Abstract
The authors investigated slopes that failed during heavy rainfall, focusing especially on those in Yamaguchi prefecture, in terms of geology, petrology, and ground engineering. In our previous paper, it was shown that many of these slopes failed due to the sliding on discontinuous planes such as joint and schistosity planes. Furthermore, irrespective of the kind of rock involved, those failures have frequently occurred along thin glossy black soil layers with very smooth surfaces that have precipitated on discontinuous planes.
Thereafter we also had the opportunity to examine six slopes in Yamaguchi prefecture which were either stable or had failed, and on whose discontinuous planes existed thin glossy black soil layers with a thickness of 0.4 to 6.0mm. As the same results as those obtained by previous study, chemical, mineralogy, and physical examinations showed that the black coloring is due to amorphous manganese oxides and hydrates having fine grains. Furthermore, direct shear tests have shown that the internal friction angle between the very smooth black soil layers is 23.3°, and the cohesion is 7.6kPa. We suggest that the strength parameter should be used for stability analyses of slopes having thin glossy black soil layers on discontinuous planes.
