Design and Verification of Support System for Underground Excavations Under Anisotropic Stress Conditions

This study addresses and demonstrates an economic support design scheme considering in-situ stress orientation and excavation direction on the basis of the work conducted in the Horonobe Underground Research Laboratory (URL) project. The URL project has been conducted by the Japan Atomic Energy Agency in Horonobe, Hokkaido, Japan to verify the technical reliability of geological disposal of high-level radioactive waste and to improve safety-assessment techniques. The project consists of shafts and tunnels to a planned depth of 500 m in sedimentary rock. Construction began in November 2005; as of the end of June 2015, the shafts and galleries have been completed to a depth of 350 m. The tunnel at the 350-m level is laid out in a loop. In construction of a loop tunnel in a rock mass under anisotropic in-situ stress conditions, the in-situ stress orientation and excavation direction are expected to affect the convergence associated with excavation-induced stress redistribution. This suggests that it is possible to achieve economic optimisation by selecting a tunnel layout, wherein only small displacement and stress are allowed on the basis of in-situ stress orientation obtained from surface investigation. Therefore, herein, a comparison between the predicted convergence used for support design and the measured convergence during excavation was conducted to validate the convergence predicted from the orientation and magnitude of the in-situ stress obtained from the surface drilling investigation. The results indicate a positive correlation between the predicted and measured convergence. The data falls within a range estimated on the basis of the difference between the calculation and measurement of deformation coefficients. These results suggest the validity and reliability of the in-situ stress obtained from the surface investigation and the support design using the in-situ stress data.