Dynamic Localized Failure of Soils via Nonlocal Poromechanics Model: A Case Study of the Lower San Fernando Dam Failure

In this article, a recently developed nonlocal poromechanics model for geomaterials is applied to analyze the dynamic response of the Lower San Fernando Dam during the 1971 San Fernando earthquake. During the earthquake, the dam experienced a major flow failure in the upstream slope. Field investigations in the immediate aftermath indicated that the horizontal movement in the upper slope of the San Fernando Dam was caused by the liquefaction and weakening of the hydraulic fill materials in certain zones in the dam. Large blocks of mostly intact soil from the upstream section of the dam moved into the reservoir, riding over on the liquefied soil. Field analysis suggested that a significant pressure buildup occurred in the central region of the dam due to cyclic loading that later migrated to the upstream “toe” where it triggered failure and subsequently a sliding flow. To demonstrate that the proposed nonlocal poromechanics is realistic and relevant to the solution of large-scale, nonlinear, and coupled analysis, we conduct a two-stage seismic analysis of the Lower San Fernando Dam. First, an elastic analysis under quasi-static condition was carried out to determine initial effective stress and water pressure distributions. Second, a fully nonlinear dynamic analysis was performed with the earthquake load modeled as a base excitation following recorded acceleration profiles. The results have demonstrated that the proposed nonlocal poromechanics model is robust in modeling the localized failure of geomaterials under seismic loading by capturing the location and mode of failure observed in the field.