Surface Vs Underground

Why is surface mining used to extract coal in Central Appalachia? The fundamental and simple answer is that coal is surface mined because that is the only method available to recover the resource.

Why do we surface mine a coal seam or group of seams instead of deep mining that reserve? The answer is generally determined by geology and topography. However, in many cases the coal reserve has been previously deep-mined and surface mining recovers what has been left. Surface mining through abandoned deep mines, recovering the blocks of coal that were left decades ago, is relatively common. Contour mining along the outer boundary of old deep mines has been a widespread practice for years. The advent of the highwall miner, which is a more productive successor to a coal auger, has encouraged this trend.

So how do geology and topography determine whether a seam is surface mined or deep mined?

Geology
Among the many geologic variables are coal thickness, expected mine roof conditions, and the vertical distance between coal seams. Some coal seams are just too thin to deep mine. Frequently, the rock overlying coal seams higher in a mountain is broken or unconsolidated, making the roof too weak to allow safe underground mining. Coal seams occurring too close together can make deep mining impossible. A coal seam that lies beneath the local valley floor (“below drainage”) will not be a candidate for surface mining in Central Appalachia but deep mining may be a possibility.

Topography
The shape of the land determines the extent of a coal reserve situated above the valley floor. The ridgelines in Central Appalachia are generally comprised of peaks and low gaps as illustrated by the topography map in Figure 1, which is taken from mapping of an actual mine project.

A coal seam that lies just a hundred feet or so beneath such a ridgetop may only measure about 400 feet to 500 feet from one side of the mountain directly through to the other side, which is not sufficient width for deep mining. The Five Block seam shown in Figure 2 is thick enough to deep mine but at this specific mine site, the ridge is only about 200 feet to 400 feet in total width at the elevation of that coal seam. There is simply not enough room to conduct deep mining in that area.

So, how much width is needed to accommodate deep mining? An underground mine will have at least five parallel entries (tunnels) that are usually 20 feet wide and spaced no less than 50 feet apart center to center, which gives an overall width of 220 feet. A barrier must be left intact along both sides of the mine to maintain ventilation, to control drainage and to support the overlying rock. Staying under at least 100 feet of solid rock (“cover”) is the general rule of thumb for mine design, so if the typical hillside slopes at 30 degrees then each of those barriers would be just over 170 feet. Adding up the numbers, a minimal deep mine configuration would require 560 feet of width. Referring back to Figure 1, one can see that deep mining a seam that lies 150 to 200 feet beneath a ridge is not practical. Only about 10 acres (shaded green) of the 108 total acres highlighted would qualify for deep mining if all other factors (seam height, etc.) were favorable.

If deep mining is eliminated as an alternative, surface mining options may include mountaintop removal, full seam recovery with restoration to approximate original contour, or a combination of contour and highwall mining. Contour and highwall mining are typically the only practical surface mining methods for seams found deeper beneath the ridge that are not amenable to deep mining (or have already been deep-mined.)