SOIL-PILE KINEMATIC INTERACTION: EFFECT OF A MIDWAY LAYER IN AN HOMOGENEOUS DEPOSIT

In last decades an increasing attention has been devoted to kinematic soil-pile interaction. Pile damage observed during recent earthquakes can be explained only if kinematic interaction is added to traditional inertial interaction. Many Technical Regulations prescribe to take into account both the types of interactions in relations to: soil conditions, area seismicity, importance of structures. Thanks to the “method of substructures” it is possible to study the effects of both kinematic and inertial interactions separately and then considering appropriately their overlap. But, inertial interaction has been studied from a very long time; while kinematic interaction has been studied from a few years. The aim of this paper is to analyze soil-pile kinematic interaction for a pile embedded in a homogeneous soil deposit within a midway layer of different stiffness exist. The majority of studies on kinematic interaction have been generally related only to soil profiles with one stiffness discontinuity. This paper is part of a research activity, carried out by the Authors for three years, which is devoted to investigate on soil-pile kinematic interaction for piles embedded in a soil deposit constituted by two or more stiffness discontinuities or by a stiffness that increases linearly with depth. In this paper, the soil-pile system is studied by a 3-D FEM transient-dynamic analysis; kinematic bending moments, which occur along the whole pile and in particular at the depths of the soil stiffness discontinuities and at the pile head, are estimated, for different values of thickness and stiffness of the midway layer. The system, characterized by a linear-elastic behavior, is subjected to different accelerograms, scaled up to an amplitude equal to 1 m/s2, and applied at the base of the model (bedrock). The results show that to not discover thin midway layers, during the soil characterization phase, can lead to a severe underestimation of bending moments at the soil discontinuity depths and at the pile head. An important role is also played by stiffness contrast of soil layers, midway layer thickness and frequency content of input motion.