y roof bolting [46], processes intended to prevent the roof from collapsing [47]. At the same time, they create ventilation openings so that dangerous gases [48] can escape and fresh air can reach the miners. Then one or more slots [49]��a few centimeters wide and extending for several meters into the coal��are cut along the face of the coal seam, also known as the wall face, by a large, mobile cutting machine [50]. The cut, or slot, provides easy access to the face and facilitates the breaking up of the coal, which is usually blasted from the seam by explosives known as permissible explosives. This type of explosive produces an almost flame-free explosion [51] and markedly reduces the amount of noxious fumes [52] in comparison with conventional explosives. The coal may then be transported by rubber-tired electric vehicles (shuttle cars) [53] or by chain (or belt) conveyor systems [54].B. Continuous Mining
Continuous mining involves the use of a single machine known as a continuous miner that breaks the coal mechanically and loads it for transport. This mobile machine [55] has a series of metal-studded rotating drums [56] that gouge coal from the face of the coal seam. One continuous miner can mechanically break apart about 1.8 metric tons of coal per hour. Roof support is then installed, ventilation is advanced, and the coalface [57] is ready for the next cycle. The method used to transport the coal requires the installation of mobile belt conveyors.
C. Longwall Mining
The longwall mining system uses a remote-controlled [58] self-advancing support [59] in which large blocks of coal are completely extracted in a continuous operation. Hydraulic or self-advancing jacks [60], known as chocks [61], support the roof at the immediate face as the coal is removed. As the face advances [62], the roof is allowed to collapse behind the remote-controlled, roof-building machinery [63]. Miners then remove the fallen coal. Coal recovery [64] is comparable to that attainable with the conventional or continuous mining systems.
D. Room-and-Pillar Mining
Room-and-pillar mining is a means of developing a coalface and, at the same time, retaining supports for the roof. With this technique, rooms are developed from large, parallel tunnels driven into the solid coal [65], and the intervening pillars [66] of coal are used to support the roof. The percentage of coal recovered from a seam depends on the number and size of protective pillars of coal thought necessary to support the roof safely. Workers may remove some coal pillars just before closing the mine.
NOTES TO THE TEXT
[1] coal mining����ú
[2] seam thickness��ú����
[3] shaft������
[4] surface mining�����濪��
[5] underground mining�����¿���
[6] open-pit mining��¶���
[7] drift mining��ƽἿ���
[8] slope mining�������
[9] contour mining��̨����
[10] auger mining�������꿪��
[11] mechanical sh
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