Study on investigation method of fracture distribution based on data obtained at the Mizunami Underground Research Laboratory, central Japan

Sasao, Eiji   

Since fractures in granite act as pathways for groundwater flow and mass transport, understanding of fracture distribution is an important subject for the disposal of high-level nuclear waste. Fracture data obtained through the borehole investigation from ground surface contains errors due to the crossing angle between boreholes and fractures. I studied method of borehole investigation that can effectively characterize the fracture distribution based on fracture information obtained from geological investigation of shaft wall at the Mizunami Underground Research Laboratory. As a result, it was found that inclined boreholes captured larger number of fractures than vertical boreholes. Therefore, inclined boreholes are preferable for efficient characterization of the fractures. Fracture orientation at surface exposure is well concordant with that at the shaft, which implies that the drilling direction should be determined based on the result of surface fracture mapping.