Bearing capacity on sand overlying clay soils: experimental and finite-element investigation of potential punch-through failure

When a jack-up spudcan foundation is installed on seabeds consisting of a sand layer overlying soft clay, the potential for 'punch-through' failure exists. This happens as a result of an abrupt reduction in bearing resistance when the foundation punches a block of sand into the underlying soft clay in an uncontrolled manner. This paper details an extensive series of 30 tests of flat circular and spudcan foundations continuously penetrated through samples of sand overlying clay, and performed under relevant stress conditions using a drum centrifuge. The large testing area of the drum centrifuge was used advantageously to produce test results that could be compared directly with tests covering a sand thickness over foundation diameter of 0.21 to 1.12. Results from retrospective finite-element analysis of the experiments are also described, with back-calculated values of the stress-level-dependent friction and dilation angles in the sand during peak penetration resistance shown to fit correlations in the literature. The back-analysis showed that larger values of peak resistance gave lower friction and dilation angles, which is consistent with gradual suppression of dilatancy under high confining stress. When compared with published results from visualisation experiments, the finite-element analysis showed a similar failure mechanism during peak resistance, with a frustum of sand forced into the underlying clay at an angle reflecting the dilation in the sand.