Evolution model of unsaturated shear strength indices over a wide range of saturation

The variation of shear strength indices of unsaturated soil with the degree of saturation S-r is complicated. In order to describe the experimental phenomena, the existing literature on the variations of cohesion, internal friction angle and dilation angle with S-r is analyzed. The results show that: (1) When S-r decreases and the pore water gradually transits from capillary water to adsorbed water, the friction angle increases. The turning point can be defined as critical degree of saturation S-rc. When S-r is greater than S-rc, the value of the friction angle is constant; when S-r is less than S-rc, the friction angle increases linearly with the decreasing S-r, and the extended Mohr-Coulomb criterion is no longer applicable. (2) The "peak effect" exists in the process of cohesion changing with S-r, i.e. the cohesion first increases exponentially with the decrease of S-r and reaches its maximum when S-r reaches S-rv corresponding to the peak unsaturated strength, and then decreases, which is the main reason for the "peak effect". (3) A linear model can describe how the dilation angle increases with the decrease of S-r and vertical load. In addition, an unsaturated shear strength model over a wide S-r range is proposed. The proposed model is verified by the comparisons between the published experimental data and the predicted results of the proposed and existing equations, which illustrate the superiority of the proposed model.