Re-examination of Slide of Lower San Fernando Dam

A major slide occurred in the upstream slope of the Lower San Fernando Dam immediately following the earthquake of February 9, 1971. Previous analyses of this slide have been based on evaluation of seismic shear stresses and the results of cyclic triaxial tests. These previous analyses fail to explain several key features of the slide. The re‐analysis presented herein, which is based on the concepts of liquefaction, steady state strength, and flow slide stability analysis, and on the data obtained during the previous analyses, is consistent with the principal features of the observed slide. The estimated in situ undrained steady state strength is substantially lower than the undrained strength measured in conventional laboratory tests due to the sample densification that normally occurs during sampling, handling, setup, and consolidation of loose sandy soils. An interpretation of the limited data on undrained strength available for the San Fernando Dam indicates that the soils that liquefied had very low undrained steady state strengths.