Seepage characteristic and fracture development of protected seam caused by mining protecting strata

Protective layer mining could effectively prevent coal mine gas problems. The protection effect was an important basis for the protection layer mining design. In order to study the crack evolution and permeability characteristics of the protected layer during protection mining,field tests were performed to analyze the development characteristics of the cracks before and after the protection layer mining,and similar permeability and pressure relief characteristics of the protected layer were simulated during the protection layer mining process. The numerical study shows that before the protective layer was mined,the number of cracks developed were small. After being affected by the protective layer mining,the number and extent of the cracks increase significantly,and the degree of crack development increases. After the protective layer was mined,the protected layer could be divided into four areas:rock stress area,supporting stress area,mining pressure relief area,and stress recovery area. The permeability of the supporting stress area was reduced,and the permeability of the stress recovery area was slightly increased. The permeability in the mining pressure relief area can reach more than 7 times before mining. After the protective layer was mined,the pressure relief of the protected layer was nearly 50%,and the pressure relief rate curve was distributed in a "ditch shape". After the protective layer was mined,the overall expansion rate of the protected layer was greater than 3‰, achieving the effect of pressure relief and permeability enhancement.