Abstract:

Stress-induced spalling and associated strainbursting can lead to significant safety hazards and costly delays through required repairs to installed support systems, ultimately threatening the viability of deeper mining projects. Arising from these experiences is that commonly used design tools are proving to be insufficient for robust prediction and can be dangerously misleading. What must be recognized is that these have been developed based on experiences dominated by shear failure, whereas brittle failure involves a component of extensile fracturing that initiates at lower stress-to-strength ratios. Furthermore, brittle failure and the corresponding bulking of the rock mass is directional in its impact on support design, whereas conventional rock behaviour models treat bulking as a volumetric attribute. These limitations have significant implications with respect to the assessment of depth of failure and bulking displacements required for support design.

Results will be presented from recent research undertaken through the International Caving Research Network (ICaRN) to develop and validate a new, purpose-built strength criterion for brittle rock. The new criterion accounts for both extensional fracturing under low confinement (e.g., at the pillar boundary) and shear fracturing under high confinement (e.g., in the pillar core). The influence of the 3D excavation shape and orientation relative to the in-situ stress state is fully accounted for in identifying excavations and pillars susceptible to spalling and strainbursting across the mine footprint. When properly parameterized, the new criterion can provide the potential depth and lateral extent of stress fracturing to assess strainbursting and bulking severity potential. These in turn can be used for excavation and pillar dimensioning and ground support design, amongst other valuable uses.

Also presented are the model validation results, which are based on a case study of the depth of failure responses measured during initial undercutting for a deep panel cave mine. High tonnage mining methods like block and panel caving result in the exposure of a larger mining footprint to both higher in situ and mining-induced stresses. The result is that support damage from spalling and strainbursting may be widespread across a large extraction-level footprint. Comparative analyses of the performance of the new criterion against conventional criteria commonly used (Mohr-Coulomb and Hoek-Brown) are reported. The model results demonstrate the sensitivity of the brittle failure and bulking processes to confining stress and the effective role that a small amount of support pressure can have on improving excavation performance. This emphasizes the need for deformation-based support design (DBSD) principles, as well as the need to explicitly model the rock support system and its sequencing relative to the mining stress path.

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Prof. Erik Eberhardt

Biography