The response of skirted foundations to combined vertical, horizontal and moment loading is important for the design of offshore installations. The vertical skirts beneath the footing interact with the soil and increase the foundation capacity, compared with a surface footing. Previous researchers have assumed that the soil within the skirts remains rigid during undrained loading, but this assumption has not been investigated rigorously. A series of plane-strain finite element analyses has been conducted to investigate directly how the skirt geometry affects the undrained strip foundation capacity under combined horizontal-moment loading and the mechanisms occurring at failure. Conditions of both uniform and non-uniform undrained strength soil have been considered. The results show that deformation of the soil between external skirts can lead to significantly less foundation capacity than that of an equivalent solid embedded foundation. Hence the specific geometry of the foundation must be considered in design. In addition, the failure envelopes for skirted foundations with different embedment ratios differed significantly. This makes general recommendations for failure envelope shapes problematical. Finally, significant increases in foundation bearing capacity may be achieved by adding an intermediate skirt to the foundation, which results in a foundation capacity that is almost equal to that of a solid embedded foundation.