Managing the geotechnical aspects of longwall face recovery

Longwall face recovery is almost certainly the most involved recurring geotechnical problem faced by operators, with major loss potential should problems occur. The outcomes of an industry sponsored (ACARP) research project on the geotechnical issues associated with conventional longwall recoveries is presented with updates the experiences gained. A number of critical features of the geotechnical environment, support design and mining geometry have a pronounced impact on ground control during take-off. A model of roof behaviour at the take-off point has been developed and validated. Key geotechnical issues include low roof competency (ie weak roof), an adverse weighting environment, geological structure and horizontal stress concentrations at the gate ends (generally the maingate). All of these are identifiable either at the support design stage or, at worst, prior to the start of powered support removal, which is the critical stage of the take-off process. Key aspects of the geometry and process are the ability to maintain powered support resistance during bolt-up and take-off, the direction in which the powered supports are removed, the impact of take-off chutes and the speed of the powered support removal process. The author presents case studies that illustrate these issues and the associated ground behaviour.