Coal-Fired Power Generation Handbook. James G. Speight
first. A ridge of undisturbed natural material is often left at the outer edge of the mined area to add stability to the reclaimed slope by preventing spoil from slumping or sliding downhill.
Once the hill slope prevents further cutting into the hillside, miners often switch to a technique known as auger mining to extract more coal along the contour.
3.2.1.4 Auger Mining
Auger mining is analogous to wood drilling; the coal is extracted from a seam (which outcrops to the surface) by the use of large-diameter augers.
This particular method for the recovery of coal was developed mainly in the period following World War II. An attractive aspect of the method is the relatively low cost of the equipment since the technique essentially involves boring a series of parallel horizontal holes into a coal seam.
The technique is frequently employed in open pit mines where the thickness of the overburden at the high-wall section of the mine is too great for further economic mining. This, however, should not detract from the overall concept and utility of auger mining as it is also applicable to underground operations. As the coal is discharged from the auger spiral, it is collected for transportation to the coal preparation plant or to the market.
In this technique, the 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 approximately 200 feet long and drill holes between 2 and 7 feet in diameter to depths of up to 300 feet in the coal seam and the coal is moved out as the drill turns farther into the seam. As these enormous drills bore into the coal seam, they discharge coal like a wood drill producing wood shavings.
Additional auger lengths are added as the cutting head of the auger penetrates further under the high wall into the coal. Penetration continues until the cutting head drifts into the top or bottom, as determined by the cuttings returned, into a previous hole, or until the maximum torque or the auger is reached. Penetration in an auger operation may vary from a few feet up to 200 feet, depending on the pitch of the coal seam, the seam thickness, and the physical characteristics of the strata immediately above the coal seam. The better the roof strata and the more level the coal seam, the deeper the penetration.
3.2.1.5 Mountain Top Removal
Mountaintop removal/valley fill coal mining is, as the name implies, the removal of mountain top to recover coal from the seam(s) underneath. In the method, power shovels are used to dig into the soil for trucks to haul away, after which a dragline is used to dig into the rock to expose the coal – explosives may also be used to make the overburden more amenable to removal. Other machines scoop out the layers of coal.
Unfortunately, the mountaintop method generates large amounts of solid waste that must be disposed in a sound environmental manner. Association of the waste with water (through heavy rains or river flooding) can cause the waste to move as a live mud stream causing serious environmental damage and danger to flora and fauna (including human life).
3.2.2 Underground Mining
To reach deeper coal beds, miners typically dig underground mines. Two or more shafts are tunneled down into a coal seam – typically, different shafts are used for the passage of miners and machinery and for the passage of mined coal. There are three types of underground mines: (i) shaft mine, (ii) slope mine, and (iii) drift mine. The type excavated in a particular case depends on the depth of the coal deposit, the angle of the coal bed, and the thickness of the coal seam.
A drift mine is one in which a horizontal (or nearly horizontal) seam of coal outcrops to the surface in the side of a hill or mountain, and the opening into the mine can be made directly into the coal seam. Thus, drift mines are used in cases where a coal seam outcrops on a hill or mountainside. A drift mine consists of a single passageway that follows the coal seam back into the mountain. Drift mines eliminate the need to tunnel through overlying rock to reach a coal deposit.
In the drift mine, mining is conducted using typically either longwall mining or room and pillar mining with continuous mining equipment. Coal is transported to the surface by conveyor belts. This method of mining is used when the coal seam outcrops at the surface, or when a bench has to be constructed on a mountain side to mine the coal. This type of mine is generally the easiest and most economical to open because excavation through rock is not necessary.
A slope mine is a mine in which an inclined opening is used to trap the coal seam (or seams). The mine opening is made by tunneling from the surface down to the elevation of the coal seam. Mining is conducted using typically either longwall mining or room and pillar mining with continuous mining equipment. Coal is transported to the surface by conveyor belts. This method of mining coal is usually utilized when the coal seam is not far from the surface, and the outcrop of the coal seam is not exposed. A slope mine may follow the coal seam if the seam is inclined and outcrops or the slope may be driven through rock strata overlying the coal to reach a seam which is below drainage.
Generally, the slope mines have not been under as much cover as shaft mines but, with the application of rock-tunneling machines, slopes can be extended to deeper coal seams. Coal transportation from a slope mine can be by conveyor or by track haulage (using a trolley locomotive if the grade is not severe) or by pulling mine cars up the slope using an electric hoist and steel rope if the grade is steep. The most common practice is to use a belt conveyor where grades do not exceed 18°.
A shaft mine enters the coal seam by a vertical opening from the surface. Shaft mines are dug to reach deep coal beds, usually at least 660 feet (200 meters) 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 crust of the Earth. 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.
In both slope and shaft mines, ground support at the opening is dependent on several factors, such as the dimensions of the mine entry, the intended lifetime and use of the mine, the water, and climatic conditions, as well as the nature of the exposed strata. In slope mines, it is common practice to use rock bolting only when the exposed rock tends to fragment but is otherwise sturdy; the general practice is to cover the sides and roof of the slope with a thin coating of cement sprayed on to say, a wire mesh. If the ground is heavy (badly fractured or overstressed), it may be necessary to support the slope with a poured concrete lining; the prevailing practice in shaft mines is to use such a concrete lining.
Once a coal deposit has been reached by a slope mine, or a 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 used at shallower depths and are allowed to collapse as the mine progresses.
Miners 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 (Speight, 2013).
In spite of modern innovations, mining remains a hazardous occupation because of roof (amongst other issues) instability and gas evolution from the coal seams (Speight, 2013).
3.2.3 Mine Decommissioning and Closure
In the United States, federal regulations for decommissioning and closure of mining operations are administered by the Mine Safety and Health Administration (MSHA), the Office of Surface Mining Reclamation