Hillside landslides frequently occur in many parts of Japan during the period of heavy rains every year. In making a hillside landslides model and investigating the stability of a mountain slope throgh this model, the results of numerical experiments will be considerably influenced by values of specific characteristics of the soil. In this study, to comprehend the vertical characteristics of soils on the mountain slope, an investigation into mountain soils was performed. As the result of this investigation, it was confirmed that the mountain slope soil consists of Two layers and that the boundary line exists approximately 50 cm under the surface of the mountain slope. Further, to improve accuracy for predicting hillside landslides through a numerical simulation model, the Two layer hillside landslides model, that considers the surface soil layer on a montain slope as a Two soil layer structure, was proposed. This Two layer model was prepared through improvement of an earlier One layer model that had been proposed. This Two layer model was applied to the basin where many hillside landslides occurred and the following matters were confirmed: a) The Two layer model is available to study in detail the behavior of groundwater level and slope stability and the time when hillside landslides occur, on a specific slope. b) On the other hand, the One layer model is available to study generally the site of hillside landslides and the final distribution of hillside landslides.
Debris-flow development processes were observed with a small flume. Through the experiments, debris-flows were classified into two kinds in terms of their longitudinal shapes. The first, steep slope debris-flow, develops in height and length as it flows. The other, mild slope debris-flow, experiences only length increases and no change in height. The steep slope debris-flow appears in areas in which the gradient is steeper than the critical gradient for the stability of saturated slopes. The mild slope debris-flow is found in regions flatter than the critical gradient. Steep slope debris-flow develops while keeping its shape, a similar triangle. Based on this fact, the shape of debris-flow at the downstream end of the developing reach, that is the steep slope, can be predicted after estimating total debris-flow volume and the width of torrents. The applicability of this method was proved by applying the method to actual cases of debris-flows.
Mass concrete has been used as the matrial to construct sabo dams in Japan. Various types of sabo dams, for example slit sabo dams, are currently designed against bed load, debris flow, pyroclastic flow and lava flow. Reinforced concrete may be better than mass concrete for these sabo dams in certain cases. In Austria, most sabo dams are made of reinforced concrete and the history, types and design methods of reinforced concrete sabo dams have been investigated. The design method was compared with the Japanese present standard. Throgh the studies it was found that various types of reinforced concrete sabo dam could be introduced to Japan and that they could be economical in some cases.