Print7.3.2: Contour Mining
It would be great if all coal were found in conditions similar to those in the Powder River Basin: relatively flat and wide-open space, thick coal seams, and shallow cover (overburden). Mother Nature was not so accommodating! A lot of high quality thermal and metallurgical coal is found in rugged terrain, in areas such as the Appalachian Coal Field, which runs from Pennsylvania down to Alabama. Contour mining methods evolved to allow the economic extraction of coal using smaller and more agile equipment with more flexible mining patterns.
As with area mining, it is necessary to remove the vegetation and remove the overburden to gain access to the coal seam. There are two ways to handle the removed overburden: haul it all the way down the hill and put it into a chosen valley in horizontal layers; or to place it in the void of the cut, where the coal has been already removed. Mining will proceed by advancing into the hillside until reaching the break-even stripping ratio. At this point, the highwall will remain. Under certain conditions, we may choose to take advantage of the exposed coal seam, and develop an underground mine or practice highwall mining, which will be discussed later in this lesson.
Now, let’s take a look at a schematic view of contour mining approach and then a photo of a contour mining site. The diagram shows more than one coal seam. In fact, it is not uncommon for multiple seams to be recovered. It creates a small sequencing and scheduling challenge because you have to remove the interburden between the seams, and place it, but it is commonly done.
Note the highwall in the photo, and the exposed coal seam.
Haulback Mining
An important submethod within contour mining is haulback mining. Haulback mining, which is illustrated nicely in this diagram from the textbook, is a method in which overburden removal and then coal extraction follows the outcrop, proceeding in a series of nearly rectangular pits. As mining is advancing at one end of the rectangular cut, reclamation is occurring on the back end. In this method, mining roughly follows the contours of the land, and the ultimate pit width is dictated by the economic stripping ratio and steepness of topography.
Haulback mining starts with developing a box cut and hauling the overburden to a suitable disposal site. Coal is mined out and mining progresses with removing the next cut of overburden, and hauling it by truck, scraper, or conveyor to fill previously mined-out cut. This method is called haulback mining, because waste moves in a direction opposite to the direction of mining, as illustrated in the diagram.
Mountaintop Removal
Mountaintop removal is another submethod of contour mining where a flat lying coal seam occurs near a mountain top and outcrops on opposite sides of a hill. When the overburden is sufficiently thin (less than 165 feet) across the hill, the deposit can be mined from outcrop to outcrop. In this method the hill is completely removed and the spoil is leveled rather than contoured to approximate the original topography, creating a flat land. This method was commonly used in Appalachia, but is very controversial, and is no longer recommended. The below picture shows a mountaintop removal operation in southern WV, prior to reclamation of the site.
