Evaluation of the Ground Response of a Pre-driven Longwall Recovery Room Supported by Concrete Cribs

Pre-driven recovery rooms allow for the safe and rapid extraction of longwall panel face equipment. Optimum support design requires an understanding of the loading mechanisms of pre-driven longwall recovery rooms subjected to large abutment pressures. This paper presents a case study evaluating the ground response of a pre-driven recovery room. The recovery room was supported by a rock bolt and cable support system in conjunction with two rows of concrete cribs. A numerical analysis of the pre-driven recovery room was conducted using the distinct element code UDEC. The numerical results were found to be in good agreement with field observations in terms of the patterns and magnitude of stress changes, roof-to-floor convergence and failure patterns. The present results suggest that the stresses carried by the outby pillar and inby fender began to significantly increase when the longwall face was approximately 20 m away. When the longwall face entered the recovery room, the stress concentration coefficient ranged from 3.0 to 3.5 in the inby fender and from 2.0 to 2.5 in the oubty pillar, resulting in spalling failure of the room ribs. The set of longwall face equipment was safely and successfully recovered. The concrete cribs, in conjunction with the rock bolts and cables, were considered effective, but conservative. It was also found that the stiffness of the concrete crib is critical to the ground response and must be considered when determining the required capacity. From the study results, design guidelines for determining the optimal support requirement of a pre-driven recovery room are proposed.