Planning for minimal impact
A mining permit requires that the mine and all reclamation efforts be fully designed on paper within the permit document and submitted to the regulatory agency. The first step in the design of a surface coal mine is known as the Approximate Original Contour (AOC) determination. This is a modeling process which:
establishes the “footprint” for the coal mine
uses site exploration data and computer modeling to design the return (reclamation) of the site to AOC
determines if there is a need for disposal of any excess material using valley fill construction, then minimizes the size of the fill(s).
The definition of approximate original contour (AOC), as found in The Surface Mining and Coal Reclamation Act of 1977 (SMCRA) is:
(3) except as provided in subsection (c) with respect to all surface coal mining operations backfill, compact (where advisable to insure stability or to prevent leaching of toxic materials), and grade in order to restore the approximate original contour of the land with all highwalls, spoil piles, and depressions eliminated (unless small depressions are needed in order to retain moisture to assist revegetation or as otherwise authorized pursuant to this Act): Provided, however, That in surface coal mining which is carried out at the same location over a substantial period of time where the operation transects the coal deposit, and the thickness of the coal deposits relative to the volume of the overburden is large and where the operator demonstrates that the overburden and other spoil and waste materials at a particular point in the permit area or otherwise available from the entire permit area is insufficient, giving due consideration to volumetric expansion, to restore the approximate original contour, the operator, at a minimum, shall backfill, grade, and compact (where advisable) using all available overburden and other spoil and waste materials to attain the lowest practicable grade but not more than the angle of repose, to provide adequate drainage and to cover all acid-forming and other toxic materials, in order to achieve an ecologically sound land use compatible with the surrounding region: And provided further, That in surface coal mining where the volume of overburden is large relative to the thickness of the coal deposit and where the operator demonstrates that due to volumetric expansion the amount of overburden and other spoil and waste materials removed in the course of the mining operation is more than sufficient to restore the approximate original contour, the operator shall after restoring the approximate contour, backfill, grade, and compact (where advisable) the excess overburden and other spoil and waste materials to attain the lowest grade but not more than the angle of repose, and to cover all acid-forming, and other toxic materials, in order to achieve an ecologically sound land use compatible with the surrounding region and that such overburden or spoil shall be shaped and graded in such a way as to prevent slides, erosion, and water pollution and is revegetated in accordance with the requirements of this Act.
Design components used for an AOC model
Mine Volumes: The volume of material (rock and coal) to be managed typically represented in cubic yards and tons.
Regrade Backfill Material: The volume of material that will be placed back onto the site to bring the new surface as close as possible to the original surface topography and in a stable configuration.
Topography: The mapped physical features of an existing location describing the shape and height of the land, represented by contours (lines of equal elevation).
Valley Fills: The footprint for excess material placed in hollows typically for permanent storage. The footprint is only allowed to be as large as calculated in accordance with the AOC determination process.
An allowance is made for voids (empty space) between fragments of material typically represented as a percentage. “Swelling” of material occurs every time you dig it up. When an AOC model is developed, the percentage of swell has to be identified and managed as a part of the mining process. A simple test for swell is to dig a hole the size of a shoe box in compacted earth and take that material and place it in the shoe box you used to measure the hole. Is some material left over? This is the result of swelling. Swell is one reason that valley fills are sometimes necessary for a coal mine.
Backfill calculations are prepared to prove that you have placed the maximum amount of material back onto the mine site before considering valley fills. If valley fills are necessary for storage of excess material, you are required to prove that you have placed additional backfill on top of the valley fill, as required by the AOC guidelines, before final design of the valley fill (to make the footprint of any fill as small as possible).
Compensatory mitigation plan
This is a United States Army Corps of Engineers (USACE) 404 (Dredge and Fill) permit requirement. During the planning stages of a newly proposed mine site, engineers carefully avoid impacting wetlands and streams. When the site conditions do not allow for a practical approach to removing coal without causing impacts to wetlands or streams, then a detailed set of instructions, designs, and accounting sheets showing how the impacted wetlands or streams will be replaced must be created. This is called the Compensatory Mitigation Plan or CMP.
Early in the planning stages of a mine, all streams and wetlands in the planned permitted area are identified and carefully mapped by scientists, in a process called delineation. The functional aspects of the streams and wetlands are also recorded in detail. This helps planners make decisions that will limit impacts.
Coal companies, scientists, and government regulators work together to understand which streams or wetlands may be affected during mining. They avoid impacts as much as possible (Avoidance), but when impacts are unavoidable, they make them as small as possible (Minimization).
This Avoidance and Minimization process is considered during the entire mine design phase. It may include things as simple as clearly marking streams and wetlands near mining areas. It may include careful engineering to limit the footprint of mine activities near streams and wetlands.
If impacts to wetlands or streams are unavoidable, engineers and scientists design plans to replace the impacted wetlands or streams. They use the best available science and information about the functions and values that are lost during the impacts. Replacing these functions and values is called restoration.
The mitigation plan, with details on how to construct or restore wetlands, is submitted to government agencies as part of a request for permission to impact stream functions and values and to replace them later. These documents summarize all of the scientific studies, engineering, and design plans that were a part of the mine planning process.