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UBC Theses and Dissertations

Empirical analysis of brittle rock mass failure in response to undercut advance for preventive support maintenance McMillan, Robert


Stress-induced brittle fracturing of massive rock near a highly stressed excavation boundary causes the volume of the rock mass to increase, known as bulking. Excessive bulking represents a safety hazard for underground workers and can cause costly production delays at underground mining operations. For block and panel cave mines, these project risks are exacerbated during cave establishment due to large-scale stress changes from undercutting and cave propagation that redistribute and magnify stresses near excavations that are critical for production. The research presented in this thesis aims to improve the identification, analysis of, and means to mitigate adverse brittle rock mass behavior in high-stress environments. This research makes use of a unique historical geotechnical monitoring database collected by PTFI from the DMLZ panel cave mine. The geotechnical monitoring database represents an initial step towards best practices for data collection at deep cave mines operating in high-stress environments during the ramp-up period. Borehole camera surveys supplemented by multi-point borehole extensometer instruments have been used to determine the depth of stress-induced brittle fracture damage in the drawpoint pillar walls. Convergence measurements and LiDAR data are used to characterize the corresponding rock mass bulking. The results presented show that the compilation and interpretation of historical monitoring data can be used to identify the long-term depth of stress-induced fracturing and bulking trends in response to undercut advance. The integration of these long-term trends shows that direct measures of stress-induced fracturing provide an early indication of excavations vulnerable to bulking. Empirical methods are used to relate the depth of stress-fractured rock to the intact rock strength and distance from the cave front. Based on the relationships derived, a methodology for generating an empirical two-factor susceptibility map to estimate the depth of stress-induced fracturing across an extraction level footprint is presented. LiDAR scanning is used to show that it is an effective method for capturing the onset of bulking and anticipating local areas that are likely to experience greater deformation demand as bulking progresses. Proactive and strategic geotechnical monitoring based on long-term depth of stress-induced fracturing trends is proposed to assist with preventive support maintenance practices.

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