Investigation of dynamic soil – pile – structure interaction in clayey soils by numerical analysis

The response of a pile-supported structures under seismic loads is generally influenced by the interaction between the surrounding soil, the supporting piles, and the structure itself (soil-foundation-structure interaction, "SFSI"). These interaction effects are typically classified into two distinct categories: inertial and kinematic. During an earthquake, the soil medium vibrates the piles and the superstructure, and when inertial interaction is significant, the accelerated/mobilized masses of the piles and superstructure shake the soil back. Kinematic interaction, on the other hand, is the collective term given to the effects of the rigidity of piles, which usually is significantly larger than that of the soil, leading to alterations in the incoming seismic waves. In this study, soil-pile-structure interaction is investigated through simulations utilizing the two-dimensional (plane-strain) finite element method (FEM). Initially, the established models are verified by a site response analysis involving elastic behavior, which is then compared to the analytical closed-form solution suggested for kinematic interaction in literature. Subsequently, parametric studies are performed. The scope of study are: (i) the kinematic interaction between soil and pile is investigated through numerical methods considering effective parameters such as the material model for soil (linear and nonlinear), the stiffness ratio between pile and soil (Ep/Es), and the slenderness ratio (L/D: pile length over its diameter); (ii) the effect of variation in phase angle of the soil's movement, which is omitted in prior studies focusing only on amplitudes, is scrutinized and a new kinematic interaction factor that considers the phase angle variations is proposed; (iii) the complete soil-pile-structure model is investigated considering elasto-plastic soil behavior in time domain. Additionally, foundation input motion at base level of the superstructure and the pile response under seismic event are examined. The important findings of this research are: (i) Nonlinear soil behavior substantially influences the kinematic interaction. The elasto-plastic properties of soil reduce the seismic motion effect, especially in terms of acceleration. On the other hand, piles have a linear behavior increasing the outcomes. Consequently, kinematic interaction factor in terms of acceleration (Ia) is larger than unity. (ii) The inertial interaction due to superstructure vibration is more dominant on the pile response; however, the kinematic interaction should not be neglected since it increases the bending moment acting on pile. (iii) The presence of piles in the ground creates a relatively stiffer area below the superstructure. This condition may increase the forces acting on the superstructure.