Relation Between Soil Profiles and Surface-Wave Survey Results for a Weathered Rhyolite Slope in Hiroshima, Japan

Abstract

We conducted microtopographic interpretation, classification of soil profiles, dynamic cone penetration tests, and surface-wave surveys for a rhyolite hillslope around slope failures caused by the heavy rain of July 2018 in Hiroshima, Japan. The dispersion curves of surface waves showed a fundamental mode on survey lines along the slope ridge and a fundamental mode（a partially include a higher mode）from the slope crest toward the upper side slope on the survey lines, but the dispersion was unclear at the bottom of a soil-creep area with tree tilting and the lower side slope. S-wave velocities from the fundamental mode were correlated with the penetration resistance（Nc value）below 25. Within the soil, the Nc value and S-wave velocity were measured to be <10 and <175 m/s, respectively. In the soil-creep area, the boundary between weathered rock and soil layer is rough, and a weathered rock mass with open cracks has been disaggregated into blocks in the soil layer. The disrupted soil structure may have obscured the dispersion curves. These results show that the thickness of the soil layer can be determined with the surface-wave survey from the slope ridge toward the upper side slope in rhyolite slopes, but may not be possible in soil-creep areas and the lower side slope.