Comparison Between Blasting and Wire Sawing Regarding Hydraulic Properties of the Excavated Damaged Zone in a Tunnel – Experiences From Crystalline Rock at the Ӓspӧ Hard Rock Laboratory, Sweden

Ericsson, L. O. (Chalmers University of Technology) | Vidstrand, P. (Swedish Nuclear Fuel and Waste Management Co) | Christiansson, R. (Swedish Nuclear Fuel and Waste Management Co) | Morosini, M. (Swedish Nuclear Fuel and Waste Management Co)

OnePetro 

ABSTRACT: In tunneling with blasting, the tunnel periphery could be damaged (the EDZ, ”Excavated Damaged Zone”) with altered mechanical, hydraulic properties and reoriented stresses. In recent underground infrastructure projects, diamond wire cutting is used as an alternative excavation method. At the Ӓspӧ Hard Rock Laboratory in Sweden investigations have been conducted to determine hydraulic properties of the EDZ at water saturated conditions. Comprehensive hydraulic testing (hundreds of injection tests), in the floor of a blasted as well as a wire sawn tunnel section has made it possible to quantitatively compare hydraulic properties of the different excavation methods. Diamond wire cutting, as expected, greatly reduces the EDZ permeability compared to blasting.

1. BACKGROUND

Any excavation in bedrock may cause some kind of disturbance or damage in the rock closest to the excavated surface. Disturbances or damages could be caused by stress redistribution due to altered geometry and/or effects of the excavation method used (McEwen, 2005). For example, spalling could be initiated if the stress concentrations around the tunnel are so high that the tangential maximum stress exceeds the strength of the rock. Damage to the rock could however also be caused by the excavation method itself even if in situ stresses are not high enough to initiate spalling (Martino and Chandler, 2004; Hudson et al., 2009).

In Scandinavia and for crystalline rock types, drill-and-blast is the traditional and most commonly used excavation method for underground construction. In recent years, full-face drilling and wire cutting have also become more common in some situations. There are different issues related to the development and properties of a damaged zone around a tunnel due to excavation (the Excavation Damaged Zone, EDZ). In traditional tunneling, stability and leakage tasks are treated while for the nuclear waste industry it is important to understand the EDZ properties for long-term safety reasons. Over the years, the Swedish Nuclear Fuel and Waste Management Co., SKB, has carried out different studies on the presence and physical behavior of an EDZ (Winberg, 1991; Backblom, 2008; Ericsson et al., 2009).