The 1981 upstream slope failure at B.F. Sisk Dam: Narrative, cause, and remediation of a dam safety incident at one of California’s largest reservoirs

ABSTRACT ONLY - B.F. Sisk Dam is an 18,600-foot long, 382-foot high earthfill embankment constructed by the Bureau of Reclamation between 1963 and 1967. The dam is located west of Los Banos, CA, and impounds a 2 million acre-foot off-stream reservoir that forms a major feature of the Central Valley Project. In September 1981, a 1700-foot long section of the upstream slope failed during a period of rapid reservoir drawdown (up to 2 feet per day) caused by an unplanned outage on the California Aqueduct. The slide occurred over a period of two months, resulting in a headscarp that came within about 35 feet of the upstream shoulder of the dam crest. By the time that movement had ceased in December 1981, the total vertical displacement of the slide mass had exceeded 40 feet, and over 1.5 million cubic yards of embankment and foundation material had been mobilized. The slide is thought to have resulted from a combination of high pore pressures and low residual strengths in the clayey slopewash material beneath the upstream shell of the embankment. This material had apparently been left in place because it was quite durable prior to construction, and because removing it would have resulted in cost and schedule impacts. Reportedly, some of the geology personnel in the field had postulated that the slopewash could weaken upon wetting, but there was no consensus about whether that could impact the stability of the dam. Although it did not exist at the time, the risk informed dam safety decision process used by Reclamation today is well suited for incorporating such information and evaluating its potential significance. This presentation gives an overview of the extent and timeline of the slope failure, provides some analysis insights into why the slide occurred, and explains how the damage was ultimately repaired. The presentation also provides a preview of the upcoming seismic modification of B.F. Sisk Dam, one of the major remediation projects currently occurring at Reclamation.