Surface Mining

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Surface mining

Surface mining techniques are used when a vein of coal or other substance lies so close to the surface that it can be mined with bulldozers, power shovels, and trucks instead of using deep shaft mines, explosive devices, or coal gasification techniques. Surface mining is especially useful when the rock contains so little of the ore being mined that conventional techniques, such as tunneling along veins, cannot be used. Surface mining removes the earth and rock that lies above the coal or mineral seam and places the overburden off to one side as spoil . The exposed ore is removed and preliminary processing is done on site or the ore is taken by truck to processing plants. After the mining operations are complete, the surface can be recontoured, restored, and reclaimed.

Surface mining already accounts for over 60% of the world's total mineral production, and the percentage is increasing substantially. Many factors contribute to the popularity of surface mining. The lead time for developing a surface mine averages four years, as opposed to eight years for underground mines. Productivity of workers at surface mines is three times greater than that of workers in underground mining operations. The capital cost for surface mine development is between 20 and 40 dollars per annual ton of salable coal, and the start-up expenses for underground mining operations are at least twice that, averaging around 80 dollars per annual ton.

The preliminary stages of mine development involve gathering detailed information about the potential mining site. Trenching and core drilling provide information on the coal seam as well as the overburden and the general geological composition of the site. Analysis of the drill core from the overburden is an important step in preventing environmental hazards. For example, when the shale between coal seams and the underlying strata is disturbed by mining, it can produce acid . If this instability in the strata is detected by analysis of the drill cores, acid runoff can be avoided by appropriate mine design. The data obtained from the preliminary drilling is plotted on topographic maps so that the relationship between seams of ore and the overlying terrain are clearly visible.

After test results and all other relevant data have been obtained, the information is entered into a computer system for analysis. Various mine designs and mining sequences are tested by the computer models and evaluated according to technical and economic criteria in order to determine the optimal mine design. Satellite imagery and aerial photographs are usually taken during the exploratory stages. These photographs serve as an excellent visual record of the actual environmental conditions prior to the mining operation, and they are frequently utilized during the reclamation process.

There are several types of surface mining, and these include area mining, contour mining, auger mining, and open-pit mining. Area mining is used predominantly in the Midwest and western mountain states, where coal seams lie horizontally beneath the surface. Operations begin near the coal outcrop—the point where the ore lies closest to the surface. Large stripping shovels or draglines dig long parallel trenches; this removes the overburden, leaving the ore exposed. The overburden excavated from the trench is thrown into the previous trench, from which coal has already been extracted. The process is similar to a farmer plowing a field in furrows. Because of the steep slopes and rugged terrain frequently encountered in area mining operations, the ore is usually recovered by small equipment such as front end bucket loaders, bulldozers, and trucks.

Area mining is also practiced in Appalachia, where many of the coal seams lie under mountains or foothills. This type of area mining is commonly known as mountaintop removal. The mountains mined using this technique generally have long ridges with underlying deposits. A cut is made parallel to the ridge, and subsequent cuts are made parallel to the first; this results in the entire top of the mountain being leveled off and flattened out. When these areas are reclaimed, the most frequent designated uses are grazing lands or development sites.

Contour mining is used primarily at the point where the seam lies closest to the surface, on steep inclines such as in the Appalachian mountains. In these areas, coal usually lies in flat continuous beds, and the excavation continues along the side of the mountain. This produces a long, narrow trench, with highwall extending along the trench—the contour line for horizontal coal seams. Recent federal regulations have outlawed many contour mining practices, such as leaving exposed highwall and spoil on the mountainside.

The auger process is used primarily in salvage operations. When it is no longer economically profitable to remove the overburden in the highwall, augering techniques are utilized to recover additional tonnage. The auger extracts coal by boring underneath the final highwall. Currently, auger mining only accounts for 4% of surface-mined coal production in the United States.

Open-pit mining is used primarily in western states, where coal seams are at least 100 ft (30 m) thick. The thin overburden is removed and taken away from the site by truck, leaving the exposed coal seam. This type of mine operation is very similar to rock quarry operations.

The equipment used in surface mining ranges from bulldozers, front-end bucket loaders, scrapers, and trucks to gigantic power shovels, bucket-wheel excavators, and draglines. Over the past ten years, technological development has concentrated on mechanization and development of heavy-duty equipment. Due to economic factors, equipment manufacturers have focused on improving performance of existing equipment instead of developing new technologies. Since the recession of the 1980s, bucket capacity and the size of conventional machines have increased. Concentration of production technology has created mining systems that incorporate mining equipment with continuous transportation systems and integrated computers into all aspects of the industry.

Surface mining can have severe environmental effects. The process removes all vegetation, destroying microflora and microorganisms . The soil , subsoil , and strata are broken and removed. Wildlife is displaced, air quality suffers, and surface changes occur due to oxidation and topographic changes.

Hydrology associated with surface mining has a major effect on the environment . Removal of overburden may change the groundwater in numerous ways, including drainage of water from the area, altering the direction of aquifer flow, and lowering the water tables. It also creates channels that allow contaminated water to mingle with water of other aquifers. Acid water runoff from the mining operations can contaminate the area and other water sources. In addition to being highly acidic, the runoff from mining operations also contains many other trace elements that adversely affect the environment.

Federal regulations require that topsoil be redistributed after the mining is complete, but many people do not consider this requirement sufficient. When soil is removed, the soil structure breaks down and compacts, preventing normal organic matter from getting into the soil. Microorganisms are destroyed by the changes in the soil and the lack of organic components. The rate of erosion in mining areas is also greatly increased due to the lack of native vegetation.

The removal of vegetation and overburden at the mining site displaces all wildlife, and a large portion of it may be completely destroyed. Some forms of wildlife, such as birds and game animals, may get out of the area safely, but those that hibernate or burrow usually die as a result of the mining. Ponds, streams, and swamps are routinely drained before mining operations commence and all aquatic life in the region is destroyed.

In addition to the short-term environmental effects, surface mining also has long-term impact on the flora and fauna within the region of the mine. Salts, heavy metals , acids, and other minerals exposed during removal of overburden suppress growth rate and productivity. Due to changes in soil composition, many native species of plants are unable to adjust. The loss of vegetation means a loss of feeding grounds, which in turn disrupts migration patterns. Displaced species encroach on neighboring ecosystems, which may cause overpopulation and disruption of adjacent habitats.

[Debra Glidden ]