Abstract:

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than S_rc, the friction angle increases linearly with the decreasing S_r, and the extended Mohr-Coulomb criterion is no longer applicable.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.

Key words: unsaturated soil, degree of saturation, shear strength indices, unsaturated shear strength model