4.2.1 San Luis Dam

The San Luis Dam (now called the B.F. Sisk Dam) was completed in August 1967. It was built on the Los Banos Creek and is used primarily for agricultural storage for the South Valley farmers. In 1981, on September 4, workers noticed rocks sliding down the upstream face of the dam. The slide occurred in deep seated failure and the failure surface extended through the engineered zones and into the native slopewash material. Investigation of the slide led to the conclusion that the wrong strength parameters for the slopewash material were used for stability analyses.

The slopewash material is deposited primarily by rain dragging small clay particles downslope from the surrounding hills. The construction specifications for excavation to the foundation were that the soil had to be stronger than the fill to be placed over it. In California’s Central Valley, the temperatures easily reach over 100 degrees for days at a time, causing significant evaporation losses in surface soils. In its desiccated state, the slopewash material exhibited strength characteristics that would seem desirable for a dam foundation. However, after extensive post failure laboratory testing on the slopewash material was performed, it was determined that the residual strength, and not the previously used “fully softened” strength should be used in stability analyses.

Fortunately, no one was hurt, and no water was lost the slide occurred. The engineering lesson to be learned from this event is that the engineer should have a good understanding of the behavior of the soil zones influenced by the dam under fully inundated conditions as well as the cyclic response of the soil mass due to drawdown.

Deep seated failure of San Luis Dam, Photo by Duncan

Slopewash: The action of water from rain or melted snow carrying (washing) soil down a slope.

Resource:
- Karl Schwartz, The 1981 Slide at the San Luis (Sisk) Dam, Dam Case History report, 2.06, 2008
- Boulanger, Ross, San Luis Dam Slide, http://cee.ucdavis.edu/faculty/boulanger/geo_photo_album/
- Duncan, J.M., & Stark, T.D. (1989), The Causes of the 1981 Slide in San Luis Dam, Raleigh, North Carolina: Dept. of Civil Engineering, Virginia Polytechnic Institute.