Text 14. Opencast workings

Sometimes, but rather rarely, extensive and fairly level deposits of coal are discovered beneath a few feet of overburden, and may readily be won by opencast workings. It is probable that opencast exploitation of outcrop coal is more successful than the mining of such coal from workings.

Opencast affords greater safety, more complete extraction and maybe less subsidence and less surface damage. Sites are worked where the ratio of overburden to coal is greater than 15 to 1, and depths may exceed 150 ft.

Conditions vary widely, and rarely, indeed, opencast workings are similar.

After the preliminary work of prospecting, drilling, trench­ing, draining and preparation of any access road, etc., the major task of removing the overburden commences.

Heavy-duty loader excavators, capable of excavating 700-800 tons per hour, are used for removing topsoil from a line of outcrop. They usually employ plough-type blades which excavate and force the soil on to conveyer belts which de­liver into bottom-dump trucks. The soil is stored separately in readiness for the final restoration of the site when it is replaced and spread by mobile equipment.

Where a considerable thickness of overburden must be removed, dragline excavators are employed. This method of extraction has been found to be very successful on steep coal measures.

The removal of the coal is usually accomplished by a skimmer scoop if the seam is thin, or by power shovel. The first differs from the second in that the scoop is hauled backwards and forwards along the jib by ropes, and picks up its load in moving towards the end of the jib.

The powerful shovel has a more positive action than a dragline. It is more suitable for digging in hard material, and deals with large lumps more readily. A recent development of opencast coal technique uses the shovel on a semi-quarrying basis, the overburden being dug by shovel and transported from the cut by 4-wheel-driven vehicles.

(1640)

NOTES:

• overburden – покрывающий пласт, наносы;

• trenching – проходка кана или траншей;

• draining – дренирование, осушение;

• plough-type blade – стругообразное лезвие;

• bottom-dump truck – вагонетка с откидным дном;

• skimmer scoop – планировочный экскаватор;

• (power) shovel – механическая лопата, экскаватор;

• jib - 1) стрела, 2) бар.

Text 15. Coal mining

  Coal has been mined for more than 1000 years, and large-scale mining was practiced as early as the 18th century.

 Coal is mined by two general methods: surface mining and underground mining. In general, surface mines extract coal deposits located up to 30 m (100 ft) below the surface, while underground mines excavate coal beds further underground.

A. Surface Mining.  In surface, or strip mining, workers remove the rocky overburden and then mechanically shovel coal up from the underlying deposit. For increased efficiency, the modern coal industry has developed some of the largest industrial equipment ever made, including shovels (part of a piece of equipment known as a dragline) capable of holding 290 metric tons of coal.

To reach the coal, bulldozers clear the vegetation and soil. Depending on the hardness and depth of the exposed sedimentary rocks, these rocky layers may be shattered with explosives. To do this, workers drill blast holes into the overlying sedimentary rock, fill these holes with explosives, and then blast the overburden to fracture the rock. Once the broken rock is removed, miners shovel coal from the underlying deposit into giant earth-moving trucks for transport.

Depending on the terrain, a mining company will build either an area mine or a contour mine. Area mining occurs on level ground, where workers use excavation equipment to dig a series of long parallel strips, or cuts, into the earth. The overburden is cleared from each cut, and the material (known as spoil) is stacked alongside the long trench. After the exposed coal is shoveled from the cut, workers dump the spoil back into the trench to help reclaim the mined area.

Contour mining occurs on hilly or mountainous terrain, where workers use excavation equipment to cut into the hillside along its contour to remove the overlying rock and then mine the coal. The depth to which workers must cut into the hillside depends on factors such as hill slope and coal bed thickness. For example, steeper slopes require cutting away more overburden to expose the coal bed.

Once the hill slope prevents miners from cutting further back into the hillside, they often switch to a technique known as auger mining to extract more coal along the contour. In this technique, miners drill a series of horizontal holes into the coal bed with a large auger (drill) powered by a diesel or gasoline engine. These augers are typically about 60 m (200 ft) long and between 0.6 and 2.1 m (between 2 and 7 ft) in diameter. As these enormous drills bore into the coal seam, they discharge coal like a wood drill producing wood shavings. Workers continue drilling until the auger drills through the coal seam or until the maximum auger length or torque (energy required to twist an object) is reached.

B. Underground Mining.  To reach coal beds deeper than about 30 m (about 100 ft), miners typically build underground mines. They create two or more shafts tunneled down into the coal seam - shafts built to allow passage for miners and machinery are typically separate from those built for passage of mined coal. Miners construct three types of underground mines - shaft, slope, or drift mine - depending on the depth of the coal deposit, the angle of the coal bed, and the thickness of the coal seam.

Shaft mines are built to reach deep coal beds, usually about 200 m (about 660 ft) or more below the surface. A shaft mine uses two vertical shafts to reach the coal bed.

Slope mines reach coal deposits that have been distorted or tilted by shifts in the earth’s crust. A slope mine uses two angled shafts to reach the coal bed. The passageways of a slope mine typically begin where the inclined coal bed outcrops on the surface and follow the incline into the ground. Some slope mines angle down through the overburden to reach the sloping coal bed, then parallel the bed into the earth. If the grade of the slope mine passageway does not exceed 18°, the coal is usually transported from the mine by conveyor. For steeper grades, coal is typically removed by trolley or mine cars. Drift mines are built where a coal seam outcrops on a hill or mountainside. The drift mine consists of a single passageway that follows the coal seam back into the mountain. Mining companies favor drift mines because they eliminate the need to tunnel through overlying rock to reach a coal deposit.

A single underground mine may use all three types of passageways (drift, slope, and shaft) to mine a coal deposit. For instance, a company might build a drift mine to excavate coal from a hillside outcrop. As the drift mine follows the coal bed into the earth, workers might dig angled passageways (slope mine) from above down to the coal bed to shorten the transportation distance into the progressing mine. If the overburden becomes too deep above the progressing mine, workers may dig more economical vertical passageways (shaft mine) to provide transportation and ventilation.

Once a coal deposit has been reached by a shaft, slope, or drift mine, workers mine the coal by one of two methods: the room-and-pillar method or the longwall method. Room-and-pillar mines extract coal at greater depths and are usually left standing when the mine is abandoned. Longwall mines are built at shallower depths and are allowed to collapse as the mine progresses.

B 1. Room-and-Pillar Method.  As its name implies, the room-and-pillar method uses rows of large pillars of coal to support the roof of a mine. Workers tunnel parallel passageways through the coal seam, and then cut 12- to 24-m (40- to 80-ft) wide pillars at regular intervals out of the separating walls of coal. The percentage of coal recovered from a coal seam mined by the room-and-pillar system depends on the economic incentive to remove as much coal as possible versus the number and size of coal pillars necessary to support the roof. Workers leave the pillars standing in areas where environmental regulations prohibit land subsidence (sinking or settling of land). In areas where land subsidence is acceptable, workers may remove some pillars just before closing the mine.

Companies use two processes, known as conventional mining and continuous mining, to remove coal from room-and-pillar underground mines. Conventional coal mining replaced hand mining (mining with pick and shovel) in the 1930s. In conventional mining, miners use power saws to slice a deep cut, 3 to 4 m (10 to 12 ft) wide, into the bottom of a coal wall. Next, they drill holes into the coal above this cut and fill the holes with explosives. Miners discharge the explosives and chunks of coal collapse from the wall. Rubber-tired electric vehicles (called shuttle cars) or conveyors carry these coal chunks out of the mine.

In continuous mining, a worker uses a machine known as a continuous miner that can be operated by remote control. This mobile machine has a series of metal-studded rotating drums that gouge coal from the face of the coal seam (known as the wall face). One continuous miner can mechanically break apart about 1.8 metric tons of coal per hour. After a wall face has been mined to a certain depth, miners stabilize the adjacent roof by bolting long rods into the mine ceiling, advance the ventilation, and begin a new continuous mining cycle.

B 2. Longwall Mining.  Instead of coal pillars, the longwall mining system uses a line of moving hydraulic jacks to temporarily support the roof in the mining area. No coal pillars are present to obstruct work, so a large coal-cutting machine cuts coal continuously along a wall face typically about 180 m (590 ft) wide. This massive coal cutting machine works like a wood power saw, shredding coal from the wall in strips about 50 to 75 cm (about 20 to 30 in) wide. As the coal-cutting machine strips layers of coal from the wall face like a meat cutter, the line of roof-supporting hydraulic jacks moves automatically behind the machine. As the hydraulic jacks move forward, the roof is allowed to collapse behind the equipment.

Longwall mining produces four to five times more coal from a given deposit than the room-and-pillar method because coal pillars are not built. But because longwall mining causes the land to sink, land use regulations prohibit this practice in many areas.

(6860)

NOTES:

• steep – крутопадающий, крутой;

• auger – змеевик, спиральный бур, ложечный бур, шнек;

• torque – момент кручения, момент скручивания;

• slope – наклон, наклонная выработка;

• room-and-pillar method – камерно-столбовая система разработки;

• longwall method – система сплошного забоя;

• hydraulic jack - гидродомкрат.