Seismic Deformation Analysis for Risk Assessment of Embankment Dams

Several failure modes, usually included in the risk analysis of an embankment dam, are related to seismic loading and include: overtopping due to embankment settlement, above crest erosion, seepage erosion through transverse cracks, and piping into a rupture of the outlet works system. All these failure modes are aggravated when the foundation soil is potentially liquefiable under possible earthquake loading. Success Dam and the Auxiliary Dam of the Isabella Lake, both in California have recently been evaluated for seismic loading and seismic risk. The dams are founded on liquefiable alluvium deposits and in one case the site has a seismically active fault that transects the dam. Seismic deformation analyses for the risk assessment were performed using the computer program FLAC and the liquefaction model UBCSAND (developed at the University of British Columbia, Canada and modified for better modeling of the liquefiable dam foundations by Dr. Michael Beaty). Correlation relationships were determined between the intensity of shaking (defined by the peak ground acceleration) and embankment deformations, in particular the crest settlement and the horizontal displacement of the slopes. The results were presented in a format adequate for easy implementation into the risk evaluation model.