Abstract

VAN DER MERWE, J.N.. Coal pillar strength analysis based on size at the time of failure. J. S. Afr. Inst. Min. Metall. [online]. 2019, vol.119, n.7, pp.681-692. ISSN 2411-9717.  http://dx.doi.org/10.17159/2411-9717/566/2019.

A major shortcoming of the statistical back-analysis of coal pillar strength is that it relies on the as-mined pillar dimensions, not taking into account time-related pillar scaling with subsequent reduction in pillar width. The paper describes an investigation where the rate of pillar scaling was applied to the pillars in the databases of failed and unfailed pillars. The reduced pillar sizes were then used in the same method of statistical back-analysis that had been used in the past. This resulted in an equation for pillar strength which predicts significantly greater pillar strength than the previous statistical analyses, which is similar to the strength which had been found earlier by direct testing of large specimens underground. It is concluded that the lower strengths found by previous statistical analyses are due to the incorrect pillar width being used and that by adjusting the pillar sizes to compensate for scaling, more credible results that correspond to direct testing are obtained. As the newly derived strength equation is time-dependent, it follows that the safety factors and probabilities of failure are likewise time-dependant. An equation to determine the probability of failure is also developed. This results in the safety factor being close to unity at a probability of failure of 50%, which is aligned to statistical expectation.

Keywords : coal pillars; time dependency; pillar failure; failure probability.

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