Factors – Mining

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As the magnitude of vertical stress increases with the depth of mining, it seems reasonable to say that these stresses will increase the probability of outburst at greater mining depths. However, this does not exclude the chance of outburst at small depths. While many parts of the world have not experienced outbursts at depths less than 180 metres, the Moura mine in the Bowen Basin, Australia has experienced three gas outbursts at a depth of approximately 130 metres. These outbursts were attributed to shear zones present at that depth and it is accepted that with the existence of certain geological conditions, outbursts may occur at depths as little as 100 metres (Lama & Bodziony, 1996). There is also little record of outbursts occurring when the depth of mining exceeds 500 metres. This is probably due to a change in the rank in coal as depth increases. Therefore, it can be surmised that while mining depth plays a role in the majority of outbursts, there can be no general rule found for the likelihood of outburst at a given depth.

Mining operations cause the stress to concentrate at the corners of the solid coal, the edges of the workings and at the pillars. Room and pillar mining is more prone to outburst than longwall mining due to the stresses inherent in the pillars. However, the removal of support pillars is nearly always completely safe as the gas has diffused from the pillars over time. As stresses concentrate at the ends of longwall faces, longwall mining should use a straight working face, complete extraction of the coal and rapid stowing.(Lama & Bodziony, 1996)

As coal deposits form a fairly brittle ore body, when they are subjected to major tectonic disturbances such as faulting, folding and shearing, they can suffer a high degree of fracture with low shear resistance. To support this, outburst studies show that the presence of fractures and shear zones coexist with outburst prone areas. The shear zones indicate the presence of excess stress and planes of weakness and also the existence of pockets of gas and pulverised coal (Doyle, 2002).

All fatal outbursts in Australia (except Leichhardt Colliery) have occurred on faults. This is not to say that all faults will show the occurrence of outburst. The types of faults most likely to be associated with outbursts are low angle reverse faults and strike-slip faults. Between the faults, mining conditions are generally good. The magnitude of the fault often bears no relationship to the magnitude of the outburst.

No standard can be set conclusively as to whether thick or thin seams are more outburst prone. In theory, seams that are thick and/or steep should be more prone to outburst due to the effects of gravity causing the gas to locate in a specific area. However, evidence points to thin seams being equally prone to outburst, with 573 out of 1,032 outbursts in the Donbass Mine, Ukraine, occurring in seams less than one metre thick. There is also no evidence pointing to seams thicker than 2.3 metres being prone to outburst. (Lama & Bodziony, 1996)