Projects per year
© 2014, National Research Council of Canada. All Rights Reserved. Difficulties are encountered, in soft clay and silt sediments, during extraction of the spudcan foundations of “mobile” jack-up rigs. This paper reports results from centrifuge model tests undertaken to provide a simple, effective, and practical means for easing spudcan extraction in soft normally consolidated and lightly overconsolidated clay. The tests were carried out on model spudcans of two different geometries typically used in the field. The spudcans were extracted from penetrations of 2~3 diameters and after an operation period of ~19 months. The measured pore pressure at the bottom face of the spudcan during extraction process provided insight into the development of suction at the interface between the spudcan base and the underlying soil. The tests were carried out with and without a sand layer placed locally in between the spudcan and the mudline at the onset of penetration. In the field, a gravel layer can be placed by means of a flexible fall pipe system. The presence of a sand layer (with a thickness less than the spudcan tip length) beneath the penetrated spudcan assisted in easing leg extraction difficulties allowing the base suction to be reduced by 73%~83% and hence the breakout force by 31%~32%. For a given penetration depth, the installation resistance was also increased by 24%~26%. This yielded a 15%~17% reduction of spudcan penetration depth under a given preload and hence the breakout force as much as 50%. Guidelines and cautions for using the proposed measure are provided. Additional advantages of the new recommended method in enhancing the efficiency of the conventional measures of lessening spudcan extraction difficulties are discussed.