Two-directional effects in seismic soil-pile-structure interaction in soft clay

The evaluation of the performance of deep foundation systems under seismic loading is a key issue during the design of important structures located in earthquake regions. With few exceptions, the analysis of seismic soil-pile-structure interaction (SSPSI) problems is oversimplified and the effects of two-directional shaking are ignored. Nevertheless, two-directional shaking may significantly increase the amount of non-linear behavior in the near field, intermediate and free field domain. In the near field, two-directional lateral movements of the pile can cause radial degradation of the soil stiffness, and can create radial gapping. This effect reduces the confinement of the pile, leading to an increase in bending moments on the structural members. In the intermediate and free field domains, the generation of pore pressure due to two-directional loading can reduce the soil stiffness and increase the permanent deformations.; For the free field, a simplified plastic constitutive model for two-directional site response analyses was described. The model can simulate hysteretic soil behavior as well as the accumulation of plastic deformation due to cyclic loading. The effect of the generation of pore pressure in the soil stiffness degradation is taken into account directly in the formulation. The constitutive model was implemented in a three-dimensional 8noded brick finite element. This dissertation includes the demonstration of the predictive capabilities of the proposed model through the analyses of some case histories.; A new testing methodology is proposed to gain insight into the multidirectional p-y behavior of the near field is introduced. In particular, the research effort focuses on the characterization of the soil-pile interface response for multidirectional pile trajectories, using model clay as a testing material. The model clay is an artificial mixture created to reproduce the mechanical properties of San Francisco Bay Mud for the 1-g physical modeling test in the large shaking table. The multi-directional pile transit paths considered during the testing program included circular, elliptical parabolic, and figure-8. The test results obtained from these tests fill the gap in experimental information regarding multi-directional p-y curves for soft clays. These tests provide information regarding the force-displacement relationship dependence on the loading path, the evolution of the soil stiffness during multi-directional cyclic loading, and the effect of loading history on the near field soil stiffness. The experimental data gathered were used to formulate, calibrate and validate a simple discrete element model to describe the response of clay-pile interfaces to multidirectional loading paths.