ABSTRACT

The development of an excavation damaged zone, EDZ, due to a blasting impact and stress redistribution after an excavation, can influence the long term stability, economy, and safety of an underground excavation. In this study, the size and characteristics of an EDZ around an underground research tunnel, which was excavated by a controlled blasting, in KAERI were investigated. From in situ tests at KURT, it was possible to determine that the size of an EDZ was about 1.5m. goodman jack tests and laboratory tests showed that the rock properties in the EDZ were changed by about 50% compared to the rock properties before a blasting. The size and property changes in the EDZ were used for a hydro-mechanical coupling analysis. In the modeling, the rock strengths and elastic modulus in the EDZ were assumed to be decreased by 50% and the hydraulic conductivity was increased by 1 order of a magnitude from the original values. From the analysis, it was possible to conclude that the displacement was increased while the stress was decreased because of an EDZ. When an EDZ was considered in the model, the tunnel inflow was increased by about 20% compared to the case without an EDZ.

1 INTRODUCTION

Because of a blasting impact or stress redistribution after an excavation, the original properties of a rock or rock mass near a tunnel wall are changed. The determination of the characteristics of an Excavation Damaged Zone (EDZ) is important, because such a disturbance can significantly influence the mechanical, hydraulic, and thermal characteristics of a rock mass and the overall performance of an underground space. In the case of underground facilities such as an underground radioactive waste repository, which requires higher safety criteria, an investigation of the influence of an EDZ on the thermal, hydraulic, and mechanical behavior of the rock mass around a repository is essential. In the DECOVALEX project, which is for understanding the complex THMC coupling behaviour around radioactive waste repositories, an EDZ is an important research item. For instance, during the last phase of the project, DECOVALEX-THMC, 3 of 5 tasks were related to an EDZ.

It is expected that the development of an EDZ will influence the performance of underground excavations. The understanding of the influence of an EDZ can help in selecting an adequate tunnel geometry, excavation method, and tunnel support depth, blasting technique, rock condition, in situ stress, and tunnel shape should be investigated. Furthermore, the development of simulation techniques for modeling the influence of an EDZ on a thermo-hydraulic-mechanical behavior is also required.

To investigate the size of an EDZ and the characteristics of it, in situ tests have been carried out in many URLs including the Aspo Hard Rock Laboratory and Stripa mine in Sweden, the Kamaish mines in Japan, the URL in Canada, the Mol in USA, the Olkiluoto research tunnel in Finland, and the Mont Terri in Switzeland (Dale and Hurtado, 1996; Backblom and Martin, 1999; Sato et al., 2000; NEA/RWM, 2002; Bossart et al., 2002).

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