Shinano Landslide

Location

Shinano, Misato-Son, Oe-Gun, Tokushima Prefecture, (34_ 01'N; 134_ 15'E).

Size of Slide

Slope Length: about 300 m; Width: 200 m; Depth: 20-30 m.

Damages

The slide consists of three blocks. The initial failure occurred within the Block-B in 1949, and intermittently further cracking and failures developed. In particular, Block-A reactivated in 1971, followed by movement of Block-C in 1974. Block-C is located along the upper slopes of the slide, and has resulted in damages to 5 residential structures (one was completely destroyed, and two have been moved). In 1989, sediment runoff from the failures near the foot portion of the slide caused damages to the local roadways. Slide activity increased in 1994, were numerous tension cracks developed near the upper slope area causing differential settlement along the local road.

Geology, Mechanism of Failure and Type of Movement

Geology at the site area consists of rhyolite crystalline schist of the Sanbagawa Belt, and includes muddy schists and basic schists. The slide planes of the Shinano District Landslide are typically found within the weathered zone of relatively stable-appearing beds. The slide planes are not necessarily in the clay beds, but rather they are found in the upper portions of the moderately fractured and mildly weathered beds. As far as the morphology of the landslide development is concerned, the slide is a retracting-type landslide that originated along the north-flowing Kawada River, located about 500 m east (relief of about 300 m) of the site area. However, the boundaries where the mitigation measures were implemented are located on the slopes with higher elevations than 300 m, and are limited to the area where active movement has occurred since 1949. The contributing cause of sliding is suspected to be ground water, however, due to the quickly severed exploratory boreholes by the slide the movement, and the nature of the ground water movement within the bedrock formation, the movement is not clearly understood yet.

Mitigation Measures

This slide is very large, and because of its headword migration, it is considered very active. Consequently, the mitigation measures required an immediate effect, and therefore soil removal works and anchor works were implemented.