Relationship between the distribution points of sand containing difficult-to-settle fine particles and the geological structure in an area upstream from a dam: The case study on Hitotsuse Dam Basin, Miyazaki

Through a soil survey, the authors collected soil samples from different strata at locations where landslip had occurred in the upstream area of a dam reservoir suffering from long-term turbidity. And soil specimens that might be responsible for accelerating the turbidity were obtained from the soil samples, using a simple suspension method. The soil specimens were categorized into four risk levels, indicating their degree of turbidity. Further, we prepared a detailed geological block diagram, which specifies different geological structures based on metamorphic grades. As a result, it was found that there was a significant correlation between the distribution of geological structure and the locations where high-risk-level soil samples were collected. This information is useful for determination of the distribution of soil structure at specific locations where there is soil causing turbidity, contributing to effective countermeasures for long-term turbidity in dam reservoirs.