Strip mining is a surface excavation technique used to extract mineral deposits that lie relatively close to the Earth’s surface over large, continuous areas. This method systematically involves removing overlying soil and rock, known as overburden, to expose the desired resource beneath. It is commonly employed for extracting commodities such as coal, lignite, phosphate, and gypsum that form in shallow, sedimentary layers. The specific procedure used is determined primarily by the geographic characteristics of the deposit being accessed.
Primary Methods of Strip Mining
The choice between area mining and contour mining is dictated by the terrain. Area mining is utilized in regions characterized by flat or gently rolling terrain, such as the expansive coal fields of the Western United States. This method progresses in a series of long, parallel excavations, or strips, that can extend for hundreds of meters. As one strip is mined, the overburden removed from the next active strip is deposited directly into the void left by the preceding, extracted strip.
Contour mining is adapted for use in mountainous or hilly regions where the mineral seam outcrops along the side of a slope. The operation begins by following the natural contour of the exposed mineral layer around the hillside. This process involves stripping the overburden only from the outer edge of the seam, creating a bench or terrace into the slope. Mining continues inward until the depth of the overburden makes further excavation economically unfeasible, leaving a steep, vertical wall known as a highwall.
Contour mining often employs “haulback mining,” where the spoil material is trucked and placed back into the mined-out cut behind the advancing operation. This minimizes the amount of material cast down the hillside. This method is often an early step toward reclamation.
The Step-by-Step Operational Sequence
The complete process of strip mining begins with meticulous site preparation to clear the area and organize the necessary infrastructure. Crews first remove all vegetation and salvage the topsoil, which is the biologically active layer of the ground. This topsoil is carefully segregated and stored separately for later use in the final restoration of the land, a practice mandated by modern regulations. Access roads and utility corridors are also established during this phase to support the heavy machinery and material transport.
The next sequence is overburden removal, the defining characteristic of this mining technique. If the rock layer above the mineral seam is too consolidated or hard to be efficiently dug by mechanical means alone, drilling and blasting is initiated. Holes are drilled into the rock, loaded with explosive agents, and detonated in a controlled manner to fracture the material into manageable sizes. This fragmentation prepares the overburden for removal.
Massive draglines and power shovels are brought in to excavate the blasted rock and soil. A dragline, characterized by its enormous bucket suspended from a long boom, scrapes the overburden and systematically deposits the material to the side of the cut, creating spoil piles. In area mining, this spoil is typically cast directly into the previously mined strip, starting the process of backfilling. This removal directly exposes the mineral seam for extraction.
Once the mineral deposit is fully exposed, the final stage of resource extraction commences. The exposed material is often loosened by smaller shovels or specialized equipment, though minor blasting may be required if the seam is particularly hard. Front-end loaders or hydraulic shovels scoop the resource and load it directly onto large, off-highway haul trucks. These trucks transport the raw material out of the mine pit for processing or direct shipment, completing the operational cycle.
Mandatory Land Reclamation Procedures
Land reclamation is the final phase of the strip mining operation, often running concurrently with active extraction to minimize the duration of environmental disturbance. The initial step is backfilling and grading, where the previously set aside spoil piles are strategically moved back into the excavated trenches and cuts. This material is placed to eliminate the sharp highwalls and steep ridges created during mining.
Regrading is performed to contour the replaced spoil material to a configuration that closely approximates the land’s original topography, known as the Approximate Original Contour. If flat land is a premium, the grading may be altered to an approved post-mining land use, such as a gentle slope suitable for agriculture. This recontouring establishes stable slopes that resist erosion and blend visually with the surrounding terrain.
Following the shaping of the land, the salvaged topsoil layer is redistributed across the newly graded surface. The topsoil is spread to a specified depth, typically between 10 and 50 centimeters, to provide a viable medium for plant growth. This layer contains the necessary organic matter, nutrients, and microbial communities required to support a sustainable ecosystem.
The final step is revegetation, which involves planting native and approved species of grasses, legumes, shrubs, and trees. Specific seed mixtures are chosen to stabilize the soil against wind and water erosion and to return the land to its approved post-mining use, which may be wildlife habitat or pasture. Reclamation standards often require a certain percentage of vegetative ground cover to be achieved within a set number of growing seasons.