Analytical Nonlinear m-θ Curves for Monopiles in Clay

Abstract

Dependable predictions of monopile foundation response to lateral loads are crucial to the efficient design of offshore wind turbine foundations. For squat monopile foundations, it is important to incorporate the distributed nonlinear moment-rotation response with depth, known as m-θ curves, in addition to traditional p-y curves and lumped force-displacement curves at the pile base. Recognizing the limited number of m-θ curves available in the literature, this paper develops new theoretical curves using a rational two-dimensional horizontal pile/soil “slice” model to obtain improved representations of the stress and displacement fields in the soil around the pile. First, this model undergoes validation through comparisons with available linear-elastic solutions. Subsequently, it is employed in conjunction with a numerical discretization of the pile circumference to obtain nonlinear m-θ curves accounting for both soil yielding and slippage between pile and soil. The resulting curves are compared with a novel simplified solution based on an approximate distribution of vertical shear stresses at the pile/soil interface that can be derived in closed form. The new solutions are developed for undrained conditions considering elastic-perfectly plastic soil material behavior and properly accounting for slip at the pile/soil interface.