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|Discussion: A. Hazell (Associate Member) and T. J. Kotze (Graduate Member): The authors must be congratulated on the development of a versatile computer programme for determining the stresses and displacements resulting from mining operations. As a design tool it will no doubt take its rightful place together with the analogue computer. Union Corporation made use of this digital computer programme whilst still in its development stage to determine the stress distributions which were causing the unexpected failure of the Marievale sub-incline shaft. Two features arising from this investigation may be of interest to members, namely: A. The solution to the problem of stabilizing a rectangular inclined shaft, which turned out to be contrary to accepted mining practice, and B. A comparison between the results obtained with the pure digital system and the analogue computer. A. The shaft is nine ft high and 20 ft wide, dipping at 9° to the horizontal and crossing the bedding planes. Fig. 1 is a plan and Fig. 2 is a section of the sub-incline shaft pillar area showing the relative positions of the shaft and reef as well as their respective dips. The deepest point in the shaft is 3 300 ft below surface and the requirement of a stress free ground surface may not be satisfied. It also means that the original gravitational stresses are low. A 900 ft wide pillar was left to protect the bottom section of the shaft, the top section being understoped years ago. Extraction of the pillar was started at the beginning of last year, mining operations being carried out in the accepted manner, starting beneath the shaft and mining outwards. The approach did, however, turn out to be the wrong one, because failure of the shaft was noticed soon afterwards. Even small movements were serious in this case due to the low clearance between the skips and the shaft supports. During October, 1968, failure between benchmarks 8 and 10 (see Fig. 1) became so excessive that hoisting speeds had to be reduced. Failure continued to occur and, by the end of March, 1969, failure had propagated up the shaft as far as benchmark 7, and down as far as benchmark 12. As this shaft had a tight hoisting schedule it was imperative that a quick solution be found to arrest the failure.|