ABSTRACT

An in-depth conceptual understanding of the hydraulic conditions nearby and in the surroundings of an underground facility is a multi-disciplinary task. The Äspö Hard Rock Laboratory (HRL), Sweden provides an opportunity to explore the parameters of importance for understanding the development and characteristics of the Excavation Damaged Zone (EDZ).

Detailed characterisation and modelling of the geometry of the blast induced fractures in crystalline rock that forms the flowing system indicate weakly connected flow paths. However, there may be special hydraulic effects in relation to the open boundary of the tunnel wall. Even though the focus of a repository for nuclear waste deals with long term saturated flow in the post-closure phase, there is a need to understand the physical and chemical evolution in the bedrock in a broad perspective. The hydraulic conditions in the vicinity of the tunnel are dependent on the excavation method, as well as the construction materials used during tunneling. Recently developed methodologies for designing grouting schemes regarding hydraulic testing, determination of fracture transmissivities and fracture connectivity provide guidance on how to model the hydraulic aspects of the EDZ.

1 INTRODUCTION

The Äspö Hard Rock Laboratory (HRL), Sweden provides opportunity to explore parameters of importance for understanding the development and characteristics of the Excavation Damaged Zone (EDZ). Two tunnels, named the Q and S tunnels at the 450-m level, have been used for this study, Figure 1.

2 SITE CONDITIONS

The crystalline rock mass is dominated by diorite and is cross-cut by two sub-vertical fracture sets, trending NW and NE. The Q and S tunnels are aligned at a large angle to the NW joint set, which is also found to be the most water bearing set. In addition, there is a gently dipping joint set.

The major horizontal stress is in the order of 28 – 30 MPa at the 450-m level, trending NW – SE, sub-parallel to the NW joint set and, consequently, perpendicular to the two tunnels. The minor horizontal stress and the vertical stress are both close to the gravitational stress, 12 – 13 MPa. (Andersson, 2007).

The mean uniaxial strength of the diorite is 211 MPa and the Young's modulus is in the order of 76 GPa.

(Figure in full paper)

3 TUNNEL EXCAVATIONS.

Both the Q and S tunnels were excavated with the drill and blast method (D&B). The Q-tunnel was excavated in 2003. The tunnel was designed with an unusual shape, primarily because of an experimental need to concentrate high stresses in the circumference of the tunnel, especially under the floor. This was achieved by excavation of a pilot drift and a bench with a half-circular floor. The blast design, blast sequences and follow-up with the in-depth investigation of slots cut in the tunnel wall and the floor of the Excavation Damaged Zone (EDZ) were reported by Olsson et al. (2004). This included cutting slices of rock out of the tunnel wall for visual inspection of the blast damage.

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