Simulation experimental study on fracture movement and stress evolution of overlying strata under thick hard roof

Thick and hard roofs with high strength and intact rock mass are widely distributed in the mining areas of southwestern China. Under mining disturbance, large-area suspended roofs easily form, inducing dynamic disasters such as gas over-limit, large-scale roof weighting, and roof water inrush. Taking the 40-m-thick basalt key stratum of Working Face 113203 of Songhe Coal Mine in Guizhou Province as the engineering prototype, two similar material simulation test models were constructed with two comparative working conditions: an unweakened roof and a roof weakened by borehole pre-splitting. The fracture characteristics, crack evolution patterns, and stress distribution differences of the overlying strata during working face advance were compared and analyzed. The results show that for the unweakened roof, the initial caving interval of the immediate roof is 33 m and that of the main roof is 55 m, and the periodic caving intervals of the immediate roof and the main roof are 17 m and 30 m, respectively, resulting in a large suspended roof area. After roof weakening, the initial caving interval is greatly shortened, and the roof undergoes periodic fracture along the pre-split paths for every 10 m of working face advance, effectively eliminating the hazard of large-area roof suspension. In terms of stress evolution, without pre-splitting weakening, the peak vertical stress in the coal seam reaches 0.78 MPa, with stress concentration occurring 30 m ahead of the working face. After pre-splitting weakening, the caved gangue fills the goaf in a timely manner and improves the load transfer mode of the overlying key strata; the peak vertical stress in the coal seam decreases to 0.72 MPa, and the advance influence range extends to 35 m. The research results provide experimental evidence and theoretical support for roof control and the parameter design of hydraulic fracturing pre-splitting weakening technology for working faces with basalt-type thick hard roofs in southwestern China.