Study On Distribution Characteristics Of Coal Spontaneous Combustion Danger Area In Overlying Gob Area Under Shallow Seam Group Mining

In this paper,based on the research background of shallow and close coal seam mining,theoretical analysis,field measurement and numerical simulation are used to study the distribution characteristics of overlying rock surface fissures,dynamic evolution rules and ground fissure air leakage characteristics under repeated mining conditions of coal seams.The effective air leakage cracks on the surface and the surface air leakage range are analyzed,and the flow field distribution characteristics of the upper gob under the condition of surface air leakage are analyzed,and the risk of spontaneous combustion of coal left in the upper gob under the influence of complex air leakage(air leakage from the lower coal seam and air leakage from surface fissures)is predicted area.When the lower coal seam is repeatedly mined,it is calculated that the development height of overlying rock fracture zone in close distance coal seam mining has reached the surface directly.The field measurement verifies the existence of new surface fractures,and the existing fractures expand in the original development degree.There are three types of fractures distributed on the surface:tensile fractures,collapse fractures and sliding fractures.The maximum development width of collapse fractures can reach 0.4m,and the maximum drop can reach 0.5m.The fractures are distributed parallel to the working face,and occur periodically,with straight and arc shapes.They usually connect the surface and goaf.The fracture development morphology and scale parameters show dynamic changes with the progress of the working face,It goes through a complete evolution cycle from generation to expansion and then to closure.The measurement of surface air leakage shows that the air leakage of ground fissures is mainly concentrated on the surface near the two roadways,and the air leakage near the middle area of the working face is relatively weak due to the influence of surface compression deformation.The air leakage capacity changes correspondingly with the dynamic development of the working face,and its development shape and scale parameters control the surface air leakage,When the crack opening amount reaches 0.1m～0.2m,the first air leakage occurs.When the crack is 10m～25m behind the working face,the width reaches the maximum,and the maximum air leakage velocity reaches 0.65m/s.It is the most serious stage of surface air leakage and the key area of surface air leakage prevention and control,The convergence speed gradually slows down and the air leakage speed is reduced to the minimum or there is no air leakage phenomenon.Numerical simulation results show that the air leakage velocity in the upper goaf decreases first and then increases from the inlet side to the return air side of the working face,and the air flow in the middle of the working face is significantly smaller than the two sides,and the air leakage from the surface and the air inlet lane The air flow is mainly concentrated on the return air side of the goaf with relatively low pressure energy,which replenishes a large amount of oxygen on the return air side of the upper goaf,which was originally at the end of oxygen consumption of the coal,and enlarges the influence range of the goaf seepage.Due to the pressure difference The air intake side of the upper goaf is relatively weakly affected by air leakage.When the lower coal seam face has different air distribution,the spontaneous combustion hazard area in the upper goaf is mainly concentrated on the return air side,which is similar to a fan ring with the return air corner as the center.The radius of the outer ring is about 45m,the radius of the inner ring is about28m,when mining with low air volume(1072m~3/min),the radius of the outer ring is about65m,the radius of the inner ring is about 30m,when mining with normal air volume(1191m~3/min),and the radius of the outer ring is about 68m,the radius of the inner ring is about 54m,when mining with higher air volume(1310m~3/min).Only a small spontaneous combustion hazard area appears on the air inlet side during high air volume recovery.