Geoscience Reference
In-Depth Information
In this chapter, the effective stress concept will be compared to the results of an
experimental study on a compacted silty sand. The following points are examined:
- the behavior before failure, from very small to large strains;
- the influence of the capillary pressure on the maximum strength;
- the validity of an effective stress approach based on a simple microstructural
model to interpret the results.
6.2. Microstructural model for unsaturated porous media
The model is based on a medium composed of elastic spherical isodiametral
particles. Gravity is not considered and the pressure in the air phase is atmospheric
(p
a
= 0). Characteristic behaviors can be brought to light in two cases [BIA 89;
BIA 93; BIA 94; FLE 95; TAI 94]:
- when the water phase is discontinuous (permeability to water
w
k
), water
forms torical menisci at the points of contact between particles (Figure 6.3). The air
phase is continuous and its pressure plays a role, as does the water pressure, in the
creation of the capillary strength; the water pressure inside the meniscus is negative
with respect to that of the air (
=
0
p p
<
. In this case, the intergranular forces due to
water are perpendicular to the tangent planes at the contact points between the
particles and, therefore, do not produce any rearrangement of the particles or
variation in volume. However, these forces play an important role in the strength of
the material. From a practical point of view, such conditions are observed for water
contents below the shrinkage limit;
w
a
a
k
=
), air forms
isolated bubbles inside the pores. The water phase is continuous and completely
wets the solid particles. Since no contact occurs between air and the solid particles,
water pressure does not play any role in the strength of the material. However, the
presence of air bubbles in the pore fluid makes it more compressible.
- when the air phase is discontinuous (permeability to air
0
When the particles arrange themselves randomly, the inter-granular forces due to
external loading are generally not perpendicular to the tangent planes and can,
therefore, induce volumetric strains.
The passage from a discontinuous to a continuous medium is made by
considering regular arrangements of particles. Four types of arrangements were
studied, with densities ranging from 0.83 (tetrahedral) to 1.81 (dodecahedral)
(Figure 6.4).
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