Disaster mechanism induced by overburden structure and control techniques for close-distance coal seam group

Compared to single-seam mining, the close-distance coal seam group is strongly influenced by the movement of overburden structure resulting from upper seam mining. The mining of the lower seam will trigger a “repetitive” movement of the overburden structure, resulting in more complex ground pressure behavior and introducing substantial uncertainty to the control of surrounding rock in lower seams. Based on the occurrence characteristics of interlayer hard strata and the evolution of bearing structures under close-distance coal seam group mining conditions, four types of overburden structural models were proposed: fully bearing-type, partially degraded bearing-type, partially failed bearing-type, and degraded-failed combined bearing-type. The dynamic evolution of these four structures during the initial and periodic weighting stages was analyzed, and the influence mechanisms of different mining stages on working face support and roadway stability were clarified. The evolution law of the fracture arch extension angle and its influencing factors were further analyzed. By comprehensively considering horizontal thrust between key blocks, mining height, interlayer spacing, and the bulking characteristics of the overburden, the mechanical and engineering transformation conditions for these four structure types were proposed. Taking the close-distance coal seam group in the Shuangma No. 1 Mine as a case study, the overburden structure type was identified by similar material tests and numerical simulation. The deformation and failure of the surrounding rock in the lower coal seam, as well as the associated energy evolution, were further analyzed. Based on the relationship between the total energy absorbed by the surrounding rock and its peak energy threshold, the instability mechanism of the surrounding rock in the lower coal seam was revealed. Moreover, considering the additional stresses in the lower seam induced by different overburden structural movements, a method for calculating the instability identification coefficient was provided, and an energy criterion for surrounding rock instability was proposed. Results shown that the instability identification coefficient for the roof of the roadway at the Ⅰ0104₂05 working face is 0.246, while the value for the sides is 0.962, indicating that an instability risk exists. On this basis, a progressive control technology system of “far-field pressure relief - near-field reinforcement” suitable for different overburden structure types was proposed. After application in the haulage roadway of the Ⅰ0104₂05 working face, the force on the anchor cables was reduced by 17.3%, and the convergence of the roof-to-floor and both sides decreased by 60.5% and 61.9%, respectively, indicating significant control effects on the surrounding rock. Finally, the relationship between various influencing factors and the overburden structure types in close-distance coal seam group was discussed.