Dynamic soil stiffness in torsional vibrations of a floating pile

Abstract

An analysis is presented here to study the soil impedance on the torsional vibrations of a loating pile. It is a continuation of an earlier study by the authors, of the vertical vibrations of the pile. As in the latter analysis, the soil is assumed to be a homogneous elastic half space and the slenderness aspect of the pile is used to assume the pile action to be due to a line distribution of couples along the pile axis. This leads to a linear equation for the torsional couple acting along the length of the pile, in terms of its rotation. The expression for the consequent dynamic soil stiffness is a modification of the one given by Novak and Howell. Whereas the latter expression assumes that only horizontally propagating torsional waves are generated by the vibrating pile, the present analysis incorporates axisymmetric waves generated by the pile of finite length. When the pile length is made infinite, the present result leads to the expression of Novak and Howell. A numerical study of the expression shows the possibility of significant departure in values of the stiffness at low frequencies. By the present analysis, the variable stiffness, as opposed to uniform in the Novak and Howell formula, can be computed along the length of the pile, save for small portions near the ends.