Re-Evaluation of the Lower San Fernando Dam: A Seismic Performance Case History Involving Liquefaction Flow Failure Conditions

Seismic performance of earthen dams subject to potential liquefaction hazard is of concern in both seismic risk assessment and design mitigation for dams. Seismic deformation analyses are increasingly utilized in evaluating and mitigating these hazards. It is important to assess different types of analytical approaches by means of their application to well-documented field case histories. The performance of the Lower San Fernando Dam (LSFD) during the 1971 San Fernando earthquake is a well-documented seismic performance field case history involving a liquefaction-induced flow slide failure of the upstream side of the embankment. The dam was in close proximity to the fault rupture during the M_W≈6.7 event, and it was strongly shaken. The dam had 11.1 m (36 ft) of freeboard prior to the earthquake, and the nearly catastrophic upstream slide reduced freeboard to within approximately 1.5 to 1.8 m (5 to 6 ft) from overtopping. This unusually well-documented field performance case history has been foundational to the development of the field of modern seismic dam engineering, and to the inception of the U.S. national seismic dam safety programs ongoing today. It is important to periodically assess the strengths and reliability of analytical models and approaches employed in engineering analyses of these types of critical facilities. The LSFD case history was re-analyzed in these current studies by means of a series of fully nonlinear seismic deformation analyses, employing (1) a suite of four different analytical models, (2) a suite of three cyclic pore pressure generation models, and (3) a suite of three post-liquefaction residual strength relationships.