富士山の雪代に関する基礎的研究 (故大草重康教授追悼号）

Slush avalanches and debris flows often occur on the slope of Mt. Fuji. It is common that the shear plane exists in the upper thawed soil layer on the frozen soil layer. As the permeability of the frozen soil layer is extremely low in comparison to the upper thawed layer, ground seepage can easily develop during rain fall. Therefore, the instability condition of the slope is obtained mainly due to the seepage force developed in the thawed soil layer. This report describes the summary and results of a model experiment in relation to the mass flow, using soil samples obtained from the slope of Mt. Fuji. The result show that the slope stability problem of Mt. Fuji can be solved with the conventional analysis being used in geotechnical engineering, if the seepage forces are properly estimated. For the prediction of slush avalanches, the following factors are at least needed: (1) thickness of thawed soil layer and depth of frozen soil layer, (2) thickness of snow deposits, (3) water level in thawed layer, and (4) slope angle.