Critical Conditions and Energy Transfer Characteristics of the Failure Process of Coal-Rock Combination Systems in Deep Mines

Abstract

With the steady increase in the size, intensification, and modernization of coal production enterprises, the deep coal resources in large coal bases are gradually entering the mining stage. When the coal mining reaches the deep zone, the interactions between various underground dynamic hazards begin to occur. These interactions are affected by the engineering geological environment and can lead to the occurrence of severe compound hazards. When coal and gas outbursts occur and destabilize the mining area, the high geostress causes the multiphysical coupling effect of the laminated overburden system to become more pronounced. Therefore, we analyzed the development path of a coal-rock system under instability conditions from the perspective of coal–rock coupling, constructed a model of the coal-rock combination system’s structure, and proposed three directions (i.e., strain softening, limit equilibrium, and dynamic instability) for the development of coal-rock system instability. Then, we established a model for the critical conditions of the system’s failure process and elucidated that the release of the rock’s elastic energy promoted the instability of the coal. Furthermore, we verified the established critical conditions through laboratory tests on a coal-rock combination structure and obtained the patterns of the rock energy transferring into the coal seam during the instability failure process of the coal–rock combination structure. When the coal–rock combination structure failed, the rock strain reached its maximum value and the strain rebound phenomenon occurred. The stored elastic strain energy released by the rock into the combination system accounted for 26% to 53% of the accumulated energy in the rock itself, and the released elastic energy and the new surface area of the crushed coal sample followed a logarithmic relationship. The findings of this study provide theoretical support for the identification and quantitative analysis of instability due to the dynamic hazards of coal-rock gas in deep mines.

Mechanical Characteristics and Energy Dissipation Trends of Coal-Rock Combination System Samples with Different Inclination Angles under Uniaxial Compression

“…The peak strains of the pure coal and pure rock samples were 0.878 and 1.026, respectively. The peak strain of the CRCS sample is less than that of the pure coal sample and pure rock sample, mainly because the rock mass part of the CRCS sample experienced no significant deformation under the uniaxial compression load [24,25]. The brittle failure after the peak of the pure rock sample was strongly pronounced, and the stress dropped, while the CRCS and pure coal samples exhibited good continuity after the peak.…”

Research on Mechanical Properties and Energy Evolution Law of Coal–Rock Assemblage with Different Gas Pressures

“…When the coal-rock composite specimen is unstable and damaged, the coal mass is damaged under the combined action of the external load and the elastic strain energy release of the rock mass. According to [15], the stress-strain curve of coal-rock assembly, rock mass, and coal mass can be drawn as shown in Figure 7.…”

“…When the coal-rock composite specimen is unstable and damaged, the coal mass is damaged under the combined action of the external load and the elastic strain energy release of the rock mass. According to [15], the stress-strain curve of coalrock assembly, rock mass, and coal mass can be drawn as shown in Figure 7. According to Figure 7 and [39,40], the total energy of rock mass pre-peak absorption (ER), the total energy of coal mass pre-peak absorption (Ec), the total energy of composite body pre-peak absorption (E), the releasable elastic energy of unloading after rock mass peak (EeR), the dissipated energy after coal mass failure peak (EdC), and the dissipated energy after composite body failure peak (Ed) can be acquired, as shown in Equation (1).…”

“…The elastic energy stored in the coal-rock mass with shock tendency is an important energy source for coal-rock dynamic disasters [12,13]. The accumulation, transmission, and dissipation of elastic energy of coal seams and rock layers directly affect the inoculation, occurrence, and development of coal-rock dynamic disasters [14,15]. Therefore, it is particularly important to carry out research on the mechanical properties of the coal-rock system composed of interlayers of coal and rock, and the law of energy transfer between rock and coal, which has important practical significance for the prevention and control of deep coal-rock composite dynamic disasters.…”