Three-Dimensional Lower and Upper Bound Analyses of Pile Systems

The objective of this paper is to present simplified methods for determining the three-dimensional (3D) system capacities of pile foundations based on the lower and upper bound theorems of plasticity. The failure surface for a typical long offshore pile subjected to 3D head loads is derived based on the lower bound (LB) theorem of plasticity, and the LB capacity of a pile system is then proposed. Using the failure mechanism determined from the elastic solution in the LB analysis of a pile system, the upper bound (UB) capacity of the pile system is obtained from the energy-balancing equation. An actual offshore platform is selected as a case study to demonstrate the proposed methods. The current methods extend the planar limit analysis by incorporating global torsion and out-of-plane failures and can be useful in designing and assessing pile systems.