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UBC Theses and Dissertations

Reliability analysis of discrete fracture network projections from borehole to shaft scale discontinuity data Brueckman, Christina


When listing the risk factors that may impact the feasibility and success of a block cave operation, Brown (2003) highlights the adequacy of the geotechnical data available as a primary risk. Detailed data on the major structures, rock mass properties, and in situ stresses are necessary to assess the caveability of the orebody, and the excavation stability on the operating levels below, including the potential for fault slip and rockburst hazards when mining in a higher stress environment. The source of this essential data, especially at feasibility-level design, is almost always limited to borehole data. This is emphasized by Laubscher (2000) who notes that most block cave mines are designed solely on borehole data. When restricted to borehole data, significant effort is expended on obtaining oriented core and/or televiewer logs to derive critical data regarding the frequency and orientation of discontinuities and the presence of major faults. Subsequent analysis of the spatial relationships between discontinuities is facilitated by the use of Discrete Fracture Network (DFN) modelling. The value of DFN models for assessing in situ fragmentation and rock mass strength identifies a critical limitation of borehole data. Required DFN inputs include the orientation, intensity, and size distributions of the discontinuities to allow the stochastic generation of a representative fracture network. The evaluation of the discontinuity orientation is relatively easy, intensity or spacing is possible with sufficient effort, but the discontinuity size is not possible given the small “observation window” of a borehole. This thesis reports the results from research carried out to compare analyses of discontinuity data sampled across different spatial scales to improve our understanding and reduce uncertainty in the characterization and projection of discontinuity networks, specifically with respect to fracture spacing and persistence within the rock mass. This work is undertaken using discontinuity data from a deep geotechnical borehole and co-located large diameter shaft. The close proximity of the borehole and shaft provided an opportunity to ground-truth borehole projections based on traditional core logging and televiewer logs. The comparative analysis was completed with the use of DFN modelling. The improved understanding of the spacing and persistence of the discontinuities will aid in further development of guidelines for rapid geotechnical characterization.

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