Combined loading capacity of a spudcan in clay after penetrating through a sand layer

During offshore operations, the spudcan footings of mobile jack-ups are subjected to combined vertical, horizontal and moment (V, H, M) loading as a result of environmental wind, wave and currents acting on the platform. In seabeds of single-layer clay or sand it is now common to express spudcan capacity directly as a surface of allowable VHM loads. Although layered soil stratigraphies with sand overlying clay are encountered in the field, the effect of soil layering on the VHM surface is not well understood. Defining the VHM capacity of a spudcan when it has pushed a layer of sand into the underlying clay is the concern of this paper. Results from a series of centrifuge tests demonstrate that the general framework used in single layers is equally applicable in layered soils. Increases in vertical and moment capacity, compared with clay only soil, are similar in magnitude and arise from the sand plug being pushed into the clay layer by the penetrating spudcan. The most significant increase, however, was found in the horizontal capacity.