Sudden outburst

(Figure) The scheme of the sudden outburst of coal and gas from the stope into the preparatory mine working

SUDDEN OUTBURST (EN: sudden outburst; DE: plotzlicher Ausbruch; FR: degagement instantane; ES: desprendimiento instantaneo, irrupcion instantanea; RU: внезапный выброс) is the spontaneous outburst of the gas, solid useful mineral (coal, salt), or country rock into the underground mine working from the stope or near-stope zone of the massif; the duration of the sudden outburst is up to several seconds.

The first sudden outburst (of coal and gas) has been registered during the 1834 within France at the "Isaac" underground mine within the Loire basin, later within Belgium during the 1847, and by the end of the 19th century, the sudden outbursts were noted within the Ruhr, Lower Silesia, and South Wales basins, and also within Hungary and Canada. On the territory of the USSR, the first sudden outburst of coal and gas has been noted during the 1906 at the "Novaya Smolyanka" underground mine (Donbass; the "Smolyaninovsky" layer, at the depth of 711 metres), the first sudden outburst of the rock has been noted during the 1955 at the "Kochegarka" # 1-5 underground mine (Donbass) during the penetration of the main crosscut at the horizon of 750 metres. With the increasing of the depth of the development, there increase the frequency and intensity of the sudden outbursts, which often take place after several minutes after the termination of the works within the stope of the mine working (in case of the blasting breaking, the delay after the explosion is several seconds, less often several tens of minutes). During the process of the sudden outburst, within the layer of the useful minerals, or within the massif of the rocks, there is formed the cavity of usually pear-like shape (see the Figure), or of the cavern-like shape (sometimes more complicated), with the volume of up to several thousands cubic metres (for example, more than 2500 cubic metres during the penetration of the Arpa - Sevan tunnel); sometimes, there remains the pillar of the virgin rock with the thickness of 2-4 metres between the cavity of the outburst within the coal layer and the plane of the stope. The mine working is being filled with the crushed mass of the useful minerals or rock, and with the natural gas. During this event, there often takes place the destruction of the near-stope roof support, and of the underground mine equipment.

The harbingers of the sudden outburst are: the significant increase of the acoustic or seismic-acoustic activity of the massif; the delay of the separation of the layer because of the pressure, and the delay of the convergence of the lateral rocks before the moving stope; the husking and "shooting" of the rocks on the surface of the mine working; the increase of the intensity of the initial gas release and of the yield of the coal rubble during the drilling of the blastholes, and the division of the cores, which are drilled from the rock, into the discs of the convex-concave shapes.

The largest portion of the ejected mass is the particles with the sizes from several millimetres to the tens of millimetres; among the crushed mass, there may be encountered the individual large blocks of the rock or of the useful minerals. For the sudden outburst within the coal layers, there is also characteristic the forming of the very thin coal dust, of the so-called mad flour, which covers the major mass of the ejected coal; for the sudden outburst within the massif of the rocks, there are characteristic the particles of the scaly and plate-like shapes.

The characteristic of the sudden outburst is its intensity, which is measured as the quantity of the ejected rock and useful minerals. There have been registered the sudden outbursts with the intensity from several tens of kilograms of the ejected mass of the rock or useful minerals to several thousands of tonnes. The sudden outbursts with the intensity of 5-50 tonnes amount to approximately 70% of the cases, and the sudden outbursts with the very high intensity (more than 1 thousand tonnes of the ejected rock mass) take place comparatively rarely.

The amount of the natural gas, which is released into the underground mine working (with the normalization to the normal atmospheric conditions) amounts to several tens of cubic metres (per 1 tonne of the ejected mass of the useful minerals), and during the sudden outburst of the rock (the amount of the gas is evaluated according to its concentration within the mine working) it varies, as a rule, from 2% to 6% (and may reach up to 50%). The sudden outbursts take place with the participation of methane and carbon dioxide (the USSR), of carbon dioxide (the Upper Silesian Mineral Coal Basin, Poland), and there are known several cases of the sudden outbursts with the participation of nitrogen (France). The sudden outburst, which was most powerful within the world, namely, 14 thousand tonnes of the ejected mass of the coal, and approximately 600 thousand cubic metres of the methane gas (normalized to the normal atmospheric conditions) has taken place during the 1968 within the Donbass at the underground mine named after Yuri Gagarin during the unsealing of the "Mazurka" steep layer with the thickness of 1.3 metres at the depth of 750 metres using the crosscut. The crosscut has been covered with the coal throughout the length of 650 metres. During this event, the thickness of the layer of the "mad flour" on the natural slope of the ejected coal reached 40-50 centimetres.

The sudden outburst of coal and gas takes place mostly at the depth of more than 250 metres. The overwhelming majority of the sudden outbursts takes place at the layers with the thickness from 0.5 to 2.5 metres, and during these events, the more is the thickness of the coal layer, the more is the average intensity of the outburst. The rocks, which are containing the layers, are represented as the siltstones, mudstones, sandy and argillaceous shales, dense and stable sandstones, less often limestones. The sudden outbursts are often associated with the geological disturbances. The places of the coal layers, to which are confined the sudden outbursts, in the majority of cases are distinguished with the decreased strength (of the single or several packs of the coal), with the crumpling, with the absence of the explicitly expressed cleavage, with the development of the cracks of the tectonic origin, with the change of the thickness of the layer. The rock interlayers, which are separati ng the packs of the coal, often consist of the carbonaceous shales or mudstones, which have been crumpled and separated into the lenses with the mirrors of the sliding, and of the dense siltstones, which have not been crumpled. Most often the sudden outbursts take place within the layers, which are represented as the coals of the PZh, K, OS, and T marks; less often of the A, and G marks. All coal layers, within which are registered the sudden outbursts, have the comparatively high gas content (10-30 cubic metres per tonne). The major portion of the gas exists within the adsorbed state. Within the layers with the constant pressure of the gas of less than 0.6 megapascals (6 atmosperes) within the hermetically sealed monitoring boreholes, the sudden outbursts are not noted.

The sudden outbursts of rock and gas take place mostly at the depth of more than 400 metres (of the sandstones, at the depth of more than 700 metres). The layers of the rocks, as a rule, are not orderly in terms of the thickness, within these layers are observed the cracks of the tectonic origin, which, along with the pores within the granular rocks, are filled with the gas. There is traced the zoning of the sudden outburst of the coal, rock, and gas (often they are noted only at one wing of the underground mine, of the stratigraphic suite of the layers, and so on). There has been developed the complex of the activities for the forecast of the sudden outburst (see the "Outburst danger" article); they distinguish the following types of the forecast: regional (the evaluation of the danger of the outburst according to the data of the geological exploration), local (the determination of the danger according to the data from the examination of the mechanical, filtration, sorption, and petrographic properties, of the structure of the useful minerals, and of the layer as a whole), and continuous (the detection of the warning signs or harbingers of the sudden outburst, including using the seismoacoustic methods).

The major activities for the provision of the safe and effective development of the coal layers, which are dangerous because of the sudden outbursts: the advancing development of the protective layers; the prophylactic processing of the coal massif, using the method for the degassing and moistening, using the injection of the water into the layer; the pillar system for the development; the narrow-scope excavation of the coal, using the shield, plow, and combined machine; the conducting of the preparatory mine workings using the combined machines; the complete collapsing at the layers with the slight inclination, and the complete stowing at the steep layers; the hydraulic separation of the coal, the hydraulic flushing of the advancing cavities and gaps, the drilling of the advancing boreholes, the camouflet blasting, the forming of the unloading gaps, the usage of the advancing roof support; in case of the emergence of the sudden outburst, the usage of the group and individual means for the protection of the people.

There exist several ideas about the mechanism of the sudden outburst, and about the causes for its manifestation. These ideas differ mostly in the evaluation of the participation in the sudden outburst of gas, of the stress-strain state of the massif, and also of the physical-mechanical and physical-chemical properties of the rocks or useful minerals.

The essence of the mechanism of the sudden outburst, within which the major place within the process of the destruction, and of the ejection of the mass, which has been destructed, into the mine working, is given to the gas, is within the following explanation. In case of the rapid movement of the stope, or in case of the suddenly emerged crack at the place with very low permeability of coal or rock, there forms the large differential pressure of the natural gas within the very thin layer near the exposed surface. The differential pressure destructs this layer, and throws it away into the goaf space, on the account of the energy from the expanding gas. During this process, the pressure of the gas near the newly formed exposed surface falls rapidly, the differential pressure is created again, and so on. Thus, the exposed surface moves rapidly into the depth of the massif, and there is always maintained the high differential pressure, which is destructing one layer after another. Ac cording to another idea about the sudden outburst, the process of the destruction (crushing) of coal or rock is associated only with the stressed state and load-bearing capacity of the massif near the mine working, while the process of the throwing of the particles away from the rapidly destroying massif is associated with the energy of the expanding gas, which has been compressed under the high pressure. During this process, in case of the destruction of the coal, and of the throwing of this coal away, the desorption of the gas assists the forming of the flow of the gas-coal mixture within the mine working.

Since the end of the 60-ies, the sudden outburst of coal and gas is also considered as the process of the destruction of the coal, as the result of the interaction of the high pressure of the gas within the pores and microcracks, with the very rapid unloading of the rock pressure from the layer within the near-stope zone. The research of the sudden outburst is the subject of the mining geomechanics.