Salamon and Munro?s (1967) strength formula has been used successfully in South African collieries over nearly four decades. However, many post-1967 collapses that took place in old workings, led the research to concentrate on seam and coalfield specific strength formulae. Altogether, 77 collapsed and 245 uncollapsed cases were carefully compiled in the revised data base. Pillar strength was evaluated in the form of simple power, linear and non-linear formulae. Three probability distributions to describe the critical safety factor were evaluated, and it was concluded that the lognormal (originally used) distribution still provides results at least as robust and consistent as those produced by the gamma and Weibull distributions. It was concluded that much of the variation in the data is accounted for by large variations between coalfields, and consequently the use of a single overall pillar strength formula is unrealistic. Various combinations of coalfields and seams were modelled, and these combinations can now be used in pillar design in South Africa. The new strength formulae should result in safer and more economical exploitation of coal reserves. Furthermore, a new design approach based on risk analysis has been introduced in this study.


Approximately 2.5 million coal pillars have been left underground in South African coal mines since the establishment of the Salamon and Munro (1967) formula. For more than three decades, coal pillar strength in South Africa has been calculated using this well-known formula, which was empirically derived after the Coalbrook disaster (1960). Since the introduction of the formula, there have not been any disasters in South African collieries due to violent multiple pillar failure. Canbulat and Ashworth (2003) stated that, based on the number of collapses in South African collieries, the Salamon and Munro (1967) formula has been very successful, certainly in preventing catastrophic failures in operational workings.

Nevertheless, research into the design of coal pillars has continued, because more hazardous extraction of previously formed pillars is on the increase, and also because the depth of coal reserves is getting greater. Consequently, pillars outside Salamon and Munro?s (1967) empirical range, and deterioration of pillars due to time and environmental conditions, will need to be taken into account.

Therefore, a project was initiated by COALTECH X 2020, an industry initiative, to develop updated seam-specific strength formulae for South African collieries, using an updated collapsed and uncollapsed pillar data base.

In this study attention is focused on bord and pillar primary workings, since this is the only configuration where the pillar load can be simply computed without recourse to models of rock mass behaviour. However, the formulae presented in this paper can also be used in the design of pillars for pillar extraction and in the design of chain pillars along longwall panels, using more accurate load estimation techniques including numerical modeling.

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