The Concept of Effective Stress in Unsaturated Soils - Multiscale Geomechanics: From Soil to Engineering Projects - page 170

When the water content increases, a progressive change from brittle to plastic
behavior can be noted. For water contents below 8%, the stress-strain curves present
a peak and the specimens seem strongly dilatant; on the other hand, for water
contents higher than 16%, perfectly plastic and slightly contractant behaviors are
observed. However, in all the tests, the volumetric deformations remain small (1.0 to
1.5 %). In the first case, the capillary pressure in the menisci creates strong bonds
between the grains, which increase the strength of the material and prevent
deformation. In fact, localization of strains are frequently observed in the samples
under these conditions, so that the measured increase in volume is probably not
wholly representative of the real behavior of the soil, but partly due to an apparent
increase in the global volume after the formation of the discontinuity. A similar
behavior was observed by Verbrugge in a quasi-dry loam [VER 02]. When the water
content increases, the grains begin to be surrounded by water and the behavior of the
soil tends towards that of the saturated soil at the same void ratio.
The changes in pore-water pressure p w - p a during triaxial tests are shown only
for water contents greater than 9.5 %. The curves for the driest samples (between 3.7
and 9.5 %) are not shown in this figure, as the pressure remains constant during the
tests. They are at the same value as at the end of the isotropic compression tests (see
Figure 6.13). The trend is the same for all the curves: (i) first, an increase in pressure
corresponding to the contractant behavior already mentioned; and (ii) then a
reduction in pressure, characteristic of dilatant behavior. This second phase is
observed even in the nearly-saturated specimens that exhibit an overconsolidated
behavior, corresponding to what is usually observed in saturated compacted
specimens. In all cases, the changes in pressure are small (± 20 kPa or less), as a
consequence of the low compressibility of sand.
6.4.3. Small strain behavior
The small strain properties of Perafita sand were measured in separate tests, not
like those used to measure the large strain properties, but under approximately the
same water content and void ratio conditions. Two tests (at w = 6.5 and 13.2%) were
performed under several confining stresses and stress deviators, whereas the others
were performed under isotropic stress states only. The influence of several factors
was investigated. Figure 6.15 shows that, for unsaturated specimens, the changes in
normalized modulus are approximately a power law of the total vertical stress, with
an exponent n = 0.35 to 0.40. On the other hand, the curves for dry and quasi-
saturated specimens are nearly superimposed, with a larger slope ( n = 0.68). Under
the same void ratio and vertical stress, the modulus increases when the water content
decreases.