ABSTRACT

ABSTRACT

Instrumentation, installed during excavation of a shaft in granite, monitored displacements, stress changes and temperatures in the rock mass, and piezometric pressures in vertical fractures. As well, the permeability of the fractures was measured after each excavation round. The experiment demonstrated that the permeability of fractures beside the shaft decreased twofold as the shaft was excavated past the fractures. Three-dimensional finite-element modeling simulated the stress changes and displacements reasonably well, but predicted the opposite trend for permeability change. It was concluded that: a) mechanisms occurred in the fractures which are not properly understood at present, b) future experiments of this type should measure fracture displacements at the same location as the piezometric pressures and permeabilities are measured, and c) a circular shaft is preferred to a rectangular shaft for this type of experiment.

1 INTRODUCTION

Atomic Energy of Canada Limited (AECL) is responsible for developing and demonstrating a methodology for the safe, permanent disposal of Canada's nuclear fuel wastes. Research efforts are being concentrated on geological disposal deep in stable rock formations of the Canadian Shield. As part of this research program, AECL is constructing an Underground Research Laboratory (URL) near Lac du Bonnet, Manitoba. The URL provides researchers with access to a previously undisturbed granitic rock mass to conduct a wide variety of in situ geotechnical experiments (Simmons 1986). Prior to starting construction, three years were spent characterizing the geology, hydrogeology, in situ stresses and intact rock properties at the site. Between 1984 May and 1985 March, a rectangular access shaft, 2.8 m by 4.9 m, was sunk to 255 m depth, and shaft stations were excavated at 130 m and 240 m depth. (Figure 1). Excavation was done by conventional drill and blast methods; 1.8 m bench rounds were used in the shaft. Shaft sinking was halted at depths of 15 m, 62 m, 185 m and 218 m to conduct detailed measurements of the near field hydraulic and mechanical rock-mass response to shaft excavation. Figure 1 shows the locations of the instrument arrays in the shaft and Figure 2 the layout of instrumentation at the arrays.

2 GEOLOGY

The URL shaft is located within the Lac du Bonnet granite batholith. Two thrust faults (referred to as fracture zones) are present between the surface and 300 m depth in the vicinity of the shaft (see Figure 1). These dip approximately 25 ° southeast. The top one (Fracture Zone #3 on Figure 1) is intersected by the shaft at 105 m depth and the second (Fracture Zone #2) lies 20 m below the shaft bottom. These fracture zones control the regional hydrogeology of the site (Davison 1984). Several splays of the Fracture Zone #2 occur in the region of the shaft and one of these intersects the shaft at 215 m depth (see Figure 1). Between the surface and the Fracture Zone #2 splay the rock is pink granite and contains a prominent vertical fracture set striking 030°; between the Fracture Zone #2 splay and the shaft bottom the rock is unfractured grey granite. The instrument arrays discussed in this paper are located in the pink granite.

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