Synthetic Investigation for Landslides from the Standpoint of Engineering Geology

-A Case Study in the Yuya Peninsula, Northwestern Yamaguchi Prefecture, Southwest Japan-

Abstract

A number of landslides occur in the Yuya Peninsula, northwestern Yamaguchi Prefecture. This area contains landslide prevention regions where have designated by the Ministry of Construction or the Ministry of Agriculture, Forestry and Fisheries. Countermeasure projects have been conducted independently by each ministry for their respective regions. Recently, evil for the independence of the countermeasure projects have been evident, because there are differences in the interpretation of landslide mechanisms and the policy of countermeasures among the prevention regions. Macroscopically, these prevention regions are found to have geographical and geological features that are common in the Yuya Peninsula area. In this study, nine landslide prevention regions in this area were selected as case study sites, and the landslides in these regions were analyzed synthetically. This analysis indicated that a number of small landslides at shallow depths originated from a large-scale landslide that occurred in the past. The slipping surface of the large-scale landslide was estimated to be located at a depth of more than 50m in a geological cross section. The small landslides around communities which are located on gentle slopes have been investigated, and countermeasures for small landslides have been taken in each prevention region. However, it is necessary to take into account the large-scale landslide at large depths, in mitigating the natural hazards caused by landslides. Further projects for the prevention of landslides in this area should be planned from a standpoint of macroscopic engineering geology. Those projects must also be free from inter-ministry conflict regarding designation of the regions of landslide management.