The Appalachian coal region, stretching from Pennsylvania to Alabama, holds some of the nation’s most extensive coal reserves. Extracting this coal requires various techniques due to the mountain range’s rugged geology. Operators analyze the specific geological conditions of each site to determine the most effective method. The choice between deep underground mining and surface excavation depends primarily on the depth, thickness, and orientation of the coal seams.
Geological Factors Influencing Method Selection
The physical characteristics of the coal seam and surrounding rock layers determine the mining method. Seam depth is the most important element, establishing whether the operation will be deep or surface mining. Coal seams close to the surface, typically less than a few hundred feet deep, are targeted for surface methods because the overlying rock can be removed economically.
Underground mining techniques are necessary for coal seams situated significantly deeper, sometimes thousands of feet below the surface. Seam thickness is also a constraint; thin seams may not be economically viable, while thick, continuous seams are candidates for mechanized deep mining. The topography of the region also plays a role. Flat valley floors may allow for area-style surface mining, but the steep slopes of the Central Appalachians require contour-based approaches.
The structural geology of the seam, including its angle or dip, influences method selection and efficiency. Flat-lying seams are optimal for high-production underground methods because machinery moves easily along the coal bed. Seams that are steeply inclined or broken by numerous faults present challenges that limit the use of large, continuous mining equipment. These factors must align favorably for an extraction method to be employed commercially.
Deep Mining: Accessing Underground Seams
Deep mining is used when coal seams are too far beneath the surface for overlying rock removal to be practical. This approach involves sinking vertical shafts or driving horizontal entries into the mountainside to reach the coal bed. The two primary techniques used in Appalachian underground mines are room-and-pillar and longwall mining.
Room-and-Pillar Mining
The Room-and-Pillar Mining method is a traditional technique where a continuous mining machine cuts a network of “rooms” into the coal seam. Blocks of coal, known as “pillars,” are left standing to support the mine roof and prevent collapse. These pillars are strategically sized to bear the weight of the overlying strata. This method typically results in a coal recovery rate of 50 to 70 percent, as the remaining coal is left as structural support. In a final stage, called retreat mining, the pillars may be systematically removed, allowing the roof to collapse in a controlled manner to achieve greater extraction.
Longwall Mining
Longwall Mining is a highly mechanized, high-volume alternative to room-and-pillar operations. This method is used for large, continuous, and relatively flat coal seams, often lying at greater depths. The process defines a large rectangular section of coal, known as a panel, which can be up to 1,000 feet wide and several miles long. A specialized shearer machine moves across the face of the panel, mechanically cutting the coal onto a conveyor belt.
Miners and equipment operate under massive, self-advancing hydraulic roof supports, called shields, which temporarily hold up the overburden. As the shearer progresses and the shields advance, the roof strata behind the shields is intentionally allowed to collapse in a process called controlled caving. This full-extraction technique is significantly more efficient, often allowing for a coal recovery rate of 80 percent or more from the designated panel area. The continuous nature of the operation makes longwall mining a highly productive method.
Surface Mining: Excavating Near the Surface
Surface mining techniques are used when coal seams are shallow enough that the overlying rock and soil, known as overburden, can be efficiently removed. In Appalachia’s mountainous terrain, these methods are adapted to the steep slopes and narrow valleys. The approach involves sequentially removing the overburden to expose the coal seam for mechanical extraction.
Contour Mining
Contour Mining is designed for the steep hillsides of the Appalachian Mountains. The operation begins by excavating a box-cut bench into the side of the mountain, following the contour of the coal seam. Overburden is removed and placed on the downslope, exposing the coal layer. As mining progresses deeper into the hillside, it creates a permanent, vertical rock face called a highwall.
Area Strip Mining
Area Strip Mining is employed in areas with less severe topography, such as flatter plateaus or broad valleys. This method involves opening a long, linear cut into the ground to remove the coal seam. The overburden from the first cut is placed to the side. A parallel cut is then made, and its overburden is placed into the void of the first cut. This process continues across the area, creating a series of parallel spoil banks and trenches.
Mountaintop Removal (MTR)
The most extensive form of surface excavation is Mountaintop Removal (MTR), used to access multiple, layered coal seams located within the mountain ridge. The process begins by clearing all vegetation and topsoil from the summit. Explosives, such as ammonium nitrate and fuel oil (ANFO), are then used to fragment hundreds of vertical feet of the overlying bedrock.
Massive earth-moving equipment, such as draglines, scoops up the fragmented rock. This overburden is systematically moved away from the exposed coal seams. Because the fragmented rock occupies a greater volume than the solid mountain, the excess material, called spoil or mine waste, is disposed of by pushing it into adjacent valleys, creating large engineered structures known as valley fills. Once the coal seam is exposed, it is removed using shovels and loaders, allowing for nearly complete extraction of the coal.