A Microstructural Model for Soils and Granular Materials - Multiscale Geomechanics: From Soil to Engineering Projects - page 199

The influence of the K -consolidation can be seen at the beginning of the
stress−strain curves. When the specimens are consolidated under a constant stress
ratio K , anisotropy develops. Thus the initial stiffness of the specimens is influenced
by the different values of K during consolidation. The model can take into account
the influence of a stress-induced anisotropy by means of the fabric tensor A , defined
in equation [7.14], as well as by the dependency of the elastic modulus with the
stress state (equation [7.3]). The model is capable of taking into account the
existence of an initial anisotropic stress state and a good agreement between
experimental results and numerical simulations can be achieved for the stress−strain
relationship (see Figure 7.13a), as well as the effective stress path (see Figure 7.13b)
during undrained triaxial tests.
7.4.2.2. Guiche clay
The previous results correspond to materials for which the contact law between
two grains is reduced to a solid friction. If two grains in contact are also “glued”, the
change in the contact law also produces a change in the mechanical properties of the
equivalent continuous medium. Its overall behavior depends on the behavior of the
glue. The main visible change in the material behavior is the fact that the maximum
strength envelope does not intercept the axes origin and that the material exhibits a
non null strength in traction: this is usually expressed by the term “cohesion”.
However, the role of the intergranular glue cannot be reduced to a scalar
representing this cohesion and experimental results show that the global
stress−strain relationship is affected by the glue's properties. If the glue has a
stiffness similar to that of the grains, a significant decrease in the role of mean stress
on the elastic properties follows. At the same time, the force necessary to displace
two grains in contact is proportional to the normal force in the case of a simple solid
friction. If the two grains are glued, we have to add the strength of the glue; if this
strength is constant, the role of the friction is negligible provided that the normal
force is small and becomes preponderant at elevated normal forces.
For the equivalent continuous material, this implies that its behavior is strongly
dependent on the glue properties at small mean stresses, when c >> Mp' , and much
less dependent at high mean stresses when c << Mp' . A progressive damage of the
glue during a mechanical loading can modify the relative influence of the cohesion
and the friction. Generally, natural soils present a higher mechanical strength than
remolded soils.
Sometimes, the intergranular glue is ductile enough to sustain macroscopic
deformations of several percentage points. Under these conditions, the maximum
strength can remain constant after reaching the peak, which creates the equivalent of
a perfectly plastic state for a cohesive material. This is the case for the Guiche clay
that was extensively tested by Khemissa [KHE 92]. It corresponds to a silty clay