A kinematic hardening model with degradation of structure application to soft natural clays
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A KINEMATIC HARDENING MODEL WITH DEGRADATION OF STRUCTURE – APPLICATION TO SOFT NATURAL CLAYS. B é atrice A Baudet University College London. INTRODUCTION. Natural clays have a different microstructure or structure to that of clays reconstituted in the laboratory.

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A kinematic hardening model with degradation of structure application to soft natural clays

A KINEMATIC HARDENING MODEL WITH DEGRADATION OF STRUCTURE – APPLICATION TO SOFT NATURAL CLAYS

Béatrice A Baudet

University College London


INTRODUCTION – APPLICATION TO SOFT NATURAL CLAYS

  • Natural clayshaveadifferent microstructureor structure

    to that of clays reconstituted in the laboratory.

  • In most clays, particularly soft clays, this structure breaks

    with plastic strain; this is termed destructuration.

  • A simple modelincluding structure is proposed, with only

  • three new parameters which can be derived from a

  • single set of test data.


TYPICAL PROBLEMS ASSOCIATED WITH CONSTRUCTION ON SOFT NATURAL CLAYS

Failure of the Carsington dam during

construction due to poor estimation

of strength of the foundation clay

Cracking of roads due to settlement of soft soil foundation


EFFECTS OF STRUCTURE ON THE BEHAVIOUR OF SOFT CLAYS NATURAL CLAYS

undisturbed

stress

destructuration

disturbed

displacement

Schematic behaviour

soil element


SOFT NATURAL CLAYS NATURAL CLAYS

  • Shear strength < 50kPa

  • Structure: combination of - fabric (stable elements)

  • - bonding (unstable elements)


THE S3-SKH MODEL NATURAL CLAYS

  • Based on an existing model, the 3-SKH model, developed for stiff clays

    with a stable structure (Stallebrass & Taylor, 1997)

  • All effects of structure described by the size of the state boundary

    surface


SCL NATURAL CLAYS

sensitivity at yield

compression of intact clay

NCL

(S

= 1)

t

current sensitivity, s

specific volume

¢

¢

¢

2 p

2 s. p

ln

p

0

0

s

s0

sf

ep

  • s: exponential function decreasing with

  • Three new parameters: s0, sf, k that can be derived from a single isotropic

    compression test


Experimental data (from Allman, 1992) NATURAL CLAYS

Predicted data

CSL

  • Stiffness successfully simulated at small strain

  • Undrained shear strength correctly predicted

  • Post-peak, comparison of predicted and test data complicated


v NATURAL CLAYS

iso-NCL*

p*ie

p′

lnp′

  • Reversal in normalised stress path direction: definition of a

    limit surface

  • Rate of destructuration with plastic strain consistent with

    experimental results


CONCLUSIONS NATURAL CLAYS

  • A simple model including structure in the size of the state

    boundary surface is sufficient to describe the behaviour of

    natural soils in triaxial space

  • The new model (S3-SKH model) requires only three additional

    parameters to the base model which can be derived from a

    single isotropic test

  • Need to investigate how soil structure degrades in general

    stress space and whether the features of behaviour found in

    general stress space can be generated by simply extrapolating

    the model from axisymmetric to general space


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