This paper describes longwall numerical model of Tabas Coal Mine (Iran) by using commercial code FLAC3D. The coal seam was modeled as strain softening material and its parameters were established by calibrating separate test pillar models to common empirical pillar strength formulas. The longwall panel and the intersection of Main-Entry and tailgate were modeled separately. For simulating gob behavior, bulk modulus updating method, as a function of vertical strain, was used. Roof and floor remained elastic during all steps and caving was modeled by replacing coal zones with gob material in the panel. Induced vertical stresses of two models in two cross sections, in the middle of pillar and near tailgate, were investigated. The optimum size of barrier pillar was determined by avoiding superposition of stress concentrations which are produced by panel and Main-Entry extraction, on pillar in the direction of mining.
Longwall mining is an exploitation method used in flat-lying, relatively thin tabular coal seams in which a long face is established to extract the coal. In these operations, a mechanical shearer progressively mines a large block of coal, called a panel, which is outlined with development entries or gate roads (Yuan&Smith 2008). Figure 1 shows a typical panel layout. The panels are blocked out by developing panel entries or gateroads perpendicular to the main entries on one or both sides of the main entries. Sometimes, if the bleeder system contains a sufficient number of entries, no barrier pillar is developed, and the setup room is developed as part of the bleeder system. Also a barrier pillar from 300 to 500 ft (90 to 150 m) (sometimes less than 200 ft or 60 m) in width is left between the recovery room and the main entries for protection of the mains. (Peng, 2006 & Hartman 1988).