Zentoku Landslide

Location

LOCATION: Nishiiya-Son, Zentoku, Miyoshi-Gun, Tokushima Prefecture, (33_ 52'; 133_ 50'E).

Size of Slide

Maximum Length: 900 m; Maximum Width: 2000 m; Landslide Prevention Area: 220 ha.

Damages

It is speculated that the Ansei Earthquake of 1854 may have triggered the Zentoku Landslide. The Taniara Block (Z-6 Block) started to slide in the early 1870's, and continued to move after 1945. Prefecture Highways suffered significant damages, as did residential structures, retaining walls and agricultural fields.

Geology, Mechanism of Failure and Type of Movement

Geologically, the site area is comprised of the Sanbagawa crystalline schist and is sandwiches between the Median Tectonic Line and Okahoko Tectonic Line. The Sanbagawa Zone near Zentoku consists of argillaceous schist and sandy schist. Due to large scale tectonic movement, the rocks are severely fractured and highly weathered. Furthermore, the site area is in a dip slope condition facing the Iya River, which is the primary cause of sliding. Abundant ground water flows are supplied by heavy annual rainfall rates (2000 to 2500 mm annually). The landslide triggering mechanism was attributed to rapidly increased ground water levels from torrential downpour associated with typhoons.

Mitigation Measures

Based on the topography, the Zentoku Landslide is divided into 7 large blocks. From 1992 to 1994, 20 caisson piles, 280 anchors, and 5 drainage wells have been constructed on Block Z-1 (the largest amount of mitigation measures among all 7 blocks). Although less in scope, the adjacent Block Z-2 also received anchor works and drainage well works in 1988 and 1989. For Block Z-6, located near the upper reaches of the river, control measures that have been implemented include surface drainage control works, drainage wells and horizontal gravity drain works. Today, a comprehensive future mitigation plan is an important subject.