Kuchisakamoto Landslide

Kuchisakamoto Landslide

Panoramic view of the Kuchisakamoto Landslide

Location
Kuchisakamoto, Shizuoka City, Shizuoka Prefecture, (35*10'N; 138*15'E).
Date of Slide
DATE OF SLIDE: This region is underlain by numerous very large and old landslides. In the Area-A, the torrential rain associated with the early summer rainy spell in June 1988 reactivated a slide measuring the Length, about 700 m; the Width; about 300, and Area, about 21 ha. A large failure occurred at the base of the slope burring check dams. The torrential rain associated with the early summer rainy spell of July. 1993 accelerated the movement of the block measuring the Length, 120 m, the Width, 250 m, and the Area, 3 ha. Furthermore, on July 1993, large scale steps were formed along the Prefecture Highway, South Alps Park Road located at the upper portion of the slide area. In the Area-B, a concentrated heavy rain in February 1989 reactivated a slide measuring the Length 700 m, the Width, 150 m, and Area 11.8 ha creating a numerous stepped cracks on the slope. The maximum movement of that day, recorded about 200 mm at the head area and 400 mm at the toe area.
Size of Slide
Length: 1100 m; Width: 1500 m; Depth: about 50 m; Area: 180 ha; Volume: 1.5x107m3.
Geology, Mechanism of Failure and Type of Movement
The Kuchisakamoto Landslide is located along the eastern face of the drainage divide between the Ohigawa drainage basin and Abegawa drainage basin at elevation between 1300 to 1500 m. The Sasayama Tectonic Line with large scale faults runs adjacent to the eastern flank of the slope exhibiting a stepped topography which has been suspected to be structurally controlled. The geology includes marine sediments, predominantly of sandstone and shale of Mesozoic Era Cretaceous Period to Cenozoic Era Lower Tertiary. The shale beds are deeply weathered and severely fractured and the slide planes are formed along the shale beds which were altered to weak clay. The slide movement is very active with larger movement during the rain, therefore it is assumed that the increase in pore water pressure from rainfall is a contributing cause the movement.
Mitigation Measures
Because of the large size and the movement of the groundwater contributes to the landslide activity , the main thrust of the mitigation measure has been the ground water removal works including horizontal gravity drainage works, drainage well works and drainage tunnel works. In order to remove the ground water from the landslide efficiently, the dewatering system has been designed in three-dimension, i.e. multiple layers of horizontal vertical drains are drilled from the drainage wells and drainage wells are connected through the drainage tunnels.

Panoramic view of the Kuchisakamoto Landslide

Cross section of the Kuchisakamoto Landslide Area-A


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