Mechanisms of Strength Loss in Stiff Clays

A slide in the upstream slope of California's San Luis Dam on September 4, 1981, was caused by a loss of strength in the stiff clay in the foundation beneath the slope. This loss in strength, which occurred over 14 years of the dam's operation, is not explainable in terms of commonly held views of the behavior of stiff clays. An investigation showed that the shear strength of the desiccated clay beneath the dam decreases very rapidly to the fully softened strength when the clay is soaked. When the clay is subjected to cyclic loading, as it was by cycles of reservoir filling and emptying, the strength of the clay decreases gradually from the fully softened strength to its residual value. These mechanisms of strength loss appear to have been responsible for the San Luis Dam slide, and should be considered in stability evaluations whenever stiff clays are subjected to wetting and cyclic variations in load.