Fill Construction and Surface Coal Mining

Surface coal mining techniques, which account for the majority of national coal production and roughly  38 percent of West Virginia’s coal production, involve the mechanical removal of overlying native earth and rock or “overburden” to reach underlying coal seams.    While surface mining includes a variety of methods, the overall objective is the same:  coal extraction through surface removal of overburden.  Under any method of surface mining, most of the excavated overburden is returned to the mined-out areas to eliminate the contours of the mineral extraction areas.  Referred to generally as “backfilling”, this restoration eliminates the contours and appearance of the mined areas such as coal pits and highwalls and restores the approximate original contour (AOC) or the original, pre-mining topographic contours and appearance of the area.  However, not all of the overburden can be returned to the original areas due to the “swell factor” of the native rock and soil.  This “excess overburden” is typically placed in valley fills that are constructed in jurisdictional waters.    The need for fills results from the swell factor- the physical expansion of the fractured rock and soil- which can increase its volume by 15-40 percent: 

The primary reason for using valley fills is that excavation of overburden results in a greater volume of material than was present on the mine site before mining. When bedrock is broken up forming spoil, void spaces are left between the individual rock fragments, causing them to occupy a greater volume than the original, unbroken rock. This expansion is referred to as swell and typically represents a volume increase of about 40 percent. Compaction of soil during backfilling partially offsets swell as the rock fragments are squeezed together by the weight of overlying material, but this shrinkage factor will not completely return the spoil to its solid or bank volume.

Figures B-8 and B-9 illustrate swell, shrinkage and bulking factors.

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B-7

Diagram Explaining the Swell, Shrinkage and Bulking Factors of Overburden Following Excavation

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Figure B-8 

Cross Section Diagram of Mining Site that Illustrates Physical Configuration and Swell, Shrinkage and Bulking Factors  

As illustrated by the diagrams above, the backfilling and regrading process of mine reclamation to restore AOC can reduce the swell factor of overburden this “shrinkage factor”, typically by 15 percent.  The difference between the swell and shrinkage factors is referred to as the “effective swell” or “bulking factor” of the overburden.  The bulking factor of overburden can very depending on mine-specific geology but in Appalachia the industry average is about 25 percent.  This “extra” 25 percent material or excess overburden must be placed in a stable, engineered manner.  Because of the extremely steep slopes that exist in Appalachia, “over-stacking” this material to create ridges or mountains that are higher than what existed before is not possible:  

Particularly on steep-sloped mine sites, the excess spoil generated by the swell factor cannot be completely backfilled on the mine bench without construction of potentially unstable slopes or substantial deviation from AOC. The maximum amount of spoil that can be returned to the mined bench is constrained by SMCRA slope stability and design requirements (i.e., the slope at which backfills can be constructed)...

Transporting the excess overburden material to a separate location on another mountaintop or ridge is also impractical.  Such sites are generally unavailable because of property constraints and any placement of overburden on disturbed areas would significantly alter the topography resulting steeper slopes that are unstable. The physical constraints imposed by steeply sloped, rugged terrain leaves only one available option for the placement of excess overburden- valley fills.              

Fill construction methods employed in surface mining were developed to some degree prior to the passage of the SMCRA.  The technique was largely perfected by the mining industry and the pre-SMCRA mining regulatory authority in West Virginia.  This practice of controlled, engineered placement of excess overburden was subsequently recognized by Congress in SMCRA and the federal Office of Surface Mining in its corresponding regulations.  Additionally, the federal Environmental Protection Agency has recognized the environmental advantages of overburden placement in valley fills as early as 1979 and the agency conducted its first Environmental Assessment on the practice in 1984.  Valley fills have an excellent record of long-term stability as recently confirmed in a multi-agency programmatic Environmental Impact Statement.              

Valley fills (and coal refuse fills and impoundments) are also some of the most heavily regulated, reviewed and engineered man-made structures in the world.  As detailed in subsequent sections, mining-related fills are subject to host of federal and state laws and regulations that affect everything from fill site selection to construction and reclamation criteria to storm water runoff discharges.   

The size of valley fills associated with mining (surface or underground) has been the subject of much debate over the last 12 years.  Lost in most of these discussions is the fundamental fact that fills are constructed because of physical and geologic realities such as swell factor that was described in detail in previous sections.  Additionally, the size of fills associated with mining operations has been addressed in West Virginia and other Appalachian states with the adoption of formal AOC policies.  These documents provide for an engineering and mathematical analysis to demonstrate that valley fills are as small as possible by maximizing the amount of overburden that is returned to the mined area as part of AOC restoration.  Also, West Virginia’s AOC policy has been accepted by the Corps and EPA as a demonstration of fill minimization under the section 404 permit and review analysis. 

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Figure B-9

Valley Fill Construction on an Active Surface Mine

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Figure B-10

Early Reclamation of a Valley Fill - Compaction and Seeding

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Figure B-11

Reclaimed Valley Fill on an AOC Variance MTR Mine

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Figure B-12

Valley Fill on a Reclaimed Area Mine

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Figure B-13

Older Valley Fill on a Reclaimed Area Mine