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An outburst is the sudden and violent ejection of coal, gas and rock from a coal face and surrounding strata in an underground coal mine. When outbursts occur, they can be very serious events, possibly even resulting in multiple fatalities.
Outbursts may range in severity from being barely noticeable, to causing the destruction of an entire mining panel, and throwing pieces of machinery weighing tens of tonnes several metres. An outburst at Tahmoor Colliery, in NSW, Australia in June 1985 involved the ejection of 350 tonnes of coal and rock and over 3000 cubic metres of gas, resulting in one fatality. Another outburst at the nearby South Bulli Colliery in 1991 resulted in three fatalities, and yet another outburst at Westcliff Colliery in January 1994 involved 300 tonnes of coal and rock and resulted in one fatality.
Several factors predispose certain coal seams to being outburst-prone. These include:
* Coal seam gas content (measured in m3 per tonne) - generally, for a given coal seam gas composition, the higher the gas content, the higher the risk
* Geological structures, in particular faults, dykes and mylonite (reddish brown powdered coal) zones
* Coal permeability (the less permeable the coal seam, the more prone to outbursts)
* Coal seam gas composition - for a given coal seam gas content (in m3 per tonne), the seam will be much more susceptible to outbursts if the coal seam gas is predominantly carbon dioxide, as opposed to the case where the coal seam gas is predominantly methane.
* The compressive strength of coal - high strength coal seams appear to be more prone to outbursts. This may be because low strength coal tends to deform in and around the face area as mining occurs, resulting in stress relaxation (see below).
* The stress regime at the face - high stress on and around the mining face can have the effect of closing up the cleats and pores in the coal seam. This can result in a steep coal seam gas pressure gradient into the seam, (as opposed to a low stress environment where the coal seam gas drains freely into the face area, resulting in a low seam gas pressure gradient). As mining progresses, this pressure can be violently released, resulting in an outburst. Wikipedia |
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