Investigating fracture evolution mechanisms in thick coal seam mining under upper hard and lower soft overburden: a case study.

To investigate the fracture evolution mechanisms during thick coal seam mining under an upper hard-lower soft overburden structure, this study analyzed the 304 working face of the Tingnan coal mine in the Binchang mining area. A physical similarity model (1.20 m height, 1:250 geometric similarity ratio) was constructed to simulate overburden fracture propagation, complemented by numerical simulations using FLAC3D and theoretical analysis based on linear elastic fracture mechanics and Griffith's energy release rate criterion. Results indicate that in soft rock layers, fractures propagate at an average inclination of 70°, while in hard rock, they extend more steeply, reaching 80°. The height of the water-conducting fracture zone was measured at 266.4 m in physical experiments, whereas theoretical analysis predicted 225.53 m, with a relative deviation of 15.3%. Stress analysis revealed that fracture initiation followed the maximum circumferential stress criterion, exhibiting compressive-shear failure characteristics. These findings enhance the accuracy of overburden fracture height predictions and provide theoretical support for mitigating water inrush hazards in thick coal seam mining.