A numerical elastic-thermoviscoplastic model for clay behaviour

D. Kurz

In the proceedings of: GeoVancouver 2016: 69th Canadian Geotechnical Conference

Session: FUNDAMENTALS - VIII Physical & Numerical Modelling

ABSTRACT: Clays exhibit creep in both compression and shear. Viscous behaviour starts immediately after loads are applied andoften formsa large component of total deformations. Movements depend on load duration, loading rate,preconsolidationpressure,stress level, the ratio of shear stress to compression stress, strains, strain rate, and temperature. Therearealso usually small recoverablestrains,so thebehaviour can be called elastic-thermoviscoplastic (ETVP).The paperoutlines a recent ETVP model and describes simulated triaxialtestresults with different straining rates and temperatures.R•SUM•Les argiles pr†sentent un fluage ‡ la fois en compression et en cisaillement. Le comportement visqueux d†buteimm†diatement apr…s l—application des charges et constitue souvent une composante importante des d†formationstotales. Les mouvements d†pendent de la dur†e de la charge, du taux de chargement, du niveau de contrainte, durapport de la contrainte de cisaillement ‡ la contrainte de compression, des d†formations, de la vitesse de d†formation etde la temp†rature. Un tel comportement peut aussi –tre qualifi† d—†lastique-thermoviscoplastique (ETVP) d…s lors que depetites d†formations r†versibles s—y manifestent. Cet article souligne un mod…le r†cent d—ETVP et d†crit les r†sultats desessais triaxiaux simul†s avec des vitesses de d†formation ainsi que des temp†ratures diff†rentes.1INTRODUCTIONƒVISCOUS BEHAVIOURClays creep:that is, they exhibitviscous behaviour.Creepisseen in long-term settlements of foundations;insettlementsandspreading of embankments on softground;and in ongoing, often irregular, movements innatural and engineered slopes.It is common in plastic andorganic clays (Mitchell and Soga 2005).Deformations in viscous materialsdepend on theduration of constant loading, the rate at which loading isapplied, the temperature, or some combination of theseprocesses.Creep rates areinfluenced bystresslevel,stress rate, strain, strain rate, and temperature.Pre-consolidation pressures, yielding, and undrained shearstrengths are all influenced by viscosity(Graham et al.1983).Viscous behaviouris experiencedas non-recoverable strainingin compression and shear duringbothprimary consolidationand secondary compression.Viscous straininginvolvesstatistical redistribution ofmore-stressed and less-stressed inter-particle contacts,and slow movement of ⁄adsorbed water—closeto thesurfaces ofclay particles(Mitchell and Soga 2005).Thecombination of load transfer and high viscosity ofadsorbed water means that creep movements changewith temperatureand decayexponentially with time.2PARTITIONING OF STRAINSCreep in clayiscommonly thought of asa two-stageprocess; first, dissipationof excess pore water pressure(primary consolidation),followed byongoing settlements(secondary compression) (Mesri and Choi 1985).An alternative approach, whichis now favoured,divides settlements into ⁄instant— and ⁄delayed—components (Bjerrum 1967). The instant componentconsists oflargely reversible (elastic)deformations of theclay particles. Thedelayed component is non-recoverable, time-dependent (viscoplastic) reorganizationofthe inter-particle microstructure.Total strains canthenbepartitioned into elastic and viscoplastic components.That is, {•total} = {•elastic}+ {•viscoplastic} (Yin et al. 2002).Inthissecond approach,preconsolidation pressuresvary with strain rateor withthe duration of loading.Theslope of Normal Consolidation Lines(NCLs)is constant,butslowerloading rates movethe linesto lower values ofspecific volumeV(Graham et al. 1983). In theoverconsolidatedrange, unload-reloadlines (URLs) andtheir slopesCr(or†)changewith strain rate becausedeformations include both recoverable elastic strains andsmall viscoplastic non-recoverable strains that vary withtherate and duration of loading(Kelln et al. 2008).3EFFECTSOFTEMPERATUREEffects of changing temperatures must be consideredinthedesign of foundations for furnaces;transmissiontowers;nuclear waste containment;deep excavations;thermal piles for geothermal energy;andin the effects ofclimate warmingandthawing permafrost under roadways.In consolidation tests at different temperatures, valuesofCr,preconsolidation pressures, and the positions of

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