Analytical Formulation of the Ultimate Resistance for Piles in Undrained Clays Based on Threefold Failure Mechanisms

Large-diameter piles (commonly called monopiles) are extensively used to support offshore wind turbines (OWTs) for withstanding large lateral and overturning moments. Results from monotonic tests suggested that the design using the current p–y methodology in clay significantly underestimates the pile stiffness. To determine a rational formulation of the undrained clay ultimate resistance on a theoretical sound basis, a plethora of published papers on the ultimate resistance of piles in undrained clays p_u were examined first. It has been found that all proposed methods for p_u were constructed on the assumption of twofold soil failure mechanisms, which are a shallow passive wedge failure and a deep full flow failure. Study of the transition depth separating the two mechanisms revealed the existence of a third pattern of failure, where the soil deforms three-dimensionally. Taking profit of the axisymmetric geometry of the pile, which is subjected to non-axisymmetric loading, an embedded rigid disk was taken to represent the behavior of the soil around a short segment of the pile in the region where the soil is expected to behave in three-dimensional (3D) mode. An expression for the soil reaction was established first by using an analytical approach. Then, p_u resulting from the third pattern was rigorously quantified by extending the obtained formula to failure using some fundamental notions of basic soil mechanics. By joining the obtained p_u to two other expressions for modeling the shallow passive wedge and the deep full flow, the variation of the undrained clay bearing capacity factor resulting from the threefold collapse mechanism was examined as a function of the pile normalized depth and for a variety of undrained clay strength parameters. Comparisons with other rigorous methods show the rationality of the proposed approach.