Abstract: The shape of failure envelope for skirted foundations under combined loading is complex and strongly dependent on embedment ratio when the traditional load reference point, i.e., the base center, is utilized. To overcome the difficulty to obtain a closed-form equation to describe the shapes of failure envelopes for skirted foundations with a range of embedment ratios, an efficient searching procedure for the optimal load reference point is suggested and implemented in the finite element package ABAQUS. Using the optimal load reference point, the failure envelope obtained through combining swipe loading and fixed displacement ratio probing can be approximated as an ellipse in normalized moment-horizontal load space. Then a unified double parameter equation is proposed, which can be utilized to estimate the combined capacity of skirted foundations in normally consolidated clay within practically acceptable accuracy. The values of the two parameters are also presented for a range of practical embedment ratios. The rationality of the proposed equation is validated by comparing the present solutions with the experimental data from a given centrifuge model test.