2022 Volume 29 Issue 2 Pages 112-118
Since fractures in the crystalline rock such as granite can act as the pathways for groundwater flow and mass transport, understanding of the fracture distribution is an important subject for the disposal of high-level nuclear waste. Fracture data obtained through the borehole investigation from the ground surface contains errors due to the crossing angle between boreholes and fractures. Therefore, I studied method of borehole investigation that can effectively characterize the fracture distribution based on fracture information obtained from geological investigation of ventilation shaft wall at the Mizunami Underground Research Laboratory. As a result, it was found that inclined lines on shaft wall captured larger number of fractures, especially high-angle fractures, than vertical ones. This indicates that inclined boreholes are preferable for efficient capture of the fractures. However, since the fracture orientation varies with depth, it is difficult to uniquely determine the drilling direction. On the other hand, predominant orientation of fractures at surface exposure, which locates close to Mizunami Underground Research Laboratory, is well concordant with average of strike and dip of fractures observed at ventilation shaft, which implies that the drilling direction should be determined based on the result of surface fracture mapping.
