ABSTRACT

A tunnel in crystalline rock at 450 m depth in Äspö Hard Rock Laboratory, designed with an oval shape and aligned almost perpendicular to the maximum horizontal in-situ stress, was selected to investigate the extent of the Excavation Damage Zone (EDZ). Extensive information for understanding EDZ such as blast design, convergence measurements, ultra sonic measurements in several boreholes, geological mapping of tunnel and cores, 3D-laser scanning of the tunnel geometry exist in the vicinity or in section 47 of the tunnel. The mapped geological features and the tunnel geometry obtained from laserscanning were used to build a 3D model of the geology. This geological model was implemented in a numerical model to evaluate the rock mechanic response during the excavation. The modeling capability of the mechanical processes of importance for the EDZ was then tested against the measured data. From the numerical model, it was concluded that larger fractures and the uneven as-built geometry generate heterogeneous stress redistribution at different tunnel sections. This could induce local fracturing and asymmetry in the EDZ, which shows that the extent of the EDZ in a blasted tunnel is strongly dependent on local irregularities in the tunnel wall.

1 INTRODUCTION

The TASQ – tunnel (Figure 1) was developed in 2003 at the Äspö Hard Rock Laboratory (HRL), Sweden purposely for a large in-situ rock mechanics experiment, the Äspö Pillar Stability Experiment (APSE) Andersson (2007). It had an unusual shape, primarily because of the need to concentrate high stresses in the circumfencial of the tunnel, especially under the floor. This was achieved by excavation of a pilot drift and a bench with circular floor. The area is very well characterized from a geological and a rock mechanics point of view (Staub et al. 2004). 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) was reported by Olsson et al. (2004). In 2005, a part of the geometry was captured using the 3D laser-scanning technique for the DECOVALEX IV project. Furthermore, in 2006, the BGR borehole seismic equipment was tested in a section of the tunnel with the aim to estimate the extent of the EDZ in the tunnel (Schuster 2007). All of these activities were performed in the vicinity or in section 47 which is located 47 m from the start of the tunnel.

(Figure in full paper)

These different activities provide an extensive set of information of interest for understanding the EDZ. Data from convergence measurements during excavation, a slot for studying the EDZ and the borehole seismic survey was also collected within section 47. during excavation, the modeling capability of the mechanical processes of importance for the EDZ could be tested against actual set of data. Overall, this approach provided a unique opportunity to evaluate different methods used to characterize the EDZ by means of numerical simulation of the tunnel response, considering both fractures and as-built tunnel geometry.

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