Bearing Capacity of Semi-Deep Skirted Foundations-Paper

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The study investigates the vertical undrained bearing capacity of circular semi-deep skirted foundations in uniform soft clay. Using finite-element analyses in ABAQUS, the authors examine embedment ratios (D/B) from 0 to 2 and foundation diameters of 5, 10, and 20 m. The soil is modeled as elastic-perfectly plastic Tresca material, with rough soil–foundation interfaces.<br /><br />Results show that bearing capacity increases with embedment, while deeper foundations also exhibit a stiffer initial load–displacement response. The increase is nonlinear and reflects a shift in failure behavior: shallow foundations fail through general shear, whereas greater embedment promotes local shear and vertical punching. Foundation diameter has a modest scale effect; normalized capacity factors decrease as diameter increases.<br /><br />The analyses indicate that a skirted foundation and an equivalent embedded solid soil plug have nearly identical vertical capacity and failure mechanisms. Capacity can therefore be estimated as the sum of resistance at the skirt tip and shaft friction along the skirt. Shaft resistance becomes increasingly important as embedment grows, accounting for much of the capacity gain at higher D/B ratios.<br /><br />Comparisons with conventional bearing-capacity methods and published numerical and bound solutions show that many established methods underestimate total capacity because they do not adequately account for skirt friction. The authors propose bearing-capacity factors as functions of embedment ratio for estimating total, end, and shaft resistance.<br /><br />Overall, the findings support skirted foundations as an effective alternative to shallow or deep foundations, particularly where they can reduce excavation and installation costs while improving capacity. For vertical loading in undrained clay, designers may treat the soil enclosed by the skirt as a trapped plug and calculate capacity from tip resistance plus external shaft friction.

Keywords

skirted foundations

undrained bearing capacity

soft clay

embedment ratio

finite element analysis

vertical loading

shaft friction

tip resistance

Tresca material

soil plug