Prevention Works for ‘Namasu landslide’ in Gunma Prefecture and Behavior of the Landslide During the 2011 Earthquake off the Pacific Coast of Tohoku

Abstract

A large ancient landside, called Namasu, is formed in North Western region of Gunma prefecture Japan. This landslide is 600 m wide, 600 m long, and the maximum thickness is 90 m. During 1982, the landslide mass moved and the bridge, that crosses the river along the toe of the landslide, deformed. The drainage works, as the main prevention measures, started in 1989 and have been completed in 2010. The drainage wells that have been installed during the last 5 years had been planned and designed by seepage analyses, using the FEA. This paper presents assessment of effectiveness of the drainage works by monitoring of the ground water and the seepage analysis. Effect of the drainage works on the behavior of the landslide, during the 2011 earthquake off the Pacific coast of Tohoku, is also examined.