Outbursts and geological structures in coal mines: A review

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Novel aspects of stresses, gas and geological structure are integrated in a worldwide review of the problem of outbursts of coal and gas in coal mines. Based on a study of the literature and recent geological studies of outburst sites, geological structures have been identified as a prime factor in outburst occurrence. Thrusts, strike-slip faults and recumbent fold hinges are especially outburst prone. Such structures have been commonly referred to in the past as ‘tectonic disturbances’ or ‘soft coal’, and are the loci of highly fractured coal and gouge material. The nature of soft coal and gouge is not fully understood, but microscopic techniques are available for their study. In and close to geological structures anomalous stress and gas conditions apparently exist. At such sites mining-induced fracture systems, microseismic noise and abnormally high gas emissions have been recorded. Present knowledge of in situ stresses, gas regimes and geological structures at outburst sites is insufficient to enable an accurate mechanistic model to be formulated for outbursts. The most appealing mechanism advanced to date is called the ‘dynamic theory’, which emphasizes the role of stresses in the events leading to failure of the coal and control of the gas regime. A conceptual model based on this theory is proposed, which amalgamates all the factors known to influence outburst-prone coal extraction. The identification of geological structures, together with measurements of the various parameters of the gas and stress systems, may be the key to much more successful prognosis of outburst-prone zones than has been the case so far.