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.

Pile foundations for offshore wind turbines are subjected to large lateral loads. By mounting wings on the perimeter of regular monopiles, winged monopiles have shown better performance in resisting deformation under horizontal loading. However, the hazardous effect of local scour on the lateral bearing capacity of winged monopiles installed in the sandy seabed has not been systematically evaluated. In this study, a modified Mohr–Coulomb model considering the pre-peak hardening and post-peak softening behavior of dense sand is adopted to simulate laterally loaded winged monopiles in the locally scoured sandy seabed, using three-dimensional finite element analyses. The effect of local scour depth on the lateral capacity of winged monopiles is examined and explained by soil failure mechanisms. The enhancement of lateral capacity with wings attached to the monopile is demonstrated to be more effective than extending pile embedment length. The effects of the relative density of sand and the wing load orientation are also discussed. Finally, the wing efficiency is evaluated to determine the optimal configuration of winged monopiles.

Foundations are commonly situated on layered soil deposits, where the elastic stiffnesses may differ from values calculated using solutions for single-layer elastic media. This paper reports elastic stiffness coefficients for a rigid circular footing subjected to vertical, horizontal and moment loads bearing on a clay layer above a stiffer sand layer. Three-dimensional finite-element analyses considered various shear modulus ratios of top to bottom soil layers, as well as the thickness of the top layer below the footing. The elastic stiffnesses are presented as dimensionless coefficients and are bounded by published solutions for circular footings on a single-layer elastic half-space. Analytical expressions that capture the numerical results well are also proposed. The results can be used in the assessment of the elastic response of circular foundations on clay overlying sand, with applications in soil–structure interaction analyses of offshore structures among others.