Panoramic view of the Zentoku Landslide
- 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.
- 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
- 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.
Conditions following the completion of the
mitigation measures for Block Z-1
Cross section of Block Z-1