ht. J. Rock Mech. Min. Sci. Vol. 34, No. 7, 1065-1073, pp. 1997 0 1997 Elscvier Science Ltd. All rights reserved Printed in Great Britain PIk !30148-9062(m2%-9 0148-9062/97 S17.00 + 0.00 Ob tkms of Brittle Failure Around a Circular Test Tunnel C. D. MARTINtS R. S. READY J. B. MARTINOf In 1987, AECL proposed several experiments for its Underground Research Laboratory. Among these was the Mine-by Experiment, the main focus of which was to investigate the formation and characteristics of the excavation disturbed zone adjacent to an underground opening. The extent and nature of this zone are important concerns for both the operational safety and the sealing of a vault to control the migration of radionuclides. The excavation response phase of the Mine-by Experiment, which involved the excavation, without explosives, of a 46-m-long, 3.5m-diameter test tunnel in brittle unfractured granite, was carried out between January and July 1992. State-of-the-art instrumentation comprising both geomechanical and geoI.rhys- ical instruments was used to monitor the progressive failure process around the circular tunnel, and the in situ experiment was supplemented by extensive laboratory tests. This paper provides a detailed record of the brittle failure process that was observed during excavation of the tunnel. 0 1997 Eisevier Science Ltd INTRODUCTION In Canada, as in many countries that employ nuclear power, the underground disposal of nuclear fuel waste is being investigated. The Canadian disposal concept considers the plutonic rock of the Canadian Shield as a potential host medium for a disposal vault located at a depth of between 500 and 1000 m. In the Canadian Shield ground water is essentially at or near the surface, and hence the disposal vault will be located below the ground water table. As part of the assessment of this concept, Atomic Energy of Canada Limited (AECL) has constructed an Underground Research Laboratory (URL) approximately 120 km northeast of Winnipeg, Manitoba in the Lac du Bonnet granite batholith. Constructed between 1983 and 1989, the URL provides a well-characterized in situ environment in a previously undisturbed volume of rock in which to conduct experiments that address important issues in assessing the Canadian disposal concept, and to demonstrate the feasibility of the proposed disposal technology. The present facility consists of major working levels at depths of 240 and 420 m below ground surface, with additional access at depths of 130 and 300 m (Fig. 1). In 1987, AECL proposed several experiments for the URL, which were reviewed by an expert group [l]. tAECL, Whit&e11 Laboratories. Pinawa. Manitoba. Canada ROE 1LO. $Present ad&a&x Geomachanics Research Centre, Laurentian Univcrsty, Sudbury, Ontario, Canada P3E 2C6. Among these was the Mine-by Experiment, the main focus of which was to investigate the formation and characteristics of the excavation disturbed zone (EDZ) [2]. The EDZ represents the zone of disturbed and damaged rock that forms adjacent to an underground opening as a result of the redistribution of stresses and has the potential to provide a contirmous pathway from the vault to the surface. The extent and nature of this zone are important concerns for both the operational safety and the sealing of a vault. The Canadian disposal concept utilizes excavated rooms and/or boreholes to contain the nuclear waste. Seals will be used to control the migration of radionuclides from these rooms and boreholes, hence the factors that affect the formation of the EDZ are important in the design of a sealing system. As a result of the expert group’s recommendations, the Mine-by Experiment was carried out at the 420 Level of the URL where the extent of the EDZ would be the largest (Fig. 1). The Mine-by Experiment comprised three phases: (1) the excavation response phase, which was designed to investigate the formation of around under- ground openings; (2) the connected permeability test phase, which investigated the hydrauhc characteristics of this damaged zone; and (3) the heated-failure studies phase, which examined the effects of thermal loading on the formation of the damaged zone. The excavation response phase, which involved the excavation, without explosives, of a &-m-long, 3.5-m- diameter test tunnel in brittle unfractured granite, was carried out between January and July 1992. The 1065