Capacity of rectangular mudmat foundations on clay under combined loading

Abstract

Rectangular shallow foundations known as mudmats are commonly used in the offshore oil and gas industry. They are often subject to large horizontal forces that can also give rise to overturning and torsional moments. This paper focuses on analysing the ultimate capacity of a rigid rectangular mudmat resting on the surface of, or embedded in, homogeneous undrained clay. Realistic three-dimensional load combinations are applied, based on the combined horizontal, moment and torsional loading generated by a horizontal force applied within the envelope of the mudmat and the equipment that it supports. Analytical lower- and upper-bound plasticity calculations are performed for a surface footing under combined horizontal and torsional loading, while embedded footings and more complex loading arrangements are considered using three-dimensional finite-element limit analysis. The undrained soil is modelled as a rigid–plastic von Mises material, and the interface between the external sides of the mudmat and the soil is modelled with both unlimited and zero tension capacity. Design charts are presented and relationships between loading combinations and failure mechanisms are explored.