ABSTRACT:

The Underground Research Laboratory (URL) situated in southeastern Manitoba is a major research facility in the Canadian nuclear waste management program undertaken by Atomic Energy of Canada Ltd. The development of the shaft at the URL has been associated with wall failures and observations of over breaks. A microcomputer-based microseismic system was used to record whole waveform signals due to crack propagations induced by shaft sinking between 324 m and 443 m of depth. Processing of the data showed that cracking occurs mainly around the bottom and the walls of the shaft. In the horizontal plane, the activity is located within 1 m to 2 m of the walls of the shaft. A preferential NE-SW orientation was observed for some periods. This orientation is compatible with that of over breaks and the direction of minimum stress in the horizontal plane. Investigation of failure mechanism based on first motion of P waves and the ratio of radiated P and S wave energies indicates that shear failure is the dominant mechanism of cracking.

1 INTRODUCTION

Failure of rock in the vicinity of the walls of deep underground excavations is a major concern, because of safety and/or production considerations, in a variety of engineering situations: deep mines or tunnels, deep repositories for disposal of nuclear waste and deep gas and oil wells. A better understanding of the process of initiation and propagation of cracks and particularly the location and the mechanism of rock failure could help in the selection of techniques/ support systems in order to control this process. Laboratory tests have been used by some researchers to simulate the behaviour of rock around underground openings. In the uniaxial tests of Rummel (1971) and Lajtai (1971) tensile cracks were formed at an early stage of the test but became stable during the loading process. Secondary extension or shear fractures were observed prior to shear failure. Gay (1976) and Hoek and Brown (1980) have reported results from biaxially loaded samples. The fracture patterns are complex but again tensile cracks were observed during the initial loading and shear cracks were formed at a later stage. In situ investigations of failure mechanisms based on visual observations have been reported by some authors (Ortlepp et al. 1975, Hoek and Brown 1980. Maury 1987, Martin 1988); the mode of failure has been attributed to shearing, extension, or a combination of both. Analytical and numerical methods based on continuum mechanics can adequately describe the onset of inelastic behavior, generally associated with initiation and stable propagation of cracks, but cannot predict rupture mechanisms (Kaiser et al., 1985). Maury (1987) states that theoretical models based on an elastic-plastic behaviour are convenient for analytical processing of the problem but ignore the physical reality of the situation: the presence of discontinuities. The experiment described in this paper is based on using microseismic techniques as a tool for monitoring propagation of cracks in the walls of a circular shaft excavated in a granitic rock mass.

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