The presence of clay crust overlying soft clay in parts of Southeast Asian Sunda Shelf may result in punch-through hazard during jack-up spudcan installation due to the potential for reduced (or softening) bearing resistance upon failure of the crust layer. In view of the above, a series of centrifuge model tests have been undertaken on circular spudcans with prototype diameter of 10 m on layered clay samples. The thicknesses of the upper crust layer adopted in the tests ranged from about 0.16 to 0.71 spudcan diameter while the strength ratio of the lower layer to the upper layer is 0.2. This paper presents experimental results obtained in terms of 1) the development of bearing resistance upon spudcan penetration in the layered clay samples, and 2) the development of soil deformation and failure mechanisms upon spudcan penetration. The shape of bearing resistance versus penetration curves changes from ductile (non-softening) to brittle (softening) profiles as the crust thickness increases.
Today's jack-up rig foundation consists of three independentlyinstalled legs each supported at the base by a spudcan, which is a steel footing of conical wedge configuration having diameter ranging from 10 m to slightly over 20 m. Spudcans are installed by penetration to some depth below the seabed surface under incremental preloading, which is usually provided by a combination of self weight and water ballast. SNAME (2002) recommended the total preload to be larger, by a factor of safety, than the maximum expected working load of the leg under extreme conditions. Given the considerable amount of preloading pressure required, typically in excess of 300 kPa, the penetration depths of spudcans may vary from several meters for cases with stiff granular seabed to as much as several tens of meters for soft clay seabed.
