The points of countermeasures for landslide dam adjacent to unstable deep-seated landslide slope in which frequent debris discharge occurs|Referring to the countermeasures taken in Akadani landslide dam caused by Typhoon 12, 2011

Wataru SAKURAI, Hikoyuki GOTO, Makoto OYAMA, Yoshito OGAWAUCHI, Takayuki MAYUMI, Soichi KAIHARA and Noriko TADAKUMA

The large deep]seated landslide and landslide dam followed heavy rainfall during typhoon No. 12 in September 2011 in the Kii Mountain range. A large landslide dam resulted from deep]seated landslide in Akadani area, Gojo city, Nara pref., therefore, Kinki Regional Development Bureau, Ministry of Land Infrastructure, Transport and Tourism, began to construct the temporary overflow channel as a part of urgent countermeasures in September 16th, 2011. Although just before its completion, large landslide occurred again, in June 19th, 2012, by heavy rainfall during typhoon No. 4. The temporary overflow channel was covered with thick debris, therefore, we had to abandon the recovery of that, and to change the plan for permanent countermeasures following the urgent countermeasures. Before of this re]slide, debris discharge from the deep]seated landslide slope had occurred frequently even in the dry season, and it had affected the progress of the countermeasure works. By the results of the survey to clarify the mechanism of the frequency debris discharge, it is seemed that flowing water origin in much spring water, resulted from concentration of underground water to the slope, scoured the foot of the debris deposition on the slope, and it made remained deposition on the slope more unstable, caused frequency debris discharge. These results prove that survey for evaluation of slope stability should be done in advance of plan or design for countermeasures and it decrease reconsideration and reconstruction, enable to progress countermeasures rapidly as result. In this term, we clarified the points to evaluate stability of slope with characteristic of geology, topography, debris discharge, hydrology of the slide slope after occurrence of deep]seated landslide. And more we considered the way of the countermeasures for the landslide dam under the unstable deep]seated landslide slope.