Impact of Stress Redistribution on Stability of workings during Depillaring

Extraction of underground coal during depillaring operations in India Poses several ground control problems. Understanding the behaviour of pillars and overlying roofs and sides in underground coal mines in important for design of efficient bord and pillar extraction system. Hence, knowledge of development of stresses, their magnitudes and redistributions with face advancement become critical. The analyses in this article have been undertaken for a mine to predict stress development and redistribution in and around pillars and galleries during depillaring which eventually assessed the ground stability to provide design guidelines for making ground control decisions in advance. Simulation of virgin state, developed panel and depillaring operations with one-, two- and three- pillars extraction have been carried out. The simulation results reveal that the maximum principal stresses is developed on the immediate rib pillar and there is 66% increase in the maximum stress level after three pillar extraction when compared to after one-pillar extraction when compared to after one-pillar extraction. The safety factor values of the rib pillars, slice galleries, and immediate pillars are decreased after three-pillar extraction. However, after one pillar extraction, it is found that the safety factor of the rib is more than one, thus it should be judiciously reduced for minimizing stress concentration inside the goaf. The simulation results also reveal that the thickness of the unstable zone in the immediate roof is 1.5 m where the safety factor is up to one. Above 1.5 m, the immediate roof is considered to be stable. The roof bolts used in the galleries are 1.5 m of length. It may be better, if a part of the roof bolts is grouted in the stable zone (that is, more that 1.5 m thickness) for effective working of the roof bolts. The simulation results are validated through continuous remote monitoring of the studied panel. The instrumentation-based observations strongly support the simulation results