Jack-up units are extensively utilized in offshore geotechnical investigations and the recent installation of wind turbine foundations. Engineers require accurate predictions of spudcan penetration resistance to assess its final penetration depth and identify potential risks, e.g., punch-through, ensuring safety during the installation of jack-up units. Notably, for seabed stratigraphy comprising sand with interbedded clay layers, a significant discrepancy exists between the penetration resistance calculated using current industry guidelines and actual measured results. In this study, three-dimensional (3D) large deformation finite-element (LDFE) analyses were performed to model the spudcan penetration response in a seabed consisting of a sand–clay–sand soil profile. A comprehensive parametric study was conducted to examine the influence of soil strength and layer thickness on penetration resistance. The variations of the soil failure mechanisms are demonstrated and associated with the varying penetration resistance, which form the basis of the new calculation method developed for the sand seabed with interbedded clay.

This study investigates the influence of inside-trapped water and remedial grouting on the vertical bearing behaviour of suction bucket foundations in sand through 1 g laboratory model tests. The tests were designed to compare the relative responses of different trapped-water and grouting conditions under the same model scale, sand preparation procedure, and loading protocol. Two target trapped-water conditions were considered: a condition without an observable continuous water layer beneath the bucket lid and a condition with an initial trapped-water thickness of approximately 2 cm. These conditions were controlled and verified before loading using the scale attached to the transparent bucket wall and the underwater camera monitoring system. The results show that inside-trapped water modifies the vertical load-transfer path between the bucket lid and the internal soil plug. When a water layer exists beneath the lid, direct lid–soil plug contact is weakened, and the foundation resistance relies more strongly on skirt-side resistance and the resistance mobilized near the bucket rim. Under cyclic vertical loading, the trapped-water case exhibited larger cumulative displacement and a lower post-cyclic bearing response than the no-trapped-water case. The secant cyclic stiffness showed a continuous increase in the no-trapped-water case, whereas a rise-then-fall trend was observed in the trapped-water case, which may be associated with cyclic densification, soil plug disturbance, changes in lid–soil plug contact, and possible local pore pressure development. Remedial grouting filled the trapped-water space beneath the bucket lid and partially restored the lid–soil plug load-transfer path. Under the present model test conditions, the post-cyclic dimensionless bearing capacity of the grouted cases increased by approximately 13–16% relative to the ungrouted trapped-water case. The grouting cases with different bentonite contents showed similar recovery trends within the limited dataset, suggesting that the improvement was mainly related to filling and sealing the trapped-water space rather than to the intrinsic strength of the grout material.