A comparison of the combined load behaviour of spudcan and caisson foundations on soft normally consolidated clay

Experimental data from loading tests of model circular footings on soft normally consolidated clay are presented. The experiments were carried out on a drum centrifuge at a radial acceleration level equivalent to 100 times Earth's gravity, ensuring conditions of stress similitude between the model and the prototype scale. The aim of the experiments was to compare the undrained response of different offshore foundations to the same loading conditions. Two different types of foundation were targeted for investigation: spudcans and suction caissons. The spudcan, typically an inverted shallow cone, is the traditional footing for mobile drilling units (also known as jack-up rigs). An alternative foundation concept that is being increasingly considered is that of foundations skirted about the perimeter and installed by suction. A loading arm that incorporated an internal hinge was used so that combinations of load appropriate to the foundations of jack-up units could be applied to the models. Although it was anticipated that, by using skirted foundations there would be increased moment capacity, this was not found to be the case (for the caisson skirt length and soil strength profile investigated). However, there was a stiffer response and additional horizontal capacity at the foundation level. Significantly the use of skirted foundations allowed for a greater combined loading capacity under tensile vertical loads. The results have been interpreted within the framework of strain-hardening plasticity theory, and comparisons with existing yield surfaces are detailed.