Study on evolution features of high level overburden separation layer and precursor information of water inrush during coal mining

This paper focuses on the space-time evolution of high-level separation strata in Zhaoxian coal mine by using the methods of theoretical analysis,physical simulation and numerical simulation. Taking the water level change and microseismic monitoring data during the mining period as the precursor information of water inrush from high-level separated strata,the evolution characteristics of main controlling fractures (longitudinal dominant fractures and transverse separated fractures) in overlying strata and the mechanism of water disaster from separated strata in fully mechanized top coal caving mining of extra thick coal seam are revealed. The coal seam overburden in Yonglong mining area of Huanglong Jurassic coalfield has the characteristics of weak cementation,high argillaceous content and large single layer thickness. The Jurassic overburden is a Cretaceous aquifer with huge thickness and high water pressure. When mining thick and extra thick coal seams,it is easy to form high-level separated layer water above and below the Jurassic Cretaceous boundary. When the water flowing crack communicates with the high-level separated layer water,the separated layer water disaster is formed. Separated layer water disaster has the characteristics of instantaneity,large discharge and strong mine pressure behavior. The research results show that: when the working face reaches full mining,the two sides of the goaf separation fractures are relatively developed,the middle part is in the compaction area,and the maximum horizontal high separation layer is at the top of the trapezoidal failure area. The periodic breaking of the sub key strata on the working face results in the periodic weighting of the working face,the periodic expansion and closure of the high separation layer space,and the periodic abnormal decline of the water level of the Yijun formation. After stoping for a long time and resuming mining,the overburden rock mass structure instability is induced by repeatedly disturbing the overburden rock mass,and the water flowing fracture zone connects with the separated layer space,resulting in the separated layer gushing (water inrush). The sudden drop of water level,the sudden increase of water level drop rate and the microseismic large energy events can be used as the precursory information of water inrush in 1304 working face. The breaking of the key layer will cause microseismic energy events. With the rapid rotation and sinking of the key layer,the space of separated layer will be expanded,resulting in the abnormal decline of water level in Yijun formation. The repeated mining of working face results in the gradual deterioration of overburden self stability ability,accelerates the formation of separated layer water inrush channel,and gradually increases the separated layer water inrush channel. With the increase of outflow times,the time of water level falling ahead of time decreases,the energy of microseismic energy event decreases,the water inflow increases. The precursory information of water inrush from separated strata during mining the water level change of Cretaceous aquifer and the monitoring data of mine microseism verify that the simulation results are reliable.