Mechanism of stress-loaded rockburst in fully mechanized top-coal caving stope with deep overburden

The stress-loaded rockburst is a unique type of rockburst that occurs in some fully mechanized top-coal caving stopes with deep thick overburden, its occurrence mechanism is still unclear, leading to the failure to effective prevention and even serious accidents when the coal mine is faced with this type of rock burst. This study focuses on the stress-loaded rockburst that occurred in the 3304 fully mechanized top-coal caving face of the Zhaolou Coal Mine, Juye Coalfield, which lasted for 38 days and had a large range of impact warning (100 m ahead). Through material tests, theoretical analysis, numerical simulations, and field measurements, the study investigates the occurrence mechanism of stress-loaded rockburst in the fully mechanized top-coal caving stopes with deep thick overburden. The study proposes the spatial movement characteristics of overburden in the fully mechanized top-coal caving stopes with deep thick overburden through simulation tests and theoretical analysis. The contour expressions of collapse arch and pressure arch of thick overburden are derived based on the arching property and time-dependent loading property of thick overburden when the bearing bedrock is first broken. An estimation model for the transfer stress of overlying strata along the strike of the fully mechanized top-coal caving stopes with deep thick overburden is established. The mechanism of stress-loaded rockburst in the fully mechanized top-coal caving stopes with deep thick overburden is revealed through the study. When the bearing bedrock reaches the hanging limit and breaks and rotates, the thick overburden moves and sinks to form an earth pressure arch structure. The weight of the overlying thick overburden is transferred to the coal seam through the arch foot, resulting in a large range of coal seam stress in front of the coal wall that continues to rise in a short time, which can induce stress-loaded rockburst. The numerical simulation results show that after the bearing bedrock of fully mechanized top-coal caving stopes with deep thick overburden breaks, the thick overburden presents the movement characteristics of progressive breaking and collapse from the bottom to the top. The stope forms two pressure arches with the rock and soil contact surface as the boundary. The weight of the overburden pressure arches causes the high concentration of stress in the large range of coal seams in front of the coal wall. The field measurement shows that the dynamic load release level of the stope is low after the stress loading effect occurs, and the micro-earthquake concentration zone is fixed. The formation of the earth pressure arch will limit the rapid subsidence of the topsoil. The working face is located in the fulcrum area of the earth pressure arch for mining. When the working face pushes over the fulcrum loading area, the destruction of the earth pressure arch will lead to the overall large subsidence of the stope surface. The study proposes a prevention and control plan for the stress-loaded rockburst in the fully mechanized top-coal caving stopes with deep thick overburden, including underground ground joint monitoring and early warning, high-strength borehole weakening of surrounding rock, reinforcement of support system in the exposed area, and “monitoring and pressure relief” critical pushing speed control.