Brittle Failure and Rockburst Hazard Management in Deep Cave Mining
Sub-Project1: Application of Data Analysis Techniques to Identify Rockburst Mechanisms, Triggers, and Contributing Factors in Cave Mining
Sub-Project 2: Development and Validation of Brittle Failure Analysis Tools to Aid Support Design in Highly Stressed Rock
Stress-induced spalling and associated strainbursting (Figure 1) 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.
ICARN research includes developing and applying novel data analysis techniques to identify strainburst damage mechanisms, trigger mechanisms, and important controlling factors in a cave mining setting. Two distinct databases are used to track local excavation damage and the cumulative impact of widespread rockburst damage that is possible in large-scale mining. New indices are proposed including a Rockburst Damage Index and Rockburst Cluster Index that are practical and useful for data driven assessments of rockbursting. Building on these, a new methodology has been developed using multivariate logistic regression and a grid cell system to study spatial susceptibility to rockburst damage in cave mining (Figure 2). The geometry of cave mining lends itself well to spatial modelling using the proposed grid system and the selection of mechanistically meaningful explanatory variables. The approach therefore combines robust procedures for logistic regression with data processing that is suitable for the cave mining environment, leading to highly interpretable results.
Key findings from the multivariate statistical analysis based on data provided by an ICaRN partner mine identify strainbursting as the dominant damage mechanism, veining as an important control on the susceptibility and intensity of strainbursting, and the positive role of a deformation-based ground support design in suppressing low severity strainbursting.
ICaRN research has further targeted the development and validation of new, purpose-built strength and deformation criteria for brittle rock and support design. The new criteria account 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.
Validation of the model has been performed against depth of failure responses measured during initial undercutting for a deep panel cave mine (Figure 3a-c). 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. Comparisons against use of the Hoek-Brown criterion calibrated to the crack initiation strength (Figure 3d) was found to significantly under-predict the distal extent (i.e., distance from the undercut) of stress fracturing, and the Mohr-Coulomb criterion calibrated to the crack initiation strength (Fig. 3e) was found to vastly over-predict the depth of failure.
Parallel ICaRN research has been directed at using bonded-block modelling methods to both simulate the depth of stress-induced brittle fracturing and the corresponding bulking as a function of the cave mining stress path (Figure 4). The key mining stages accounted for include the excavation of the extraction-level drifts (i.e., footprint development), followed by compression of the pillars simulating the abutment stress as the undercut passes over, and slow unloading of the model to simulate the subsequent stress shadowing. Several key learnings have emerged from the models. Two of the more important findings are: i) adding even a small support pressure (as uniformly as possible) can significantly improve the performance of the excavation; and ii) in a caving operation, it is crucial to install and maintain appropriate support before and ahead of the undercut abutment stress. The difference is mostly attributable to the extra confinement the support pressure provides to the failed skin, allowing it to carry more load and thus provide more confinement to the rock behind it limiting the depth of failure and subsequent rock mass bulking.
The findings from this research 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.