For jack-up units founded upon sand, sliding of the windward leg is often the governing foundation criterion in a site specific integrity assessment Since the penetration depth in sand is relatively small, the spudcans are more sensitive to sliding. The main goal of this study was to investigate the influence of the height of the sand layer overlying stiff clay on the sliding capacity of the spud can. Additional experiments were performed to examine the punch-through phenomenon and to determine the vertical bearing capacity of different soil configurations. The tests were performed in a geotechnical centrifuge at 150g. The spudcan model used was rough and conical shaped with a diameter of 50mm. Since the tests were performed on dry sand a stress gradient was simulated which belongs to a prototype footing of 12 metres diameter. A computer controlled loading system was developed to simulate the load path a spudcan is subjected to during a storm. The test results show that the height of the sand had a significant influence on the vertical bearing capacity. Even with a height of sand of 9 metres overlying the clay, the bearing capacity was reduced to half the value of pure sand. The sliding resistance increases when the height of the sand decreases, because the spudcan penetrates deeper into the sand during the vertical preloading.
Conical shaped footings, spudcans, with a diameter of 10 m - 20 m are often used as foundation elements for mobile jack-up units. When the jack-up unit is founded on clay, the penetration depth is generally sufficient to provide resistance against sliding. However, the penetration depth in sand is relatively small, so that the spudcan foundations are more sensitive to sliding.