerousdrainageconditionsexpectedunderthegivenload.Thedrainageconditionandshearstrengtharethenasinthefollowing:

(a)Whenloadingtakesplacerapidlyonthecohesivesoilground:Becauseconsolidationprogressesandshearstrengthincreaseswiththeelapseoftime,themostdangeroustimewillbeimmediatelyaftertheloadingwhenalmostnodrainagehasoccurred.Thisiscalledtheshort-periodstabilityproblem.Theshearstrengthτfatthistimeistheshearstrengthcu thatisdeterminedfromunconsolidatedundrained(UU)testsusingthesamplebeforeloading.Theparametercu (undrainedshearstrength) isalsocalled theapparentcohesion,andtheanalysisusingcu isalsocalled the“ =0method”.Constructionsofseawallsorbreakwaterswithoutexcavation,landfill,andembankmentsonsoftcohesivesoilgroundfallinthiscategory.

(b)Whengroundpermeability is largeorwhendrainagefromconsolidated layer isalmostcompletedduringconstructionperiodbecausetheloadingiscarriedoutveryslowly: Becausedrainagefromthelayeroccurssimultaneouslywith loadingandanincreaseinstrengthof thelayer is expectedalongwith the loading, theperformanceverificationof structures shouldbecarriedoutusing cD and D determinedunderconsolidatedanddrained(CD)conditions.Constructionsofseawallsorbreakwaters,landfillandembankmentsonsandysoilbelongtothiscategory.

(c)Whenthehydraulicconductivityofthegroundispoorandtheloadisremovedtodecreasethenormalstressσontheshearplane: Inthiscase,themostdangeroussituationisafteralongtimehaselapsed,whenthesoilabsorbswater,expands,andlosesitsshearstrength,thisiscalledthelong-termstabilityproblem.AsshowninFig. 2.3.6,undrained shear strength becomes the lowest after water absorption and soil expansion when the over-consolidationratioissmall,inotherwords,σisalittlelessthanp0.Inthissituation,therefore,thecu valueshould be usedwith consideration of soil swelling. Earth retaining and excavation in clayey ground orremovalofpreloadingoncohesivesoilgroundbelongstothiscategory.Ontheotherhand,inthecaseofheavilyover-consolidatedgroundwhereσisverysmallcomparedtop0,theparameterscD and D areusedforperformanceverificationbecausetheshearstrengthunderconsolidated,drainedconditionisthesmallest.Usually,thisoftenappliestocaseswherecutearthmethodsareemployedbutitalsoappliestoconstructionworksincoastalareassuchasworkstodeeperquaywalldepthanddredgingworksonseabedsoil. In almost all cases for normal construction conditionsof port facilities, theundrained strength inUUconditionsof(a)isusedintheperformanceverificationforcohesivesoilsandthestrengthparameterintheCDconditionsof(b)isusedforsandysoils.Thefollowingequationsshowthestrengthcalculationmethodsrespectively:

1) Forcohesivesoilwiththesandcontentislessthan50%

(2.3.14)


–225–

whereτ:shearstrengthcu:undrainedshearstrength

2) Forsandysoilwiththesandcontentishigherthan80%

(2.3.15)where

τ :shearstrength σ :normalstresstoshearplane u :hydraulicpressureatthesite D :angleofshearresistancefordrainedconditions(º)

Furthermore, because soil with a sand fraction ranging from 50% – 80% displays intermediatecharacteristicsbetweensandysoilandcohesivesoil,itiscalledtheintermediatesoil.Theevaluationofshearstrengthofintermediatesoilisdifficultcomparedwiththatofsandysoilorcohesivesoil.Hence,theshearstrengthforsuchsoilshouldbeevaluatedcarefullybyreferringtothemostrecentresearchresults.Withrespecttointermediatesoilthatcanbetreatedascohesivesoil,itispreferabletoutilizeresultsoftriaxialCUtestsetc.ratherthanevaluateshearstrengthfromunconfinedcompressivestrength.

(4)ShearStrengthofSandBecausesandysoilhashighhydraulicconductivityandisregardedincompletelydrainedcondition,theshearstrengthofsandisrepresentedbyequation(2.3.15).Theangleofshearresistance D fordrainedconditionscanbedeterminedusingatriaxialCDtestunderconsolidatedanddrainedcondition.Becausethevalueof D becomeslargewhensand’svoidratiobecomessmallanditsdensitybecomeshigh,thevoidratioe0in-situshouldbeaccuratelydetermined.Therefore,itisbesttotakeandtestanundisturbedsample.Althoughthe D valuesofsandwiththesamedensitywillvaryalittlewiththeshearconditions,thevalueof D determinedbyatriaxialCDtest,whichisconductedwiththeconsolidationpressurecorrespondingtodesignconditionswithundisturbedsample, canbeused as thedesignparameter for stability analysis. However, in the caseofbearing capacityproblemforfoundation,whichismuchinfluencedbyprogressivefailure,thebearingcapacityisover-estimatedinsomecasesifthevalueof D determinedbyatriaxialCDtestisdirectlyusedasthedesignparameter. Comparedwiththecaseofcohesivesoil,samplingofundisturbedsandsamplesistechnicallydifficultandalsoveryexpensive.ThisisthereasonthattheshearstrengthforsandysoilisfrequentlydeterminedfromtheN-valueofstandardpenetrationtestratherthanfromalaboratorysoiltest.Fortheequationtodetermine DfromN-values,referto2.3.4 (4) Angle of shear resistance of sandy ground.

(5)ShearStrengthofCohesiveSoilHere, soil ofwhich the clay and silt fraction by percentage is greater than 50% is regarded as cohesive soil.Thereareseveralmethods,aspresentedbelow, todetermine theundrainedshearstrengthcu ofcohesivesoil.Anappropriatemethodshouldbechosen inconsiderationof such factorsas thepast experiences, the subsoilcharacteristicsandtheimportanceofthestructures.

① qu method:Thismethodusestheaveragevalueofunconfinedcompressivestrengthdeterminedfromundisturbedsamples.Theundrainedshearstrengthcu usedfortheperformanceverificationisgivenbythefollowingequation:

(2.3.16)

Inthisequation, uq istheaveragevalueofunconfinedcompressivestrength.Inunconfinedcompressiontests,confiningpressureisnotappliedonthetestsampleandtherefore, thestrengthresultobtainedmayberemarkablysmallduetodisturbanceofthesample.ApplicationisparticularlydifficultonclayeysoilsampledfromdepthsuchasstiffPleistoceneclayeysoilinwhichcrackscanappeareasily.Cautionisalsoneededforapplicationonintermediatesoilwithhighsandcontentaseffectivestressmaynotbemaintainedinthetestsampleandconsequently,aremarkablysmallshearstrengthmaybeobtained.Inthiscase,itispreferabletoemployothertestmethodssuchastriaxialtestordirectsheartest.

②Methodofusingstrengthbytriaxialteststakinginitialstressandanisotropyintoconsideration:Considerthestabilityanalysisofaembankmentontheclayeygroundusingacircularslip,asshowninFig. 2.3.7.Directlybelowtheembankmentshearingiscausedbytheincreaseinverticalstress,soitispossibletoevaluatetheshearstrengthcorrespondingtothisbythetriaxialconsolidatedundrainedcompressiontest(CUCtest),althoughstrictlyspeakingtherearedifferencesintheplanestrainandaxialsymmetry.Ontheotherhand,shearingoccursattheendpointofthecirculararc,inotherwordsnearthebaseoftheslope,duetotheincreaseinhorizontalstress,soitispossibletoevaluatethisbythetriaxialconsolidatedundrainedextensiontest(CUE).

–226–


Ofcourse,therearedifferencesintheplanestrainandaxialsymmetry,andthereisthemajordifferencethatincontrasttothetriaxialextensiontestinwhichtheaxialforcereduces,inthefailureofanembankmentthehorizontalstressincreases.Nearthebottomofthecirculararc,thedeformationmodeisnotcompressionnorextension.,butvirtuallyhorizontalshearingisproduced.Therefore,itispossibletoevaluatethisbyadirectsheartestorasimplesheartest. Theshearstrengthsu*usedintheperformanceverificationmaybetheaveragevalueoftheshearstrengthsucobtainedfromacompressiontestandtheshearstrengthsueobtainedfromanextensiontestasgivenbythefollowingequation

(2.3.17)

orthedirectshearstrengthsusmaybeusedastherepresentativevalue.

usu ss =*

Formostsoils,thetriaxialextensionstrengthsueisabout70%ofthetriaxialcompressionstrengthsuc.

Triaxialextension

sue suc

sus

Triaxialcompression

Box shear

Fig. 2.3.7 Stability Problem and Strength Anisotropy for an Embankment Constructed on a Clayey Ground

Disturbanceofatestspecimenduringsamplingisinevitabletoacertainextent,evenifeffortsaremadetominimizeit.Also,ithasbeensaidforalongtimethattheunconfinedcompressiontestislackinginreliability,but the performance verification methods are frequently based on them, as in the present situation othermethodscannotbeadopted.Asamethodofdeterminingtheundrainedshearstrength,themethodknownasthe“recompressionmethod”11)issaidtobethemostreliableamongthetestmethodscurrentlyproposed.Thismethodisbasedonthethinkingthatbyreproducingthesamestressstateasthesampledtestspecimenintheoriginallocation,theeffectofdisturbanceinthetestspecimencanbemadesmallerbyconsolidation. Elementswithin a ground are subject to the vertical overburden effective stressσ'v0, and the horizontalearthpressureatrestσ'h0(=K0σ'v0).Asampledtestspecimenhaszerostressunderatmosphericpressure,andanisotropicresidualeffectivestressduetosuctionremainstoacertainextent.However,byconsolidationtoσ'1=σ'v0,σ'3=K0σ'v0intriaxialtestapparatus,undrainedsheartestscanbecarriedoutwiththesameeffectivestressstateastheoriginalpositionreproduced.Theeffectiveoverburdenpressureσ'v0canbecalculatedfromtheunitweightofthesampledtestspecimens.However,aproblematthisstageishowtoobtainthecoefficientofearthpressureK0.Severalmethodsforobtainingitfromin-situtestshavebeenproposed,butitcanalsobeobtainedfromalaboratorybyaK0consolidationtestusingatriaxialcell.12)Here,theK0consolidationtestisatestinwhichthecellpressureσ3iscontrolledsothatthecross-sectionalareaofthetestspecimendoesnotchangewhentheaxialpressureσ1ortheaxialstrainε1increases.However,K0obtainedbythismethodisK0forthenormallyconsolidatedstate,frequentlyexpressedasK0NC,soitisnecessarytobeawarethatitisnottheK0forsoilwiththeagingeffectasinarealground.InJapaneseclays,K0undernormallyconsolidatedconditionsismostlyintherange0.45to0.55. Therecompressionmethodisalsopossiblewiththedirectsheartest.Inthiscase,thechangeinthediameterofthetestspecimenisconstrainedbytheshearring,sobysimplymakingtheconsolidationpressureequaltotheeffectiveoverburdenpressureσ'v0,thereisnoparticularneedtobeawareofK0.Althoughtheundrainedshearstrength(qu/2)obtainedfromaunconfinedcompressiontesthasalargeamountofvariation,theaveragevalueisvirtuallythesameastheaveragevaluevalueofsucandsueoftheundrainedshearstrengthobtainedfromtriaxialcompressionandextensiontestsbytherecompressionmethodwithconsolidationofσ'v0andK0σ'v0,which iscapableofreproducing thesamestressstateas the testspecimenin theoriginallocation.Thereliabilityofthetestresultsusingtriaxialcompressionandextensiontestsbytherecompressionmethodwhosemechanicalbasisisclearer,isslightlyhigherthanthatoftheunconfinedcompressiontests.Insection2.1 Estimation of Ground Constants,itisexpectedthattriaxialtests,fromwhichresultswithsmall


–227–

variationcanbeobtained,arepreferablefortheperformanceverification.

③Methodusingstrengthfromadirectsheartest:ThismethodusesthestrengthτDS determinedbyadirectsheartestafterundisturbedsampleisconsolidatedone-dimensionallyunderin-situeffectiveoverburdenpressure.Thedirectsheartestcanbeconductedaccordingto theJGS 0560 Method for Consolidation parameters Pressure Direct Direct shear test on Soilof theGeotechnicalSocietyofJapan.Theundrainedshearstrengthcu usedfortheperformanceverificationisgivenbythefollowingequation:

(2.3.18)

Inthisequation,0.85isacorrectionfactorrelatedtoshearrateeffect.Themeasuredvalueshavethereforeundergoneprimaryprocessingtoarriveatthederivativevalues.

④Methodscombiningunconfinedcompressivestrengthandstrengthfromtriaxialcompressiontests:Oneproblemwiththequ methodisthatthetest’sreliabilityislowinsoilwithnopastrecords,becausethetestissubjecttotheinfluenceofdisturbanceduringsampling.Toresolvethisproblem,acombinationmethodcanbeusedtodeterminethestrengthbycomparingthequ ofundisturbedsampleswiththestrengthfromatriaxialCUtestandevaluatingthequalityofthesample.Inthismethod,thesampleisisotropicallyconsolidatedbyin-situmeaneffectivestressof2σ'v0/3whenK0=0.5,afterwhichtriaxialCUtestisperformedinundrainedcompressioncondition.Theundrainedshearstrengththusobtainedmustbeempiricallycorrectedbymultiplying0.75.Inotherwords,asisthecasewiththedirectsheartest,forthistriaxialtest,measuredvaluesmustundergoprimaryprocessingtoarriveatthederivativevalues.Thismethodisusedfornaturalsoilgroundandcannotbeappliedtounconsolidatedreclaimedground.Formoredetailsseethereferences13)and14).

⑤Methodfordeterminingundrainedshearstrengthfromanin-situvanesheartest:Avanesheartestisconductedasdescribedin1.3 Selection of Investigation Methods.Theaveragevalueoftheobtainedshearstrengthcu(v) canbeusedintheperformanceverificationastheundrainedshearstrengthcu 15).Anin-situvanesheartestcanbecarriedoutrathereasilywithmobilityatafieldsite.Thetestisabletodeterminetheshearstrengthforverysoftclayforwhichanunconfinedcompressiontestcannotbeperformedduetothedifficultyinmakingaspecimenfreestanding.Itcanthusbeapplied,forexample,totheconstructionmanagementwheresoilisbeingimprovedusingverticaldrains.Althoughthetestmethodandprinciplearesimple,attentionmustbegiventotheeffectoffrictionontherod.Waysofreducingthefrictionandcalibratingitseffectneedtobedevised. Eachmethodhasitsowncharacteristics,whichmustbedulyconsideredinordertoselectthemostappropriateone.Theundrained shear strengthcu of cohesive soils increases as consolidationprogresses, and thehigher theconsolidationloadthelargerthecuafterconsolidation.Therefore,theconsolidationpressureincreaseswithdepthastheoverburdenpressureincreases,sonormallythecuofaclaygroundincreaseswithdepth,andthedistributionofundrainedshearstrengthused in theperformanceverification is frequentlyexpressedby thefollowingequation.

(2.3.19)

where cu :undrainedshearstrengthatdepthzfromthesurfaceoftheclaylayer cu0 :undrainedshearstrengthatsurfaceoftheclaylayer k :rateofincreaseofcuwithdepthz z :depthfromthesurfaceoftheclaylayer

(6)IncreaseinCohesiveSoilStrengthduetoConsolidationTheundrainedstrengthofcohesivesoilwillincreasewiththeprogressofconsolidation.Forsoilimprovementmethodssuchastheverticaldrainmethod,theratioofstrengthincreasecu/p byconsolidationisanimportantparameterbecausethestrengthisincreasedbythedrainageofporewaterbyconsolidation.Naturallysedimentedcohesivesoilgroundcanbesomewhatoverconsolidated,orevenifitisnormallyconsolidatedintermsofstresshistory, it canappear tobeoverconsolidatedwith largeconsolidationyield stresspc due toagingeffect. Forthis reason, the ratio of strength increase becomes the cohesive soil's specific parameter in the case of slightoverconsolidationthroughnormalizing,notbytheeffectiveoverburdenpressureσ'v0equivalenttotheconsolidationpressure,butbytheconsolidationyieldstresspc (m=cu/pc).Thelargerthevalueofcu/pc,whichisasoilpropertyparameterusedintheverticaldrainmethodforincreasingstrength,thelargertheincreaseratioofthestrengthandthemoreeffectivesoilimprovementareexpected.FromthepastexperiencesinthefieldandresearchresultsformarineclayinJapan,thevalueofcu/pc liesinarangeshownbythefollowingequation,regardlessofplasticity.

–228–


(2.3.20)

InviewofthefactthattheoverconsolidationratioOCRofnaturallysedimentedcohesivesoilisnormallyintherangefrom1.0to1.5,andσ'v0=pc/OCR,therefore,thedatainFig. 2.3.8 15)providessubstantiationforequation (2.3.20).

Triaxial compressionDirect shearTriaxial extension

0.4

0.3

0.2

0.1

00 4020 60 80 100

Plasticity Index Ip

c u /

σ'v0

Fig. 2.3.8 Relationship between Plasticity Index and cu/σ'v0

(7)DecreaseofStrengthofCohesiveSoilsduetoSwellingIfpartoftheloadisremovedafterconsolidation,cohesivesoilsabsorbwaterandswellwithtime,whichcausesthecu toreduce. Inaddition,thetimerequiredforswellingisconsiderablyshorterthanthetimerequiredforconsolidation.Thedrainageconditionsofthiscasecorrespondtotheleftof③asshowninFig. 2.3.6,soitisnecessarytoevaluatethelikelystrengthdecreaseafterswelling.16)Specifically,theremovalofloadattheendofconsolidationinsoilimprovementworkssuchastheverticaldrainmethodorthepreloadmethod,excavationforearthretainingstructures,17)anddredgingtodeepertheseabottom,etc.,correspondtothissituation.

(8)StrengthofIntermediateSoilSoilwithasandcontentintherangeof50%-80%isintermediatesoilbetweensandysoilandcohesivesoil.10)Forthistypeofsoil,thehydraulicconductivityanddesignconditionsaretakenintoconsiderationtodeterminewhetherthesoilissandysoilorcohesivesoil.Thentheshearstrengthisdeterminedaccordingly.Forintermediatesoilwithalargesandfractionorwithcoralgravel,thehydraulicconductivitydeterminedfromanincrementalloadingoedometertestgenerallygivesanunderestimatedvalue,becauseofthelimitationsoftestconditions.Itispreferablenotonlytoimprovethetestprocedure,butalsotoconductanin-situpermeabilitytestoranelectricalconetesttodeterminethehydraulicconductivity.19) Whenthehydraulicconductivitydeterminedbythiskindofprocedureisgreaterthan10-4cm/s,thegroundisregardedpermeable.Hence,thevalueof D determinedfromanelectricalconepenetrationresistanceoratriaxialCDtestcanbeusedasdesignparametersregarding cD =0.AccordingtoexperienceininvestigatingthepropertiesofintermediatesoilsinJapan,thevalueof D isgreaterthan30ºinmanycases.20),21),22) Whenthehydraulicconductivityislessthan10-4cm/s,theperformanceverificationoftheintermediatesoilshouldbeconductedasacohesivesoil.Becausetheinfluenceofstressreleaseduringsamplinginintermediatesoilismuchgreaterthanthatincohesivesoil,theshearstrengthdeterminedbyqu methodisunderestimated.Acorrectionmethodisusedforthestrengthofsuchintermediatesoilwithalargesandfractionbymeansofclayfractionandplasticityindex.23)However,itispreferablethatthecombinedmethodwithunconfinedcompressiontestand triaxialcompression testor thedirectshear testbeusedas themethodforevaluating thestrengthofintermediatesoil.24)

2.3.4 Interpretation Method for N Values

(1)Theangleofshearresistanceforsandysoilsiscalculatedusingthefollowingequationfromastandardpenetrationtestvalue.

(2.3.21)

where :angleofshearresistanceofsand(º)


–229–

N :standardpenetrationtestvalue σ'v0 :effectiveoverburdenpressure at thedepthwhere the standardpenetration test is performed

(kN/m2)

(2)RelationshipsbetweentheN-valueandmanysoilparametershavebeenestablishedbythedataatvarioussites.Whenusingtheserelationships,however,itisnecessarytoconsiderthebackgroundoftheirderivationandthegroundconditionsofthedataandtoconfirmtherangeoftheirapplicability.AscanbeseeninDunham’sequation,which has commonly been used formany years, the value of was determined directly from theN-valueswithoutconsideringtheeffectiveoverburdenpressureσ'v0.However,becausetherelativedensityDr varieswithσ'v0asseeninFig. 2.3.9,σ'v0mustbetakenintoconsiderationtodetermineDrfromanN-value.Thisconceptwasincorporatedinthejudgmentofliquefaction.Inthisjudgment,liquefactionresistanceisexaminedfromN65,theequivalentN-valueconvertedintoN-valuewheneffectiveoverburdenpressureσ'v0=65kN/m2.Similarly,itisknownthateveningroundswiththesame , theN-valueincreaseswiththeincreaseineffectiveoverburdenpressure.Therefore,theinfluenceofσ'v0mustbetakenintoaccountwhendetermining fromtheN-values.

0

10

20

30

40

50

40 600

80 100

'v0=200kN/m2

'v0=100kN/m2

'v0=100kN/m2

'v0=0

'v0=200kN/m2

N-v

alue

Relative density Dr (%)

σ'v0

: Terzaghi

: Gibbs, Holtz (dry sand, wet sand)

: Yanase (wet fine sand)

: " (saturated fine sand)

: effective overburden pressure

: Terzaghi

: Gibbs, Holtz (dry sand, wet sand)

: Yanase (wet fine sand)

: " (saturated fine sand)

: effective overburden pressure

Fig. 2.3.9 Influence of Effective Overburden Pressure and Relative Density on N-Values

(3)FactorsAffectingtheN-valuesThe factors that affectN-valuesmutually overlap, andmethods for quantitatively correcting for these factorshavenotyetbeenestablished.However,forunderstandingofN-values,theextentoftheeffectoftheimportantinfluencingfactorsisasfollows:

① DensityAsthedensity,relativedensity,ofthesubsoilincreases,inparticularforsandysoils,theN-valueincreases.

②WatercontentApartfromwellcompactedfinesandandsiltysoils,theN-valueincreasesintheorderofsaturatedsand,drysand,andwetsand.

③ EffectiveoverburdenpressureTheN-valueincreasesastheeffectiveoverburdenpressureincreases.

④ EffectofgroundwaterlevelAsthegroundwaterlevelfluctuates,theeffectiveoverburdenpressureandthedegreeofsaturationofthesoilvaries,sotheN-valuesvaryaccordingly.

⑤ OtherinfluencingfactorsTheN- value varies in accordancewith the soil particle shape, the grain size distribution, and themineralcompositionofthesoil.

–230–


(4)AngleofShearResistanceofSandyGroundTheangleofshearresistance isanimportantconstantfor theperformanceverificationofgrounds,similartotheundrainedshearstrengthofclaysoils.However, isacomplexvaluethatisgovernedbymanyfactors,andeventhesamesoilwillnothaveaconstantvalue.Therefore,itisnecessarytoinvestigatesufficientlythebackground toestablishing theperformanceverificationmethods,suchaswhatconditionsareassumedin theperformanceverificationmethodsusing .

(5)N-valueofCohesiveSoilGroundComparedwithsandground,theN-valueofacohesivesoilissmall,anditsreliabilityislow.Accordingtopastexperienceandtestresults,ifthequis100kN/m2orlowerthemeasurementofN-valueisdifficult.Incohesivesoilswithquofthisvalue,whensoftcohesivesoilsarefoundbytakingtestspecimenswithastandardpenetrationtesttypesamplerduringapreliminaryinvestigation,orwhentestsarecarriedouttoknowthephysicalproperties,thishassignificance,butthestrengthandothermechanicalsubsoilconstantscannotbedeterminedfromtheN-valuesonly.InthecaseofaPleistoceneclaysoilwithhighstrength,thepastdepositionenvironmentandstresshistoryhaschangedseveraltimes,soevenwithinthesamestratumthepropertiesofthesoilarenotuniform,andanoverconsolidatedstatethatisunrelatedtothepresenteffectiveoverburdenpressureisfrequentlyfound.Therefore,theN-valuesandsoilpropertieschangegreatlywithonlysmallchangesinpositionordepth.Also,techniquesforsamplingstiffsoilsaredifficult,andcrackscaneasilybefirmedinthetestspecimens.InJapanthestrengthofstiffcohesivesoilisfrequentlyevaluatedusingthequvalue,butthequvalueisveryeasilyaffectedbythequalityofthetestspecimens.

2.4 Dynamic Analysis2.4.1 Dynamic Modulus of Deformation

(1)Forseismicresponseanalysis,anappropriatedynamicmodulusofdeformationofsoilsshallbedeterminedtoprescribetherelationshipbetweentheshearstressandshearstrainofsoil.

(2)Theperformanceverificationofseismic-resistantcanbebroadlyclassifiedintothestaticperformanceverificationmethodsand thedynamicperformanceverificationmethods. Oneexampleof staticperformanceverificationmethodsistheseismiccoefficientmethodofwhichtheseismicforceisassumedtoactonthegroundorstructuresintheformofastaticinertiaforce,andstabilityisexaminedfromtheequilibriumofforces. Inthedynamicperformanceverificationmethods,ontheotherhand,dynamicmagnificationfactorsoramplificationvaluesofacceleration,speed,anddeformationofsubsoilsshallowerthanbedrockandfoundationgroundforstructuresarecalculatedtoexaminethestabilityofgroundorstructures.Asfortheseismicresponseanalysismethod,boththetimedomainanalysisandthefrequencydomainanalysisareused.Foreithermethod,therelationshipbetweentheshearstressandshearstrainofthesoilsisrequired. Normallytherelationshipbetweentheshearstressandshearstrainingroundsubjectedtodynamicactingsisdescribedbyaskeletoncurveandahysteresiscurve,asshowninFig. 2.4.1 (a).Askeletoncurvewilldisplayremarkablenonlinearityastheshearstrainamplitudebecomeslarger.Sincethedynamicmodulusofdeformationprescribes this relationship between the shear stress and shear strain, itmust be appropriately appliedwhenconductingaseismicresponseanalysis.

(3)RelationshipbetweenDynamicShearStressandShearStrainofSoilTherearemanymodelstoapplytheshearstressandshearstraincurvesofsoilintoanalysis,suchasthehyperbolicmodelcalledHardin-Dornevichmodel,andtheRamberg-Osgoodmodel.29)

(4)ExpressionMethodofDeformationPropertiesintheEquivalentLinearModelToestimatethebehaviorofgroundduringanearthquake,thenonlinearityoftherelationshipbetweenthedynamicstressandstrainofsoilforawiderangeoftheshearstrainamplitudemustbeappropriatelyassessedandmodeled.Therelationshipofthedynamicstressandstrainofsoilisexpressedwithtwoparameters:theshearmodulusandthedampingfactorintheequivalentlinearmodel.TheshearmodulusG andthedampingfactorh aredefinedwiththeshearstrainamplitudebyequation(2.4.1)andequation(2.4.2)asshowninFig. 2.4.1 (b).

(2.4.1)

(2.4.2)

where G :shearmodulus(kN/m2) τ :shearstressamplitude(kN/m2) γ :shearstrainamplitude


–231–

h :dampingfactor W :strainenergy(kN/m2) W :dampingenergy(kN/m2)

SincethevaluesofshearmodulusG anddampingfactorh varynonlinearlydependingonthevalueofγ,aG/ G0~γcurveandah ~γcurvearenormallydrawnasshowninFig 2.4.2,whereG0istheshearmodulusatγ≒10-6.

Hysteresis curve

Skeleton curve

Shear strain γ

Shea

r stre

ss τ

Fig. 2.4.1 (a) Stress Strain Curve

G : Elastic shear coefficienth : Damping constant

G

h

W

WW

A

B

1

2

0

21

-

-

W

-=

Shear strain γ

Shea

r stre

ss τ

Fig. 2.4.1 (b) Shear Modulus and Damping factor

1.0

0.5

0.3

0.2

0.1

010-510-6 10-4 10-210-3

G/G

0 G/G0

h

Shear strain amplitude γ

h

Fig. 2.4.2 Shear Modulus, Damping Factor and Shear Strain Amplitude

–232–


(5)MeasurementoftheShearModulusandtheDampingFactorTheshearmodulusand thedampingfactormustbedeterminedby laboratory testssuchas theresonance testorcyclictriaxialtest,orbythein-situtestsusingelasticwavessuchasthePSloggingmethodorthecrossholevelocitymeasurementmethod.Thelaboratorytestscanbeusedtomeasuretheshearmodulusanddampingfactorforawiderangeofshearstrainamplitudesfromtheshearstrainof10-6tothefailurealthoughundisturbedsamplesfromthefieldmustbeobtained.Thetestscanalsobeusedtoevaluatethechangeinthemodulusofdynamicdeformationduetoconstructionofstructures.Withthecyclictriaxialtest,theshearmodulusisdeterminedfromequation (2.4.3)togetherwithPoisson’sratiov.

(2.4.3)

where σa :axialstressamplitude(kN/m2) εa :axialstrainamplitudeForv, thevalueof0.33isnormallyusedforadrainageconditionand0.45isusedforanundrainedcondition.

Thedampingfactoriscalculatedfromequation (2.4.2)withW and Wobtainedfromthestress-straincurvesimilartothatshowninFig. 2.4.1 (b). In-situtestsarelimitedtomeasurementsoftheshearmodulusthatonlycorrespondsto10-6levelofshearstrainamplitude.Suchtestshavenotbeenputtopracticalapplicationtomeasuretheshearmodulusanddampingfactorforthelargeshearstrainamplitude.Butthetestspossesstheadvantageofbeingabletomeasurethevaluesin-situdirectly. Theyarealsousedtocorrect theshearmodulusobtainedfromlaboratory tests. Theelasticconstantofsubsoilisobtainedbyequations (2.4.4)to(2.4.6)fromthedataofelasticwavevelocitymeasurementsbyaseismicexplorationusingboreholes.

(2.4.4)

(2.4.5)

(2.4.6)

where Vp :longitudinalwavevelocity(m/s) Vs :transversewavevelocity(m/s) G0 :shearmodulus(kN/m2) E0 :Young’smodulus(kN/m2) ν :Poisson’sratio ρ :density(t/m3) γt :wetunitweight(kN/m3) g :gravitationalacceleration(m/s2)

Therearevariousitemsrequiringattentionrelatingtothetakingofmeasurementswhencarryingoutelasticwaveexplorationonsoft seabedground. These includevibration inductionandreceptionmethods forelasticwavessuchaslongitudinalandtransversewaves,accuracyofwaveprofilereadingsandmethodsforprotectingboreholes.

(6)SimpleEstimationofShearModulusandDampingFactorsIncaseswhereitisdifficulttodirectlymeasuretheshearmodulusandthedampingfactorsofsoilsfromlaboratorytestsor in-situ tests, therearemethodsforestimatingfromtheplasticity index, thevoidratio, theunconfinedcompressivestrength,andtheN-value.30)However,itisnecessarytobeawarethatinthemethodofestimatingfromtheN-value,thevariationintheestimatedvaluesislarge,andthecoefficientofvariationisabout0.2.Forexample,onthebasisofthevariationofN-valueandSwavevelocitiesbyImai,31)foreachgroundtype,accuracyexaminationofestimationerrorofSwavevelocity isshownforHolocenesandyandclayeysoil inFig.2.4.3.ThehorizontalaxisshowstheratioofestimatedvaluesofSwavevelocityconvertedfromtheN-valuesandtheactualvalues.ForHolocenesandysoilstheaveragevalueoftheratiois1.12withastandarddeviationof0.29,an


–233–

extremelylargevariation.ForHoloceneclaysoilstheaveragevalueoftheratiois0.95withastandarddeviationof0.32.Inbothcasesthestatisticaldistributionmayberegardedasalog-normaldistribution.32)

0 1 20

10

20

Vs_ cal/Vs Vs_ cal/Vs0 1 2

0

10

20

30

freq

uenc

yHolocenesandy soil

Holoceneclay soil

freq

uenc

y

Fig. 2.4.3 Estimation Accuracy for S Wave Velocity

2.4.2 Dynamic Strength Properties

(1)Soilstrengthagainstdynamicexternalactionsisnormallydeterminedthroughlaboratorytests.Inthiscase,thepropertiesoftheexternalforcesandthesubsoilconditionsneedtobeappropriatelydetermined.

(2)The typical dynamic external actions encountered in ports and harbors are seismic movement and waveforce. Seismicmovements are characterized by a short period and few cyclic repetitions,whilewave forcesarecharacterizedbyalongperiodandmanycyclicrepetitions.Atpresentthesedynamicexternalactionsarenormallyconvertedintostaticactionslikeintheseismiccoefficientmethod.Therearethecases,however,inwhichitisnecessarytotreatthemasdynamicloadslikeinliquefactionanalysisorinstrengthdecreaseanalysisofcohesivesoiloffoundationgroundbeneathstructuresexposedtowaves.Insuchcasesthedynamicstrengthofsoilsarenormallyobtainedbycyclictriaxialtests.Whenconductingcyclictriaxialtests,thecyclicundrainedtriaxial testmethodexplained in theSoil Testing Methods and Commentaryof theGeotechnicalSocietyofJapancanbeused.33)

References

1) Watabe,Y.,T.Ueda,H.SaegusaandM.TanakaandY.Kikuchi:Practicaldeterminationmethodofsoilparametersconstantsbasedonforperformance-baseddesign,Jour.JSCECVol.63,No.2,pp.553-565,2007.(inJapanese)

2) Ogawa,F.andMatsumoto,K.:Thecorrelationofthemechanicalandindexpropertiesofsoilsinharbourdistricts,Rept.ofPHRI,Vol.17No3,pp.31-35,1978.(inJapanese)

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Chapter 3 Geotechnical Conditions - OCDI 一般財団 ... · the investigation should continue until a stratum of engineering rock with a shear wave ... Bearing capacity ... CHAPTER