Numerical modeling of dynamic soil-pile-structure interaction

Surendran Balendra

The analysis of structures subject to earthquake ground motions must properly account for the interaction between the foundation and the superstructure. The passage of seismic waves through the foundation affects the ground motion at the base of the structure and generates stresses on foundation elements. This effect is termed kinematic interaction and its effects on the ground motion are described by a function termed the transfer function. On the other hand, the response of a structure is a function of the foundation compliance, and, in turn, inertial forces resulting from structural response affect the stresses on foundation elements. This interaction is termed inertial interaction and is captured by representing the foundation through an impedance function. In this study, numerical models using ABAQUS were developed to study both inertial and kinematic effects. The focus of this study was to perform parametric studies on the various variables that affect kinematic transfer functions and inertial impedance functions of pile foundations. The independent variables of this parametric study were material nonlinearity, soil-pile separation, pile diameter, intensity of the input motion, and the inertial force magnitude. A bounding surface plasticity soil model is used in this study to model soil nonlinearity. Issues related to the numerical modeling of soil-pile-structure interaction are discussed at length, including the application of dynamic loading as a shear stress time history, the development of user-defined pile-soil interface models, and the treatment of infinite and absorbing boundaries for the lateral and bottom boundaries, respectively. The model was validated by comparison with analytical solutions and previously published results. Results for a fixed head single pile in a plastic soil show that soil nonlinearity reduces the amplitude of the transfer function significantly for high frequencies; however, the intensity of the ground motion does not affect significantly the kinematic transfer function. Soil-pile separation has no effect on the kinematic transfer functions but has a considerable effect on the impedance function. Normal stress due to kinematic effects attains a maximum when the loading frequency coincides with the frequency corresponding to the resonant frequency of the soil column (e.g., the site frequency). The maximum kinematic stress on flexible piles occurs at the depth of a stiffness contrast between soil layers. For rigid piles, maximum kinematic stresses occur at the pile head. The presence of an interface between hard and soft soil has no effect on stresses due to inertial interaction for flexible piles and has considerable effect on inertial stresses for rigid piles. Soil-pile separation results in an increase of both kinematic and inertial stresses in the pile.

Numerical modeling of dynamic soil-pile-structure interaction

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