Fully-coupled earthquake response analysis of earth dams using a critical state sand model

Current approaches in dealing with flow liquefaction and post-liquefaction deformation problems are essentially empirical and semi-empirical in nature. While these methods reflect the current state-of-the-art, in-depth understanding of these issues is still limited and further research on these issues is necessary. Research on liquefaction-related issues is characterized by multifarious fronts. Sophisticated analytical procedures are always helpful. One of the major obstacles to such procedures, however, is the limited simulative capability of the constitutive soil models, which ideally should be able to reproduce all essential stress-strain-strength responses of granular soils during the entire process of earthquake. A two-dimensional fully-coupled dynamic finite element procedure has been developed to evaluate the response of embankment dams subjected to earthquake loading and to investigate the underlying mechanisms of flow liquefaction. The procedure is formulated based on the physical laws of balance of momenta and conservation of mass, and on separately defined constitutive laws for soil skeleton, pore fluid, and the interaction between the two phases, respectively. The validity of the procedure is confirmed by comparing the numerical results and the close form solution of a one-dimensional saturated porous elastic dynamic problem. A sand model based on the concept of state-dependent dilatancy, which is consistent with the framework of critical soil mechanics, is built into the procedure to describe the stress-strain-strength behavior of granular soils over the full range of loading conditions encountered during an earthquake. This model, in which the soil dilatancy depends on the state of stress as well as the present internal state of material, can handle both flow liquefaction and cyclic mobility responses in a unified manner. Two series of fully-coupled analyses on the Upper and Lower San Fernando Dams are conducted. A number of factors affecting the flow deformation of embankment dams, including the seepage force, the relationship between the residual strength of the liquefiable soil and the driving force acing on it, the soil conditions of berm and foundation, the soil density, the intensity of input motion, and the artificial rigid side boundaries, are investigated in detail, and their effects are addressed. Remediation measures to improve the seismic performance of the San Fernando Dams are discussed, and the effectiveness of the proposed remediation strategies is numerically verified