Kensuke Goto

A lot of slope failures occurred on sideslopes of the mountain at Dazaifu city, Fukuoka pref. due to the 2003 Kyushu heavy rainfalls. In this study, at the upper reaches of Hara River, we investigated a sideslope where two slope failures had occurred. Then we discussed three relationships between joints of basement rock and landforms, soil horizon and slope failure, and the degrees of soil creep estimated by deformation of trees and landforms. Three joint systems exist in the basement rock. We classified the sideslope into seven landforms, crest, deep side-hollow, shallow side-hollow, terrace of smooth slope, landslide scar of valley floor, cliff and convex break of slope. Thick soil overlies basement rock and is creeping from crest to valley floor. Many forms of the basement rock were also classified. They are valley, hollow and stairs affected by the joint systems, and two buried convex breaks of slope affected by climatic change. These breaks of slope do not always exist just under the convex breaks of slope on the surface. The strength of soil creep is larger where the level of basement rock drops suddenly and at the valley side of convex break of the surface than other locations. On the basement rock, the soil creep is small at the crest side and is large at the valley side of the buried convex break of slope in strength. The soil which arrives and stagnates in the valley makes the shallow side-hollow and terrace of smooth slope. The causes of the slope failures are presumed that the soil above weathered angular gravels or decomposed granite was saturated with water by the heavy rainfalls, and then the stability of the soil decreased. The joint systems on the basement rock affect the forms of sideslope, forms of basement rock and landforms, and trends of soil creep at the area. Consequently, we propose to survey not only landforms but also soil, trees, and joint systems of basement rock for the available prediction of the area induced slope failure.