Torsional piles in non-homogeneous media

Abstract

Abstract The torsional response of a pile exhibits features which are a mixture of those for axial and lateral response. At low load levels, the response is dominated by interaction with the upper soil layers and by the pile rigidity itself, similar to laterally loaded piles. However, failure will generally occur by the whole pile twisting, and so the latter part of the response incorporates the integrated effect of all soil penetrated by the pile, as is the case for axial loading. In view of the above, solutions for the torsional response of pile must endeavour to incorporate accurate modelling of the soil stiffness profile, and also pay appropriate attention to the gradual development of slip (relative twist) between pile and soil. The paper presents analytical and numerical solutions for the torsional response of piles embedded in non-homogeneous soil, where the stiffness profile follows a simple power law with depth. The solutions encompass: (1) vertical non-homogeneity of soil expressed as a power law; (2) non-linear soil response, modelled using a hyperbolic stressstrain law; (3) effect of relative slip between pile and soil for non-homogeneous stiffness and limiting shaft friction; (4) expressions for the critical pile slenderness ratio (or length) beyond which the pile head response becomes independent of the pile length. The solutions are developed using a load transfer approach, with each soil layer acting independently from neighbouring layers, and are expressed in terms of Bessel functions of non-integer order, and as simple non-dimensionalised charts. The solutions are applied to two well-documented case histories in the latter part of the paper.