Length: 600 m; Width: 1700 m; Depth: 40-50 m; Area: 100 ha; Volume:
1.96x107m3.
Damages
There had been landslide activities prior to 1940 and residents have
been evacuated from the affected areas. Recently, slide was reactivated
October 16, 1991 due to rainfall and cracks and steps were formed along
the National Highway Route 462 and local roads, residential and other structures.
Geology and Mechanism of Failure
The basement rocks of the landslide is crystalline rocks belonging
to the Sanbagawa Belt. Furthermore the Sanbagawa Belt is found throughout
the drainage basin, and landslide is wide spread. Today, recognized landslides
include large scale bedrock landslide (depth 40 to 50 m) having the slide
plane within the crystalline schist. The crystalline schist exhibits a
dip slope structure along the schistosity surfaces, joint surfaces and
fault planes and develops cracks which promotes deep chemical weathering
and over all weakening of the rocks creating the factors for the easy formation
of the slide plane. Beside the rock quality, geologic structure and other
internal factors, it has been concluded that the sliding was induced by
heavy rainfall (daily rainfall of 137 mm).
Mitigation Measures
The prefectural government has been responsible for the mitigation
works until 1994 which included the installation of 10 drainage wells for
the ground water removal. However, the National Government took over the
responsibility in 1995 and selected as nations 12th Landslide Mitigation
Works under the Jurisdiction of the Ministry of Construction. For the immediate
mitigation works, it has been decided to continue emphasis on the ground
water removal works (drainage well and drainage tunnel works as the past
dewatering system is not adequate.
