Synergistic fracture of composite key strata and energy evolution characteristics of overlying strata under mining action

Revealing the synergistic fracture response characteristics and energy evolution mechanism of composite key strata constitutes the theoretical foundation for preventing catastrophic accidents in coal seam surrounding rocks. A combination of physical similarity simulation, numerical calculation, and theoretical analysis is employed to investigate the fracture and the instability characteristics of composite key strata. On this basis, the aggregation and release patterns of overburden strain energy and gravitational potential energy under mining-induced action are examined, and the evolution mechanism of overburden energy under the excitation of key strata fracturing is elucidated. During the mining of working faces with composite key strata, the fracturing and movement of lower and upper key strata mutually restrict and influence each other, which is the intrinsic cause of severe mine pressure manifestations in such coal seams. After excavation, the instantaneous fracture of the lower key stratum produces an unloading effect on the overlying upper key stratum, whereas the instantaneous fracture of the upper key stratum imposes an impact effect on the lower key stratum. The instantaneous fracture of the high-level key stratum will form a dynamic load impact on the low-level key stratum, resulting in the formation of cutting and rotary instability of a single key stratum under mining, and the formation of synergistic rotary instability and overall cutting of the composite key stratum. There are two fracture modes, resulting in significant differences in the energy accumulation and release characteristics of overlying strata under corresponding conditions. Among them, when the low key stratum rotates and cuts off, the elastic strain energy released by the overlying strata of the stope is basically the same, but the gravitational potential energy shows the distribution characteristics that the latter is greater than the former, which promotes the overlying strata to produce 'low frequency-high energy' microseismic events. After the synergistic fracture of the low and high key strata, the energy consumed by the coal and rock in the stope is the sum of the energy released .