In this paper, a probabilistic design chart for coal pillar based on South African coal mine experience is proposed as a tool capable of estimating the pillar strength. A methodology based on the logistic regression is employed to identify failed and intact pillars with their corresponding probabilities of occurrence. The input parameters used for pillar design include the mine depth, pillar width, bord width, height of mining excavation and panel width. The results showed that 84.6% of the pillars, including stable and failed categories, were correctly classified. In addition, the probability associated with the pillar conditions was determined and plotted in form of a pillar strength graph. The results were in good agreement with the field data and existing studies. One of the merits of the current study is that the probability of having a coal pillar intact or failed can be easily determined. This is particularly useful for design purposes since a series of uncertainties are usually involved in collecting the rock mass parameters. It is concluded that the results of this study could improve the empirical design of coal pillars.
Room and pillar mining method is commonly employed in flat-lying deposits such as coal seam, where the ore is extracted creating a network of rooms and leaving pillars behind after extraction. The role of the pillars is to support the overburden weight and to protect the mine entries and crosscuts during the development and extraction processes [1]. Notwithstanding the well-established application of this method of mining and its good reliability, failure of pillars, even of those with a high safety factor, still occur. The progressive nature of such failures, due to redistribution of the load, makes such accidents more severe [2]. Moreover, the pillars left behind to support the overlying strata might be engaged in the second stage of mining to increase the recovery. In other words, pillars can be permanent or temporary, partially or completely extracted [3]. Thus, it is of paramount importance to properly design pillars and analyze the existing ones to ensure both safety and cost-effectiveness of the mining operation.
Conventionally, the Factor of Safety (FoS) is used to design stable pillars, where the pillar strength must be high enough to accommodate the pillar stress. Several factors affect the in-situ stress of the pillars including the pillar span and stiffness, mine depth, thickness of overlying rock mass and others [4]. The average stress on pillar is commonly estimated through the tributary area while the pillar strength can be determined with laboratory test, in-situ test, empirical formulas or numerical modeling [5-9]. However, it is extremely difficult to perform large-scale tests in the field to determine the input parameters for the design, which makes the use of empirical formulas acceptable for the estimation of pillar strength [10, 11].