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Circular skirted foundations are used widely in the offshore oil and gas industry and renewable energy industry to support subsea infrastructure and founding turbines. The foundations are subjected to combined vertical-horizontal-moment (V-H-M) loadings during operation. This paper describes an extensive investigation of the response of installed circular skirted foundations under operational loadings in sand-over-clay. The overarching aim was to examine the influence of the presence of a sand layer on the combined load capacity. A detailed parametric study was undertaken through small strain finite element analyses, varying the thickness of the sand layer, embedment depth of the foundation (or skirt length), and level of vertical load mobilisation. The effect of the surface sand layer was profound for thicker sand layer of Ts/D > 0.2. Normalised failure envelopes were presented in v-h, v-m and h-m spaces. The FE results on the former two spaces showed unique trend, but not on the latter. New approximating expressions for describing the failure envelopes as a function of the sand layer thickness ratio, skirt length ratio and vertical load mobilisation level were proposed.