Estimating Spudcan Penetration Resistance in Stiff-Soft-Stiff Clay

Abstract

The paper presents a new mechanism-based two-step approach for assessing spudcan penetration resistance in three-layer stiff-soft-stiff clay sediments. An alternative approach is also proposed, establishing a direct correlation between spudcan and cone penetration resistances. The approaches are developed based on the results from a parametric study through large-deformation finite-element analyses and validated against centrifuge test data and a case history. These can be taken as a “top-down” approach. The bearing capacities are evaluated at the seabed, at the depth of maximum bearing capacity in the first (stiff) layer, at the depth of triggering squeezing in the second (soft) layer, and at the second–third layer interface successively, followed by the estimation of the penetration resistance profile in the third (stiff) layer. For the mechanism-based approach, design formulae are also proposed for assessing the limiting cavity depth, variation of the soil plug height, and equivalent undrained shear strength of the second layer that accounts for the effect of the third layer. The depth of triggering squeezing in the second layer is estimated through an iterative approach to incorporate the effect of the soil plug trapped beneath the advancing spudcan, which leads to earlier squeezing. For the alternative approach, design correlations are established between cone tip and spudcan penetration resistances. A direct comparison between the estimated profiles using the proposed design approaches and the “bottom-up” approach recommended in the current international design standard confirms the benefit of using the former.