Visualization of fractures induced around the gallery wall in Horonobe Underground Research Laboratory

摘要

In the excavation of a repository for high-level radioactive waste(HLW)disposal,the hydro-mechanical characteristics of the excavation damaged zone(EDZ)induced around the gallery wall must be understood because the EDZ can lead to the migration pathway of radionuclides.We performed herein a resin injection experiment at the 350 m gallery of Horonobe Underground Research Laboratory in Japan to investigate the characteristics of the fractures induced around the gallery wall during excavation.In this experiment,we developed a low-viscosity resin mixed with a fluorescent substance and injected it to the borehole drilled to be approximately 1 m in length.We then overcored around the injection borehole.The observation on the cut surface of the overcore under ultraviolet light revealed that the fractures were distributed within 0.8 m from the gallery wall.The fractures were interconnected with one another,particularly within 0.25 m from the niche wall.Furthermore,the fractures with a large aperture(ca.~1.0 mm)were developed in that region.These observation results will provide fundamental information for understanding the fracturing process in the EDZ.