Analysis of overburden structure and evolution characteristics of hard roof mining in extremely thick coal seam

The mining of extremely thick coal seam form a large space stope, the fracture disturbance of hard roof spreads to a wide range, the working face weighting has the characteristic of “strong mine pressure”, it is obviously different from rock burst, especially when the coal seam roof “square” mining period, the stope mine pressure is more intense. Based on this, the disturbance height of overburden and fracture characteristics of hard roof in extremely thick coal seam mining are discussed by field measurement and 3D physical similarity simulation. Firstly, the fracturing law of overburden was analyzed based on the field measurement, the results showed that the hard roof was fractured advance the working face during the extra thick coal seams mining. The initial movement of the roof mainly revolved around the base point near the fault line and sinking, behind the working face for a certain distance. The observation results of ground sinking showed that the strata formed a roof groups and moved combined of extra thick coal seams, and there is a step phenomenon during strata movement. The study shows that the disturbance height of overburden in extremely thick coal seam mining is large and the fracture of hard roof has steering characteristics. With the advance of coal seam mining, the transverse “O-X” fracture of low roof layer gradually turns to the longitudinal “O-X” fracture of the high hard rock. The critical position of fracture turning of hard roof in extremely thick coal seam is the so-called “square” area of coal seam mining, which reasonably explains the phenomenon of “square” pressure in working face mining. The large longitudinal fracture size of the high roof and the wide range of disturbance influence are the main factors inducing the strong mine pressure in the stope. The rationality of the “low-medium-high” layer structure of the overburden in extremely thick coal seam after mining is determined, which provides a basis for the accurate interpretation of the large space stope size cycle and the strong mine pressure.