Nonlinear analysis of a drilled pier-soil system under static and dynamic axial loading

Realistic time history simulation of drilled pier/pile-soil system under static and dynamic loading is essential for the development of effective performance based earthquake design of deep foundations. Due to the nonlinear transient nature of the system and high computational requirements, to-date the application of nonlinear finite element analyses to this problem has been limited. Thus, an important aspect of numerical modeling is a soil model that is simple enough to be computationally efficient, yet able to capture the cyclic stress strain behavior. In particular, it is essential to account for the modulus degradation and energy dissipation characteristics during cyclic loading, which depends on the rate of loading and soil properties. In this study, a multi-axial cyclic bounding surface plasticity model proposed by Borja and Amies (1994) is developed in a general finite element framework called OpenSees ©. The model requires a minimal number of parameters that can be easily obtained through conventional site investigations. The stress point algorithm of this model is formulated in detail for finite element implementation. It is shown that the model can reasonably capture modulus degradation and hysteretic damping of nonlinear soil, and it is suitable for fully nonlinear analysis of soil-structure interaction as well as site specific response analysis. The model is used to study load-displacement-capacity characteristics of an axially loaded pier-soil system at small and large strains by simulating a set of in-situ static and dynamic axial load tests on drilled piers performed at UC Berkeley. The numerical results are in excellent agreement with the field data and show that fully coupled nonlinear soil-structure interaction analyses can be successfully performed.