Pressure relief mechanisms and effects of liberation seam mining in 1000-m-deep coal mines: A case study of the Huafeng coal mine in Tai’an, China

Liberation seam mining serves as an important approach to the prevention of deep dynamic disasters such as coal and gas outbursts, as well as rock bursts. Based on the Nos. 2613 and 2412 mining faces of a 1000-m-depth coal mine in the Huafeng Coal Mine in Tai’an, Shandong, this study investigated the engineering scientific issues such as the pressure relief mechanisms and effect evaluation of liberation seam mining through physical and numerical simulations. As a result, it determined the movement law of overburden strata and the characteristics of underground pressure after the mining of the liberation seam and ascertained the evolutionary laws of the stope stress and stratum displacement during the mining of the liberated seam. Accordingly, it evaluated the pressure relief effect of the liberation seam mining and the feasibility of the liberation seam mining based on various indices. The results are as follows: (1) During the mining of the liberation seam, the overburden strata gradually collapsed, forming a funnel-shaped, asymmetric multi-end fixed beam structure. After the liberation seam mining, quasi-cantilever beam structures, which provided permanent pressure relief protection for the liberated seam, were formed at the left and right ends of the stope, with left and right protection angles of 54° and 66°, respectively; (2) The overburden strata of the liberation seam can be divided into the protection zone of permanent pressure relief and the compaction zone of gangue in the goaf. Corresponding to the protection zone and the compaction zone, the maximum stress on the roof the of liberated seam was about 20 MPa and 36 MPa, respectively, and the maximum stress within the liberated seam was approximately 29 MPa and 24 MPa, respectively. The liberated seam was subjected to the combined effects of the two stress zones; (3) After the mining of the liberation seam, stress accumulated at both ends of its mining face, and the overburden strata in other zones were in the pressure relief state. The continuous propagation of the compaction zone affected the liberated seam, whose subsidence at distances of 50-100 m from the liberation seam roughly equaled the mining height; (4) During the mining of the liberated seam, the overburden rock showed a palling index (f) of 0.5, which was less than the critical value 0.7, indicating that the dynamic disaster of high underground pressure was unlikely to happen in the liberated seam. Moreover, the overburden strata’s disturbance range varied slightly compared to that before mining. In combination with the failure morphology of the overburden strata, it can be concluded that the liberated seam was always within the pressure relief range during its mining. Therefore, due to the sufficient pressure relief of the liberation seam, the mining of the liberated seam is feasible.