Nonlinear dynamic behavior of pile foundations: Effects of separation at the soil-pile interface

During strong ground motion, pile foundations are subjected to two special effects. First, behavior of the soil surrounding the piles is nonlinear. Second, large inertial forces are generated in the soil around the pile heads, causing separation between the soil and pile. In this paper, a new approach is presented to overcome material nonlinearity of the soil as well as geometrical nonlinearity arising due to separation. The analysis is performed in two steps. To account for material nonlinearity, equivalent linearization is used in conjunction with a hyperbolic model of the soil. The hyperbolic model defines the nonlinear stress-strain relationship of the soil. To deal with separation, a Winkler soil model is used. The dynamic stiffness reproduced by the soil model is changed according to the degree of separation. Depending on the level of excitation, different cases of separation arise which are investigated with skeleton curves. It has been found that due to separation, dynamic response of the soil-pile system increases whereas the dynamic stiffness decreases significantly.