Large-Deformation Simulation of the 1971 Lower San Fernando Dam Flow Slide Using the Material Point Method

A finite difference method (FDM) to material point method (MPM) transitioning model is applied to the well- known case study of the Lower San Fernando Dam (LSFD) failure during the 1971 San Fernando earthquake. This model uses an effective stress analysis in FDM during shaking, then a post-shaking total stress analysis in MPM to simulate undrained largedeformation response. Two advances in the Berkeley Geomechanics MPM code used for this case study are a nonconforming traction boundary for hydrostatic pressure and an adhesion boundary for modeling the interaction between the sliding mass and the reservoir bottom. The MPM runout model captures large deformation failure characteristics. Important quantitative metrics include remaining freeboard, crest loss, and upstream runout into the reservoir. Important qualitative aspects include shear banding, blocky features, and toe mass separation and bulging.