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

slide2

INTRODUCTION

  • 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.
slide3

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

slide4

EFFECTS OF STRUCTURE ON THE BEHAVIOUR OF SOFT CLAYS

undisturbed

stress

destructuration

disturbed

displacement

Schematic behaviour

soil element

slide5

SOFT NATURAL CLAYS

  • Shear strength < 50kPa
  • Structure: combination of - fabric (stable elements)
  • - bonding (unstable elements)
slide6

THE S3-SKH MODEL

  • 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

slide7

SCL

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

slide8

Experimental data (from Allman, 1992)

Predicted data

CSL

  • Stiffness successfully simulated at small strain
  • Undrained shear strength correctly predicted
  • Post-peak, comparison of predicted and test data complicated
slide9

v

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

slide10

CONCLUSIONS

  • 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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