In the aftermath of a major collapse of coal pillars in a South African mine, research was initiated in 1963 to develop a reliable method of pillar design. To progress expeditiously, the research had been pursued in an empirical direction aiming to rationalize the accumulated experience of miners. Some 125 mining case histoties, including 27 failed and 98 unfailed examples, had been accumulated. The critical safety factor of pillars was assumed to be a random variable, quantified by the lognormal distribution with its median at unity. Constants in a strength formula were derived from the data, using the tributary area pillar load, a simple three-parameter formule for pillar strength and the statistical method of the maximum likelihood. The results were published in 1966 and the design method based on them has been utilized almost exclusively in the intervening 32 years. Finally, some weaknesses and outstanding problems are discussed.


On January 21, 1960 the strata overlying an area of approximately 320 ha, comprising the eastern sector of the room and pillar workings in the No. 2 Seam at the Coalbrook North Colliery in the Republic of South Africa, failed disastrously. The event caused the loss of 437 lives. It is estimated that some 900 pillars collapsed. The magnitude of surface subsidence over the central part of the effected area reached 1.5 1.8 m, while the magnitude of the average depression was about 0.5 m. Seismic evidence indicated that the dramatic part of the failure took place over a period of five minutes starting at 7:26 p.m. This activity, however, was preceded by collapses on December 28, 1959 and on the day of the disaster, some three hours before the main event.

The magnitude of the tragedy shook South Africa profoundly. The leaders of the government and the coal industry decided to start a research program to ensure that no similar disaster would occur again. The research started effectively in early 1963. The primary objective of the program was to develop a methodology for the rational design of room and pillar workings in situations where the pillars are left behind as permanent support.

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