OBSERVATION AND PREDICTION - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 138

Fig 9.2a shows a hardening stress path A-B-C of the Modified Cam-Clay model.
When loading starts (point A), the original yield surface is the smaller ellipse,
characterised by the preconsolidation pressure p 0 . The corresponding over-
consolidation ratio is OCR = p 0 / p' A . At point B the material starts yielding and the
corresponding yield surface grows, until the stress path reaches the critical state
(failure), point C. Fig 9.2a.b shows the v -ln p space, where characteristic lines
becomes straight. Fig 9.2a.c shows the corresponding shear stress-strain hardening
during yielding.
Fig 9.2b shows a softening stress path A-B-C of the Modified Cam-Clay model.
When loading starts (point A), the original yield surface is the larger ellipse,
characterised by the preconsolidation pressure p 0 . The corresponding over-
consolidation ratio is OCR = p 0 / p' A . The first part of the stress path A-B is elastic.
At point B the material starts yielding and the corresponding yield surface
diminishes, until the stress path reaches the critical state (failure), point C. Fig
9.2b.b shows the v -ln p space, where characteristic lines becomes straight. Fig
9.2b.c shows the corresponding shear stress-strain softening during yielding.

Constitutive model for peat
Peat in particular behaves anisotropic related to the occurrence of fibres (organic
remains) and depending on the rate of humification. Fibres are expected to mainly
align horizontally, and therefore cause structural anisotropy. Since peat is a soft
and very compressible, material loading may cause also induced anisotropy in
stiffness. The compressibility of organic material, the main 'solid particles' in peat,
is another aspect that differs from sand and clay where the solid particles are
relatively incompressible. This will cause a decrease of the inter-particle stress and
since equilibrium must hold, it may give rise to an additional pore pressure
increase. It also affects the principle of effective stress, which becomes = ' +

(1 the ratio between particle compressibility and bulk compressibility

(Biot's coefficient). Furthermore, the large flexibility of peat may have a
pronounced effect on pore pressure development during consolidation, because the
permeability will change significantly with the pronounced compression.
Therefore, in triaxial tests, horizontally retrieved samples may reveal differences
compared to vertically retrieved samples (Zwanenburg). Conventional treatment in
laboratory experiments of peat is therefore not sufficient. The constitutive
behaviour of peat requires a different approach than provided in current
constitutive models (Den Haan and Kruse).

2 ) u , with 2

Other constitutive models
A familiar elastic-plastic model is the hyperbolic model or the Duncan-Chang
model, which adopts an incremental nonlinear stress-dependent concept, based on
stress-strain curve in drained triaxial compression test of both cohesive and
cohesion-less soil, approximated by a hyperbola (power law). The failure criterion
is based on Mohr-Coulomb. The Duncan-Chang model is widely used as its soil
parameters can be easily obtained directly from standard triaxial test. It is a simple
enhancement of the Mohr-Coulomb model. It does not cover dilatancy, unloading
behaviour and full plasticity.