Coal and rock failure migration characteristics of fully mechanized caving face in steeply dipping and extra-thick coal seam based on mining dip angle effect

• Steeply-dipping face 3D coal–rock migration morphology revealed. • Analyzed failure features in different zones of the longwall face. • Clarified 3D morphology differences under varied dip angles. • Quantified failure features for varying conditions and zones. In the fully mechanized mining of steeply dipping coal seams, the intense movement of coal and rock in the mining area makes the evolution pattern of fractures extremely complex. Taking the working face of Wangjiashan Coal Mine in Jingyuan, Gansu Province as the research object, a research method combining 3DEC discrete element numerical simulation, Matlab calculation of fracture fractal dimension, and on-site monitoring was adopted to analyze the collapse morphology and fracture development characteristics of the mining area under different coal seam inclinations. The results show that: as the coal seam inclination increases, the non-equilibrium filling along the strike of the mining area becomes more obvious, and the stable area of the upper and middle roof of the mining area significantly increases. The development height and range of fractures along the strike of the mining area continuously decrease. As the seam dip angle increases from 35° to 45° and 55°, the fracture-network fractal dimension drops from 1.525 to 1.400 and 1.336, corresponding to sequential reductions of 8.2 % and 4.6 %. From the perspective of strike distribution, the fractal dimension of fractures in the inclined upper part of the mining area increases with the increase of coal seam dip angle, while those in the inclined middle and lower parts exhibit a decreasing trend. Fractures are mainly distributed in the inclined upper part of the mining area, and the variation of coal seam dip angle exerts a significant influence on the number of fractures in the upper and lower parts.