Comparative Numerical Interpretation of Uplift Mechanisms of Surface and Skirted Shallow Foundations on Clay

Uplift of shallow foundations from clay seabeds is governed by the generation and dissipation of negative excess pore pressure beneath the foundation. Previous centrifuge tests have revealed markedly different uplift responses between surface and skirted foundations, but the underlying mechanisms remain unclear. This study investigates these mechanisms using coupled finite-element analyses with a hydro-mechanical interface model, validated against published centrifuge data. The analyses examine uplift resistance, suction evolution, gap opening, soil deformation, and stress paths under different uplift rates. Results show that uplift resistance for both foundation types is primarily generated by suction beneath the top plate, with negligible tensile contribution from the soil skeleton. For the surface foundation, suction dissipates mainly through tangential flow along the plate–soil interface, accompanied by progressive gap opening from the edge towards the centre and eventual interface breakaway. In contrast, the skirted foundation mobilizes a deeper reverse bearing mechanism and a more uniform displacement field within the confined soil plug. Under partially drained conditions, suction dissipation in the skirted foundation is accompanied by marked stress redistribution within the confined soil plug. Although the present formulation does not explicitly simulate progressive failure within the plug, the numerical pre-breakaway response, together with the centrifuge observations, suggests that skirted foundation breakaway cannot be interpreted simply as detachment at the plate–soil interface. These findings clarify the fundamental differences in uplift and breakaway mechanisms between surface and skirted foundations.