Numerical investigation of bearing capacity of spudcan foundations in clay overlying sand under combined loading

The evaluation of the foundation capacity under combined vertical, horizontal, and moment loading is an essential element of the site-specific assessment that ensures mobile jackup rigs are safe to operate in large storms. However, the prediction approaches available in design guidelines are restricted to jackup spudcan foundations on homogeneous sand or clay. The aim of this study is to examine the ultimate bearing capacity for spudcan foundations in clay overlying sand under combined vertical, horizontal, and moment loading. This study investigates the effects of the underlying sand layer on the three uniaxial capacities and on the combined failure envelope, with results derived from three-dimensional finite-element analyses. The findings show that, as the spudcan nears the underlying sand layer, the absolute horizontal capacity remains relatively constant while the vertical and moment capacities increase at around the same rate. A closed-form expression for the failure envelope is proposed. It allows for easy assessment of the bearing capacity of a spudcan that remains in clay overlying sand. This new expression can be used during site-specific assessments of jackup platforms.