In this chapter, the probabilistic porous-solid model is used to determine the
mean effective stress of soils at failure. The plot of the deviator stress against the mean
effective stress shows a unique failure line for a series of triaxial tests performed at
different combinations of the confining net stress and suction for both wetting and
drying paths. This result confirms that the proposed effective stress equation is adequate
to predict the shear strength of unsaturated soils. It also results in different strengths for
wetting and drying paths as the experimental evidence indicates.
Keywords: Shear strength, effective stress, net stress, triaxial tests, confining
stress, axis translation technique, constant volume test, friction angle, wetting
path, drying path, porous-solid model, soil-water retention curve, logarithmic
normal distribution, critical state, pore size distribution, grain size distribution.