Abstract: The soil profile of “hard crustoverclay” is prevalent in offshore area of Chengdao, where mobile jackup rigs are used frequently. Different to sand and clay soils, the “hard crust” mainly consists of silty sand or silt. Therefore, the existing numerical method and design formula may not be suitable for analyzing the potential for spudcan punchthrough in this type of soil profile. In this paper, large deformation finite element method is used to simulate centrifuge tests that have been carried out for spudcan penetration in hard crust layer overlying clay. In the numerical analysis, the hard crust is modelled as sand or stiff clay layer, using MohrCoulomb (MC), modified MohrCoulomb (MMC) and modified Tresca models. The results show that when the hard crust is modelled as sand layer, the penetration resistance of spudcan in “hard crustoverclay” is significantly underestimated by MC and MMC models. By contrast, the numerical analyses, where the hard crust is modelled as stiff clay layer, provide a better prediction for the peak resistance in the hard crust as punchthrough happens. For the hard crust soil used in the centrifuge tests of this study, which was collected from Chengdao Sea with an effective unit weight of 11 kN/m3, an equivalent undrained shear strength of about 80kPa is suggested.

Key words: Hard crust, numerical simulation, large deformation, spudcan, punch-through