Characteristics of cross section and occurrence site of the first-activated type landslides in the area of Shimajiri-mudstone, Okinawa Island: For the risk evaluation of landslide

This paper discusses the characteristics of cross section of slide and c'-tanφ' diagram of the landslides observed in the area of Shimajiri-mudstone of Okinawa, Japan. In addition, the pre-slide conditions of unstable slopes were given due consideration and an attempt was made to propose an advanced typological classification of first-activated type landslides. Obtained results are as follows: 1. Length and displacement of Shimajiri-mudstone landslides were found to increase with decreasing average gradient of the slip-surface. The values of tanφ'-intercept of the c'-tanφ' diagrams of the slides showed a tendency to increase as the average gradient of the slip-surface increases and vice-versa. 2. The occurrence of first-activated type landslide is influenced by the release of in situ stress, which is brought about by the displacement of a previous landslide or human-induced land alteration, such as cutting away at slope toes. The displacement of a landslide became larger when the slip-surface of a previous slide transformed into being a part of the slip-surface of a succeeding slide. 3. Pre-sliding slopes of first-activated landslides were classified into three types based on the land features and the geotechnical conditions: unstable slopes with a previous slide located in the lower part of slope, unstable slopes with a previous slide located in the middle part of slope and, thirdly, unstable slopes with cut away portions.