KINEMATIC SEISMIC RESPONSE AND BENDING OF FREE-HEAD PILES IN LAYERED SOIL

The paper studies the kinematic response of free-head piles. Such pile deformation has triggered structural damage in many strong earthquake s. In this Paper dimensionless parametric graphs for pile bending mome nts are presented which pertain to characteristic two-layer soil profi les. The results are derived by using an existing rigorous dynamic fin ite-element code, and by implementing a realistic beam-on-dynamic-Wink ler-foundation formulation specifically developed for the kinematic re sponse of piles in layered soil. The Winkler model is shown to reprodu ce quantitatively even detailed trends observed in the finite-element results; a simple analytical expression is thereby developed for estim ating the Winkler stiffness in terms of the local soil Young's modulus and key dimensionless pile/soil parameters. The study concludes that even relatively flexible piles may not exactly experience the wavy and abruptly changing ground deformation of the free field. The critical region of pile distress due to such kinematic loading is shown to be a t or near the interface between alternating soft and stiff soil layers . The magnitude of the bending moment at such critical interface locat ions depends mainly on the stiffness contrast of the two layers throug h which the pile penetrates, the excitation frequency and the relative rigidity of the pile. A constraining cap may exert an important effec t on such kinematic deformations.