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The South African squat coal pillar strength formula, developed by researchers from the Chamber of Mines Research Organization in the 1980s, predicts an exponential increase in coal pillar strength once a critical width-to-height ratio of 5 is exceeded. The arguments that have been proposed in favour of this formula are discussed critically in this paper.Field experience with squat pillars in the USA and evidence from published physical, analytical, and numerical model pillar studies corroborate the fact that a squat effect is unlikely to occur in coal pillars at width-to-height ratios less than 10. Furthermore, an exponential increase in peak strength of coal pillars does not exist. An alternative design criterion for squat pillars in South Africa is therefore suggested. squat pillars, coal pillar strength, bord-and-pillar mining.

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An underground bord-and-pillar coal mine can, over a lifetime of a few decades, result in several tens of thousands of pillars being left behind in its abandoned workings. For South Africa, the total number of coal pillars in abandoned workings is estimated to be in the region of 6 million (van der Merwe and Mathey, 2013a).Currently, there are 98 cases known in which isolated but large groups of pillars have collapsed (van der Merwe and Mathey, 2013b). Consequently, large subsidence troughs occur on surface, potentially endangering infrastructure, housing, and ultimately lives. The prediction of long-term instabilities in underground workings therefore becomes imperative for regional development planning, and is also compulsory for mining companies in order to obtain mine closure certificates.The challenge in assessing pillar stabilityrelated risk is to take into account the great number of pillars, their individual dimensions, geometries, and deterioration over time. One can imagine that an unfavourable combination of these factors may cause localized areas of weakness in the underground panels. Those areas can eventually trigger a collapse, which subsequently may spread towards stronger regions within the same panel. Such examples have been discussed in the literature (Mathey, 2010(Mathey, , 2013Mathey and Schulte, 2011).It is apparent that an extensive risk assessment for abandoned mine sites is required that allows fast and detailed access to spatial information. Furthermore, the longterm stability of underground panels needs to be analysed in high resolution, i.e. on an individual pillar-by-pillar basis. In this paper the use of a geographic information system to address issue is proposed.The principal methodology underlying the pillar stability analysis is to pick up mining dimensions, geometries, and dates from the original survey plans and process the data in a quasi-3D digital mine model, making use of a geographic information system (GIS). Within this model, individual safety factors are determined for each pillar in the underground workings.Empirical equations are implemented to predict pillar scaling, giving the expected deterioration of pillar size over time. Thus a dynamic risk assessment for pillar-related stability problems is obtained. All static and dynamic, measured and inferred information related to a pillar is stored in a relational database.Finally, the analytical results are projected back onto maps in order to facilitate the use of stability-related information by mine personnel. Surface structures can be included on these maps, so as to provide a comprehensive and detailed basis for assessing the risk involved with abandoned Modelling coal pillar stability from mine survey plans in a geographic information system by M. Mathey A practical method for analysing the individual stability of large numbers of pillars in bord-and-pillar workings based on mine survey plans is outlined. The procedure aims at modelling pillar safety factors and failure probabilities under tributary area loading as...

The application of the Monte Carlo technique to production planning and everyday economic decisionmaking in mine production management is demonstrated. The logic is detailed using an example of underground production with continuous miners (CMs) and truck haulage. It is argued that availability of equipment and personnel are the predominant variables influencing mine output and productivity and that those availabilities may be well represented by binomial probability distributions. The probabilistic model is implemented in a standard Excel® spreadsheet with Palisade's @Risk add-on to facilitate simulations. Starting from model calibration against data obtained from a mine's annual reports, some general interdependencies of availability, utilization, productivity, and costs of production processes are outlined. Finally, several possible options and their consequences as regards production improvements are explored.