Rock Mechanics Considerations for Roof Bolt-Based Breaker Line Design

Application of roof bolts to support in developed galleries during underground coal mining is a common practice. Mechanised depillaring (MD) applies these roof bolts to support the galleries at the goaf edge also. Goaf edge of a depillaring panel inherits a number of openings, which provides least resistance path for the caving roof strata to encroach the working areas. Influence of strata caving, inside the goaf, make it difficult to apply conventional roof bolt support norms in these openings at the goaf edge. Basic rock mechanics considerations find that the mining induced stress development improves the efficacy of roof bolt at the goaf edge. Application of, relatively, high density of bolts as roof bolts-based breaker line support (RBBLS) in the existing openings at the goaf edge to arrest the goaf encroachment is found to be working satisfactorily only if the surrounding natural supports are stable. The stability of a natural support, in and around the goaf edge, is found to be diluted due to side spalling/loosening caused by the mining induced stress. Different field experiences found that a high value of the stress causes side spalling/loosening in natural supports, standing in and around the goaf edge. Under the existing geo-mining conditions of some of the recent MD operations in Indian coalfields, the extent of spalling varied from 0.5 m to 2 m only. Considering an acceptable dimension of the rib/snook for these site conditions, the position of RBBLS is shifted out-bye from the goaf line as per the extent of spalling, which performed satisfactorily in the field. Presenting basic working principles of a RBBLS at the goaf edge; this paper discusses different field experiences for a suitable positioning of the RBBLS at the goaf edge.