Modeling large-deformation features of the Lower San Fernando Dam failure with the Material Point Method

Abstract

The Material Point Method (MPM) is used to model the well-known case study of the earthquake-induced Lower San Fernando Dam failure. This upstream flow slide, leaving only 1–2 m freeboard, emphasized the importance of seismic hazard analyses of earthen dams. This study expands on the Chowdhury (2019) nonlinear seismic deformation analysis, which successfully captured liquefaction initiation and engineering failure characteristics. It could not, however, fully capture the large-deformation behavior and post-failure equilibrium cross-section due to mesh entanglement limitations of the Finite Difference Method. This MPM model initializes post-shaking and simulates the large-deformation undrained behavior of both liquefied sands and shear-softened clays. Several novel features implemented in the MPM advance modeling capabilities, including a nonconforming traction boundary to model reservoir pressure, and an adhesion boundary to model the reservoir bottom sliding interface. Compared to previous studies, the current study predicts improved kinematic behavior and post-failure cross-sectional features. Qualitative highlights include well-defined shear banding, sharp heel scarp features, blocky features, a toe bulge, and shear separation of the leading slide mass. Quantitative highlights include 1.5 m remaining freeboard, 20 m crest loss, and 66 m upstream toe runout, versus field observations of 1.2–1.8, 20, and 65 m, respectively.