Simulation analysis of pressure relief effect of upward mining in the lower protective layer (group) of deep extra-thick coal seam

In this paper, an examination of the upward mining of multi-protected layers through simulation is conducted in a mine in Dongsheng Coal Field, China, as an example. FLAC3D numerical simulation software was used to calculate the mining scheme of the lower protected layer (group) and analyze the stress changes and pressure unloading characteristics of the protected layer during the mining process. Similar simulation experiments were conducted to provide additional explanations on the characteristics of fissure development in the overburden rock layer and the expansion and deformation characteristics of the protected layer during the mining process. The test results show that the stress distribution curve of the protected layer can be divided into four parts: the original rock stress area, the stress concentration area, the stress reduction area, and the stress recovery area. The bending deformation of the coal seam is evident after the completion of mining the lower protected seam group, and the expansion deformation area undergoes three processes: increasing, decreasing, and stabilizing. Overall, the unloading rate and length of double-protected seam mining are much larger than those of single-protected seam mining, providing an engineering basis for the mining of deep extra-thick coal seam protected seam (group).