Pore Pressure Development under Offshore Gravity Structures

One of the important geotechnical considerations related to the stability of offshore gravity structures founded on sand deposits is that of potential loss of supporting capacity of the sand due to the pore pressures induced in it by severe storm loadings. In this paper a procedure is presented for evaluating wave induced pore pressures. The method takes into account the distribution of cyclic shear stresses in the profile, and the important factor of pore pressure dissipation. The analysis provides the complete time history of pore pressure response of the soil underlying the tank during the storm, and shows clearly that failure to include pore pressure dissipation effects would lead to radically conservative design. The results also provide a basis for evaluating the stability of the tank foundation on an effective stress basis and suggest that critical support conditions are most likely to develop around the outer edges of the structure.