Keywords: soil-pile-structure interaction, inertial interaction, radiation damping, soil-pile gap.

Soil-Pile-Structure Interaction (SPSI) has an important effect in the dynamic analysis and seismic design of massive or stiff structures and pile foundations. The effects of the inertial interaction on seismic structural response and hence the performance of the pile foundations are presented. Using a three-dimensional finite element model of a structure supported on a single pile foundation, both linear and nonlinear analyses in the time domain are performed to provide an accurate method for assessing the seismic performance of a soil-pile-structure system with inertial interaction included. The Drucker-Prager soil plasticity model is employed to represent the nonlinear behavior of the soil and an infinite element boundary condition is used to simulate radiation damping. Interface elements are introduced to consider soil-pile gapping and slippage caused by nonlinear soil behavior and the development of plastic strains. Both harmonic and specific seismic excitations are considered.

The dynamic characteristics of a soil-pile-structure system are studied and the dynamic performance of a single pile foundation and a superstructure simplified as a lumped mass due to a harmonic excitation sweep is presented to illustrate the methodology. An uncoupled analysis and a fully coupled analysis are used to compare the response of the soil, pile foundation and superstructure. The proposed model is validated against available experimental data and existing results of numerical analyses.

Two extreme cases of the lumped mass are considered to represent flexible and stiff structures. Natural frequency extraction for two systems is performed first with the consideration of the effects of separation between the pile and soil. This study shows the contribution of the superstructure to the entire system is small. Then the effects of inertial interaction on the dynamic response of the soil-pile-structure system are verified by a harmonic excitation sweep analysis. The dynamic responses including acceleration, soil displacement and structural relative displacement from uncoupled analysis and coupled analysis are compared and it is found out coupled analysis is necessary for an accurate result. Then the effects of soil-pile gapping and nonlinear soil behavior on a soil-pile-structure system under harmonic excitation are studied, respectively. On the one hand, the separation between the pile and soil decreases the energy transmitted from the soil-pile system to the superstructure. On the other hand, the structural relative displacement is increased dramatically. With the consideration of soil nonlinear behavior as well, it is found that the effects of soil nonlinear are insignificant to the dynamic response of the entire system. Finally, the seismic response of a soil-pile- structure system under El Centro excitation is illustrated. This study further verifies the energy filter effects of soil-pile separation on the seismic response of the superstructure. It is shown that the effects of soil plastic behavior are significant on the soil-pile gap but small on the structural acceleration response.

This study shows that inertial interaction in seismic SPSI can be significant. It is found that the soil-pile foundation system plays a critical role in structural seismic response. The comparison of uncoupled and fully coupled analyses suggests that a fully coupled analysis is necessary in many cases to obtain accurate results. The effects of soil-pile separation are significant on structural relative displacement but small on structural acceleration response. Soil plastic behavior has local effects only.

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