Abstract
In the search for enhanced alternatives to the conventional footings of jack-up platforms, the offshore industry has turned to novel types of shallow foundations. A recent hybrid foundation concept was proposed that merges a skirted footing with a deeper caisson, with the motivation to mobilise higher bearing capacities and increase the foundation stiffness. Defining the behaviour of this hybrid foundation under combined vertical, horizontal and moment loadings is crucial to understanding the overall response of the offshore structure. This paper presents a study of the combined bearing capacity of a hybrid foundation system both experimentally and numerically. Centrifuge tests were performed at an acceleration of 200 times that of Earth's gravity on three different hybrid foundations resting on soft clay and subjected to combined vertical, horizontal and moment loads. Combined loads were applied using a novel apparatus, which enables independent vertical, horizontal and rotational movements of the footing. Then numerical studies were conducted by adopting the soil properties measured in the test. Experimental results revealed that the central caisson's dimension plays an important role in the determination of the combined bearing capacities. It also showed that for skirted foundations, the combined bearing capacity is nearly identical once the footing has been pushed to the depth of its skirt, even if the vertical capacity is not fully preloaded. When comparing the numerical and experimental prediction of the combined bearing capacity, a reasonable agreement was achieved across all hybrid footing types.