Abstract
There are discernible hydrothermal alteration zones along the Median Tectonic Line in central and western parts of Shikoku. The hydrothermal alteration zones mainly consist of smectite bearing clay around intrusive rhyolite body of the middle Miocene. The rhyolite body is not available in north east Shikoku, but in this part also, the hydrothermal alteration along the Median Tectonic Line consequently ensued lot of smectite bearing clay minerals in rockmass. The bedrock deterioration by the hydrothermal alteration was overlooked in the previous geological investigations, because it had been interpreted as the fault gouge and crushed materials. In this study, various problematic sites of hydrothermal alteration along the Median Tectonic Line were investigated in terms of clay mineralogy, shear strength and slope stability. The soils consist of smectite and smectite bearing mixed-layer clay minerals collected from fault gouge as well as clay veins possesses low shear strength. Therefore, due to continue reduction of soil strength, the landslides having low angle slip surface were observed even in the cut slope that were established several years ago.
