WhentoUseImmediateSettlementinSettle3D

Mostengineersagreethatsettlementismadeupofthreecomponents:immediate,primaryconsolidationandsecondaryconsolidation(orcreep).Mostengineersalsoagreethatforspecificscenarios,oneortwoofthesecomponentsmaybemissingornegligible.InSettle3D,youhavetheabilitytoturnonoroffeachofthesetypesofsettlement.Usersgenerallyknowwhetherornottheirmaterialiscreeping,butthequestionremains,whenshouldyouturnonimmediatesettlement,whenshouldyouturnonprimaryconsolidationandwhenshouldyouturnonboth?Thisarticleattemptstoanswerthisquestionthroughconsultationofcommontexts(seeBibliography)alongwithuserscommentsandsomeofourowninsights.Wehopethatthisarticlewillhelptosettlethismatter.

ComponentsofSettlementVirtuallyallreferencesagreethatthetotalsettlementofafoundationiscomposedofthreecomponents: ST=Si+Sc+Sswhere: STisthetotalsettlement Siistheimmediatesettlement Scistheconsolidationorprimarysettlement SsisthesecondarysettlementorcreepSomeofthesecomponentsmaynotbepresent,ortheeffectsmaybenegligibledependingonthesoiltype.Hence,theabilitytoturneachcomponentonandoffinSettle3D.Butthequestionremains,whendoIuseeachtypeofsettlement?Letsmaketwoassumptions:

Thesoilgrainsareincompressible Waterisincompressible

Neitheroftheseistrueofcoursebutthestiffnessofthesoilgrainsandthewaterisgenerallysolargethatwecanignoretheeffectsoftheircompressibility.Withthesetwoassumptionsinmind,theonlywaythatsettlementcanoccuristhroughrearrangementofthesoilgrainsand/orcollapseofporespace.Thebasicmechanismforsettlementisthereforeessentiallythesameforallthreetypesofsettlement,andthedivisionintothreecomponentsisbasedonotherfactors.

SecondaryConsolidationLetsfirstconsidersecondaryconsolidation.Thesecondarysettlement,Ssrepresentstimedependentsettlement,orcreep,thatoccursunderaconstanteffectivestress.Thiscomponentofsettlementisimportantinorganicsoilssuchaspeat,butcanoftenbeomittedforothersoiltypes.Itisfairlyeasytoseparatethiscomponentofsettlementfromtheothertwosinceitoccurswheneffectivestressdoesnotchange1.Thereforewewillnotdiscussthiscomponentfurther.

1 There is research that suggests that creep occurs simultaneously with consolidation settlement, but traditionally, engineers adopt the definition of creep occurring after consolidation settlement. This is the approach adopted by Settle3D.

ImmediateSettlementTheimmediatesettlementisalsosometimescalledvolumedistortionsettlement.Essentially,thisistherearrangementofgrainsduetochangingstress,resultinginareductioninvoidratioandinstantsettlement.Inmosttexts,immediatesettlementisconsideredtobeanelasticprocess.Thereforethesettlementcanbecalculatedfromelastictheory(seeforexample,Day2005,equation8.9).Settle3Dalsouseselastictheorytocalculateimmediatesettlement.So,byturningonimmediatesettlementinSettle3D,youareessentiallycalculatingthevolumedistortionduetostresschanges.Oneimportantconsiderationisthattheremaybeinelasticprocessesatwork(plasticflow),meaningthatthisimmediatesettlementcalculationwillunderestimatetheactualsettlement.HoltzandKovaks(1981)state,Theimmediate,ordistortion,settlement,althoughnotactuallyelasticisusuallyestimatedbyusingelastictheory.ItisrecommendedthatalowervalueofEbeusedtoaccountfortheplasticflow.Therearemanyothermethodsthattrytosimplifytheelasticcalculation,especiallyintheestimationofEfromlaboratoryorfieldtests.Someofthesemethodstakeintoaccounttheplasticflow,loadrigidityandloadshape.AllarebasedonempiricalsettlementdataandarethereforegroupedunderempiricalmethodsinSettle3D(e.g.Schmertmannmethod).Thesecanalsobeusedtocalculateimmediatesettlement.Notehoweverthatthesemethodsaregenerallynotasrobustinthattheycanonlyconsidersimpleloadinggeometries.Also,whenplottingtotalsettlement,theempiricalsettlementsarenotincluded.Youmustaddthemmanually.

OR

Sowhendoyouneedtoconsiderimmediatesettlement?Immediatesettlementisgenerallyassumedtobethesettlementthatoccursquicklyaftertheconstructionofyourfoundationorembankment.Howquickly?Bowles(1996)suggeststhatimmediatesettlementsarethosethattakeplacewithin7daysof

loading.Adoptingthisdefinitionallowsustoconsidermanypossiblescenariosthatcouldleadtoimmediatesettlement.Thesewillbediscussedinthenextsection.

PrimaryConsolidationTheprimaryconsolidationsettlement,Scisthegradualrearrangementofgrainsaswaterisexpelled.Thiscomponentofsettlementisusuallydominantinfinegrainedsaturatedclays.Butconsolidationalsooccursinothersoiltypes.Hence,thereissomegrayareaastowhentouseimmediatesettlement,whentouseprimaryconsolidationsettlementandwhentouseboth.ThefollowingsectionoutlinesdifferentpossiblescenariosandhowtosimulatetheminSettle3D.

SettlementScenariosInthissection,weconsiderdifferentsettlementscenariosandhowtodealwiththeminSettle3D.Firstwewilldescribeasimpletestmodelusedtoillustrateeachsituation.

TestModelConsideraflexiblecircularloadofmagnitudeq=10kPaandradiusa=1montopofasoillayerwiththicknessh=10m.Wewillusethismodeltoexaminedifferentscenarios.Lookingatthescreencapturebelow,aquerypointisplacedatthecentreofthecircleforcalculationofsettlements.

Scenario1:UnsaturatedMaterialInthiscase,thereisnoconsolidationandallofthesettlementisimmediate.However,thisdoesntnecessarilymeanthatyoushouldturnonimmediatesettlementandturnoffconsolidationsettlement.Ifyouturnonconsolidationsettlementforanunsaturatedmaterial,thenthesettlementwilloccurimmediatelysincethereisnotimedependentdissipationofporepressures.Thistechniquewouldbeusefulintwopossiblescenarios:

Yourmateriallayerispartiallybelowthewatertable.Inthiscase,youcanturnonconsolidationsettlementandturnoffimmediatesettlement.Thesoilthatisabovethewatertablewillcompactimmediatelyandthesoilbelowthewatertablewillcompactgradually.

YourmaterialisnonlinearandyouwishtousethenonlinearparametersCcandCr.Inthiscase,youmustuseconsolidationsettlementandnotimmediatesettlement.

Fortheunsaturatedmaterial,itdoesntreallymatterifyouchooseimmediateorconsolidationsettlement,aslongasyoudontchooseboth.

Infact,ifyouchooseconsolidationsettlementwithalinearmaterial,mvisjusttheinverseofEthatisusedforimmediatesettlement.Soamodelwithmv=0.001m2/kNwillgiveexactlythesamesettlementasamodelwithE=1000kPa.

Scenario2:PartiallySaturatedMaterialInthiscasetherewillbesomeimmediatesettlementasairisquicklypushedoutoftheporespaces.However,aswiththeunsaturatedcase,itisnotnecessarytoturnonimmediatesettlement.InSettle3D,youcanaccountforthiseffectbysettingtheBbarlessthan1(Bbarrangesfrom0foranunsaturatedmaterialto1forasaturatedmaterialseetheTheorymanualformoredetails).Whenyoudothis,aproportionoftheappliedstressistransferredimmediatelytothesoilsothatyouseesomeimmediatedeformation.

Scenario3:FullySaturatedSandIfyouaredealingwithsand,orsomeothercoarsegrainedmaterialthathasahighpermeability,thenporepressuresdissipatequicklyandsettlementisessentiallyimmediate.InSettle3D,youcansimulatethisinseveralways:

TurnonImmediateSettlementandturnoffPrimaryConsolidation TurnoffImmediateSettlement,turnonPrimaryConsolidationandseta

largevalueforcvork. TurnoffbothImmediateSettlementandPrimaryConsolidationanduse

oneoftheEmpiricalMethods.Thefigureonthefollowingpageshowsacomparisonofresultsforthefirstmethodandthesecondmethod.

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Comparisonofsettlementresultsforthecircularloadmodelusingdifferentmethodstoaccountforrapidporepressuredissipation.

Scenario4:FullySaturatedClayThisisprobablytheonlysituationinwhichyoumaywanttousebothImmediateSettlementandPrimaryConsolidation.Settle3DusesonedimensionalconsolidationtheorytocalculatethePrimaryConsolidationsettlement.However,mostloadingscenariosareactuallythreedimensional.Thereforeevenwithanincompressiblefluid,somevolumedistortionsettlementwilloccurimmediatelyinasaturatedclay.Elastictheorycanbeusedtoestimatethissettlementwithoneimportantconsideration:youshouldusetheundrainedYoungsmodulusandPoissonsratio.TheundrainedPoissonsratiois0.5(alowernumbercanbeusedifnot100%saturated).TheundrainedYoungsmoduluscanbecalculatedby:

draineddrained

undrainedEE 12

3

so,foramaterialwithaPoissonsratioof0.2,Eundrained=1.25Edrained.However,LambeandWhitman(1979)statethatWithactualsoils,theratioofEundrained/Edrainedistypicallymuchlargerthanthistheoreticalvalue.Valuesof3or4arenotuncommonfornormallyconsolidatedclays.Itisalsoimportanttonotethatthisisathreedimensionaleffect,sotheamountofimmediatesettlementdependsonthegeometryofthesituationandthedrainedPoissonsratio.Foraloadwithverylargearealextent,orlowdrainedthesituationisclosetoaonedimensionalloadingscenario,sotheimmediatesettlementwillbenegligible.Poulos(2000)presentsgraphsshowingtheimportanceoftheseparametersincalculationofimmediatesettlement.

Theoreticalabilityofonedimensional(PrimaryConsolidation)analysistopredicttotalfinalsettlement.FromPoulos(2000). Forsoft,normallyconsolidatedmaterials,ImmediateSettlementshouldbeaddedtothePrimaryConsolidationsettlement.Therefore,thefollowingequationapplies:ST=Si+SC Soft,normallyconsolidatedclaysHoweverPoulos(andothers)suggestthatfornormallyconsolidatedclays,theimmediatesettlementasaproportionofthetotalsettlementisoftennegligible.Thisisillustratedonthefollowingpage.

Normally consolidated clay: Eundrained ~ 3Edrained Si is small

Forstiffoverconsolidatedmaterial,PouloswarnsthattheimmediatesettlementshouldnotnecessarilybeaddedtothePrimaryConsolidationsettlement.InsteadtheimmediatesettlementaccountsforpartofthePrimaryConsolidationSettlementandthatifyouaddthemtogether,youwillbedoublecounting. InSettle3Dtherefore,youwouldhavetosomehowreducethecalculatedconsolidationsettlementtobalancetheimmediatesettlement. InSettle3D,thesethreedimensionaleffectscanbemoreeasilyaccountedforinanotherway.GotoProjectSettings,choosetheAdvancedtabandselectUseMean3DStressasshownonthenextpage:

Settle3Dwillnowsettheinitialexcessporepressuretobeequaltotheundrainedmeanstressratherthantheverticalstress,wheretheundrainedmeanstressis:

5.032131

undrainedM SeetheTheoryManual,section4.2andDas(2007)fordetails.Usingthemeanstressinsteadoftheverticalstressmeansthatwhenaloadisapplied,partoftheloadisinstantlycarriedbythegrainskeleton.Thisresultsinsomeimmediatesettlement.Sincethecalculatedmeanstressisafunctionoftheloadgeometry,thereisnoneedtoapplyafudgefactortoaccountforthesizeoftheload.Theplotbelowshowshowimmediatesettlementissignificantforasmallloadonathicklayer,yetinsignificantforalargeloadonathinlayer.Thesubsequentplotalsoshowshowamodelthatdoesnothavemeanstressturnedonproducesthesamelongtermsettlementasthemodelwithmeanstressturnedon.Thisshowshowtheimmediatesettlementisnotbeingdoublecountedwhenmeanstressisturnedon.

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Settlementcalculatedforoverconsolidatedclaywithmeanstressoptionturnedon.

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Settlementcalculatedforthe10mloadwithmeanstressturnedoncomparedtothesettlementcalculatedwithverticalstressonly.

SummaryThetablebelowsummarizeshowtomodeldifferentscenariosinSettle3D.Thisisonlyaguidelineandspecificsites,loadingscenariosandacquireddatamaydemandadifferentapproachforyoursettlementproblem.Asalways,youshoulduseyourengineeringjudgmentwhencreatingandanalyzingthesenumericalmodels.

Material Possible Methods

Unsaturated 1. Immediate on*, primary off or 2. Immediate off, primary on

Partially saturated 1. Immediate off, primary on with B-bar < 1

Sand 1. Immediate on*, primary off or 2. Immediate off, primary on, cv or k is large or 3. Immediate and primary off, use empirical method

Saturated normally consolidated clay

1. Primary on +

(optional) immediate on* with undrained parameters**

Saturated overconsolidated clay

1. Primary on, immediate off, mean stress on or 2. Immediate on* with undrained parameters** +

primary on + correction to account for double-counting

*Elasticsolutionsdonotaccountforplasticflowandmaythereforeunderestimatetheactualsettlement**Undrainedstiffnessparameterscalculatedfromtheorymaybetoosoft,thereforetheimmediatesettlementmaybeoverestimated

Bibliography Bowles,J.E.(1996).FoundationAnalysisandDesign5thed.,McGrawHill,1175pp.CanadianGeotechnicalSociety(2007).CanadianFoundationEngineeringManual4thed.,CanadianGeotechnicalSociety,488pp.Das,B.M.(2007).PrinciplesofFoundationEngineering6thed.,Thomson,750pp.Day,R.W.(2005).FoundationEngineeringHandbook:DesignandConstructionwiththe2006InternationalBuildingCode,McGrawHill.DepartmentoftheNavy(1982).SoilMechanicsNAVFACDesignManual7.1,DepartmentoftheNavy,Alexandria,VA.,378pp.Holtz,R.D.andW.D.Kovacs(1981).AnIntroductiontoGeotechnicalEngineering,PrenticeHallInc.,733pp.Lambe,T.W.andWhitman,R.V.(1979).SoilMechanics,SIVersion,JohnWiley&Sons,553pp.McCarthy,D.F.(2007).EssentialsofSoilMechanicsandFoundations7thed.,PrenticeHall,850pp.Peck,R.B.,Hanson,W.E.andT.H.Thornburn(1974).FoundationEngineering2nded.,JohnWiley&Sons,514pp.Poulos,H.G.(2000).FoundationSettlementAnalysisPracticeVersusResearch,TheEighthSpencerJ.BuchananLecture,https://ceprofs.civil.tamu.edu/briaud/buchanan%20web/lecture_series.htm

Components of SettlementSecondary ConsolidationImmediate SettlementPrimary Consolidation

Settlement ScenariosTest ModelScenario 1: Unsaturated MaterialScenario 2: Partially Saturated MaterialScenario 3: Fully Saturated SandScenario 4: Fully Saturated Clay

SummaryBibliography