A study on the effective extraction layer of overburden fracture zone in goaf based on key layer theory

The effective extraction layer of the overburden fracture zone in goaf is the basis for arranging high-level extraction boreholes to treat adjacent layers and gas in goaf. Based on the key layer theory, a mathematical model for the effective extraction layer in fracture zones is established, and the upper and lower boundaries of the effective extraction layer are determined. The lower boundary of the effective extraction layer is the first key layer above the collapse zone of the goaf, and the upper boundary is the first key layer below 10 times the mining height of the overburden layer in the goaf. The effective extraction layer includes the lower boundary rock layer and does not include the upper boundary rock layer. According to the mathematical model of the effective extraction layer of the fracture zone, it is calculated that the effective extraction layer of the fracture zone in the 8+9 coal seam of Duanwang Coal Mine is from the medium sandstone at 12.6 m above the coal seam roof to the No. 4 coal at 39.3 m. According to the drilling and observation results of the overburden fracture zone in the goaf, the fracture angle of the working face is about 62°. The height range of the fracture zone is 11.5-40.5 m above the coal seam roof. A high-level drilling and extraction test is conducted at Duanwang Coal Mine. It is found that the actual effective extraction layer of the fracture zone is from medium sandstone at 13.9 m above the coal seam roof to sandy mudstone at 37.4 m. The results of drilling observation analysis and high-level drilling extraction test have verified the accuracy of the mathematical model of effective extraction layer in the fracture zone. The research results can provide theoretical basis for the design of high-level extraction engineering in high gas and coal and gas outburst mines.