ABSTRACT: Trials were undertaken in a narrow-vein mine of Northern Quebec, Canada, to monitor stress changes in sill pillars due to mining. Trials were carried out in active mine areas using vibrating wire proving rings and short extensometers, installed in boreholes to monitor radial and axial displacements in three orthogonal directions. Trials took place at intermediate depths, 600 meters underground, in areas with extraction ratios exceeding 80%, and lasted several months. The present paper focuses on a successful trial, showing regular stress changes within the sill pillar all along its loading curve. Stress changes were relatively slow during the first half of the experiment. Changes increased significantly during the second half, resulting in the definition of a clear inflection point when the stope reached about 75% of its final height. Mining was stopped when a rock burst resulted in complete sill pillar failure, which prevented any return to the burst area. Monitoring and numerical modeling records were reviewed. These show reasonable agreement and validate the trial and the data gathered during the period of monitoring. This experiment provides insights into the unpredictability of rock burst events, and ways to prevent or alleviate their damage.
1 INTRODUCTION
Mining and excavation of rock material inevitably produce a redistribution of stresses around mine openings, within pillars and abutments. Loading of pillars and abutments is limited by the strength of rock materials. Once this limit is reached, rock material will either deform plastically, in the case of soft rocks, or fail violently, in the case of hard brittle rocks. Intermediate rock materials show an intermediate behavior, with failure progressing through limited ductile deformation and relatively small bursts (Labrie et al. 2002). Trials were done in a narrow-vein mine of Northern Quebec, Canada, to monitor stress changes taking place within pillars during mining.
