A large re-slide occurred on a deep-seated landslide slope on September 21, 2013, 5 days after Typhoon No. 18

Wataru SAKURAI, Ryo SAKAI, Hikoyuki GOTO, Yoshito OGAWAUCHI, Hidetomo TATSUMI, Soichi KAIHARA, Tadakazu UTO and Yasumasa FUJIWARA

During Typhoon No. 12 (September, 2011), a large landslide dam resulted from a deep]seated landslide in Akadani area, in Gojo, Nara Prefecture. Subsequently, Kii Mountain District Sabo Office, Ministry of Land Infrastructure, Transport and Tourism, developed countermeasures. However, frequent debris discharge from the deep]seated landslide slope has disturbed the progress of the countermeasure works. On September 21, 2013, although no rainfall was observed, a large re]slide, with an estimated collapsed soil volume of 420,000m3, occurred suddenly after 5 continuous days of fine weather following the passage of Typhoon No. 18. Slope stability analysis using airborne laser scanner data obtained on September18, just before the re]slide, showed that its mechanism involved gully erosion at the foot of the remaining collapsed soil, which made it unstable. As it occurred before morning, no people were injured. Nevertheless, conducting countermeasures safely became more difficult, even in fine weather. Therefore, we plan to conduct a slope stability analysis using airborne laser scanner data before resuming work after a heavy rainfall.