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.

1. Introduction

In the excavation of a repository for the high-level radioactive waste (HLW) disposal, fractures are induced around the gallery wall because of the resultant stress redistribution. This fracture zone is called the excavation damaged zone (EDZ) (Tsang et al. 2005). The hydraulic conductivity in the EDZ can increase and then lead to the migration pathway of the radionuclides from the disposal facilities (Japan Nuclear Cycle Development Institute, 2000). Therefore, it is important to understand the fracture characteristics causing the enhancement of the hydraulic conductivity in the EDZ for the performance of the comprehensive HLW disposal risk assessment.

Many papers have reported the results of in-situ surveys and numerical analyses to study the hydro-mechanical behavior of the EDZ in various underground research laboratories (URLs) (e.g., Nuclear Waste Management Organization, 2011). The visualization of fractures in the EDZ is one of the best methods of investigating the fracture characteristics. For example, in the Mont Terri rock laboratory in Switzerland and Meuse/Haute-Marne URL in France, some researchers visualized the fracture network in the EDZ through a resin injection experiment and studied the results as supporting data to make a conceptual model of EDZ fractures induced around the gallery (Armand et al. 2007; 2014; Bossart et al. 2002; 2004; Kupferschmied et al. 2015).

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