[H-DS07] Landslides and related phenomena

[HDS07-01] Effects of narrow alteration zones with clay veinlets on soil layer structure of rain-induced slope failures within rhyolite areas in Hiroshima, Japan

*Yasuto Hirata¹ (1.Central Research Institute of Electric Power Industry)

Keywords:shallow landslides, clay veinlets, shear strength, halloysite

Clay veinlets associated with hydrothermal alteration in igneous rocks are known as a geological structure that regulates the shape of slope failures, but it is unclear whether they enhance occurrence of rain-induced shallow landslides. In this study, the author investigated the distribution of alteration and weathering in a weathered mantle area of the Late Cretaceous Takada Rhyolites in Hiroshima, Japan, where slope failures were densely triggered due to heavy rainfall in July 2018, and compared two different soil profiles of slope failures in the area: One is on a few meters wide and vertical alteration band with clay veinlets (altered rock). The other is on unaltered but strongly weathered rock (weathered rock). In the heads of those slope failures, soil and surface weathered rock were sampled to measure bulk density, porosity, saturated hydraulic conductivity, and shear strength. The slid material in both altered and weathered rock areas was mainly 1–2 m thick of soil and the scar heads exposed piping holes in the lower part of the soil layers. Soils have similar ranges in saturated hydraulic conductivity (10⁻⁴–10⁻⁶ m/s) and porosity (40–55%). However, vertical changes in hydraulic conductivity and grain size distribution and shear strength were different. In the soil profile of the altered rock, saturated hydraulic conductivity and clay content decrease in the depth direction. In that of the weathered rock, saturated hydraulic conductivity increases slightly, and the clay content is constant. The shear strength of the soil samples under water in the direct shear box was greater in the uppermost part of altered rock (3.0 kPa, 38.8°) than in that of weathered rock (2.4 kPa, 31.8°). The altered rock showed a large strength difference between the peak value and residual value, whereas the weathered rock made no difference. These results indicate that the uppermost part of the altered rock loses clay and strength to become soil, and that of the weathered rock has already been washed out and disintegrated. Despite these differences in soil structure, the number ratio of the two soil types in all observed landside scars that occurred during the heavy rainfall event was almost the same as that of outcrop types showing the altered rock and weathered rock in the survey area. This suggests that the presence of clay veinlets in the weathered mantle of rhyolite had no significant effect on the landslide density caused by the heavy rainfall.