Length: 800 m; Width: 350 m; Area of Landslide Prevention Zone: 149.12
ha; Volume: over 6.0x107m3.
Damages
DAMAGES: There has been landslide movement in past history however,
activity has increased in recent years. Accelerated movement occurred after
torrential rains in June, 1992. Immediate damage included destruction of
a small check dam, deformation of 8 small check dams, and tension cracks
that developed along the slope. Further movement of the slide mass damaged
residential structures and the National Highways, and buried Kashio River
which created the potential for a large scale debris flow on the central
area of Ohshika Village (located downstream of the slide area).
Geology, Mechanism of Failure and Type of Movement
The Median Tectonic Line traverses along the west side of the landslide.
A fragmented synclinical axis extends along the regional structure on the
upper slopes where the bedding dips into the slope. The site area is underlain
by high pressure metamorphic rocks while the lower slopes of the hillside
are underlain by muddy schists, basic schists, and siliceous schists of
the Sanbagama Belt with intrusive bodies of serpentine. The upper slopes
are underlain by geenstone, peridotite and serpentine of the Mikabu Belt.
As a whole, the schistose rocks are severely fractured and create favorable
conditions for argillization. The currently active landslides consist of
multiple slide blocks, and the slide planes of the overall blacks coincide
with the deep seated slide plane of the Upper Block. The slide plane has
a maximum depth of 70 m. The extension of this slide plane is connected
to the slide plane of the Lower Block. The maximum depths to the slide
planes of the Upper Block and the Lower Block are 30 m and 50 m, respectively.
Mitigation Measures
By 1994, the slide movement has slowed down due to the effective mitigation
measures. However, the Upper Block shows an annual movement of 300 mm,
therefore mitigation works must be continued.
