In high-level radioactive disposal projects, it is important to investigate the extent of the excavation-damaged zone (EDZ) for safety assessment because EDZ can provide a migration pathway for radionuclides from the facility. To investigate the quantitative differences between EDZs formed because of blasting and mechanical excavation, we studied the characteristics of fractures induced by excavation based on fracture mapping performed during shaft sinking (V- and E-Shafts). As a result, it was found that blasting excavation can lead to the formation of a large number of newly created fractures (EDZ fractures) compared with mechanical excavation. In addition, the seismic velocity (P-wave velocity) measured during blasting excavation (E-Shaft) was lower than that measured during mechanical excavation (V-Shaft). Furthermore, we found that the support pattern that reinforces forward rocks to be appropriate for limiting damage to the shaft wall.

1 Introduction

The construction of underground facilities induces fractures in the surrounding rock mass because of the resultant stress redistribution. This has particular implications for projects related to the disposal of high-level radioactive wastes (HLW), where fracture development creates an excavation-damaged zone (EDZ) that increases the permeability of the surrounding rock mass and potentially creates a pathway for the migration of radionuclides from storage facilities (Tsang et al. 2005). Therefore, in the construction of a repository for HLW disposal, it is important to understand EDZ development and the behavior of rock mass around the facility.

A HLW disposal repository is spread over a few square kilometers, and the associated tunneling length is around 300 km (JNC, 2000). Therefore, it is necessary to apply an efficient excavation method to reduce the time required for and the monetary cost of excavation.

To this end, blasting excavation is applied in various tunneling projects. However, it has been reported that the extent of EDZ in blasting excavation is considerably larger than that in mechanical excavation by using a roadheader or a tunnel-boring machine (e.g., Sato et al. 2000; Siren et al. 2015; Tokiwa et al. 2018). This is one of the disadvantages of blasting excavation in the construction of a repository for HLW disposal. Therefore, it is important to investigate an efficient excavation method that limits EDZ development.

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