The Concept of Effective Stress in Unsaturated Soils - Multiscale Geomechanics: From Soil to Engineering Projects - page 174

300

200

100

Dry & quasi-saturated tests :
E v0 = 200 ( σ ' v /p a ) 0.68
r 2 =0.87

50

10

20 30 50

100

200 300

Effective vertical stress σ ' v = σ v + σ ' cap (kPa)

Figure 6.18. Interpretation of the results of the small strain
triaxial tests using the micromechanical model
for determining effective stress

6.6. Conclusions

A significant number of tests were performed on Perafita silty sand, under many
different conditions of water content and density, isotropic and deviatoric stress,
using non-standard devices for measuring very small strains or capillary pressures.
The results from all these tests provide a fairly broad picture of the properties of the
sand, from nearly dry to nearly saturated, from the elastic to the perfectly plastic
domain. The residual material exhibits original properties with the grain size
distribution of a fine sand, on the one hand, and noticeable plasticity, on the other
hand that results in large changes in capillary pressure (from 3 to 2,000 kPa) in the
domain of the water contents studied (7 to 21%).

Mechanically, the increase in capillary pressure results in an increase in maximal
strength, stiffness and brittleness of soil. In fact, it is observed that, for soils
compacted left of the modified Proctor optimum water content, volumetric changes
due to alterations in isotropic or deviatoric stress become negligible. When the
strains are very small, the curves relating the modulus values to the total stress do
not cross the y-axis at 0, but present a positive ordinate for σ v = 0. Moreover, when
plotted in a bi-logarithmic coordinate system (see Figure 6.15), the paths seem fairly
linear and parallel, but with a different slope from that of dry and quasi-saturated
specimens. This type of behavior is systematically found in all partially saturated
granular materials [TAI 08]. For the driest specimens, the increase in stiffness with