1971 Failure of Lower San Fernando Dam, CA

The year 2021 marks the 50th anniversary of the failure of the upstream slope of Lower San Fernando Dam (LSF Dam, see Figure 1). A major slide developed about 40 seconds after shaking stopped from the 1971 San Fernando earthquake. There were no fatalities associated with the dam failure but over 80,000 people were evacuated until the reservoir was lowered to a safe level. Seismoscopes located on the abutment and on the crest of the embankment indicated that peak accelerations of about 0.55 to 0.6 g had occurred. The slide was a result of liquefaction of a zone of hydraulic sand fill near the base of the upstream shell of the dam. Several extensive investigations of the slide have been performed by Seed et al. (1973, 1975a), Lee et al. (1975), and Castro et al. (1989, 1992). The failure continues to be evaluated by various investigators working to establish the undrained residual (also referred to as steady state) shear strength of the hydraulic fill (e.g., Weber, 2015).
This case history reviews the construction of the dam, recounts the earthquake and failure event, summarizes various investigations of the failure, and notes important lessons that have been learned. Estimates of the undrained strength of the contractive hydraulic fill materials from several investigators are presented and reviewed relative to alternative in-situ and laboratory methods available to assess undrained steady state strength.