Liquefaction phenomena underneath marine gravity structures subjected to wave loads

The foundation of a vertical breakwater or offshore gravity platform may fail due to wave loading. The question is discussed whether liquefaction phenomena in a sandy subsoil, such as the generation of residual excess pore pressures and cyclic mobility, may significantly contribute to such failure. Theory, descriptions of prototype failure cases, and several scale tests are analyzed. It is concluded that the most spectacular failure type "liquefaction flow failure" is only possible in the case of a subsoil of very loose sand or silt combined with a low drainage potential, e.g., by the presence of a clay layer or large structure dimensions. Three other potential failure types are more likely in other cases: stepwise liquefaction failure, stepwise failure, and wobble failure. Their relevance increases with decreasing relative density and decreasing drainage potential. Brief recommendations for assessment of each failure type are presented.