The bearing capacity envelopes for a spudcan foundation in single layer sand or clay soil are well-documented based on model tests and finite element analyses. In reality, layered soils are common in the field and in such conditions it is also important to consider the combined vertical, horizontal and/or moment bearing capacity envelopes. The sand over clay soil stratigraphy is commonly encountered for many offshore oil and gas operation sites. For jack-up installation in such soil stratigraphy, the spudcan foundation normally punches through the top sand layer with a sand plug trapped underneath and together they form a composite foundation in the underlying clay layer. This paper reports a series of finite element analyses to examine the bearing capacity envelopes of a spudcan foundation in the underlying clay soil after penetrating through the top sand layer. The evolution of the envelopes with the shear strength of clay, embedment depth as well as sand plug thickness is investigated. Findings about the combined failure envelopes are discussed.
A jack-up platform is a mobile offshore structure used in the oil and gas industry for exploration and production and in the renewable energy industry to construct offshore windfarms. As shown in Fig. 1, it comprises a hull supported by legs (generally three or four) with foundations called spudcans which bear on the seabed. Prior to installation of a jack-up at a site, the penetration of the spudcan into the seabed and the corresponding ultimate bearing capacities, i.e., the loads above which the soil starts to fail, have to be assessed in order to determine whether the seabed is sufficiently strong to support the platform during installation and environmental storm conditions. This requires understanding of the bearing capacity of each spudcan under combined vertical (V), horizontal (H) and moment (M) loading, as well as their interaction with the jack-up structure.