Deformation and micromechanics of granular materials in shear zones [Elektronische Ressource] : investigated with the discrete element method / by Ingo Kock

Ingo Kock

Description

Deformation and micromechanics of granular materials inshear zones - investigated with the Discrete ElementMethodDissertationzur Erlangung des Doktorgrades der Naturwissenschaften am Fachbereich Geowissenschaftender Universität Bremensubmitted for the doctoral degree in natural sciences at the Faculty of Geosciences of BremenUniversityvorgelegt vonbyIngo KockBremen,Januar/January 2007Tag des Kolloquiums:11.05.2007Gutachter:Prof. Dr. Katrin HuhnProf. Dr. Achim KopfPrüfer:Prof. Dr. Michael SchulzProf. Dr. Heinrich VillingerGeneral outlineGeneral outline and remarksThis thesis consists primarily of three manuscripts that have already been published or arecurrently under review by international scientific journals. These are Chapter I to Chapter IV.Furthermore, Chapter V is a published manuscript which was prepared in collaboration withProf. Dr. K. Huhn and Prof. Dr. A. Kopf, and to which I contributed.In the following sections and chapters, nomenclature is kept consistent. There is a distinctionbetween ‘grain’ and ‘particle’. Grain is used in connection with natural samples andconditions, whereas particle refers to numerical specimens.Chapter I outlines the general motivation for this thesis. The concepts of friction, failure ofsoil and rocks, and laboratory methods to evaluate shear strength are briefly introduced.

Informations

• Publié par: universitat_bremen
• Publié le: 01 janvier 2007
• Nombre de lectures: 16
• Langue: English
• Poids de l'ouvrage: 9 Mo

Extrait

Deformationandmicromechanicsofgranularmaterialsin
shearzones-investigatedwiththeDiscreteElement
Method

Dissertation

zurErlangungdesDoktorgradesderNaturwissenschaftenamFachbereichGeowissenschaften
derUniversitätBremen

submittedforthedoctoraldegreeinnaturalsciencesattheFacultyofGeosciencesofBremen
yUniversit

gvorgelevontby

IngoKock

en,BremJanuar/January2007

Tag

desKolloquiums:

11.05.2007

ter:Gutach

Prof.Prof.DDr.r.AKcatrhiminHKuophnf

Prüfer:

PProf.rof.DDr.r.HeMiinchricahelVSillchinulzger

outlinelraneGe

Generaloutlineandremarks
Thisthesisconsistsprimarilyofthreemanuscriptsthathavealreadybeenpublishedorare
currentlyunderreviewbyinternationalscientificjournals.TheseareChapterItoChapterIV.
Furthermore,ChapterVisapublishedmanuscriptwhichwaspreparedincollaborationwith
Prof.Dr.K.HuhnandProf.Dr.A.Kopf,andtowhichIcontributed.

Inthefollowingsectionsandchapters,nomenclatureiskeptconsistent.Thereisadistinction
between‘grain’and‘particle’.Grainisusedinconnectionwithnaturalsamplesand
conditions,whereasparticlereferstonumericalspecimens.

ChapterIoutlinesthegeneralmotivationforthisthesis.Theconceptsoffriction,failureof
soilandrocks,andlaboratorymethodstoevaluateshearstrengtharebrieflyintroduced.One
maingoalofChapterIistoreviewstate-of-the-artscientificconceptsnecessarytoplacethis
thesisintoawiderframeworkandbeyondthescopeofasinglemanuscript.Onlythencan
specificobjectivesofanindividualmanuscriptbeoutlined.Lastly,thenumericalmodelling
techniqueisbrieflypresented,followedbythemodelandinterpretationtechniquesIhave
elf.symdeveloped

Thefactthattheroughnessofcountryrockandofgeotechnicalsheartestwallsdoeshavean
influenceonlocalizationpatternsandshearstrengthofsoil/faultgougeisdemonstratedin
ChapterII.Whenroughnessisscaledbythemaximumparticlesizeofasample,weshow
thattherearethresholdlevelsofroughness,belowandabovewhichshearbecomesextremely
localized.Inbetweenthesethresholds,shearisdistributedthroughoutthespecimen.

ChapterIIIexplorestheeffectsofparticleshapeonthedeformationbehaviourofa
numericalmodelled‘clay’sample.Twokeyfactors,sphericityandroughness,couldbe
identifiedasrelevanttofrictionalstrength,shearzonedevelopment,andparticlerotationwith
sphericitydominatingoverroughness.

InChapterIVtheconceptofChapterIIIisbroadened.Here,thedeformationbehaviourof
micro-sizedlayeredstratigraphicunitsofmodelled‘silt’and‘clay’isanalyzed.Weshowthat
differentdeformationmodes(sliding,rollingandrotationofparticles)leadtodifferentoverall

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frictionalstrengthandlocalizationphenomena.Theabilityofsingleparticlestoslide,rollor
rotateisinfluencedbyparticlefrictioncontrast.

ChapterVisacomparativestudyofnumericalmodellingandgeotechnical/analoguetests.
Thepresenceandtheconcentrationofclaymineralsaremajorfactorscontrollingtheshear
behaviourofsedimentarymaterials.

ThelastChapterVIsummarizesresultsandconclusionsofpreviouschaptersandconnects
theseaspectstosomeconceptsofChapterI.Then,advantagesandlimitationsofthe
numericalmethodarediscussed.Lastly,anoutlookofnecessaryfutureworkispresented.

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Abstract
Thisthesiswasinspiredbythelackofdetailed(i.e.particlescale)knowledgeconcerning
deformationprocessesofmechanicallyweaksediments,especiallyintrinsicallyweaklayers
onthebasalshearplaneofsubmarinelandslides.Ithasbeenknownforsometimethatmany
differentparametersinfluenceshearstrengthandlocalizationfeatures.Thisistruenotonly
forsediments,butalsoforotherkindsofweaklayers,suchasfaultgouge.Theseparameters
includeforexamplemineralogy(e.g.smectite,illite,quartz),sedimentcomposition(clay,
silt),sedimentstructureandtexture(microfabric),grainsizedistribution,excesspore
pressure,magnitudeofeffectivestress,anddeformationhistory.However,todateithasnot
beenpossibletorankortoquantifytheinfluenceofeachoftheseparameters.Themaingoal
ofthisstudyistoanalyzetheinfluenceofsomeoftheseparametersand,ifpossible,rankand
hem.tyquantif

Standardmethodstoexamineshearstrengthofsedimentsandfaultgougearevarious
geotechnicalshearexperiments.Inthese,asampleisshearedunderdefinedconditionsand
resultingcoefficientoffriction,voidratiochange,andothermeaningfulparametersare
analyzed.Unfortunately,itisnotpossibleto‘look’insideashearboxduringatestandto
analyzegraindeformationbehaviouronamicroscopicscale.Therefore,thisstudyemploysa
differentapproachtospecificallyaddresstheproblemofmicroscopicdeformationprocesses.
Here,anumericalmodellingtechnique,theDiscreteElementMethod(DEM),isused.

TheDEMisanumericaltoolbasedonthebehaviourofgranularmaterials.Withinsome
limitations,soilsandfaultgougecanbeconsideredasgranular.Thus,theDEMallows
simulatingdeformationbehaviourofweaklayers.TheDEMprincipleisbaseduponsimple
physicalcontactandmotionlawsandcanreproduceawiderangeofgrainfeaturesand
behaviour.Thetechniquehasalreadybeenusedtomodelotherkindsofgranulardeformation
processessuchaslargeandsmallscaledeformationprocesses.

UtilizingtheDEManumericalshearbox,verysimilartogeotechnicalringsheartests,was
designed.Insidethisbox,avarietyofdifferentnumerical‘samples’weregenerated.These
‘sediments’,or‘faultgouges’,weredesignedwithclosespecificationsineachstudy
(manuscript),respectively.Duringthenumericalexperimentsamultitudeofmicromechanical
parameters(particledisplacementandrotation,microfabricevolution,coordinationnumber,

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slidingfraction,contactforcedistributionandorientation)weremeasured.Thesealso
encompassclassicalgeotechnicalmeasurements,suchascoefficientoffriction,voidratioor
e.ghancvolume

Itwasnotpossibletostudyallparameterswhichmightinfluencedeformation.Therefore,the
manuscriptsconcentratedonthestudyof:
(1)Surfaceroughness.Thisissuspectedtohaveatleastasecondorderinfluenceonfrictional
strength.Butsofarithasbeenextremelydifficulttostudysurfaceroughnesssystematically,
mainlyduetoirreproducibleboundaryconditionsinthelaboratory.
(2)Clayparticleshape.Geotechnicaltestssuggestthattheshapeofclaymineralshasastrong
impactonthedeformationbehaviour.Themanuscriptfocusesontwoaspectsofparticle
shape:sphericityandroughness.
(3)Clayandsiltstratification.Itisunknownwhereexactly,singlegraindisplacement
accumulatesinasampleduringdeformationinacomplexlayeredsetting.Thus,adetailed
investigationofmicromechanicalparticledeformationinsuchasettingwascarriedout.
(4)Clayandsiltmixing.Awell-knowngeotechnicalrelationshipbetweenclayfractionand
frictionalstrengthwastestednumericallytovalidateandassesthecomparabilitybetween
laboratoryandnumericalshearboxexperiments.

Comparisonbetweennumericalandgeotechnicalexperimentsimpliedthatresultsfrom
numericalmodelscouldbeappliedtonaturalconditions,albeitwithsomelimitations.As
geotechnicaldatareflect,parameters(1)-(4)didhaveaninfluenceonthemicroscopic
deformationbehaviourofsedimentsandfaultgouge.
InChapterII,thresholdlevelsforboundaryroughnessarepresented.Belowathreshold
boundaryroughnessof0.25,lowroughnessledtolowboundaryandsamplefrictionfollowed
bycompletemechanicdecouplingoftheboundary.Aboveathresholdlevelof4,high
roughnessledtolowboundarybuthighsamplefriction.Theboundarycouldnotdecouplebut
aconcentratedshearzonedevelopedinitsvicinity.Intermediateroughnessbetweenthese
levelsledtointermediatefrictionalstrengthofthesample,buttohighestfrictionforthe
boundary.Deformationinsidethesamplewasdistributed.
Inthenextchapter(ChapterIII)particlesphericityandroughnesscouldbeidentifiedaskey
parametersfordeformationbehaviourandlocalization.Microfabricdevelopmentiscontrolled
byparticleinterlockingandrotation.Sinceincreasingroughnesscausedincreasingparticle
interlocking,microfabricbreakdownduringshearwaspreventedandshearzonedevelopment

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precluded.Incontrast,breakdownofcomplexinitialmicrofabricwasenhancedbydecreasing
particlesphericity.Thiscausedparticlerotationtopreferredangles,whichresultedinlow
frictionandincreasingshearzonelocalization.
Subsequently,twotypesoflayeredspecimenswerestudiedinChapterIV:(1)A’silt’-’clay’-
’silt’assemblageand(2)a‘clay’-‘clay’-‘clay’package.Here,thecontrastbetweenparticle
frictionoftwolayerswasdiscoveredtohaveahighimpactondeformationbehaviour.
Differencesinparticlefrictioncontrastcontrolledwhetherthedominantmodeofgranular
deformationwasparticlesliding,rollingorrotation.Whendeformationmodechangedinside
thesample,localizationfeaturesalsovaried.Overalllocalizationstructureswereverysimilar
tofeaturesfoundinnatureandlaboratory.Y-,P-andR1typeshearplaneswereparticularly
abundantinthenumerical‘clay’wherealsoadistinctcorrelationbetweenslipplanes
localizationandparticlealignmentcouldbefound.
ThecomparisonofnumericalandgeotechnicalexperimentsinChapterVrevealedclaysize
fractiontobeanimportantcontrolonfrictionalstrength.Withbothmethodsathresholdvalue
of50%couldbeidentifiedastheminimumamountofclaysizefractiontoinducea
significantdecreaseinfrictionalstrengthinasediment.Here,itwasdemonstratedthatthe
combinationoflaboratoryandnumericalexperimentsallowtoclassifytheinfluenceof
parameterscontrollingfrictionalstrength,particularlysinceshearplanemonitoringwiththe
DEMallowedadirectcontroloftheshearlocalizationinallmaterials.

Attheendofthisthesisarankingorderofeverysingleparameterispresented.Inthis
ranking,itisimportanttodistinguishbetweenpurelynumericallyderivedconclusionsand
theirimplicationfornaturalmaterials.Hence,withinmodellimitations,grainroughnessand
claysizefractionexhibitmaximuminfluenceonfrictionalstrengthandlocalizati