During the operational phase, the spudcan foundations of a mobile jack-up rig are subjected to combined vertical, horizontal and moment loading. Although previous research has indicated a substantial increase in vertical bearing capacity when a spudcan penetrates through a soft clay layer towards a sand layer, the response of spudcan foundations subjected to combined loadings in such stratification has yet to be understood. This study investigates the effect of the underlying stronger sand layer on the undrained VHM capacity of a spudcan foundation using three-dimensional small-strain finite element analysis. Results show the significant increase in vertical and moment capacity, whereas the horizontal capacity is minimally affected. The soil failure mechanisms are identified and changes in the size and shape of failure envelopes, accounting for the sand layer underneath, are quantified. An analytical expression is proposed to predict the combined capacity of a spudcan foundation in clay overlying sand.

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