OBSERVATIONS OF BRITTLE FAILURE AROUND A CIRCULAR TEST TUNNEL

In 1987, AECL proposed several experiments for its Underground Researc h Laboratory. Among these was the Mine-by Experiment, the main focus o f which was to investigate the formation and characteristics of the ex cavation disturbed zone adjacent to an underground opening. The extent and nature of this zone are important concerns for both the operation al safety and the sealing of a vault to control the migration of radio nuclides. The excavation response phase of the Mine-by Experiment, whi ch involved the excavation, without explosives, of a 46-m-long, 3.5-m- diameter test tunnel in brittle unfractured granite, was carried out b etween January and July 1992. State-of-the-art instrumentation compris ing both geomechanical and geophysical instruments was used to monitor the progressive failure process around the circular tunnel, and the i n 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. (C) 1997 Elsevier Scienc e Ltd.