Underground » Coal Burst
Bursts are events that expel material at velocity at the time of ejection. This is usually coal but is sometimes rock. Unlike falls where the velocity is gained through gravitational acceleration, the sources of energy that drive the material outwards are either strain energy of the rock or coal or energy derived from expanding gas.
Whether a burst will occur or not is very difficult to predict with any precision as it depends on local variations in conditions that may convert a stable situation into one where it becomes unstable and material is ejected. Rather than attempt to set predictive thresholds of whether a burst will take place or otherwise, the approach taken here is that it is possible to predict how fast that ejection will be and how far the material may travel should it occur. Personnel should not be within the range of moving material and need to be withdrawn to a safe standoff distance during mining. Another limit on the velocity of the ejected material is the damage that it may do to equipment.
Where the velocity of ejection is likely to be too high then methods need be taken to reduce it. These can include gas drainage and destressing.
The approach described above is considered to be quite tenable for bursts that do not involve gassy, finely ground coal or coal that may disintegrate into fine particles. The outbursts associated with these are quite different in character as they are frequently not a single step process but occur in multiple stages. These are the most dangerous type of burst as they can eject thousands of tonnes of material and gas volumes that may be of the order of a million cubic metres. Mining through fault gouge requires a completely different and much more rigorous approach as the consequences of an outburst associated with this material are much more serious and the range of ejection well beyond any practical safe stand-off distance.
Determining what kind of event may take place is crucial and this involves measurement and interpretation of conditions so that a correct assessment of the situation may be made. This applies especially to the determination of the location and size of gouge zones in gassy coals. In this respect, there needs to be development of in-seam drilling practice to determine such structures and to determine zones of high stress.
While the safe stand-off distance concept is good while mining is taking place, there will be a need for personnel to return to the face at some point and a decision must be made as to whether it is safe to do so or not. As all burst events are associated with microseismic noise this is seen as the area where development is needed albeit in a much more operator friendly form than is currently in use.
This report consists of a 19 page synopsis and a main report. The synopsis of the main report presents the findings without the same detail of case studies and without the mathematical derivations.