Study On The Overburden Fracture Evolution Law Of Repeated Mining In Coal Seam Group Of Hanjiawan Coal Mine

The mining area in northern Shaanxi is a special shallow coal seam group occurrence mining area in western China.Due to the repeated mining of shallow coal seam group,the overburden rock in the mining field experienced multiple breaking and instability,which made the evolution law of overburden rock fractures more complex.The phenomenon that the goaf hydrops in the upper coal seam flow into the lower coal seam working face often occurs.Therefore,the evolution law of overburden rock fractures in repeated mining of shallow coal seam group has become a major problem that needs to be solved urgently in various mines in the mining area.Based on the mining of shallow coal seam group in Hanjiawan Coal Mine,the evolution law of rock fracture in repeated mining of shallow coal seam group was studied by means of similar simulation,numerical simulation,theoretical analysis and field measurement.The specific contents and research results are as follows:The physical and numerical simulation experiments showed that when the lower coal seam of the coal seam group are mined,the upper coal seam overburden rock at 10～30m before and after the working face generate secondary activation,and the fracture undergo six dynamic cycle stages of generation,expansion,closure,re-generation,coalescence and re-closure.The fracture development width near the goaf side is large.The central part of the working face is mainly transverse fractures,and the two sides of the working face are mainly longitudinal fractures developed along the fracture angle.The "trapezoidal" fractures zone is formed during the first mining,and the semi-enclosed "M" fracture zone is formed under the secondary and multiple mining.Theoretical analysis showed that the main influencing factors of overburden rock fractures evolution in repeated mining of shallow coal seam group are bedrock thickness,loose layer thickness,direct top thickness,key layer thickness and position height,mining height,advancing speed of working face,interlayer rock thickness,key layer between layers thickness and position height.The 3DEC numerical simulation software was used to study the development characteristics of overburden fissures under different occurrence conditions of shallow buried coal seam groups in northern Shaanxi.The research showed that when the mining height is reduced or the interval between layers is increased,the evolution of overburden rock fractures can be inhibited in a certain extent,which is conducive to weakening the superposition effect of the fractures development in coal seam group mining.The development height of fractures was obtained by empirical formula,key layer and composite beam principle,and the key layer position was determined.Through theoretical analysis,similar simulation and numerical simulation,it is concluded that the repeated mining fractures of coal seam group in Hanjiawan Coal Mine have penetrated the bedrock layer and the interlayer rock and developed to the surface,and the maximum height of fractures theoretical development is 151.73m.The field measurement results show that the fractures development height is 152.6～164.5m,which is similar to the fractures theory development height,and the reliability of the study is verified.The research results of this paper can provide certain theoretical guidance for the subsequent mining of working face in Hanjiawan Coal Mine,and also provide certain reference value for the study of shallow buried coal seam group mining overburden rock fracture.