Abstract
Slope failures occurring near earthquake centres have attracted much attention since the 2004 Niigata-ken Chuetsu Earthquake. The 1995 Hyogoken-nambu Earthquake affected large modern cities and urban infrastructure, but hilly and mountainous areas suffered heavy damage from the Niigata-ken Chuetsu Earthquake. During that earthquake, numerous landslides occurred in Koshi of Nagaoka city (formerly Yamakoshi village). Social problems developed when many towns became isolated because landslides cut off traffic and public service lifelines. Soil from landslides closed river channels and formed natural dams along the Imo River, which flows north to south in Koshi. The natural dams submerged some towns and emergency measures were promptly undertaken to prevent debris flows caused by natural dam breaks. In addition, many surface slides also occurred at steep slopes. The endogenous and exogenous factors of those landslides must be clarified because of their few precedents. For that purpose, topography and geology in this region are arranged according to each failure type. In addition, triaxial tests of saturated and unsaturated soils obtained from a slope failure site were conducted to examine the soils' strength properties. This research is intended to propose estimation items that can help to judge the risk of natural slope failure.
