ICGCM Papers:
Fundamental Research Studies
 
 
How to Mitigate Coal Mine Bumps Through Understanding of the Violent Failure of Coal Specimens
32nd International Conference on Ground Control in Mining
How to Mitigate Coal Mine Bumps Through Understanding of the Violent Failure of Coal Specimens
by
Gamal Rashed, West Virginia University, Morgantown, United States
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[Conference] 32nd International Conference on Ground Control in Mining
[Price] Free  [Comments] 0
[Topical Area] Fundamental Research Studies
[Author] Gamal Rashed, West Virginia University, Morgantown, United States
[Abstract] 
Key Conclusions:
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There is a very strong positive correlation between the unconfined compressive strength and the violent failure of coal specimen such that bump liability increases with increasing UCS of the coal specimen. From laboratory results, softening the coal specimen through drilling holes in the rib zone does not significantly reduce the UCS of the coal specimen, so the coal specimen still has the ability to store more elastic strain energy. This is why softening the rib zone is not an effective way to mitigate the violent failure. Conversely, from laboratory results, softening the core zone of the coal specimen reduces the UCS, such that the elastic strain energy decreases and the likelihood for violent failure decreases as well. The reduction in the UCS resulted from reduction in the induced horizontal stress. Rib zone failure is not the main cause for violent failure.
Key Findings:
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Both laboratory and numerical simulation results prove that potential for violent failure increases with increasing UCS of coal specimen, because the elastic strain energy increases with increasing UCS. Most of the elastic strain energy is stored in the core zone of the coal specimen. From laboratory results, softening the core zone is the best way to mitigate the violent failure, where the specimens fail gradually and smoothly with minimum amount of ejection and noise during failure and the specimens do not lose strength suddenly. Computer modeling shows everything being equal that there is reduction in the induced vertical and horizontal stresses due to softening of the coal specimen. Failure of the rib zone is not the main factor for the violent failure of the coal specimen. Conversely, failure of the core zone is the key factor for the violent failure
Objective of the Paper:
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The purpose of this research is to understand experimentally in the laboratory and through numerical simulation the key factors responsible for coal mine bumps for coal specimens having different width to height ratios. How does the strength of the coal specimen control coal mine bumps? How to mitigate the violent failure of coal specimen through both softening the coal specimen completely or partially?
Problem Statement:
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Coal mine bumps which are sudden, violent failures of one or more of highly stressed pillars are still a big problem that faces miners today. When coal mine bumps happen, coal fragments from the pillars are ejected into the mine entries, mine equipment, standing supports, or both may be damaged or destroyed, and mine personnel may be injured or even killed. Therefore, mitigating coal mine bump is necessary for today’s highly competitive coal industry which requires a safe workplace. When a coal specimen fails, different zones can be recognized on the top surface of the failed coal specimen. The first zone is completely crushed, and located near the edges of the coal specimen where the constraint and strength are minimum. The second zone is the intact core located near the central portion of the coal specimen where the constraint and strength are maximum. The third zone is a transition zone between those two zones. From the intensity of damage point of view, these zones can be divided into two zones, elastic zone, usually called the core zone, which experiences little or no damage, and yielded zone, usually called the rib zone. In this research, three hypotheses have been assumed for violent failure of coal specimen. The first hypothesis is that violent failure is restricted to the rib zone only. The idea here is that if the ribs of the pillar are soft, the ribs will fail gradually and gently, and the load is transferred to the deeper core until the whole pillar fails. Conversely if the ribs of the pillar are so stiff that they do not fail gradually, but keep storing more and more energy. At the moment of failure, they have the least confinement so they fail with so much energy that is released suddenly causing violent failure. The second hypothesis is that violent failure is restricted to core zone. The core does not yield because of high confinement, and hence it can store so much energy that could be released suddenly and violently at the moment of failure. The third hypothesis is that violent failure occurs both at the rib and core zones. In order to determine which one of these hypotheses is true, the coal specimen needs to be softened either in the rib zone or the core zone, or both and then study its mode of failure to see whether it is violent or not. Softening the core and rib zones was conducted through drilling holes at specified locations in these zones. While softening the coal specimens completely was conducted through immersing them in water for 24 hours, and then testing them directly. 3D computer models have been conducted to examine the effect of UCS on the potential for violent failure. Moreover, the effect of softening on the variation of the vertical and horizontal stress distributions have been studied for coal specimens with and without softening, where the potential for violent failure increases with increasing the vertical and horizontal stresses in the core zone.