Extraction of Spudcan Foundations in Single and Multilayer Soils

Abstract

Difficulties are often encountered in a variety of seabed sediments during extraction of spudcan foundations of mobile jack-up rigs. This paper reports results from centrifuge model tests undertaken to provide insight into spudcan foundation behavior during vertical extraction through single, double, and multilayer soils. The model tests included half-spudcan tests against a transparent window, allowing the soil flow to be captured continuously by a digital camera and subsequently quantified through particle image velocimetry (PIV) analysis and separate full-spudcan tests to measure the uplift resistance. The observed soil-failure mechanisms provide insight with respect to the profile of extraction resistance. The mechanisms reveal evolving failure modes at different spudcan extraction depths and, in particular, changes in the failure patterns because of varying layer geometry, soil types, and properties. For single, double, and multilayer clays, suction at the spudcan base, shearing along a truncated cone above the spudcan, and weight of the soil above the spudcan constitute the extraction resistance. Maximum extraction resistance occurs in the stiffer soil layer. The extent to which suction is sustained at the spudcan base is critical in determining the extraction resistance. The presence of a sand layer beneath the penetrated spudcan, for example as part of the soil plug pushed down during spudcan penetration, allows the base suction to be released within a short distance of uplift and results in relatively low extraction resistance. Improved understanding of spudcan extraction mechanisms through layered soils resulting from this study will allow development of analytical solutions to model spudcan extraction behavior, assisting jack-up operators to plan properly for leg extraction based on seabed conditions.