Rigorous lower and upper bounds for the generalised failure loads of pile groups

A renewed philosophy for the ultimate limit-state design of pile groups is based on the adoption of multiaxial strength envelopes. In this view, this study proposes a limit analysis-based framework providing high-fidelity predictions for the generalised failure loads of piled foundations. This is accomplished through: (a) the development of an upper-bound solution that better approximates the exact solution compared to existing ones and (b) the integration of a recently developed lower-bound solution with a formulation accounting for pile-soil-pile interaction at failure. The comparison between these complementary methods proved to be satisfactory on many load combinations. The proposed framework is readily applicable in design, at a minimal computational effort, and in a more refined assessment of structures, as it provides the ultimate limit-state surface for a plasticity-based macroelement representation of deep foundations. The framework is used herein to investigate the horizontal efficiency of a well-documented piled foundation, and theoretical predictions are compared with results of fully coupled numerical analyses. The resulting strength envelopes point out the effect of the vertical load on the mutual interaction between piles, which is an indicator of the contribution of the pile's axial capacity in the activation of multiaxial failure modes.