Experimental study on similar simulation of overburden fracture evolution in shallow-buried coal fire zones

To investigate the fracture evolution law of overlying strata in coalfield fire zones, physical similarity simulation tests and high-temperature triaxial mechanical tests on siltstone were carried out according to the geological conditions of shallow buried fire areas in Baitata Coal Mine, Shenmu City. An equivalent numerical model was established by means of UDEC to systematically explore the influences of coal seam thickness, damage advancement rate and temperature on the development of overlying fractures, as well as the evolutionary characteristics when fractures penetrate the ground surface to form air leakage channels. The results indicate that in physical similarity simulation experiments, primary fractures penetrate through to the ground surface to form air leakage channels with a length of 62.1 cm when the coal seam damage advancement distance reaches 160 cm. Moreover, the caving interval after the formation of surface-penetrating fractures is larger than that before penetration. During the damage propagation, the overlying strata undergo continuous dynamic subsidence. Measuring lines farther from the simulated ground surface record larger displacement magnitudes with smoother variation trends, and the maximum subsidence of the simulated ground surface is approximately 73.2 mm at a damage advancement distance of 280 cm. High temperature induces fluctuating variations in both the elastic modulus and peak stress of siltstone. Poisson’s ratio varies within the range of 0.1–0.3 and reaches its maximum lateral deformability at 300 °C. The mechanical properties of rock fluctuate with temperature, rendering the rock prone to deformation and failure. Numerical simulation results indicate that a greater coal seam thickness, slower coal seam damage advancement rate, and higher temperature lead to earlier formation and greater length of surface-penetrating fractures. For a given overburden height, the coal seam damage advancement rate exerts a more significant influence on the coal seam damage advancement distance required for overburden fractures to develop to the surface. Physical similarity simulation revealed surface-penetrating fracture formation at 160 cm coal destruction. Combustion propagation rate was identified as the dominant factor governing overburden fracture breakthrough. Greater coal thickness, slower burning rate, and higher temperature accelerate surface air leakage pathways. Physical similarity simulation reveals overburden fracture evolution and air leakage pathway formation. Thermal damage alters siltstone mechanical properties, reducing strength and elastic modulus above 400 °C.