Failure Propagation of Pillars and Roof in a Room and Pillar Mine Induced by Longwall Mining in the Lower Seam

Underground mining using the room and pillar method leaves pillars to support the overlying strata. When longwall mining is used to extract a lower coal seam beneath the room and pillar mine, the bending of the interburden strata can induce pillar failure and roof caving. In this study, two physical model experiments with different pillar width/height ratios were performed, and a non-contact displacement measurement technique was used to record and analyse the deformation and failure processes. The failure mechanism of pillars and the roof in a room and pillar mine induced by longwall mining in a lower coal seam was investigated. The results show that, during the longwall retreat, the immediate roof of the longwall panel collapses and the main roof bends resulting in load redistribution among a series of overlying pillars. Eventually pillar failure and unsteady strata movement are induced. Physical model tests indicate the entire deformation and failure process can be divided into two stages, i.e. steady deformation and failure propagation. In addition, it is observed that different width/height ratios of pillars lead to different failure patterns in the second stage. For model #1 with higher pillar width/height ratios, load redistribution is induced by the bending of the main roof of the longwall mine, but most panel pillars remain stable. The unsteady movement of the main roof of the longwall causes an upward failure propagation in the overburden strata. For model #2 with lower pillar width/height ratios, the failure propagates downward and it is triggered by the progressive collapse of the panel pillars during the load redistribution process.