Study on the Fracture Evolution Characteristics of Overlying Strata in a Fully Mechanized Mining Face with a Large Mining Height Based on a Three-Dimensional Large-Scale Physical Simulation Experimental System

To investigate the evolution characteristics of overlying rock fractures, based on a geological prototype of a large-height comprehensive mining face in Shanxi, a three-dimensional large-scale physical similarity model was established. The experiments were carried out using microseismic monitoring and physical model cutting methods to study the activity and fissure evolution of the overburden rock. Model cutting revealed that, approximately 65 m from the bottom of the coal seam, delamination occurred, marking the top of the overburden rock fissure zone and the bottom of the bending and sinking zone. At 25 m from the coal seam bottom, the rock layer was highly fragmented, forming the collapse zone, which was 4.8 times the mining height. Between 25 and 65 m from the bottom, a fissure zone existed, which was 12.5 times the mining height, with abundant delamination fissures at the top of the fissure zone. Significant microseismic events were observed as the coal face advanced to 45 m, with notable increases in the concentrations and distribution ranges of these events in both the strike and height directions of the coal seam. The subsidence range of the overlying rock layer expanded from the top to the bottom, with the subsidence slope area extending gradually and the central compaction area remaining relatively flat. The overall shape presented an irregular ellipse, with peripheral uplift phenomena observed in the subsidence area. At 39 m from the coal seam bottom, the maximum subsidence of the rock stratum was 4.0 m, with subsidence amounts decreasing with increasing stratum height. Fissure density along the coal seam inclination and direction exhibited a double hump pattern, with fissure areas on both sides showing high densities and the central compaction areas having low densities. Coal seam mining caused stress redistribution in the surrounding rock layer, and the stress in front of the work was divided into the stress reduction zone, dynamic influence zone, mining influence zone, and unaffected zone. Coal rock porosity under high stress was less sensitive to stress changes, resulting in smaller changes in fissure permeability and fissures remaining mostly closed.