Confining layer influence on capacity of offshore caisson foundations

Regarding to offshore structures, such as jack-up platforms and wind turbines, one of the main concerns is about their foundations. In the British shore, for instance, the sand layer overlying the rock is not so deep, which can be an issue for the installation of shallow foundations. The traditional bearing capacity theory assumes an infinite depth of soil, but the presence of a hard layer underneath can affect the load. Then, this research project aimed to investigate the influence of this hard layer on the installation of offshore shallow foundations. To do so, loading tests were undertaken using a reduced scale model of a caisson foundation. This kind of foundation is positioned on the seabed and it is driven into the sand by the suction of the confined water. Then, the loading tests consisted on the penetration of the caisson model into 3 samples of sand using a press. The samples were placed inside a barrel, which was reinforced underneath with a birch plywood plate. An additional loading test was done, but without the plate. As a result, higher loads were observed with the decrease of the specimen heights, considering the same settlement of the caisson. In addition, higher loads were noticed for the tests with the plate compared with the test without the plate, and the difference between the loads for both cases increases with the settlement of the foundation. For future work, it is advisable to adapt the model in order to consider the suction effects.