Water permeability, water retention and microstructure of unsaturated compacted Boom clay

EngineeringGeology54(1999)117–127Waterpermeability,waterretentionandmicrostructureofunsaturatedcompactedBoomclayE.Romero*,A.Gens,A.Lloret´cnicadeCatalunya,JordiGirona1–3,DepartmentofGeotechnicalEngineeringandGeosciences,UniversidadPoliteMod.D-208034Barcelona,SpainAbstractThreeclassesofexperimentsareconsideredinthispapertoprovideinformationfortwoartificiallypreparedBoomclayfabrics:mercuryintrusion/extrusiontests;mainwetting/dryingpaths;andwaterinflow/outflowtransient(permeability)tests.Thesetests,whichareusuallytreatedseparately,arejoinedinacommonreferenceframetoprovideinformationaboutthemorphologyoftheporousmediumandfactorsinfluencingBoomclayunsaturatedhydraulicstateswithreferencetowaterretentioncurvesandrelativewaterpermeabilityvalues.Themainobjectiveistointerpretmercuryintrusionporosimetryresultsinordertodefineanentranceporesizeregionatca130to180nmseparatingintra-aggregateandinter-aggregatezones.Thisporesizeregionisfurtherassociatedtoadelimitingzoneintheretentioncurveseparatingregionsof‘intra-aggregategoverningsuction’atgravimetricwatercontentslowerthan13–15%(gravimetricwatercontentisnotaffectedbymechanicaleffects)and‘inter-aggregategoverningsuction’(gravimetricwatercontentissensibletomechanicalactions).Thiswatercontentisfurtherusedtodefineathresholdzonearoundarelativewaterpermeabilityofk/k=0.01delimitingazoneofgreaterwaterrelativepermeabilitywwsfromothersthatpresentarestrictedflowinageneralisedDarciansense.Alltheseresultsareconsistentwiththeexistenceoftwomainporesizeregions:anintra-aggregateporositywithquasi-immobilewaterthatislittleaffectedbyloadingprocessesandaninter-aggregateporosityforwhichtheloadingmechanismresultsinareductionofinterconnectedmacroporesaffectingfreewater.Testingresultsshowthatintra-aggregatewaterrepresentsbetween54and59%ofthetotalvolumeofwaterinsoilinalow-porositypackingcompactedatadryunitweightof16.7kNm−3,whereasitcorrespondstoca28and38%inthecaseofahigh-porositypackingcompactedatadryunitweightof13.7kNm−3.©1999ElsevierScienceB.V.Allrightsreserved.Keywords:Clay;Mercuryporosimetry;Poresizedistribution;Retentioncurve;Suction;Waterpermeability1.IntroductionTheporesizedistribution(PSD)obtainedbymercuryintrusionporosimetry(MIP)isanessen-tialfabricelement,whichisrelatedwithsomesoilbehaviourcharacteristics:water,airandheatcon-ductivityproperties;adsorptionanddesorption*Correspondingauthor.Fax:+34-93-4017251.E-mailaddress:dromero@etseccpb.upc.es(E.Romero)isotherms(capillaryphenomena);anddistortionandvolumetricdeformations(rearrangementoffabricunits).PredictiveequationsforsaturatedpermeabilitybasedonPSDsarepresentedinGarcia-Bengocheaetal.(1979)(capillaryandhydraulic-radiusmodels)andJuangandHoltz(1986)(probabilisticmodel).DelageandLefebvre(1984)usedtheporosimetrytechniquetoinvesti-gatefabricchangesofasensitivenaturalclayduringconsolidationandtodefineanentrapped0013-7952/99/$–seefrontmatter©1999ElsevierScienceB.V.Allrightsreserved.PII:S0013-7952(99)00067-8118E.Romeroetal./EngineeringGeology54(1999)117–127andfreeporosityonclaystructure,GriffithsandJoshi(1989)studiedchangesinPSDsduetoconsolidationofdifferentclaytypes,andAl-Mukhtar(1995)analysedporespaceresultsonakaoliniteclayrelativetotheeffectsofmechanicalandhydraulicstates.Prapaharanetal.(1985)predictedthesoilmoisturecharacteristiccurveofacompactedclayuptoamatricsuctionvalueof0.7MPabasedontheporosimetrydata.MIPdataisusedinthispapertoprovideinformationaboutthemorphologyoftheporousmediumrelatedtoentrappedandfreeporosityproportionsandaboutfactorsinfluencingBoomclayunsaturatedhydraulicstateswithreferencetothesoilwaterretentioncurveandtherelativewaterpermeability.Constantvolumemainreten-tioncurvesobtainedbyvapourequilibriumandairoverpressuretechniques,aswellasMIPdataassimilatedtothedesorptionpathofthesoilmoisturecharacteristiccurve,areusedtoexaminethedensityfunctionoftheporestructureforthedifferentartificiallypreparedclayfabrics.Furtherresultsobtainedundercontrolledmatricsuctionusingtransientinflow/outflowdataandinterpretedwithadiffusionequationarecomparedtorelativewaterpermeabilitydeterminationsusingMIPdatacalculatedbyintegrationoverthecontributionsfromfilledpores.Relativewaterpermeabilityisstudiedasafunctionofthedegreeofsaturationandfordifferentconstantporosityvaluestodefinearegionseparatingazoneofgreatervaluesfromothersthatpresentarestrictedflow.2.Materialandtestprocedures2.1.SoilcharacteristicsandsamplepreparationLaboratorytestswereconductedonartificiallyprepared(drysidestaticallycompacted)powderobtainedfromnaturalBoomclay(Mol,Belgium).Thismoderatelyswellingclay(20–30%kaolinite,20–30%illite,10–20%smectite)hasaliquidlimitofw=56%,aplasticlimitofw=29%and50%ofparticlesL<2mm.InpreparingPspecimens,powder(passingNo.40–425mm)isleftinequilib-riumwiththelaboratoryatmosphereatanaveragerelativehumidityof47%(totalsuctionofcay=100MPa)toachieveahygroscopicwatercontentofsomewhat<3%.Forsamplesusedwithintheairoverpressuretechniqueformatricsuctioncon-trol,therequiredquantityofdemineralisedwatertoachieveapredeterminedgravimetricwatercontentof(15.0±0.3)%wasaddedtothepowder,previouslycuredatarelativehumidityof90%.Afterequalisation,itapproximatelycorrespondstoaninitialtotalsuctionofca2.3MPa.Thisinitialwatercontentrepresentsareasonablevaluefortheworkabilityoftheclaypowderonthedrysideofoptimumanddoesnotinvolvealargesuctionchangewhenimposingthestartingmatricsuctionvalueof0.45MPa.Avariablepeakstress–constantstrokestaticcompactionprocedurehasbeenfolloweduntilaspecifiedfinalvolumeisachievedunderconstantwatercontent.Netverticalstressoverairpressure(s–u)anddryunitweightrelationshipsfiedvforspeci-wateracontentsareplottedinFig.1.Eachcurveterminatesataspecificdegreeofsaturationandwhenreachingnearsaturatedstates,thestaticcompactiontestisnolongerpossibleandisstoppedbeforethebeginningofconsolidation.Totalsuctionvaluesdeterminedfromvapourequi-Fig.1.Staticcompactioncurveswithcontoursofequalsuction.E.Romeroetal./EngineeringGeology54(1999)117–127119libriumtechniqueandmatricsuctioncomponentsimposedbyairoverpressuretechniquearealsorepresented.Astrikingaspectisthevariationofthedegreeofsaturationuponloadingwithoutappreciablechangeinsoilsuction,asitisobservedintheverticalcontoursofequalsuctionrepre-sentedinthisfigure,wherenoimportantwatercontentchangeisobserveduponloadingatsuctionvalueshigherthany=3MPa.Theloadingmecha-nismaffectsmainlythemacroporositythatdoesnotcontainfreewater,becausemoistureatthesefabricationhumidityvaluesismainlyadsorbedandheldinthelessbondeddiffuse-layers(i.e.intra-aggregatewater,withsmallquantitiesatinter-aggregatescale,asfurtherdiscussed).Thisway,degreeofsaturationchangesareonlyassoci-atedtomacroporositychangesunderconstantintra-aggregatewatercontent.Asmatricsuctionvaluesapproachthesaturationline,contoursofequalsuctiontrytoinclineinordertoconvergetothislimitcondition(theloadingmechanismaffectsinter-aggregatewater).Thetestingprogramhasincludedtwomainsoilpackingsofclayaggregatesfabricatedatamould-Fig.2.ImageanalysisofSEMphotomicrographsforthedifferentsoilpackings.Theporesareblackandtheparticlesarewhite.ingwatercontentof15%:high-porositywithcol-lapsibletendencystructureatadryunitweightofc=13.7kNm−3andalow-porositystructurewithswellingdtendencyatc=16.7kNm−3,assug-gestedfrommatricsuctiondcontrolledtestsreportedbyRomero(1999).Fabricationprecon-solidationnetverticalstressisca4.5MPaforthehigh-densityfabricandca1.2MPaforthelow-densitypacking,asindicatedinFig.1.Aqualita-tiveobservationwithimageanalysisofscanningelectronmicroscopy(SEM)thatcomplementstheoveralldescriptionofbothpackingsisgiveninFig.2,wherelocalenlargementsrepresentedinblackandinterconnectingclaybridgesbetweenaggregatesareobserved.Theconstrictedporosityarisesfromtheseclayeybridgesandirregularityoftheaggregates,whichactas‘ink-bottle’necksentrappingtheinterconnectedporosity,assug-gestedbyDelageandLefebvre(1984).Athighercompressivestresses,theaggregatesarefusedduetointer-aggregateporositycollapseasshowninFig.2(a).2.2.ExperimentalmethodsandproceduresThreemajorclassesofexperimentswereconsid-eredforbothsoilfabrics:MIPtests;mainretentionwetting/dryingpaths;andwaterinflow/outflowtransienttests.MIPtestswerecarriedoutona‘Quantachrome-Autoscan33’porosimeter,wherethemercurypressureisraisedcontinuouslybetween0.17and227MPa(apparentporediame-tersfrom7nmto9mm)atapressurebuilt-uprateof7MPamin−1,inconjunctionwitha‘QuantachromeAutoscan’fillingapparatusmeas-uringapparentporediametersupto135mm.MIPtestsrequireddehydratedsamplesmeasuring<3×103mm3(limitedbythesampleholderandthecellstemvolume).Soilsampleswerepreviouslypreparedandcompactedatthedesireddryunitweightsandatconstantwatercontentof(15.0±0.3)%.Fromeachcompactedsample,MIPspecimenswerecarefullytrimmedintocubes,sub-sequentlyfreeze-driedtoremovetheporewaterandthenkeptinadesiccatoruntiltesting.Aplotofthecumulativeporevolumedistributionnormalisedbysampledryweightinanintrusion/extrusioncycleforbothsoilfabricsis120E.Romeroetal./EngineeringGeology54(1999)117–127presentedinFig.3(intrusionisrepresentedbythefilledsymbols).Duetothelimitedcapacityofthemicroporosityunittoenterthesmallestpores,therearesomedeviationsfromthetheoreticalporosityvaluescalculatedforthedifferentcom-pactedpackings(Dn=0.020forthe13.7kNm−3packingwithanestimatedporosityofn=0.482andDn=0.029forthe16.7kNm−3fabricowithnofo=0.371).Thisfeatureconfirmsthepreservationthesoilstructureduringfreeze-drying.Totalsuction–gravimetricwatercontentanddegreeofsaturationrelationshipsunderfreeswell-ingandshrinkingconditionswithnochangeinboundarystresseshavebeenmeasuredasafunc-tionoftheinitialdrydensity(reflectingthedifferentpackings),followingamulti-stagepro-cedureequilibratedundervapourequilibriumtech-nique(vapourpressurecontrolwithdifferentsalinesolutions),wherethesamesoilspecimenundergoesamainwettingphasefollowedbyamaindryingstage.Mainwettingpathreferstotheslowwatersorptionofaninitiallydrysampleathygroscopichumidity,whilefirstmonotonicgradualdesorptiondepartingfromaminimumsuctioneverexperi-encedisreferredtoasmaindrying.Suction–moisturecontentrelationshipswerecomplementedinthelowersuctionrange(between15and2MPa)usingpsychrometerdata(bothtransistorandther-mocoupleinmaindryingpaths).RelationshipsbetweentotalsuctionandwatercontentatfixedFig.3.Cumulativemercuryintrusion/extrusionporevolumenormalisedbythesampleweight.drydensitieshavebeeninterpolatedfromfreeswellingandshrinkingdataassuminganoncon-strainedintra-aggregateswelling.Thistermreferstoadouble-porositynetwork,whereenoughspaceisleftataninter-aggregatescaletoallowthisswelling(usuallyfordrydensities<1.7Mgm−3,where>23%oftheporevolumecorrespondstoentranceporesizes>1mmasobservedinFig.3).Mainwettinganddryingdataatlowsuctionvalues(under0.50MPa)areobtainedfromcon-stantvolumeswellingpressuretestsusingtheairoverpressuremethodology.Startingfromamatricsuctionofca2MPathesoilismonotonicallyhydratedupto0.01MPaandthenismonotoni-callydriedto0.45MPawithoutexceedingtheair-entryvalueofthepacking,inordertoavoidshrinkagethatcouldalterthenullvolumecondi-tion.Uponwetting,theswellingpressuretestsimplycertaininter-aggregateporositydecreaseatexpenseofsomenonconstrainedintra-aggregateswellinginordertomaintaintheoverallconstantvolumecondition.Areasonablecorrelationhasbeenfoundbetweenthedifferentvaluesobtainedusingaxistranslationandvapourequilibriumpro-cedures.BothtechniquescanbeoverlappedinmainwettinganddryingpathsshowingtheoverallFig.4.Mainwettinganddryingretentioncurvesforthedifferentpackingsatconstantporosity.‘Intra’and‘inter-aggre-gategoverningsuction’zones.E.Romeroetal./EngineeringGeology54(1999)117–127121Fig.5.Suction–degreeofsaturationretentioncurvesatfixedpackings(mainwettinganddryingpaths).retentioncurveindicatedinFig.4.Suction–degreeofsaturationretentioncurvesatfixedpackingsrepresentedinFig.5canbeestimatedbymeansofvolumetricphaserelations,wheretheremark-ableinfluenceofclayeysoilstructureonhysteresisbehaviourisobserved.Measurementsofthehydraulicconductivityhavebeenperformedatdifferenttemperaturesusingoedometercellsundercontrolledmatricsuc-tion(bothwettinganddryingpaths)andunderconstantvolumeconditions.Astepmatricsuctiondecrease/increaseisappliedtothesoilandthetransientinflow/outflowofwateriscarefullymea-suredwithtimeandinterpretedwiththeresolu-tionofRichards’sdiffusionequationtakingintoaccountmembraneimpedance(KunzeandKirkham,1962).Experimentalproblemsrefertoimpedanceeffectsfromceramicdiscclogging,diffusionofairintothemeasuringsystemandfreewaterevaporation(Romero,1999).3.MIPresultsandinterpretationsTwotypesofporositycanbedeterminedwithanintrusion/extrusioncycle(refertoFig.3).Thefirstintrusionfillsalltheaccessibleandintercon-nectedporespace,givingthedistributionoftotalporosity,whereasonthecompletereleaseoftheintrusionpressureitwillletonlysomemercuryoutfromthenonconstrictedpores(DelageandLefebvre,1984).Asecondintrusionwillfollowapproximatelythesameextrusionpath,thusdefin-ingthefreeporosity,whichseemsnottobesignificantlychangedwiththeincreasingcompac-tioneffortasobservedinthepreviousfigure.SimilarresultshavebeenreportedbyGriffithsandJoshi(1989)studyingPSDchangesduetotheconsolidationofclay.Thedifferencebetweentheintrusionandextrusioncycleistheentrappedporosity,whichaccountsforthemainreductionintotalvolume.DelageandLefebvre(1984)pro-posedthatthefreeornonconstrictedporosityofthereintrusionvolumecorrespondstotheintra-aggregateporespace,whiletheinter-aggregateporespacecorrespondstotheentrappedorcon-strictedporosity.Theintra-aggregateporositydetectedwhenpressureisreleasedrepresents28%ofthetotalporosityinthelow-densitypackingandaccountsfor54%ofthetotalporosityinthehigh-densityfabric,whichcorrespondstoanentranceporesizeofca130to180nmintheintrusioncurveasobservedinFig.3.Thisporesizerangecanbeacceptedforthedelimitingregionseparatinginterandintra-aggregatezones.Thisporesizeregionisfurtherutilisedtodefinethe‘intra-aggregategoverningsuction’zoneoftheretentioncurve.MIPresultscanbeusedtodeterminematricsuction–saturationrelationshipsatconstantpack-ings.Themercuryintrusionprocedurecanbecomeassimilatedtoadesorptionpathofthesoilmois-turecharacteristiccurveinthematricsuctionrangebetween0.01and45MPabyapplyingtoanini-tiallysaturatedsampleincreasingexternalairpres-sure(nonwettingfluid)tograduallydrythesoil.Thustheinjectionofnonwettingmercurywithcontactangleh−1=is140equivalent°andinterfacialtothetensions=0.484NmnwwaterHgejectionoffromthepores(desorptioncurve)bythenonwettingfrontadvanceofairwithh=forthesamediameterofporesbeingintruded.nw180°Thevolumeofporesnotintrudedbymercury,thatis,Sr=1−Srnw(Srnw=n/nostandsforthe122E.Romeroetal./EngineeringGeology54(1999)117–127nonwettingmercurydegreeofsaturationorintrudedporositynnormalisedbythetotalporos-ityn),shouldbeusedtoevaluatethedegreeofsaturation,oSr,correspondingtotheequivalentappliedairoverpressure.However,thislastexpres-siondoesnotconsiderthehygroscopicwatercontentofstronglyattractedadsorbedwatertothemineralsurfaceandtheequivalentresidualwatercontentcorrespondingtothenonintrudedporosity.Thisresidualwatercontentcorrespondstocaw=5%,whichissimilartothatobtainedinFig.4forramatricsuctionvalueof45MParelatedtothemaximumintrusionpressureof227MPa.Inordertotakeintoaccountthisresidualwatercontent,anewexpressionisproposed:Sr=(1−Srsaturatednw)+wgravimetricrSrnw/wsat,wherewstandsforthewatercontent.satFig.6repre-sentsthemainwettinganddryingpathsforthedifferentsoilpackingscomparedtotheMIPresults.Ingeneral,quiteagoodagreementisobservedwithrespecttotheshapeoftheretentioncurves,thusbeingfeasible,asafirstapproxima-tion,tousethecumulativeintrusionvolumetodeterminethesoilmoisturecharacteristiccurve.SimilarresultswerereportedbyPrapaharanetal.(1985)whenanalysingonalowsuctionrangetheFig.6.Themainwettinganddryingpathsonadegreeofsatura-tionbasiscomparedtotheMIPresultsforthedifferentpackingsatconstantporosity.retentioncurvebasedontheMIPdata.However,theMIPdatafollowsmorecloselythewettingpathswhentheyshouldhavecomenexttothedryingpaths,whichisthemoisturecurvebeingpredicted.Noconclusiveexplanationshavebeensuggestedforthesedifferences,whichcouldariseduetothedifferenteffectsthatwateranddissolvedsaltsproduceonclayfabriccomparedtoalessactivemercuryintrusion.However,therapiddesa-turationofthehigh-porositypackingcouldberelatedtothefissurelikestructuredetectedatca10mm(refertoFig.7),whichhasalsobeenobservedbyinterpretingtheporoussurfacedimen-sionusingafractalscalingoftheporosityadmit-tingself-similarityofthehierarchicalvoidstructure(Romero,1999).OtherusefulinformationthatcanbeobtainedfromtheMIPtestsistheporesizedensityfunction,whichisaderivativeofthecumulativeporevolumerepresentedinFig.3.Toovercomedistortionsandemphasisedporesizes,anequallyspacedclasswidthonthelogarithmicdiameteraxisof0.023

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