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Ng and Chiu [NG 03] also performed several tests, consisting of wetting
specimens of the same silty sand at a constant stress state. After an isotropic
consolidation up to 100 kPa, a specimen was sheared at constant net mean stress and
constant suction,
s
= 40 kPa, until it reached a net stress ratio
η
= q/p
= 1.4. Then,
the specimen was wetted by reducing the suction while the deviatoric stress and net
mean stress were kept constant. Figure 7.21 shows the evolution of the axial strain
and the volumetric strain during the wetting phase as a function of suction. A
continuous increase in the axial strain can be noted throughout the process. At the
beginning of the wetting, this increase remains limited; toward the end of the test,
the rate of increase accelerates. The same kind of evolution can be observed for the
volumetric strain. At first, a very limited volumetric contraction takes place, but, as
the suction continues to decrease, toward the end of the test the rate of increase in
volumetric contraction also increases. The authors attribute this increase in the rate
of volumetric strain to the collapse of the soil skeleton.
Figure 7.21.
Evolution of axial and volumetric deformations during a wetting test
(p
0
= 100 kPa) (experimental results from [NG 03])
A numerical simulation of the same wetting test was undertaken. The shearing
phase was conducted at a constant capillary stress in order to reproduce the
condition of constant suction as closely as possible. The numerical results are
presented in Figure 7.22. An increase in both the axial and the volumetric strain was
obtained, as in the experiment, with a continuous increase in the strain rate when the
capillary stress decreased. However, the evolution was more progressive than in the
experimental results, the specimen starting to strain as early as the beginning of the
wetting phase. This could be due to the choice made for the relationship between
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