Evaluating the penetration resistance of spudcan foundations in clay overlying sand

Safe installation of mobile jack-up platforms requires accurate prediction of the vertical load-penetration curves of their large ~20 m diameter spudcan footings. Difficulties can arise due to a change of soil strength from a soft to a stiff layer. A substantial increase in the ultimate bearing capacity occurs when a spudcan penetrates vertically through a soft clay layer towards a sand layer. This is because the softer soil that is trapped beneath the spudcan base starts to be squeezed horizontally as the vertical load begins to be borne by the stronger underlying sand layer. The theoretical solution of the bearing capacity of a soil layer on a rigid base has conventionally been used to predict the spudcan penetration resistance in soft over strong layers. However, recent centrifuge experimental studies suggested that the design approaches recommended by current industry guidelines ISO (2016) and SNAME (2008) are not adequately accurate in predicting the sharp increase in the resistance prior to penetration into the sand layer. In this paper, large deformation finite element (LDFE) analyses that capture the evolving soil failure mechanisms during spudcan penetration in a clay layer overlying sand, and explore relationships for the penetration resistance, are reported. The Coupled Eulerian-Lagrangian method was used to simulate the continuous penetration of the spudcan foundation. The clay layer was modelled with the elastic perfectly plastic Tresca model, while the sand layer was modelled by the Mohr-Coulomb model using a nonassociated flow rule. The findings show that the increased capacity is associated with the squeezing mechanism, which starts at the depth where the bottom of the soil failure mechanism first touches the underlying sand layer. This is approximately a third of the spudcan diameter above the clay-sand interface. A simplified expression is proposed to predict the spudcan penetration resistance between the depth at which the squeezing prevails and the clay-sand interface.