Evaluation of excavation response in sedimentary rock mass

Sugihara, Kozo*; Ninomiya, Yasuo*

Among many R&D categories for the performance assessment of geological disposal, it is very important to develop the techniques to measure and to evaluate the disturbed zone due to excavation. This study aimes to investigate the disturbed zone around the drift excavated in the sedimentary rock formation and to evaluate the accuracy, reliability and effectiveness of existing techniques for excavation, measurement and analysis for the use of the repository construction. For this purpose, a drift (2.52.5m, L=30m) was excavated by NATM. Rock mass displacement, rock bolt stress, permeability and pore water pressure were measured in and around the drift. Using this drift as measuring drift, a test drift (3.03.0m, L=20m) was excavated parallel to the measuring drift. As the supporting system of this drift only rock bolting was employed. In-situ measurement of rock displacement, elastic wave velocity, permeability and initial stress have been carried out. These results were compared with the predicted results to investigate the effectiveness of the existing evaluation techniques. Laboratory test results and geological mapping data were also used to understand the rock mass behavior and the disturbed zone around drift. These investigations revealed that low velocity zone of elastic wave was generated around the drift, and in the zone, permeability decreased and displacement increased. Rock mass movement was predicted roughly by numerical analysis based on the laboratory test results. And detailed numerical analysis considering the initial stress data and disturbed zone around the drift, provided good simulation of rock mass movement. But measurement technigues of mechanical and hydrological properties of narrow zone adjacent to the drift wall and understanding of relationship between mechanical property change and hydrological one and phenomenon of disturbed zone generation remained as future study subjects.