Geoscience Reference
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constitutive parameters. Thus the underlying adopted model is crucial, in this
respect. The same holds for predictions of structural behaviour performed by
numerical simulations, as the outcome in terms of deformations and stresses
depend on model chosen, corresponding parameter values adopted and the
robustness of the computational algorithm. In general five basic aspects of soil
behaviour can be recognised:
- the influence of pore water and pore pressures (permeability, consolidation)
- the influences the soil stiffness (stress and strain level, stress path, anisotropy)
- irreversible deformation (plasticity, creep, preconsolidation)
- strength (loading rate, density, drained-undrained)
- compaction (contractancy, dilatancy)
p
p
q,
q,
volume strain
G
G
p
p
1 - sin
cut off
2 sin
elastic
elastic
F
F
vertical strain
p',
p',
v
p
v
p
(a) flow rule F and plastic potential G (b) dilatancy (triaxial test)
Figure 9.1 Failure, plasticity and dilatancy concepts
Plasticity
It is assumed that plastic deformations can be represented by deformation rates
t
and that these rates are zero in the elastic range and non-zero at the edge of
the elastic range. A flow potential or yield surface, fully defined by effective
stresses, describes the elastic range, e.g.
F
(
ij
/
ij
'
) < 0. At the edge of the elastic range
F
= 0. It is further assumed that the strain rates that develop at
F
= 0 are related to a
plastic potential
G
fully defined by effective stresses, e.g.
ij
'
. If the
flow potential
F
is identical to the plastic potential
G
the model behaves associated.
If not, the behaviour is non-associated (Fig 9.1a). In soil mechanics, an associated
flow rule is commonly used to model the behaviour of normally consolidated clay.
A non-associated flow rule is frequently used to describe the behaviour of sands,
particularly for dilation and contraction.
In geotechnical engineering many constitutive models have been suggested.
Here, commonly accepted constitutive soil models are shortly outlined: for sand the
Mohr-Coulomb model and for clay the Cam-Clay model. Some remarks are given
for peat behaviour and a few other constitutive models are mentioned briefly.
ij
/
t =
G/
Mohr-Coulomb model for sand
The Mohr-Coulomb model (see Chapter 7) is one of the most popular
constitutive models for soil behaviour (failure). The model adopts the maximum
and minimum stress in the plane of failure, such that the stress in the orthogonal
direction falls somewhere in between both. It is a (linear-)elastic perfectly-plastic
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