Jinmyono Landslide

Location

Tanokuchi, Tochio City, Niigata Prefecture, (37*25'N; 139*39'E).

Date of Slide

December 26, 1989

Size of Slide

Length: 300 m; Width: 120 m; Depth: 25 m; Area: 3.6x105m2; Volume: 4.0x106m3.

Damages

1.5 ha of cultivated field, 790 m of roadways, and 5 reservoirs.

Geology and Mechanism of Failure

The site region is underlain by alternating beds of muddy sandstone and mudstone. Outcrops around the slide area expose mostly gray mudstone, and around the Mizushirizawa River, located near the northeastern corner of the slide, expose gray mudstone of relatively high lithification. Moderately weathered grayish brown mudstone interbedded with thin tuffs are observed in the head portion of the slide, and shows the development of joints that are oriented perpendicular to the bedding planes. The primary causes of landsliding include: 1) the bedrock is composed of mudstone with high a swelling potential; and 2) due to tectonic movement the bedrock is highly jointed and easily weathered. The contributing cause of landsliding is infiltration of prolonged rain water prior to the addition of snowfall and snowmelt. The past history of slide activity is unknown, however, there has been some erosion during every snowmelt season. Although it has been very slow, the slide has showed signs of instability since 1985. The current landslide was initiated near the head portion of the slide at the ridge areas where scattered reservoirs are located. The sliding debris reached the Mizushirizawa River, temporary ponding the upstream areas. There are 7 reservoirs of various size within the slide that have lost water storage capacity due to the development of cracks. Also, the growing capacity for rice paddies has been completely lost.

Mitigation Measures

Drainage channel works including drainage wells, drainage borings and interceptor under drains as have been constructed. Two years after the completion of the mitigation works, some leaks have been noticed in the head portion of the slide. However, it is assessed that the drainage control works are functioning effectively as a large discharge of ground water has taken place in the drainage borings. Furthermore, the results of the movement detection instrumentation indicate that stability has been achieved.