Design method for the undrained capacity of skirted circular foundations under combined loading: effect of deformable soil plug

The response of skirted circular foundations with rough and smooth soil–skirt interface to combined loading is investigated. The shape and size of the failure envelopes of the skirted circular foundations under a practical range of embedment ratio, soil strength heterogeneity, soil–skirt interface and level of vertical mobilisation are compared to those of solid embedded circular foundations and skirted strip foundations. The results show that both the foundation geometry and soil plug inside the skirt compartment significantly influence the uniaxial capacity and the shape and size of the failure envelopes. Approximating expressions for solid embedded foundations do not capture the shifting eccentricity of the failure envelopes of the skirted circular foundations. A new approximating solution for describing the failure envelope as a function of embedment ratio, soil strength heterogeneity and soil–skirt interface is proposed. Scaling factors accounting for the decreasing uniaxial capacity of skirted foundations with smooth soil–skirt interface are also defined.