On September 14, 1981 a major slide was discovered in the upstream slope of San Luis Darn, located about 100 miles southeast of San Francisco, California. The slide occurred at the end of a period of rapid drawdown of the reservoir. Although this was the longest and fastest drawdown in the life of the dam, 180 feet in 120 days, there had been seven previous cycles of drawdown, some nearly as severe as the one that preceded the slide.

Field measurements showed the slide was caused by the clayey slopewash material left in the foundation of the dam during construction. Although the slopewash was dry and extremely strong when the embankment was built, it apparently was weakened considerably when submerged beneath the reservoir and its strength was further degraded by cyclic loading effects as the reservoir level was raised and lowered during the 14 years preceding the slide.

The objective of this research was to gain a better understanding of the mechanisms of strength loss in the slopewash that resulted in the 1981 slide at San Luis Dam. This was accomplished using laboratory tests on undisturbed samples of slopewash, analyses of seepage through the embankment and foundation, finite element analyses of stresses in the dam during construction and operation of the reservoir, and conventional equilibrium slope stability analyses.

The laboratory tests showed that the shear strength of the slopewash decreases very quickly when the desiccated material is wetted. Wetting causes immediate reduction in shear strength to the fully softened value, and there is no lasting effect of consolidation by drying. After wetting the highly desiccated slopewash has the same strength as in the remolded, normally consolidated condition.

Tests that simulated cyclic changes in normal stress and shear stress like those during drawdown and refilling of the reservoir showed that further strength loss results from cyclic loading of the slopewash. Cyclic loading at stress levels below the fully softened peak strength result in continual shear displacement, and eventually, when the cumulative horizontal displacement reaches approximately ten inches, the shear strength is reduced to its residual value.