The transfer and evolution of stress among rock blocks directly change the void ratios of crushed rock masses and affect the flow of methane in coal mine gobs. In this study, a Lagrange framework and a discrete element method, along with the soft-sphere model and EDEM numerical software, were used. The compaction processes of rock blocks with diameters of 0.6, 0.8, and 1.0 m were simulated with the degrees of compression set at 0%, 5%, 10%, 15%, 20%, and 25%. This study examines the influence of stress on void ratios of compacted crushed rock masses in coal mine gobs. The results showed that stress was mainly transmitted downward through strong force chains. As the degree of compression increased, the strong force chains extended downward, which resulted in the stress at the upper rock mass to become significantly higher than that at the lower rock mass. It was determined that under different degrees of compression, the rock mass of coal mine gobs could be divided, from the bottom to the top, into a lower insufficient compression zone (ICZ) and an upper sufficient compression zone (SCZ). From bottom to top, the void ratios in the ICZ sharply decreased and those in the SCZ slowly decreased. Void ratios in the ICZ were 1.2–1.7 times higher than those in the SCZ.
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The failures of surface vertical wells (SVWs) for methane extraction during mining are known to be the main causes of the shortening of the highly efficient drainage periods of the SVWs. Therefore, in order to improve the stability of the SVWs, it is of great significance to accurately investigate the failure characteristics when SVWs are severely destroyed and can no longer extract methane. In this research, a physical simulation experiment was carried out for the purpose of investigating the most possible failure periods, as well as the highest possible failure locations and failure modes of SVWs. This study’s results showed that the displacements of the SVW were constantly changing under the influences of mining stress. After the mine working faces had past the SVWs by more than 157 m, the total displacements reached the maximum, and the SVWs were in the most possible failure periods. During those periods, failure positions of the SVWs are prone to occur at the interfaces between the hard and soft strata; within the thick rock layers; and within the thick-thin-thick strata combination. Among these, the SVWs may suffer from shear slippage at the interface of the hard-soft combined rock layers with relatively large thickness differences; the SVWs may suffer from horizontal shear at the interfaces of the hard-soft combined rock layers with small thickness differences; the SVWs within the thick rock layers may become destroyed by shear slippage actions; and the SVWs in the thick-thin-thick combined rock layers may become distorted by shearing at the interface between the rock layers or blocking within the intermediate thin rock. The results of this research study were verified through the field tests which were conducted in China’s Sihe Coal Mine.
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