Stability of submarine slope subject to repeated loadings

It is known from the results of investigations into the collapses of submarine slopes experienced in the past that such collapses are attributable to reasons such as earthquakes, waves, abrupt deposit of soil and construction of offshore structures. Concerning collapses of submarine slopes caused by repeated application of the external force such as earthquakes and waves, landslides of the submarine slopes are considered to have taken place as a result of reduction of the ground strength caused by liquefaction in the case of loose sand layers and by excess pore water pressure resulting from repeated loadings in the case of clay and silt layers. When we look into the cases of submarine slides in the world, we can see the case of Grand Banks (1929) where the submarine slide was caused by earthquakes in spite of the sea bed slope gradient of 1/152 and the case of the collapse of the Huanghe delta submarine slopes (1987) caused by waves. For calculation of the stability of submarine slope, there are many methods proposed, but no reliable method has been established so far. The paper describes the results of examinations of the stability of submarine slope against earthquakes and waves and the setting method of soil constants used for basic design of the foundation of the submerged floating tunnel projected in the Uchiura Bay, Hokkaido. The examinations were made by calculating the excess pore water pressure, ground surface displacement and response acceleration in a ground using the seismic response analysis method which is based on the effective stress analysis and is showing a rapid progress of late, and the safety factors against submarine slide were obtained in time series.