Review of current method to determine outburst threshold limits in Australian underground coal mines

Desorption rate index (DRI) was presented to the Australian underground coal industry in 1995 as a means for determining outburst threshold limits for Australian coal seams. DRI is a measure of the gas volume released from a coal sample in the first 30 s of crushing during the Q3 stage of gas content testing, multiplied by the ratio between measured Q3 and QM. Relationships were identified between QM and DRI for both CO₂ and CH₄ rich coal samples collected from the Bulli Seam at West Cliff Colliery and that identified relationship was referred to as the Bulli Seam Benchmark. The outburst mining gas content threshold limit values specified for the Bulli Seam at that time, when applied to the QM-DRI Bulli Seam benchmark, was shown to closely align with a DRI value of 900 (DRI900), for both CO₂ and CH₄ rich seam gas conditions. The Australian coal industry adopted the DRI900 as the basis for determining outburst gas content TLV for Australian coal seams. Outburst mining experience in Australia has shown that gas content is not the only significant factor that impacts outburst risk, as all significant outburst events have been associated with abnormal geological conditions, such as faults and dykes. Therefore, assessing the potential application of additional outburst risk factors, to accurately define outburst risk zones, set safe mining threshold levels, and determine appropriate mining controls, warrants further investigation. Several Australian coal mines have implemented mining procedures enabling mining to continue in areas with gas content greater than the TLV determined using the DRI900 approach, without inducing an outburst. There is a broad lack of understanding among Australian coal mine operators as to the procedure and calculations used to determine DRI. Also, there has been growing concern regarding the accuracy and validity of the DRI900 method for determining outburst TLV. A comprehensive set of gas data has been collected from Australian coal seams, including the Bulli Seam, and this data has been used to investigate the DRI, Bulli Seam Benchmark, and the applicability of using DRI900 as the basis for assessing outburst risk and determining gas content TLV. The results are presented and discussed.