This paper investigates the applicability and operational characteristics of different partial pillar extraction methods for a room-and-pillar coal mining project located in the region of southern Africa. Three partial pillar extraction methods, i.e. the Pillar Splitting method, the Duncan method and the Lift Left-Right method are evaluated using three-dimensional finite element analysis. Multiple 3D models were developed for each method by varying the geometry of the remnant pillar during partial pillar extraction. Pillar strength was estimated using the Salamon and Munro equation and pillar stress was obtained from the numerical models. Surface subsidence was also calculated for each room-and-pillar panel. All three methods meet the stability factor requirement for safe secondary extraction. The Lift Left-Right method exhibits the highest stability factor while the Pillar Splitting method is the most favorable in terms of surface subsidence.
The Room-and-pillar (R&P) method is a widely utilized underground mining method used in coal, limestone and metal mines. The basic premise is that a series of rooms (i.e., long horizontal openings) will be extracted, while leaving behind pillars of ore. Pillars constitute the main support elements of the underground openings, while this type of mining may require ground support systems, such as roof and/or cable bolting. The cross-sectional area of the pillars needs to be minimized in order to maximize the extraction ratio. In addition, the room dimensions (entry span) must be large enough to accommodate the traffic of mining equipment, but not so large that room stability is compromised. The coexistence of these two basic elements (i.e., rooms and pillars) makes it necessary to evaluate their safety jointly.
Since coal is a nonrenewable resource, it is important to improve the initial extraction ratio by recovering pillars during a secondary mining stage. Pillar extraction (or recovery or retreat mining) is the practice of partially or fully extracting some or all of the pillars developed during the development stage of an R&P operation. Full or partial pillar recovery helps increase resource recovery, since any coal left in place that is not recovered at the time a particular mine section is mined, will most likely never be recovered.
