Underground » Coal Burst
The first stage of this project reported the findings of experimental and numerical modelling approaches to investigate the burst tendency of coal. With the help of experimental and theoretical approaches, a new burst index for coal bursts was proposed. The numerical framework was applied to conduct field scale bursting evaluation, and the stress states, as well as resultant deformations at the excavation faces, were investigated. The results obtained were promising and further refining results were expected to extend the project to more detailed experimental investigations as Stage 2.
Stage One Report
Rock in depth is subjected to stresses due to overlaying burden and tectonic activities. When a mining activity is undertaken, the stress fields are disrupted resulting in induced stresses around the openings. To manage the rock/coal burst issues, accurate information on the stress magnitudes and their orientation is important. However, the measurement of in-situ stresses in deep mines is challenging as the conventional methods like hydraulic fracturing and overcoring suffer from inherent complexities and are difficult to apply in great depths and remote regions at a reasonable cost, making them impractical for many mining scenarios. Research endeavours to address this pressing issue by developing and enhancing Non-Destructive Techniques (NDT), such as Deformation Rate Analysis (DRA) and Acoustic Emission (AE) for in-situ stress measurement from oriented cored rocks.
These methods of stress measurement have been used for the many decades, however many researchers have rightly questioned the viability of these methods because they believe that stress memory is affected by the time delay. Doubts have been raised about the applicability of method under triaxial conditions.
The main objectives of the first stage of this project were to:
- Perform experiments on the memory effect of rocks, with a specific focus on coal and coal measured rocks, to obtain reliable data for scientific scrutiny;
- Improve the experimental procedures and develop new analysis methods for better interpretation of experimental results;
- Measure in-situ stresses from oriented cored rocks and compare the results with the conventional methods for better stress management.
Experiments were performed on different rock types with specific focus on coal and coal measured rocks.
Stage Two Report
This research investigates the mechanism of coal burst, studying the energy concept and fracturing characteristics under true-triaxial loading/unloading conditions. It presents a comprehensive study on the influence of initial stress ratios (ISR) and drill hole orientations on coal burst characteristics and the burst failure mechanism. The research employed true triaxial unloading coal burst tests, high-speed camera (HSC) imaging, and acoustic emission (AE) systems to analyse the coal burst phenomenon and associated microcracking processes.
The project focused on understanding the effects of ISR on coal burst and AE characteristics. Four groups of initial stresses were considered, simulating stress conditions at different depths.
The impact of drill hole orientations on the burst failure mechanism was investigated. Two designs of drill hole orientations were examined: Upper Hole (UH) and Centre Hole (CH) aligned with the minor principal stress, and Top Hole (TH) and Side Hole (SH) aligned with the major and intermediate principal stress directions. It was found that the drill holes effectively reduced the risk of coal bursts, with the SH-type exhibiting the most significant reduction in burst intensity, followed by the CH-type. Both types of drill hole patterns mitigated coal bursts, and the order of effectiveness in reducing burst intensity was CH, UH, TH, and SH. The orientations of the drill holes had minimal influence on the failure mode (splitting). The AE activities during coal bursts noticeably decreased in coal samples with drill holes, particularly in the UH and CH specimens. The entropy of AE amplitude increased initially and then decreased during coal bursts, and the drill holes limited the upper limit of AE entropy. Among the drill hole patterns, CH-type drill holes were recommended for field application due to their superior effectiveness.
Overall, this new approach in coal burst mechanism investigation will provide reliable, cost effective data for direct use in underground mine design and geotechnical monitoring. The whole experimental fracture evolution can be treated for valuable training and testing data for machine learning to better predict the fracture propagation process during bursting.