Division method and fractal characteristics of overburden gas slow permeability zone in up-dip fully mechanized face of inclined thick coal seam

In order to study the division method and fractal characteristics of the overburden gas slow permeability zone in up-dip fully mechanized face of inclined thick coal seam, based on the fractal theory and grey theory, the plane physical similarity simulation experiment was used to study the crack evolution law of the overburden gas slow permeability zone in up-dip fully mechanized face. The dynamic expansion law of the overburden gas slow permeability zone was obtained, and then the division method of the overburden gas slow permeability zone was constructed. The grey correlation analysis method was used to determine the maximum correlation degree between the horizontal direction (the advancing direction of the working face) and vertical direction (perpendicular to the upward direction of the coal seam floor) fractal dimensions of the overburden gas slow permeability zone and the separation amount, the fracture density and the stress concentration coefficient. The results show that the overburden gas slow permeability zone is initially formed after the first periodic weighting. After each periodic weighting, the caving angle on both sides of the overburden gas slow permeability zone decreases continuously and the width and height increase continuously. Specifically, from the initial formation of the slow permeability zone to the full development of the slow permeability zone, the caving angles of the open-off cut side and the working face side of the slow permeability zone are reduced from 68.3° and 76.2° to 44.7° and 53.5° respectively. The width and height of the slow permeability zone increased from 16.3 m and 19.2 m to 52.1 m and 38.4 m respectively. According to the established division criterion and process of the overburden gas slow permeability zone, combined with the grey correlation analysis method, it is obtained that the variation of fractal dimension along the horizontal and vertical directions of the overburden gas slow permeability zone has the strongest correlation with the variation of the separation amount ( r_1 =0.93, r_1^ * =0.91), and the accuracy of theoretical calculation is verified by experiments. Therefore, when the final hole (roadway) of pressure relief gas extraction borehole (roadway) is determined in the later stage, the boundary of the overburden gas slow permeability area can be judged by field observing the separation amount. The research results have certain guiding significance for accurate and efficient extraction of pressure relief gas in goaf.