Two coal mining experiences from the Sigma Colliery in South Africa are described. The first was an attempt to mine a longwall out into a pre-developed tunnel ahead of the face, which failed in spite of detailed prior investigation, because the significance of a single water bearing joint was not appreciated. The second was longwalling underneath highly stressed pillar remnants on an overlying seam and through a pre-mined, backfilled dyke. This attempt, although characterised by higher risk than the first example, succeeded. It is concluded that, in mining, what is predicted is more important than the accuracy of the prediction.
Deux axperienoes dans la domaine de l'exploitation du charbon sur la Houillère Sigma en Afrique du Sud sont decrites. La première etait unc tentative d'exploitation d'une taille longue (Iongwall) dans une galerie pre-developpee en avance de la paroi frontale, tentative qui a echouee en depit d'etudes prenlahles detaillees du fail que l'importance d'un simple joint porteur d'eau n'avait pas ete reconnue. La deuxième experience etait une taille en dessous des resles de piliers assujettia à de fortes contraintes sur une couche sus-jacente et à travors un filon lithique dejà exploite et remblaye. Cette deuxième tentative, bien que plus rinquee que la premièew, a reussi. On peut done conclure que, dans Ie domaine de l'exploitation minière, la prediction elle-même est plus importante que son exactitude.
Zwei Bergbau-Erfahrungen der Sigma-Kohlengrube in Suedafrika werden beschrieben. Die erste war ein Versuch einen Streb zu einem Stollen vor dem Ortsstoß auszubauen. Dieser Versuch misglueckte trotz einer detaillierten Voruntersuchung, weil die Bedeutung einer einzigen, wasserfuehrenden Stoßstelle nicht in Betrach gezogen wurde. Die zweite Erfahrung war ein Versuch, einen streb unter den besonders beanspruchten Pfeilerresten einer höherliegenden Ader und durch ein aufgefuelltes Flöz auszubauen. Obwohl das Risiko bei diesem Versuch viel höher war, glueckte er. Daraus kann schließen, daß beim Bergbau die richtigen Indikatoren wichtiger sind als die Genauigkeit einer Voraussage.
The primary goal of rock engineering application is to ensure the safety of people working underground, but the rock engineer also has to contribute to profits. This is achieved by inter alia assisting with the development of more efficient mining methods. Rock engineering application in mining requires knowledge of the rock properties, application of techniques to predict the reaction of the rock environment and knowledge of the capabilities of the people, mining equipment and support units. Finally, some or other form of monitoring the performance is required. The success of a design is defined by the degree to which performance matches prediction. This paper describes two experiences at a colliery in South Africa, which demonstrate the importance of balancing all the elements which are required for successful performance. The first experience was a failed attempt at shortening the time required to move a longwall face installation. The second experience was the successful attempt to extract coal under very difficult conditions.
The mining depth varied from 60 m to 160 m below surface and the average mining height was around 3 m. The mining methods were longwalling, bord and pillar mining and pillar extraction. Annual production was around 6 million tonnes. There are three minable seams, numbered from the bottom up as the No's 2A, 2B and 3 seams. The 2A and 2B seams are separated by 0,5 m to 1,0 m siltstone. Mining both the 2A and 2B seams in -the same are has not been attempted. The number 2B and 3 seams are separated by 8 to 14 m of siltstone (which is the roof of the Number 2B seam) and a micaceous sandstone (the floor of the No 3 seam). The roof of the Number 3 seam is a weak, rapidly weathering mudstone. The overburden consists of layers of shale and sandstone, the sandstone making up about 60% of the total. The situation is often complicated by the presence of a strong intrusive dolerite sill of varying thickness and position in the overburden.
