Role of soil inhomogeneity in pile radiation damping for SSI applications

Most studies of pile response to dynamic head loads are restricted to homogeneous or horizontallylayered soil. This work considers a soil medium, which is inhomogeneous both vertically (with respect to depth) as well as in the horizontal direction (radial distance from pile centreline). Vertical heterogeneity typically arises from sedimentation of soil particles and from overburden effects. On the other hand, radial inhomogeneity may arise from soil disturbance due to pile installation and from material nonlinearity due to loading effects. In this study, the problem is treated in the realm of linear elastodynamic theory by means of a rigorous finite-element formulation developed by the Authors. The study is concerned with the effect of this double soil inhomogeneity on: (1) pile head stiffness, (2) pile radiation damping, (3) modulus of equivalent Winkler springs and dashpots along the pile shaft. Swaying, rocking, and vertical oscillations are considered, arising from concentrated harmonic forces and moments imposed at the pile head. It is shown that ignoring soil inhomogeneity may substantially underestimate pile head stiffness and overestimate damping. On the other hand, modelling a vertically inhomogeneous deposit as a stiffness-equivalent homogeneous layer may grossly overestimate radiation damping leading to unconservative predictions of dynamic pile movements. Implications of radiation damping effects in design are discussed.