Earthquake response of a nearshore gravity foundation sitting on a soft rock is studied with a special attention to sliding at the foundation base. Shake table tests are conducted for, a gravity foundation model partially submerged in water in order to observe the effects of sliding on the earthquake response of the foundation. In addition, large scale direct shear tests are carried out to investigate the characteristics of the friction between the foundation base and rock bed during the cyclic response of the foundation. Reviewing the direct shear test results, the coefficient of friction during the cyclic motion is formulated. The proposed formulation is first verified with the shake table test results and is used in the parametric studies of earthquake response of a nearshore gravity foundation which allows for sliding at the base. Various important and interesting features are found.
Construction of a large floating structure in the nearshore may be one of the attractive ways of ocean space utilization. Such a structure is particularly advantageous in the active seismic areas mainly because water can not transmit the earthquake ground motions to a floating structure. However, the safety of such a structure requires special consideration of the problems which are unique to the ocean environment. A gravity foundation may be used for anchoring a large floating structure. Since a partially or fully submerged foundation in the ocean is subjected to buoyant forces, it may easily slide at the base during earthquake excitation. Therefore, it is important to evaluate the earthquake safety of an anchor gravity foundation with consideration of sliding at the base. "The study presented herein considers this problem for a partially submerged gravity foundation placed directly on a soft rock seafloor.