抄録
Many slope failures were induced by Typhoon 10, 2003 in Hidaka district, south Hokkaido. Two types of failures occurred on mudrock slopes: (1) smaller slope failures (soil slips) with slip depth of 35-120cm on slopes underlain by Miocene tuffaceous mudstone (Tm) and (2) larger slope failures with slip depth of 320-370cm on slopes underlain by Cretaceous mudstone and siltstone (MSi). To elucidate the difference in the scale and the form of these two types of failures, bedrock properties for weathering susceptibility and soil structure on slopes were investigated. A laboratory experiment of wet-dry weathering shows that Tm rock is disintegrated into finer materials than that of MSi rock. X-ray diffraction analysis shows that the alteration of minerals from rock to soil is not recognized, i.e., chemical weathering is weak, on both slopes. These weathering characteristics lead to the following mechanism of slope failures on the Tm slopes. The finer slope materials are produced on the surface of the slopes reflecting the grain size of the slaking materials. These finer materials having a lower permeability of the soil layer restrict the downward movement of soil water and the deeper front of wet-dry weathering, and therefore the soil layer does not become thicker. In this slope, rising of groundwater surface toward the slope surface during heavy rainfall causes the soil slips. In contrast, in the MSi slope, the higher permeability of the soil layer resulting from the coarser materials promotes the downward movement of soil water and the wet-dry weathering, and therefore the soil layer becomes thicker. Piping arising from deep water table triggers the larger slope failures.
