Geoscience Reference
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synthesis can also be found in Constitutive Modeling of Soils and Rocks , edited by
P.Y. Hicher and J.F. Shao [HIC 02].
Let us briefly recall the essential mechanical characteristics of soil behavior:
1) In the presence of water, the total stress alone cannot adequately describe the
behavior of soils. This can only be described by the effective stresses that are obtained
from the total stress and pore pressures. In undrained conditions and saturated soil, the
behavior conforms quite well to the principle of Terzaghi's effective stress, assuming
that the grains are incompressible. By working in effective stress, we can also eliminate
the apparent cohesion for many soils. Real cohesion, as far as we are concerned, is the
shear strength (in Mohr's plane) of a saturated soil under zero mean effective stress and
not the linear extrapolation of strength at the origin, obtained from non-zero confining
stresses. In unsaturated soils, capillarity can create a fictive (or apparent) cohesion,
which is modeled by a pore pressure below air pressure (suction). The behavior of
unsaturated soils will be examined in section 9.3.5.
2) Behavior of soils is highly non-linear. This non-linearity is due to four main
mechanisms:
- the reversible deformation of granular particles at contacts,
- irreversible shear at intergranular contacts,
- the irreversible deformation of particles themselves,
- the irreversible deformation due to the breakage of particles.
In current applications of soil mechanics, due to the intrinsic rigidity of the soil
particles, the third type of deformation is negligible. This is not always the case for
some granular materials.
3) Effective confinement influences significantly both the stiffness and the soil
strength.
4) Porosity (or void ratio or density), at a given level of confinement, also modifies
the behavior of a soil. This change can be observed at the stiffness level, both for the
initial value and during the loading (Figure 9.1).
5) Dilatancy refers to the ability to produce volumetric strains under the effect of
shear, even at constant mean effective stress. It is dependent on both the porosity and
the ratio between the average shear and mean effective stress.
6) During shearing, the material reaches a state where, in drained conditions, the
deformation takes place under constant stress and without volume change; in undrained
conditions, the pore pressure no longer changes and deformation occurs under constant
effective stress. This state, called the critical state , occurs at a void ratio which depends
only on the stresses. Triaxial tests show that the relationship between the critical void
ratio and confinement is unique for a given material.
7) Although sands and clays have very different morphologies and structures, their
macroscopic mechanical behavior is very similar. It must be said, however, that soil
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