NUMERICAL PROCEDURES FOR NONLINEAR TRANSIENT ANALYSIS OF TWO-PHASE SOIL SYSTEMS (GEOTECHNICAL, FINITE-ELEMENT METHOD)

A general and efficient numerical procedure is proposed for analysis of the dynamic response of geotechnical structures, which are considered as both nonlinear and two phase systems. The equations for two phase soil structure pore fluid systems are reviewed and the appropriate coupled dynamic field equations and boundary conditions for the response of a soil system are developed. Then, the finite element spatial discretization of the field equations is described and time integration for the resulting nonlinear second order semidiscrete equations is discussed. A constitutive model for geomaterials is developed. The model is based on a multi-yield surface plasticity theory and includes two plasticity mechanisms, the first controlling yielding in shear and the second yielding under hydrostatic compression. A stress-point algorithm is developed for the integration of the resulting nonlinear constitutive equations. A large amount of computational effort is expended in the iterative phase of the solution of the global nonlinear system of finite element equations. The performance of three iterative procedures is examined here: Newton Raphson, Modified Newton Raphson and Quasi-Newton methods, including BFGS and Broyden updates. A procedure for locating the free surface in partly saturated soil structures is developed. Finally, the tools developed are used with a finite element computer program to solve the elasto-plastic earthquake response of a two phase nonhomogeneous earth dam. The results of the numerical calculations are compared to the recorded response of the dam.