Yielding of unsaturated soil.

Considerable efforts have been made in recent years to develop a better
understanding of the mechanical behaviour of unsaturated soils in terms of elastoplastic critical state constitutive models. These models are defined in terms of four
independent stress state variables : mean net stress, deviator stress, suction and
specific volume. An important feature of the models is the suggestion of the existence
of a yield surface in mean net stress : deviator stress : suction space.
Suction-controlled triaxial tests were performed to investigate the shape of the
yield surface for unsaturated compacted speswhite kaolin with a particular stress
history. The tests were conducted in a Bishop-Wesley triaxial cell with suction applied
by the axis translation technique. The soil samples were instrumented with local strain
gauges for measuring the sample volume change.
Ten samples were tested, and in each test the location of the yield surface was
fixed by isotropically consolidating the sample to a mean net stress of 400 kl'a and a
suction of 100 kPa. This procedure effectively erased the one-dimensional stress
history produced by the compaction process. After unloading to stress states inside the
yield surface, samples were re-loaded either isotropically or by drained triaxial
shearing. Re-loading stages were conducted at three different values of suction.
Yield points in the re-loading stages were most easily identified from plots of
specific volume against the logarithm of mean net stress (even for shear tests), whereas
it was often difficult to identify a clear yield point from a plot of deviator stress against
shear strain. The yield points identified from the re-reloading stages were used to
define the shape of the yield surface in a stress space with axes of deviator stress, mean
net stress and suction. Constant suction cross-sections of the yield surface were
approximately elliptical in shape, with one axis of the ellipse coinciding with the mean
net stress axis. As expected, the size of the elliptical constant suction yield curves
increased with increasing suction Soil elastic indices obtained from swell-back and suction-change stages showed
non-conservative behaviour, indicating that the behaviour of the soil was not truly
elastic even for stress states inside the yield surface. Plastic strain increment vectors
were plotted for the post-yield behaviour, and these were consistent with an associated
flow rule.