San Luis Dam Case History: Seepage and Slope Stability Analyses and Lessons Learned

Rapid drawdown is an important condition controlling the design of the upstream slope in embankment dams and levees. This paper presents the upstream slope failure at San Luis Dam as a case study for performing effective stress drawdown stability analyses. In effective stress analyses, stress-dependent drained shear strengths were used for the fine-grained core and slopewash materials. The seepage-induced pore-water pressures were estimated in the slopewash and fine-grained core by first calibrating the saturated and unsaturated hydraulic parameters with piezometers installed after the slide. The shear-induced pore-water pressures due to the changes in total stress were estimated from Skempton's A coefficient. The stability analysis methodology incorporates stress-dependent drained shear strength envelopes and shear-induced pore-water pressures to determine the factor of safety (FS). The FS determined at various reservoir levels and slopewash shear strengths are used to provide recommendations for calibrating unsaturated soil properties, integrating seepage-induced, and shear-induced pore-water pressures in stability analyses, and use of commercial software for drawdown stability analyses.