Research on stress release and pressure relief mechanism of underlying coal and rock under protective layer mining

To study the effect and mechanism of coal-rock underground impact pressure prevention and control under the influence of upper protective layer mining. In this paper, the research is carried out based on four factors, namely, the mining stress environment, the mechanical strength of coal and rock, the dynamic load disturbance effect of mining, and the energy loss space of coal and rock. Theoretical calculation and analysis, rock mechanics test, physical similar simulation test and on-site industrial experiments are adopted to study the stress distribution of the coal rock below the influence of the mining of the upper protective layer, overlying strata migration, and the law of mine pressure behavior. The results show that after the mining of the upper protective layer, the vertical stress of the coal rock body under the mined-out area increases with the depth, and the horizontal compressive stress in the shallow area gradually decreases with the increase of depth. In contrast, the horizontal tensile stress in deep areas gradually increases with the depth increase. The uniaxial compressive strength and impact energy index of the coal rock both decrease with the increase of the mining intensity of the upper protective layer. The mining of the upper protected layer will weaken the overburden structure of the mining field of the protected layer, and will reduce the breaking span and disturbance intensity of the key layer in the protected layer. The mining coal rock body in the process of upper protected mining experienced three stages of advanced stress concentration, the rapid growth of pressure relief, and the slow recovery of pressure relief, and the stress release rates all decreased with the increase of depth. The results of the study can provide an important reference value for the prevention and control of underground impact pressure and other disasters in the upper protective layer mining of closed-distance coal seam groups.