Shallow Slides in Compacted High Plasticity Clay Slopes

Shallow slide failures in embankments constructed of high plasticity clays create costly maintenance problems on highway projects and can threaten the integrity of water-retaining earth structures. This paper investigates the mechanisms of stability degradation that lead to these slope failures. The failure mechanism involves moisture infiltration into the slope surface that leads to decreases in suction and soil shear strength. Both the degree and time rate of strength loss are investigated based on stability and moisture diffusion analyses, respectively. Stability analyses indicate that the failures are associated with destabilizing hydraulic gradients in the pore water, and the suction level at the surface of the slope declines to a limiting suction of about u = 2 pF when exposed to moisture. Moisture diffusion analyses indicate that the time rate of strength degradation is controlled by the depth and spacing of desiccation cracks that form in the soil mass and the moisture diffusion properties of the soil. The stability and moisture diffusion models described above were evaluated in light of 34 documented shallow slides in Texas high plasticity clays.