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gas drainage/outbursts |
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outburst control |
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Outburst Control
Early methods of mining in locations liable to outburst tended to involve giving the face a "shake" (by bumping it with the miner cutting head or using explosives) to initiate an outburst at a time when the crew was prepared for it, should one be imminent. While there is some validity in this, the degree of protection to personnel if a miner was used was minimal – the term "bomb squad" was sometimes used, with good reason. For a period efforts were concentrated on protecting personnel at the face in the event that an outburst did occur (utilising a fully enclosed cabin for the miner driver with protected air supplies, all other personnel having retreated outbye while cutting was in progress). Meanwhile work was progressing on ways to prevent an outburst occurring at any time.
The result of all the experimental and development work has led to the present methods of outburst control involving drainage of gas from the coal to reduce the gas content to below "threshold levels" at which outbursts will not occur, before normal mining is permitted. These threshold levels vary according to the gas composition. If it is found to be impossible (or impractical) to reduce the gas content sufficiently, then mining can only be carried out by means which do not require personnel to be at the face while coal is being cut. Such methods may involve remote operation of the equipment using CCTV to observe what is happening, or using "grunching" ( shotfiring the coal off a solid face i.e. no cut in the face to provide a second free face to shoot into). These methods are slow and cumbersome but do at least allow development to proceed safely, the slow rate usually being acceptable for a period.
Outbursts are rarer on longwall faces than on development, most probably because the extended face length and the high front abutment loads which cause the face coal to fracture, reduce the likelihood of any structure acting as an effective dam. Nevertheless outbursts have occurred on longwall faces and drainage to threshold levels should still be practiced, though the threshold values may be somewhat higher than those for development.
Threshold values appear to be consistent within a given seam in a given area, but it may be necessary to develop such values for other seams and locations. It may well be that threshold values are in fact far more variable, but those used are very conservative and cover such variations. If this is the case it is unlikely to change unless somebody can develop a valid system for calculating more accurate values. The consequences are too great for any risks to be taken in this area.
It is not safe to say that the outburst problem has been totally overcome, but it can be said that there have been no outburst incidents, certainly no outburst injuries where the present control systems have been used.