Contour strip mining is a surface excavation method designed to extract mineral seams that lie along the slopes of hilly or mountainous terrain. This technique is primarily used for deposits like coal that are near the surface and follow the natural contours of the land. Unlike area strip mining, which operates on flat land, contour mining creates a series of terraces that wrap around a hillside. This method provides a practical approach for resource extraction in rugged areas.
The Step-by-Step Mining Process
The operation begins by clearing vegetation and removing the upper layers of soil, known as the overburden, in a narrow strip along the mineral seam’s outcrop. The overburden is drilled and often blasted to loosen the material. Specialized heavy equipment, such as excavators and bulldozers, then removes this material, exposing the underlying coal seam.
The exposed mineral is extracted, and the flattened area is called the bench. The vertical cliff face left behind on the upslope side of the bench is known as the highwall. The process advances into the hillside by creating successive parallel strips, with the overburden from the new cut often placed into the void of the previous, mined-out strip.
Mining continues until the depth of the overburden becomes too great, making it economically unfeasible to remove any more material. The highwall marks the final limit of the strip mining operation. A secondary technique called augering is sometimes employed to recover additional coal by drilling horizontally into the exposed seam at the base of the highwall. This method allows for limited resource recovery without requiring the removal of massive amounts of additional overburden.
Distinctive Environmental Consequences
The primary environmental issue unique to contour strip mining is the handling of the spoil material (excavated overburden). Historically, the spoil was often cast down the steep slope below the bench, leading to significant instability. This practice creates massive erosion and downslope movement of sediment that can clog and degrade waterways below. The loose material alters the natural drainage patterns of the area and increases the risk of localized flooding.
The creation of the highwall presents a long-term hazard, as the exposed vertical rock face is prone to instability and landslides. Exposing the underlying mineral seams and rock strata to air and water can lead to the formation of Acid Mine Drainage (AMD). The weathering of sulfide minerals, such as pyrite, generates sulfuric acid, which then leaches toxic heavy metals from the surrounding rock. This acidic runoff contaminates groundwater and surface streams, degrading the water quality.
Mandated Land Restoration
Federal law requires that land disturbed by contour strip mining must be reclaimed to a condition capable of supporting its previous use. The primary physical requirement for restoration is the elimination of the highwall and returning the land to its approximate original contour. This is accomplished by backfilling the bench with the spoil material, which may require special handling if the material is acid-producing or toxic.
The backfilled slopes are then graded to ensure stability and reduce the potential for future erosion and runoff. Topsoil, stored separately during the initial clearing phase, is redistributed over the graded area. Finally, the land is revegetated using native species to establish a permanent vegetative cover that stabilizes the soil. Operators are required to post a financial bond to ensure that reclamation obligations are met.