1 0 0 0 OA 透水性の異なる砂岩と泥岩からなる丘陵地における斜面崩壊の発生機構と発生位置
To study the effect of bedrock permeability on hill morphology, we performed hydrological measurements and a slope stability analysis in the hilly terrains underlain by unconsolidated sandstones and consolidated mudstones around Mt. Kanozan, Boso Peninsula, central Japan. The sandstones provide high round-top hills with the low drainage density, and the mudstones comprise low rugged hills with the high drainage density. The permeabilities of sandstone and mudstone were 2 × 10^<-3> cm/s and 5 × 10^<-6> cm/s respectively. Slope failures in both areas have distinctive features: (1) failure-generating areas in the sandstone hills are confined to the lower parts of steep slopes adjacent to valleys; and (2) the slope failures in the mudstone hills take place almost on convergent hollows locating at the upper part of the hill slopes. The rainfall-runoff characteristics differed in the two areas. No increase in discharge was observed in the sandstone areas even during rainstorms. This implies that vertical percolation and deep groundwater flows are dominant in the sandstone hills. In contrast, rapid runoff and a large peak discharge responsive to rainfall were found in the mudstone areas. This indicates that subsurface storm flows mainly contribute to the discharge in the mudstone hills. A stability analysis on slipped slopes in both sandstone and mudstone areas revealed a difference in their instability mechanisms. Slope failures in the sandstone hills were caused by reduction of soil cohesion as a result of increasing moisture content in the regolith. Slope failures in the mudstone hills result from increasing pore water pressure derived from a rising subsurface water level in the regolith zone. These differences appear to be a result of the different behaviour of percolated water according to the different permeabilities of the two bedrocks. Morphology in the hilly terrains around Mt. Kanozan, such as the relative heights of the ridges, must be controlled by these slope instability mechanisms and locations of the slope failures.