The coal–roof/floor combination system is a special geological structure consisting of both coal and roof/floor materials, which often leads to the occurrence of impact failure. The instability and failure of the coal seam are not caused by a single disturbance, but often occur under repeated loading and unloading processes under the action of mining stress. Thus, research on the impact failure, especially the energy evaluation of coal–rock combination materials under cyclic loading and unloading, is of great significance for understanding the rock burst mechanisms of deep coal mines. This paper focuses on the combination material formed by coal seam and roof and floor rock. Taking the #9 coal seam and roof and floor sandstone from the Zhangshuanglou Coal Mine as the test subjects, conventional triaxial compression and cyclic loading and unloading tests were conducted on sandstone, coal, and coal–rock combination material, respectively. Based on the strain energy evolution characteristics, the failure behavior of the coal–rock combination materials throughout the entire process of energy accumulation, dissipation, and release during cyclic loading and unloading are discussed. The research finds that the macroscopic failure behavior of the tested coal–rock combination specimens is dominated by the weaker coal component, and there are marked differences between the tested sandstone and coal components in their respective energy storage capacities, release characteristics, and dissipation behavior. On the basis of these measured differences and of CT-confirmed failure localization within the coal layer, it is inferred—as a mechanistic working hypothesis rather than a directly demonstrated result—that the main driving energy for the impact failure of the coal component may originate from the elastic strain energy stored in the roof/floor sandstone components, released preferentially toward the coal through their interfaces. For mining and excavation at high-in-situ-stress mining areas, the essence of the impact failure of surrounding rock is the non-coordination of energy storage and release between the roof and floor rock materials and the coal seam. The key to preventing impact failure is to eliminate the differences in energy storage and release between different rock materials in the coal seam.
P. Yin, Chun Liu, Peng-Xiang Wang et al.· Applied Sciences· 0 citations
The development characteristics of primary fractures in coal serve as a prerequisite, influencing reservoir stimulation outcomes. The propagation of experimentally induced tensile cracks in the reservoir is closely related to the coal’s tensile fracture properties and the distribution of natural fractures. The effectiveness of reservoir stimulation directly determines the productivity of coalbed methane (CBM) wells, with clear variations in natural fracture development observed across different macroscopic coal components. Therefore, accurately evaluating the tensile fracture characteristics of different macroscopic coal components and the interaction patterns between experimentally induced and natural fractures is of great importance for deep CBM resources. This study focuses on vitrain, clarain, and durain from the deep #8 coal seam in the Daning-Jixian Block, located in the southern part of the Jinxi Flexural Fold Belt on the eastern margin of the Ordos Basin, which exhibit varying degrees of natural fracture development. Using the Brazilian splitting test and the centrally grooved three-point bending test, the tensile strength and Mode I fracture toughness of different macroscopic coal components were investigated. The study reveals the propagation behavior of pure tensile cracks and the resulting fracture network morphology in coal specimens with developed natural fractures. The presence of natural fractures reduces the tensile strength and fracture toughness of coal while increasing its brittleness. Compared to durain specimens (with average tensile strength of 2.09 MPa and fracture toughness of 0.338 MPa·m0·5), vitrain (avg. 0.92 MPa, 0.234 MPa·m0·5) and clarain (avg. 1.40 MPa, 0.185 MPa·m0·5) exhibit clearly lower tensile strength and fracture toughness, along with more pronounced brittle characteristics. Differences in geomechanical parameters among macroscopic coal components lead to distinct fracture behaviors: high-strength, high-fracture-toughness durain specimens require higher pressure to initiate fractures, which then propagate in a relatively regular manner; in contrast, low-strength, low-fracture-toughness vitrain and clarain fracture more easily, but their fracture paths are clearly influenced by natural fractures, promoting the formation of complex fracture networks. This study quantitatively characterizes the deflection and arrest behavior of experimentally induced tensile cracks interacting with dense natural fractures, providing a mechanistic basis for understanding fracture network complexity in deep coal.
