Explanation of Geological Structure and Movement of the Taninouchi Landslide in Relation to Slope Evolution Processes

Abstract

Slope Evolution of a deep-seated landlide is investigated by using the data of borehole survey, groundwater tracing and seismic prospecting at the Taninouchi Landslide, southwestern Japan. Explanation of geological structure and movement of the present landslide which is located at the lower part of the whole landslide slope are also tried through consideration of slope evolution processes.Double ridges at the head of landslide slope were formed by an ancient large-scale rock slide which formed the whole landslide slope. Because there are thin oxidized detritus or fragmental rock layers just under the slip plane at the toe part of landslide slope, the rock slide mass covered the ancient ground surface, bulldozing clayey schalstein layer along the slip plane. Displacement by the rock slide amounts around 150 m. Longitudinally S-shaped slip plane behind the rock slide-covered ground is considered to have been formed by passive rupture of the toe part of landslide at the beginning. River channels were also shifted and thrust up to the present locations.Discontinuity of the schalstein layer between the toe part of landslide mass and the surrounding slopes is explained by the displacement of softened schalstein toward downslope along slip plane.Geological structure and feature of slip plane (Figs. 2, 8) along which the present landslide occurs can be understood through consideration of the slope evolution processes described above. Surfacial distribution of the present landslide movement namely tension and compression is correspondent to the feature of the slip surface. Present landslide phenomena should be understood in relation to landslide slope evolution processes.