Jizukiyama Landslide

Location

Nagano-City, Nagano Prefecture, (36*40'N; 138*14'E).

Size of Slide

Length: 700 m; Width: 500 m; Depth: about 60 m; Area: 25 ha; Volume: 3.5x106m3.

Date of Slide

July 26, 1985; around 5 P.M.

Damages

25 deaths, 4 serious injury, 50 residential structures destroyed, 5 half destroyed, 9 partially destroyed. Other damages include destruction of forest, roadways, water distribution system and other infra structures.

Geology, Mechanism of Failure and Type of Movement

The site area is underlain by the Upper Tertiary Late Miocene rhyolitic tuff and the investigations following the sliding revealed that the rock has unique characteristics of alteration and rupturing. As for the history of slide movement, a small movement was observed in 1981 and investigation has been conducted by Nagano Prefecture Enterprises Bureau which administer the tall road in the area. The slide movement has been accelerated by an early summer rainy spell of 1985 which produced unusually heavy rainfall. Finally, in the early evening of July 26 the large scale landslide occurred. The slide mass maintaining the unity failed rapidly. The debris flowed to the Yudani Subdivision located to southeastern direction and Shojuso, a home for aged and Bogakudai subdivision located to south.

Mitigation Measures

Immediately following the sliding, landslide mitigation measures and restoration works have been implemented. The mitigation measures included large diameter cast-in-place pile works, anchor works, pile works as the central measures; and construction of drainage wells and drainage tunnels as restraint measures. The works were started in 1986 and completed in 1987.