Underground coal mining, or deep mining, extracts coal seams lying too far beneath the surface for open-pit methods to be practical. This technique involves creating a complex subterranean network of tunnels and passages to reach the coal deposit. Because the coal can be hundreds of feet below the earth, specialized engineering and a detailed, multi-stage approach are necessary to ensure safe and efficient extraction. The entire operation is defined by carefully designed access points and sophisticated removal methods that navigate the surrounding rock strata to reach the coal seam itself.
Planning the Mine Site
Before physical excavation begins, a rigorous planning phase establishes the viability and layout of the mine. This process starts with geological surveys to map subsurface rock layers and identify the precise location, thickness, and quality of the coal seam. Techniques such as seismic reflection and gravity surveys create a detailed three-dimensional model of the deposit.
Exploration drilling programs follow, where core samples are extracted to analyze the coal and surrounding rock directly. This step determines the seam’s depth, methane content, and the mechanical properties of the roof and floor rocks necessary for designing support systems. Logistical planning addresses provisions for power, water, ventilation systems, and the infrastructure needed to transport personnel and the mined coal, which dictates the choice of access method and overall mine design.
Constructing the Access Points
The physical connection between the surface and the deep coal seam is established through one of three primary types of mine entries, each selected based on the local geology and the seam’s depth.
Drift Mine
A drift mine is established when the coal seam is exposed on the side of a hill or mountain, allowing for horizontal access. The entry tunnel, or adit, is driven straight into the hillside, following the coal seam. This horizontal orientation simplifies coal removal, as gravity assists with drainage and haulage, making it typically the least expensive access method.
Slope Mine
When the coal seam is relatively shallow but not exposed on a hillside, a slope mine provides inclined access from the surface down to the deposit. This angled passage allows for heavy equipment transport in and out of the mine. The slope is also commonly used to install conveyor belts, which efficiently move the extracted coal up to the surface.
Shaft Mine
For deep coal seams, a vertical shaft mine is the necessary solution. This method involves sinking a vertical tunnel straight down through the rock layers until it intersects the coal seam. Shafts are the deepest type of access and are designed to handle high-capacity elevators, or cages, used for lowering personnel and equipment, and for raising coal and rock. They are also frequently used to provide ventilation and act as auxiliary exits.
Methods for Coal Removal
Once the coal seam is accessed, two main underground techniques are used to remove the material, each employing a different approach to excavation and roof support.
Room and Pillar Mining
Room and pillar mining uses a network of horizontal tunnels, known as “rooms,” excavated into the coal seam. Between these rooms, large columns of unmined coal, or “pillars,” are deliberately left in place to support the overlying rock strata. This grid-like pattern is adaptable for mechanized equipment, such as a continuous miner. The method is less efficient in resource recovery, often leaving 30% to 50% of the coal in the ground as supportive pillars.
Longwall Mining
Longwall mining is a highly mechanized technique that achieves a recovery rate of about 80% of the coal in a panel. This method involves cutting a long, single face of coal, often hundreds of feet wide, using a machine called a shearer. As the shearer cuts across the face, the coal is immediately loaded onto a conveyor belt running parallel to the wall.
A system of movable hydraulic roof supports, called shields or chocks, protects the working area. As the shearer advances and the shields move forward, the roof behind the shields is intentionally allowed to collapse into the mined-out space, known as the “goaf.” This process is ideal for large, uniform coal deposits and is considered the most economical form of underground mining.