This paper summarizes the results of an experimental investigation of pore pressure buildup in the subsoil underneath a caisson breakwater subject to rocking motion. In the experiments, soil was silt with d50 = 0.045 mm. Foundation of the caisson was simulated by a rectangular plate, slightly buried in the soil. Pore water pressures were measured. With the rocking motion of the caisson, the pore-water pressure first builds up, reaches a maximum value and begins to fall off, and is eventually dissipated. The effect of amplitude and period of the caisson motion on pressure buildup was investigated in detail. The size and shape of the caisson were also investigated.
Two of the foundation failure modes of caisson breakwaters are (1) slip surface failures and (2) excess settlement. In the former, the hydrodynamic loading on the subsoil creates stresses which exceed the strength of the soil (the so-called general shear failure). In this process, the strength of the soil may be influenced by pore pressure buildup due to cyclic shear deformations in the soil. In the second failure mode, vertical settlements occur when soil consolidation takes place. Here, too, consolidation may be affected by the buildup of pore pressure in the soil. (Coastal Engineering Manual, 2006, Chapter 2, Part 6, can be consulted for a detailed description of failure modes of caisson breakwaters.) In their review on buildup of pore pressure underneath marine gravity structures subjected to wave loads, De Groot et al. (2006) consider various case histories. They report that pore pressures in the subsoil under a model caisson breakwater reached as much as 50% of the initial effective stress values in a large-scale model experiment (Kudella et al., 2006), one of the case histories considered.
