Outburst Risk Determination and Associated Factors

This report covers an investigation into the factors that lead to an outburst. It also covers the processes of measurement to arrive at the quantification of these and the shortcomings thereof. As part of the study a large number of outburst cases have been examined. This data base has come from international cases as well as those from Australia. This information has come from the Russia, China, Kazakhstan, New Zealand, Turkey and the UK. Unfortunately the majority of information is little more than anecdotal and extracting real values from it is difficult. This has meant that the emphasis for really well documented cases has fallen back on to the Authors' data that they have measured in their work.

One important finding from overseas is that other national systems regard coal toughness and structure as a key factor. Coal toughness is a key to mining seams that would otherwise outburst because of the potential energy they contain. Another finding is that mining of outburst prone seams is avoided as much as is possible and maximum use is made of mining less outburst prone seams to de-stress and degas adjacent seams, whether these are above or below. An important key to determine whether a seam will outburst or not, is its toughness.

The findings show that outbursts are failures which are accompanied by significant energy release. Energy release does not occur without failure. The failure mode and fragmentation that accompanies it are key to the energy release that follows. If coal has already failed by reason of the fact that it has been structurally altered, as in fault gouge, then the sudden release of energy from expanding gas is greatly facilitated and a major outburst can be expected. Outbursts from such situations are by far the most common, and can lead to thousands of tonnes of material and more than a hundred thousand cubic metres of gas being released if major structures are intersected.

The less certain case is when the coal is apparently solid but actually contains a multitude of small scale structures which will lead to its failure and fragmentation. These have been the subject of major study in this report. The approach has been to examine coals in polished section for such structures which have indeed been found to exist. Also coals were tested by sudden desorption by a new method and by using a Protodyakanov drop hammer as used in Russia and China. This work has established that some coals will indeed fragment.

Failure in an outburst situation is greatly facilitated by the removal of material from the face of the outburst by gas. This unloads the face and prevents the failure choking off on material that is broken.

Once the fragmentation process has occurred the prime energy release to drive the outburst is provided by desorption of gas. If gas is present in significant pressure this is generally more than an order of magnitude greater than either gravitational or strain energy. This energy is dependent on the gas content and pressure as well as the diffusion coefficient of the coal and importantly the fragment size that is generated by prior fracturing. In addition to desorbing gas there is an unknown component that may come from gas held in free pore space. The petrographic studies have shown that this pore space may be significant. Short of targeted petrographic examination it is at present difficult to quantify. If this pore space is readily connected the gas associated with it will be lost in a coring operation and not accounted for in gas content measurements as currently conducted.

The question that this study poses is whether or not mining should proceed under conditions of high gas content on the basis that the coal will not fail. If so, the determination of what constitutes failure is very important. This becomes immaterial when structures that contain a significant mass of broken material exist. If they are breached by mining and they contain gas then they are likely to outburst. Thus, detecting such structures becomes critical.

If mining is to proceed on the basis that the coal may contain undetected structures that contain gouge, or other altered material that has a high propensity to disintegrate, then the risk posed by such events can be expected to be related to the energy that may be released. This will predominantly be gas, either from desorption or from pore space. This report provides a basis for estimating this energy release, though a less than certain one for the case of gas contained in pore space. It should be considered on the basis of energy per unit volume and energy over the size of (un)expected structure. This value will depend on the diffusion coefficient, gas content/pressure and particle size that is likely to exist or be generated.

The report highlights serious deficiencies in some of the common procedures to measure gas content and casts doubts on the validity of isotherms. It is also at issue with some of the adopted outburst thresholds.

This document is intended to be revised with time as knowledge develops.