Pillarless protection of the mine workings
PILLARLESS PROTECTION of the mine workings (EN: non-pillar maintenance of mine workings; DE: Grubenbausicherung ohne Kohlenpfeiler; FR: maintien des voies sans piliers de charbon; ES: explotacion de minas sin pilares; RU: бесцеликовая охрана горных выработок) are the methods for the ensurance of the operational status for the underground mine workings, which are conducted without keeping the protective coal pillars, on the account of the special measures and roof supports.
The pillarless protection is widely used within the USSR, at approximately 50% of all the retreat mining stopes (1981). The gist of the pillarless protection comprises the situation of the mine workings (mostly of the preparatory excavation ones) within the zones with the decreased rock pressure, or at the boundaries of these zones, which creates the favourable conditions for their maintenance.
They divide the methods for the pillarless protection of the preparatory mine workings into the three major groups.
The first group comprises the protection of the mine workings, which are supported at the boundary with the goaf space for the repeated usage (Figure # a), by the strengthening of the roof support within the influence zone of the retreat mining works by the portable metal struts (friction or hydraulic ones), with the help of the artificial barriers (breaker props roof support, chock roof supports, crushed stone strips, reinforced concrete blocks, and others), by the preliminary weakening of the laboriously collapsing rocks of the roof before the long wall stope with the help of the drilling-blasting works.
The second group comprises the methods for the pillarless protection of the mine workings, which are conducted rock-adjacent to the goaf space with partial preservation of the old mine working, without keeping the pillar (Figure # b), or with the pillar with the width of up to 5 metres between these mine workings and the long wall stope.
The third group comprises the pillarless protection of the mine workings, which are conducted, formed, or restored within the goaf space (it is used in limited quantities prevalently during the continuous development system).
The methods for the pillarless protection depend on the mining-geological and mining-technical factors, and are distributed in accordance with the relative volume of usage: 35% is accounted for the protection of the mine workings for the repeated usage, 35% for the rock-adjacent ones, 10% for those which are formed behind the long wall stope, 20% for the protection by the crushed stone stripes. The pillarless protection of the repeatedly used mine workings (which are maintained at the boundary with the goaf space) is used prevalently at the layers with the thickness of up to 2 metres with any angle of the dip, with easily collapsing roof, with non-heaving or slightly heaving (up to 1.3 metres) rocks of the soil.
The pillarless protection of the mine workings, which are conducted rock-adjacent to the goaf space, is performed at the layers with the thickness of 1.5-3.5 metres with the angles of the dip of up to 35 degrees primarily in case of the presence within the roof of the layer of the weak stratified rocks, with any watering of the long wall stopes, and with the heaving of the rocks of the soil of up to 0.6 metres; in this case they emplace the mine workings outside the boundary of the fault of the rocks of the major roof. Such scheme is especially convenient during the simultaneous working of the contiguous layers in descending order; in case of the ascending order, they usually avoid the under-workings of the rock-adjacent mine workings. The third group of the methods for the pillarless protection is performed in case of the complicated conditions during the maintainance of the mine workings, in case of the heaving and intensely heaving rocks of the soil, at the layers with the thickness of 1.5-2 metres and with the angles of the dip of up to 35 degrees. The pillarless protection substantially decreases the operating losses of the coal, and in case of the certain mining-geological conditions improves the effectiveness and safety of the mining works. The major directions for the perfecting of the schemes for the preparation and working of the excavation places with the pillarless protection are: the creation of the special types of the roof support for the excavation mine workings, which are preserved within the goaf space, the improvement of the effectiveness of the special measures for the weakening of the laboriously collapsing roofs, and others.
Abroad (Czechoslovakia, Poland, West Germany, Belgium, and others), the pillarless protection methods are used prevalently in case of the continuous development systems. They understand as the pillarless protection also the pillarless maintenance of the mine workings, pillarless preparation, pillarless technology, pillarless working of the layers.
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