There are five main types of surface mining as detailed below.
Strip mining is a
bucket-wheel excavator used in strip mining.
Strip mining is the practice of mining a seam of mineral, by first removing a long strip of overlying soil and rock (the
overburden); this activity is also referred to as overburden removal. It is most commonly used to mine
coal and
lignite (brown coal). Strip mining is only practical when the ore body to be excavated is relatively near the surface and/or is mostly horizontal. This type of mining uses some of the largest machines on earth, including
bucket-wheel excavators which can move as much as 12,000 cubic meters (16,000 cu. yd.) of earth per hour. There are two forms of strip mining. The more common method is area stripping, which is used on fairly flat terrain, to extract deposits over a large area. As each long strip is excavated, the overburden is placed in the excavation produced by the previous strip.
Contour mining involves removing the overburden above the mineral seam near an
outcrop in hilly terrain, where the mineral outcrop usually follows the contour of the land. Contour stripping is often followed by
auger mining into the hillside, to remove more of the mineral. This method commonly leaves behind
terraces in mountainsides.
Open-pit mining located near
Silver City, New Mexico is an open-pit copper mine.
Open-pit mining refers to a method of extracting
rock or
minerals from the earth through their removal from an open pit or
borrow. This process is done on the ground surface of the earth. It is best suited for accessing mostly vertical deposits of minerals. Although open-pit mining is sometimes mistakenly referred to as "strip mining", the two methods are different (see above).
Mountaintop removal Mountaintop removal mining (MTR) is a form of coal mining that mines coal seams beneath mountaintops by first removing the mountaintop overlying the coal seam. Explosives are used to break up the overburden (rock layers above the seam), which is then removed. The overburden is then dumped by haul trucks into fills in nearby hollows or valleys. MTR involves the mass restructuring of earth in order to reach coal seams as deep as below the surface. Mountaintop removal replaces the original steep landscape with a much flatter topography. Economic development attempts on reclaimed mine sites include prisons such the
Big Sandy Federal Penitentiary in
Martin County, Kentucky, small-town airports, golf courses such as Twisted Gun in
Mingo County, West Virginia and Stonecrest Golf Course in
Floyd County, Kentucky, as well as industrial scrubber sludge disposal sites, solid waste landfills, trailer parks, explosive manufacturers, and storage rental lockers. This method has been increasingly used in recent years in the
Appalachian coal fields of
West Virginia,
Kentucky,
Virginia, and
Tennessee in the United States. The profound changes in topography and disturbance of pre-existing ecosystems have made mountaintop removal highly controversial. Proponents of mountaintop removal point out that once the areas are reclaimed as mandated by law, the technique provides premium flat land suitable for many uses in a region where flat land is rare. They also maintain that the new growth on reclaimed mountaintop mined areas is better able to support populations of game animals. Critics contend that mountaintop removal is a disastrous practice that benefits a small number of corporations at the expense of
local communities and the
environment. A
U.S. Environmental Protection Agency (EPA)
environmental impact statement finds that streams near valley fills sometimes may contain higher levels of minerals in the water and decreased aquatic
biodiversity. The statement also estimates that of Appalachian streams were buried by valley fills from 1985 to 2001. Blasting at a mountaintop removal mine expels dust and fly-rock into the air, which can then disturb or settle onto private property nearby. This dust may contain sulfur compounds, which some claim corrode structures and tombstones and is a health hazard. Although MTR sites are required to be reclaimed after mining is complete, reclamation has traditionally focused on stabilizing rock and controlling erosion, but not always on
reforesting the area. Quick-growing,
non-native grasses, planted to quickly provide vegetation on a site, compete with tree seedlings, and trees have difficulty establishing root systems in compacted backfill.
Erosion also increases, which can intensify
flooding. In the eastern United States, the Appalachian Regional Reforestation Initiative works to promote the use of trees in mining reclamation.
Dredging Dredging is a method for mining below the water table. It is mostly associated with gold mining. Small dredges often use suction to bring the mined material up from the bottom of a water body. Historical large-scale dredging operations often used a floating dredge; a barge-like vessel that scoops material up through a conveyor belt on the bow, filters out the desired component on board, and returns the unwanted material to the water via another conveyor belt on the stern. In gravel-filled river valleys with shallow water tables, a floating dredge can work its way through the loose sediment in a pond of its own making.
Highwall mining Highwall mining is another form of mining sometimes conducted to recover additional coal adjacent to a surface-mined area. The method evolved from auger mining but does not meet the definition of surface mining since it does not involve the removal of overburden to expose the coal seam. CERB final report No. 2014-004 "Highwall Mining: Design Methodology, Safety, and Suitability" by Yi Luo characterizes it as a "relatively new semi-surface and semi-underground coal mining method that evolved from auger mining". In highwall mining, the coal seam is penetrated by a continuous miner propelled by a hydraulic pushbeam transfer mechanism (PTM). A typical cycle includes sumping (launch-pushing forward) and shearing (raising and lowering the cutterhead boom to cut the entire height of the coal seam). As the coal recovery cycle continues, the cutterhead is progressively launched into the coal seam for . Then, the PTM automatically inserts a rectangular pushbeam (screw-conveyor segment) into the center section of the machine between the Powerhead and the cutterhead. The pushbeam system can penetrate nearly (proven in 2015 till today) into the coal seam. One patented highwall mining system uses augers enclosed inside the pushbeam that prevent the mined coal from being contaminated by rock debris during the conveyance process. Using a video imaging and/or a gamma-ray sensor and/or other geo-radar systems like a coal-rock interface detection sensor (CID), the operator can see ahead projection of the seam-rock interface and guide the continuous miner's progress. Highwall mining can produce thousands of tons of coal in contour-strip operations with narrow benches, previously mined areas, trench mine applications, and steep-dip seams by utilizing a controlled water-inflow pump system and/or a gas (inert) venting system. Recovery with tunneling shape of drives used by highwall miners is much better than round augering holes, but the mapping of areas that have been developed by a highwall miner are not mapped as rigorously as deep mined areas. Very little soil is displaced in contrast with mountaintop removal; however, it is comparatively more expensive to own and operate a highwall miner. Mapping of the outcrop, as well as core hole data and samples taken during the bench-making process, are taken into account to best project the panels that the highwall miner will cut. Obstacles that could be potentially damaged by subsidence and the natural contour of the highwall mine are taken into account, and a surveyor points the highwall miner in a line (theoretical survey plot-line) mostly perpendicular to the highwall. parallel lines represent the drive cut into the mountain (up to deep (2015 records), without heading or corrective steering actuation on a navigation azimuth during mining results in missing a portion of the coal seam and is a potential danger of cutting in pillars from previous mined drives due to horizontal drift (roll) of the pushbeam-cuttermodule string. Recently highwall miners have penetrated more than (2015 ongoing records into the coal seam, and today's models are capable of going farther, with the support of gyro navigation and not limited anymore by the amount of cable stored on the machine. The maximum depth would be determined by the stress of further penetration and associated specific-power draw (torsion and tension in screw transporters string), but today's optimized screw-transporters conveying embodiments (called pushbeams) with visual product development and discrete element modeling (DEM) using flow simulation behavior software shows smart-drive extended penetrations are possible, even so under steep inclined angles from horizontal to more than 30 degree downhole. In case of significant steep mining the new mining method phrase should be "directional mining" (commonly used technologies as valuable synergy directional drilling and directional mining are categorized in "surface to in-seam" (SIS) techniques), dry or wet, dewatering is developed or cutting and dredging through screw transporters are proactive in developing a roadmap of the leading global highwall mining engineering company. ==Transport==