Evaluation of different methods of pillar recovery in coal mining by numerical simulation: a case study

The room-and-pillar technique is a self-supporting mining method in which part of the ore is left unmined as pillars. To increase the mining efficiency, the pillars are later recovered partially or completely. This research aims at evaluating and comparing four methods of pillar recovery, namely, ‘pocket and wing’, ‘open ending’, ‘split and fender’, and ‘Christmas tree’ which are of particular importance. The performance of these methods of pillar recovery in the Tabas coal mine was evaluated by numerical modeling. To do this, a series of three-dimensional simulations were carried out in which the pillars were excavated according to the sequences specified by each method of recovery. The behavior of chain pillars, barrier pillars, and the roof was monitored and compared for the different cutting sequences. Using the shear strength reduction technique, the safety factor values were calculated during the different recovery methods until the verge of roof caving. The results revealed that the method of split and fender leads to a softer drop of the safety factor, and consequently, more controlled caving of the roof.