ABSTRACT:

The Underground Research Laboratory (URL) access shaft was excavated from the surface to about the 185-m depth in fractured pink granite. Below this depth to the 443-m depth the shaft was excavated in massive grey granite. The grey granite is essentially unfractured, except for a major low-dipping thrust fault (Fracture Zone 2) and associated minor splays (Fracture Zones 2.5 and 1.9). Observations (time-dependent behaviour, shaft-wall failure, and core discing), hydraulic fracturing and overcore measurements indicate that unusually high in situ stresses may be associated with large volumes of massive unfractured granite at fairly shallow depth. Interpretation of observations and measurements indicate that the maximum horizontal stress (σhmax) could range from 40 to 100 MPa below Fracture Zone 2. The database of in situ stress measurements collected at the URL indicates that major geological features, such as thrust faults, can act as boundaries for in situ stress domains, and that both the magnitude and direction of the in situ stress state can change when these geological features are traversed.

1. INTRODUCTION

Atomic Energy of Canada Limited (AECL) is conducting geomechanical research at its Underground Research Laboratory (URL) in southeastern Manitoba, Canada, (Figure 1) to assess the feasibility of nuclear fuel waste disposal deep in a plutonic rock mass. One component of the geomechanical program at the URL is the characterization of the far-field in situ stress state surrounding the URL. Overcoring, hydraulic fracturing, back-analysis and observations have been used to develop an in situ stress model for the URL extending from the surface 10 the 443-m depth. Over 800 overcoring measurements have been made at the URL, about four times as many measurements as contained in the Canadian Shield database. The measurements and observations made during URL shaft sinking indicate that major geological features, such as thrust faults, can act as boundaries for in situ stress domains, and that both the magnitude and direction of the in situ stress state can change when these geological features are traversed. The failure observations made as these stress boundaries were crossed during shaft sinking support the recent concepts describing failure mechanisms around circular openings (Maury, 1987). This paper summarizes the measurements and observations made at the URL and describes the in situ stress model developed to date.

2. SITE DESCRIPTION
2.1 Site Geology

The URL is located within the Lac du Bonnet granite batholith, which is considered to be representative of many granitic intrusions of the Precambrian Canadian Shield. The batholith trends east-northeast and its elongated body is about 75 by 25 km in surface area (Figure 2). The batholith is a relatively undifferentiated pink and grey massive porphyritic granitegranodiorite. The massive, medium- 10 coarse-grained porphyritic granite is relatively uniform in texture and composition over the batholith, although locally it displays subhorizontal gneissic banding. Variations within the batholith, referred to as phases, are attributed to contamination by assimilation of country rocks or to stress during intrusion (McCrank, 1985). The batholith is post-tectonic, Late Kenoran age (2680 Ma) and has a faintly foliated core and foliated margin (Cherny et al.,1980).

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