Outburst (of Gas)
Outburst (of Gas) : An outburst is defined as a violent, simultaneous release of gas(es) and comminuted rock material into a working face or the interior of a borehole. In general, an outburst event has the following phases (1): 2 (1) A stressed volume of rock containing gas(es) is exposed to a rapid change of confining stress. This rock volume has been highly fractured as a result either of some preexisting geologic disturbance (such as a fault) or of mining induced stress concent ration. (2) Gas(es) adsorbed in or contained in sandstone or evaporite rocks are rapidly released into the fractures , which already contain free gas. When more gas enters the fracture space than can be transported away through the less permeable rock body, a state of stress due to gas pressure may be reached where the rock body cannot contain the increasingly stressed fractured rock volume. (3) When the rock body can no longer contain the stressed and fractured rock volume, containment ceases and the fractured rock mass and gas(es) undergo movement as they are driven by the gas into a pressure sink, e.g. , a mine opening or borehole. (4) After the movement of the fractured rock and gas(es), there may be continued gas flow from the fractured but in-place rock that forms the outburst cavity. This gas flow generally decreases over time. Two major theories--the "pocket" and the "dynamic" theories--can describe the basis of the coal outburst mechanism. The pocket theory holds that there exist certain volumes of "soft" or crushed coal enclosed by "harder" or less fractured coal that form reservoirs of gas contained in the fracture void space. These crushed coal volumes are associated with faulted or sheared zones and with intensely folded strata. This comminuted coal has little unconfined compressive strength and is separated from the mine opening by an intact zone of coal under sufficient stress to become a "permeability dam." When mine development approaches a "soft coal" region, an outburst can result if the region is not sufficiently drained of free gas and/or the stresses in the region are not dissipated (2). The dynamic theory holds that a volume of relatively gassy coal, which is highly stressed and penetrated by mining-induced fractures, is outburst prone. When a mine opening and induced stresses approach such a coal volume, the coal fractures, releasing high-pressure desorbed gas, and the coal face fails, resulting in an outburst (2). Common to both theories is high-gas-content fractured coal that is able to desorb gas rapidly upon release of confining pressure. This rapid desorpt ion feature of outburst-prone coal is the basis for a rather extensive set of predictive methods, which are detailed later in this report. Other aspects of outburst-prone coal include low in situ strength due to fissuring, high free-gas pressure, and association with geologic structures such as fracture zones and igneous dikes. These aspects are also the basis of a variety of predictive methods (2). Outbursts in coal mines represent considerable hazards. The most immediate hazard is the unexpected inundation of the ventilation systems with asphyxiating volumes of gas. When methane is the released gas, an explosive hazard can be created, possibly exacerbated by ejected coal dust. The force of the released gas and displaced material can be sufficient not only to disrupt mine ventilation but to debilitate stoppings and ground control structures such as arches and posts, and to injure or kill mine personnel. Additionally, an outburst zone presents a ground control problem due to the fissile nature of the rock that forms the remaining outburst cavity. Furthermore, gas may continue to be emitted, and without appropriate ventilation can accumulate in the outburst cavity. While the scientist and researcher would prefer to describe the mechanics of coal-gas outburst in very exact quantitative terms, the mining geologist and engineer need to reliably foresee the preconditions and precursors. The body of literature concerning coal-gas outbursts has abundant works (i-?) that represent overviews of the outburst phenomena at both national and international levels. Case studies of outbursts are extensive, and the bibliographies of the aforementioned references contain numerous examples. An overview of some of the more commonly practiced coal-gas outburst prediction and prevention methods used was compiled as a result of Bureau of Mines research. (An OSH glossary used in safety and health at work which is, adopted by ILO
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