Undrained bearing capacity of spudcan in soft-over-stiff clay after penetration

Abstract

Abstract As the boundary condition of jack-up systems, the foundation model of offshore structures affects the vibration mode and structural behavior of leg structure. Typical simple foundation models such as pinned and linear spring do not reflect either plastic failure or the resulting plastic behavior of the soil. Alternatives, for example Model B for clay, derived for consideration of the nonlinear behavior of soil, have been studied continuously improved until recently. Additionally, research on the foundation models has been continued in multi-layered soils, especially stiff-over-soft layer soils that present a risk of punch-through. In the present study, a yield envelope analysis is performed for the situation wherein the jack-up leg penetrates deeply through the soft clay and meets the stiff clay. In such soft-over-stiff clays, both the upper soft clay and the lower stiff clay affect the spudcan, thereby complicating the use of a foundation model of single clay. The numerical analysis used in this study is based on the Coupled Eulerian-Lagrangian (CEL) analysis with strain-dependent nonlinear soil properties. Installation effect and corresponding soil disturbance are considered in the following yield envelope analysis. Undrained bearing capacities of the single clay are calculated at the deep embedment for checking the suitability of the analysis, and continuous results have shown to the existing theories of single clay. In the soft-over-stiff clay case, the lower stiff clay has a significant effect on the capacities in the vertical, horizontal and rotational directions. The squeezing effect does not affect the vertical tensile capacity but increases the compressive vertical, horizontal, and rotational capacities. The stronger the underlying clay and the deeper the embedment, the asymmetrically larger the yield envelope becomes. The calculated yield envelopes are normalized to the increased capacities, and a yield envelope expression for the soft-over-stiff clay is suggested for the normalized shape. The effect of the maximum capacity on the yield envelope equation is examined, and it is found that the largely calculated capacity can make the yield envelope close to the paraboloid.