Coal mining can be broadly classified into the following two categories:
- surface or opencast mining
- underground mining
Although the geotechnics of the mining region and the geology of coal deposits play a key role in determining the mining method, the ultimate decision maker is the economics of mining under given constraints. The mining method has a direct influence on the productivity of a mine.
Surface or opencast mining
Surface mining is carried out when the coal seams are lying close to the surface, as the amount of overburden to be removed for extracting each ton of coal mined (or the stripping ratio) will be at a minimum. According to the World Coal Association estimates, coal recovery in surface mining in the US is more than 90%.
Mountain top removal mining
There are many ways to extract coal lying closer to the surface. Of all surface mining methods, mountaintop removal mining in the central Appalachian region attracted the most attention for its impact on the environment.
This method involves removing the top of a mountain to recover coal contained beneath its surface. The excess spoil generated in the process is placed in valley fills on the sides of the former mountain.
According to the EPA’s report on effects of mountaintop mining and valley fills on the aquatic ecosystem of central Appalachian coalfields, intermittent and perennial streams are permanently lost with the removal of the mountain and from burial under fill. Moreover, the process also elevates the concentration of harmful chemical ions downstream.
The Office of Surface Mining Reclamation and Enforcement (or OSMRE) has proposed a Stream Protection Rule under the Surface Mining Control and Reclamation Act.