Reliability Assessment of the San Fernando Dam During the Northridge Earthquake

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9416469 Lacy During the Northridge, CA earthquake, the upstream face of the Lower San Fernando dam spread laterally upstream several feet. Surface cracks and sand boils developed. The documentation of the behavior is during of this during the Northridge earthquake provides an opportunity to validate an existing state-of-the-art method to analyze the seismic stability of earth dams, including the prediction of liquefaction and permanent displacements. Data collected at the Lower San Fernando dam during the Northridge earthquake will be used to assess the reliability of the dam during and after an earthquake event. This will be achieved by generating a sequence of possible realizations of the ground motions and soil properties and analyzing each deterministically using a general two-phase nonlinear finite element procedure. The ground motion at the base of the dam is simulated according to a spectral content derived from measurements at nearby bedrock sites. The material properties pertinent to the occurrence of liquefaction and strength are treated as spatially varying random fields with mean, variance and, where possible, spatial correlation structure derived from actual measurements at the site. The saturated soil in these problems is treated as a two phase continuum of soil skeleton and pore fluid. the porewater and soil skeleton behavior are fully coupled, thereby accounting for the interaction between the two media as the soil deforms. The nonlinear elasto-plastic behavior of the soil skeleton is modeled by multi-yield surface plasticity. for each analysis, time histories of soil response are produced, including accelerations, displacements and pore water pressures. The resulting sequence of soil responses is processed statistically to yield estimates of reliability of the earth dam system. ***