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Finite-element limit analysis is used to identify the critical internal skirt spacing for the undrained failure of shallow skirted foundations under conditions of plane strain based on the criterion that the confined soil plug should ideally displace as a rigid block, such that optimal bearing capacity is realized. General loading (vertical, horizontal, and moment) is considered for foundations with skirt embedments ranging from 5 to 50% of the foundation breadth in soil having either uniform strength or strength proportional to depth. The results explicitly identify the number of internal skirts required to ensure soil plug rigidity under arbitrary combinations of horizontal and moment loading expressed as a function of the normalized skirt embedment and the maximum expected level of vertical loading as a fraction of the ultimate vertical bearing capacity. It is shown that fewer internal skirts are required with increasing normalized foundation embedment, but more internal skirts are required with increasing soil strength heterogeneity. The results also indicate the potential for a significant reduction in capacity if insufficient skirts are provided, such that plastic deformation is permitted to occur within the soil plug. [ABSTRACT FROM AUTHOR]