Analytical and Numerical Modeling Approaches for Estimating the Optimum Line of Extraction in Continuous Miner Workings with Field Observations

The line of extraction in Bord and Pillar (B&P) or Room and Pillar methods plays a paramount role in the extraction of coal pillars in underground mines. It is an important factor that has to be decided at the premining stage, and its optimization not only boosts productivity but also enhances the stability of underground operations. In Indian coal mines or in other B&P mines around the world, the quest to determine the effective line of extraction remains elusive. In Indian B&P coal mines, both diagonal and straight lines of extraction have been widely adopted. In one of such mines, Mine-A, situated in the southern part of India, several coal panels were successfully extracted in the middle seam using a continuous miner (CM) technology in straight and diagonal lines of extraction. However, local geomining effects, such as the influences of top seam panel goaves, barrier pillars, delays in main fall, surges in abutment loading, and increased convergence in roadways caused difficulties throughout the coal extraction in some panels of the middle seam CM workings. This underscores the need to evaluate the optimum line of extraction for successful pillar extraction in B&P workings. Hence, a study was undertaken with numerical modeling and extensive field observations. Strata monitoring instruments such as stress cells, rotary telltale, auto warning telltale, and 4-anchor extensometers were extensively utilized in the field to observe the stress distribution and roof displacement in both lines of extraction. Comparative analysis using field data and numerical modeling with FLAC3D (version 5.0) exhibited that in the diagonal line of extraction, incidents such as delays in major and main fall occurrences, surge in induced stress distribution, and increased rock load development were more prevalent than in the straight line of extraction. This analysis unequivocally demonstrated that a straight line of extraction offers better safety and more effectively reduces ground instabilities in CM panels. Furthermore, the principles of plate theory were used to interpret the roof deflection during the depillaring of the panels, and the modeling results were corroborated with these theoretical findings.