The elastic stiffness of spudcan foundations in stiff-over-soft clays exhibits changes similar to “punch-through” failure, creating significant uncertainty for jack-up platform operations. This study conducted a three-dimensional small-strain finite element analysis on this specific topic to discretely simulate the spudcan elastic stiffness profile in stiff-over-soft clay. The influence of the soil surface, layered interface, and their coupling effects were isolated and separately evaluated, and a simple semi-theoretical framework for the influence zone was proposed. The key parameters of layered soil (thickness ratio, shear modulus ratio, soil heterogeneity coefficient, and backflow) affecting the influence mechanism of spudcan elastic stiffness were evaluated and analyzed. It was found that the effects of the soil surface and layered interface competed with each other. The vertical deformation mechanism of the spudcan reduces the “punch-through” failure risk of elastic stiffness by transferring more of the soil deformation to the bottom soft clay layer. Based on the findings from the parameter study, a simplified profile is proposed to predict the variation of the spudcan elastic stiffness. The proposed prediction method provides a comprehensive view of elastic stiffness in stiff-over-soft clay for offshore in-site assessment.

Determining the elastic stiffness coefficient of spudcan foundations is a challenging task due to the natural variability of soil properties coupled with the complexity of loading conditions. Uncertainty in the elastic stiffness coefficient is a potential risk factor for the safe and stable operation of offshore jack-up platforms, and there are no guidelines for operators to assess its confidence level. This paper conducted a study on this specific subject using the three-dimensional random finite element method. The influence of soil strength heterogeneity, the coefficient of variation and the scale of fluctuation of the shear modulus, embedment depth and soil backflow on the deformation mechanism and elastic stiffness coefficient of spudcans were systemically discussed. The results suggest that the above parameters, combined with the soil's spatial variability, can alter the deformation mechanism, thereby the elastic stiffness coefficient. If spatial variability is ignored in platform design, the safety probability of spudcans is only 50%. So, a matrix algebraic expression is proposed to accurately predict the elastic stiffness coefficient in spatially variable clay at any given confidence level. The study findings may facilitate incorporating spatial variability into elastic stiffness research and supplement the calculation method in the existing jack-up assessment guidelines.