Overburden Breakage Characteristics And Fracture Field Evolution Of Extra-thick Seam Mining Of The Jurassic And Carboniferous Mining

Datong mining area is a typical mining area,where the Jurassic and Carboniferous coal seams are mining in China.At present,the Jurassic coal mining have been almost finished,and the Carboniferous 3-5# extra-thick coal seam is the key mining zone in Tongxin mine.The mining thickness of 3-5# extra-thick coal seam is 14-20 m,which increases the overburden breakage height and fracture development height.When the fracture filed in overburden communicates with the mining gob in upper Jurassic coal seam,the accumulated water and CO and other harmful gases in the gob are released to the working face in Carboniferous 3-5# extra-thick coal seam,through mining-induced fracture channel,leading to the water and gas drainage disaster.Therefore,it is of great significance to accurately grasp overburden breakage characteristics and fracture field evolution under different conditions in extra-thick coal mining,which could provides reference for the prevention and control of water and gas drainage disaster.Based on the mining geological conditions at 8309 working face in Tongxin mine of Datong mining area,the overburden breakage characteristics and fracture field evolution under different conditions in dual system of extra-thick coal mining are studied,by means of numerical simulation,similarity simulation and field measurement.By analyzing the variation characteristics of mining-induced fractures in different zones,the relationship between overburden breakage and fracture field evolution in dual system of extra-thick coal mining under different conditions is obtained.The research results and conclusions are as follows:(1)When the dual system of extra-thick coal seam is mined,the mining-induced fracture field is divided into three zones in the transverse direction,namely the influence zone of advanced fracture,the development zone of key fracture channel,and the recovery zone of lagging fracture.Meanwhile,it is divided into five zones in the longitudinal direction.From the bottom to the top,the zones are the lower caving zone,the middle macroscopic fracture zone,the higher abscission zone,the higher microscopic fracture zone,and the Jurassic coal floor failure zone,respectively.The overburden fracture openings in dual system of extra-thick coal seam mining have obvious zoning characteristics,and the development zone of key fracture channel is the disaster-causing zone for water and gas drainage disaster.(2)the original overburden occurrence characteristics are changed,after the key strata in higher position is ruptured by the ground fracturing technology;besides,the rupture in horizontal direction has the greatest influence on the original occurrence of key strata in higher position and the mining-induced fracture field development.When the fracturing fragmentation is below 20 m,an artificial cushion structure is formed in the key strata and the upper overburden,which slows down the deformation and movement in overlying stratum;then,the coordinated deformation tends to occur,reducing the mining-induced fracture field development.(3)The three zones of mining-induced fractures in extra-thick coal mining in the transverse direction are further verified by the monitoring results of borehole water level at 8309 working face,and the key fracture channel development are closely related with the breakage characteristics of the key strata in higher position.