Investigation Study on Slope Failure in Kitakyushu City Due to Torrential Rainfall in July 2018

From June 28 to July 8, 2018, concentrated torrential rains occurred in a wide area nationwide, mainly in western Japan, including Hokkaido and the Chubu region, due to Typhoon No. 7 and the rainy season front, etc., and caused serious sediment disasters, including failure slope failure in Kitakyushu City, Fukuoka Prefecture, located in the northern part of Kyushu region. To protect the lives and property of residents from such slope disasters, we believe it is necessary to clarify the risk level of slope failure and the mechanics of slope failure to construct efficient and effective Slope Disaster Prevention measures. Therefore, this study was conducted on six slopes where failures occurred in July 2018 in the Kitakyushu City area to investigate the topographic and geological conditions that contribute to slope failures in detail through field observations, UAV topographic observations, and surveying. The Kitakyushu area is characterized by a relatively steep mountainous terrain with diverse geological compositions and complex geological structures, and we were able to investigate and study the failure slopes that occurred in several geological units, including granitic and paleo sedimentary rocks. The results showed that the six failures were shallow failures classified as surface failures, and all of them occurred in a strongly weathered zone. Although there were no significant differences in the failure modes due to differences in topography and geology, we were able to elucidate the mechanism of each failure.