How to mitigate coal mine bumps through understanding the violent failure of coal specimens

Coal mine bumps have presented serious mining problems for many years. These sudden violent failures around underground mine openings have compromised safety, ventilation and access to mine workings. There is general agreement among researchers that both geologic and poor/inappropriate mining conditions that create high stresses in the pillar are the main causative factors for coal bumps.;The main objective of this research is to understand bump mechanisms in laboratory settings through examining the violent failure of coal specimens and to test three hypotheses to mitigate that violent failure. Also, man-made concrete specimens were prepared and tested to simulate failures in underground mines of several geological conditions. To understand and mitigate coal mine bumps, the role of five factors on the potential for violent failure was examined: First, the role of the mechanical properties of coal was investigated. The main reason behind studying the role of the coal itself is that, a debate was found in the literature about that role. From the literature, some researchers think that strong and stiff coal is necessary for coal bumps. However, others think that bumps would happen irrespective of coal strength or stiffness. Hence, the role of the coal was investigated to help put an end to that debate. Second, two of the most commonly proposed bump mechanisms which are sudden loss of constraint and sudden impact load were examined with the aid of Finite Element Method. Many researchers believe that coal bumps occur because of a sudden loss of constraint between pillar/roof & pillar/floor or the hammering effect due to breaking of thick, massive strata above the coal seam, which creates a sudden impact load on the pillars and causes them to bump, little work has been done to explore and understand the consequences of these proposed mechanisms for coal bumps.;Third, the interface friction and the width-to-height (W/H) ratio of coal specimens were determined where bump would occur. The W/H ratio ranges from 1 to 10 while the interface friction is either 0.1 or 0.25. It is believed that understanding how the mode of failure of the coal specimen changes with changing interface friction and width-to-height (W/H) ratio is the corner stone to mitigating coal mine bumps. In the author\u27s point of view, the W/H ratio of a pillar is the most convenient way to control coal mine\u27s bump hazard. Fourth, three hypotheses were examined to mitigate the violent failure of coal specimens. These hypotheses depend on softening coal specimens either partially or completely. It is believed that softening coal specimens would minimize the elastic and kinetic energies released at the failure.;Fifth, man-made concrete specimens were assembled in a three layered approach to investigate the mode of failure for eight different geological conditions. The main reason for using man-made concrete specimens to simulate different field conditions is that the ability to examine and determine how a specific geologic condition contributes to bumps using actual coal and rock specimens is very difficult, because there is uncertainty about their strength. For man-made concrete specimens the strength is well known before running the tests. The three layers simulate roof, pillar and floor. The proposed strength for the assembled concrete specimens is 2000 & 4000 psi for weak and strong coal respectively, while the strength is 4000 and 9000 psi for medium and strong-rock respectively.;According to the laboratory and numerical modeling results, the coal itself plays an important role in bumps. However, that role alone is not sufficient for the occurrence of bumps. However, the interface friction and the width-to-height ratio generally control the mode of failure for coal specimens. Every interface friction is associated with a specific threshold W/H ratio above which the mode of failure is non-violent, and below which the failure would be either sudden & violent or sudden & non-violent. The core zone for the failed coal specimens is neither elastic nor intact; however it is damaged to some extent. Softening the rib zone is not effective in mitigating the violent failure of coal specimens. However, softening the core zone would help mitigate that violent failure. This result gives an indication that the energy stored in the core zone of a coal specimen is the main causative factor for the violent failure. Sudden impact load and sudden loss of constraint are associated with instantaneous changes in the vertical stress, and sudden release of elastic and kinetic energy, both the elastic and the kinetic energies are needed for the occurrence of bumps