Non-linear analysis of pile groups under general loading conditions
A computer program (called PGROUPN) has been developed for the non-linear analysis of pile groups subjected to any combination of vertical loads, horizontal loads and moments. The code is based on a complete boundary element approach and may be regarded as a generic descendant of the program PGROUP (Banerjee & Driscoll, 1976) which has been extended in order to make the analysis numerically efficient for practical problems and to include the effects of soil nonlinearity by means of a stepwise linear incremental procedure. One of the main advantages of a non-linear analysis system over a linear elastic approach is that it has the desirable effect of demonstrating a reduction of the corner loads in larger groups in both the vertical and horizontal senses. This observation is of basic importance in practice because it offers the prospect of tangible improvements in design procedures and potential saving of materials. The non-linear analysis put forward in this thesis may be applied to large pile groups embedded in cohesive soil, specifically fully saturated clay under undrained conditions. The soil is modelled as an elastic-perfectly plastic material, which is assumed to behave linearly elastic at small strain levels, but fails when the stresses at the pile-soil interface reach certain limiting values. The analysis only requires the definition of three soil parameters whose physical meanings are clear, ie the (initial tangent) Young’s modulus Es, the Poisson’s ratio vs and the undrained shear strength Cu. This represents a significant advantage over more common load-transfer approaches which are based on either empirical parameters or the results of full-scale pile load tests. The validity and accuracy of the proposed PGROUPN solution have been verified by comparison with alternative numerical analysis for single piles and pile groups subjected to axial and lateral loads. Benchmark solutions in the linear and non-linear range are presented, and the critical question of estimation of soil parameters is addressed. Finally, two published case histories are described which demonstrate the applicability of the method to practical problems.