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prone to scaling, and thus are progressively reduced in size and consequently subjected to increased load and reduced strength. The effective factor of safety therefore reduces and it is quite possible for failure to occur after a long time interval. In fact, scrutiny of the database of failed pillars indicates that failure occurred after periods ranging from 2 to 49 years, and not immediately after mining. The factor of safety for pillars at the time of failure is not the same as that immediately after mining. The concept of the safety factor is based on the pillar strength obtained by statistical analysis of pillar dimensions immediately, or within a month after mining, when the surveys are conducted. Strictly speaking, the safety factor as commonly used is valid only immediately after mining, and it continues to reduce thereafter. The ideal situation would be to base analyses of pillar strength on pillar dimensions at the time of failure, but that is simply not possible because the real dimensions cannot be measured at that time. Due to the time-related reduction in the stability of pillars, databases of failed and stable pillars cannot be regarded as definitive. They are merely snapshots of the situation at any given point in time. This is just one argument in favour of periodic reviews of the databases as well as the derivation of pillar strength considerations based on empirical analyses of the databases. When Salamon and Munro (1967) published their groundbreaking work on coal pillar strength, they had only 27 collapsed pillar cases in the database. That number has now grown to 85 (van der Merwe and Mathey, 2013a). Of the new collapse cases, eight were mined prior to 1965 which means that at the time of the original analysis they would have been regarded as stable cases. Fifty of the new collapses were mined after the introduction of the 1967 safety factor, indicating that collapses have not been completely eliminated by designing according to the safety factor. The effect of time has to be included in any consideration of long-term pillar stability. The previous attempt at estimating the expected lifespan of coal pillars was described by van der Merwe (2003). More than a decade has passed since then and more data is now available – therefore a review of the procedure is required. The methodology that was followed in this review is essentially similar to that used Review of coal pillar lifespan prediction for the Witbank and Highveld coal seams