2012-06-20_PERMIT FILE - C2010089 (87)

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Hypothetical Coal Mining Situations in Alluvial !'alley Floors In order to better describe the amount of strippable coal affected by restric- tions on mining of alluvial valley floors, this section investigates s ted hypo- thetical mining situations in alluvial valley floors. The situati amined in which maintenance of pit wall stability adjacent to alluvial vs o may re- quire setting aside additional amounts of coal. Also exami . a.'{ aL s in f affected which hydrologic complexity appears to preclude success alluvial valley floors. It must be remembered that the ` follow are based not on exhaustive field investigations, but rather s to generalize surface and subsurface conditions according to principles .rology and geology. While there is confidence that situations such as these exi =t, field investigations may show them to be rare, oversimplified, or even too complex. But whatever the con- ditions are, they cannot be ignored or their analysis postponed since judgements as to their compatibility with surface mining of coal must be made now. We have assumed for purposes of calculations that an "average" alluvial valley floor is 200 meters wide and, as noted above, that any coal under the floors must be left undisturbed. In certain cases, coal may be removed up to the border of the alluvial valley floor. Such cases might exist if the coal were relatively thin (three to six meters for western coal), covered by thin yet sufficient overburden to permit regrading to a similar elevation and topography, and in relatively simple hy- drologic situations such as a thick alluvial aquifer that can be reestablished through selective placement of spoiled overburden. In other cases, pit wall stability or perhaps maintenance of a stream may require leaving a coal barrier outside the allu- vial valley floor, thereby increasing the amount of "lost" coal. We have assumed the pit wall stability situation to be one where pit wall stability requires main- tenance of a 45 slope (1:1) on the active highwall such that 60 meters of the coal seam extending away from each side of the alluvial valley could not be mined. The 60 -meter value is obtained by assuming that the mine pit will be 60 meters deep and that a 1:1 highwall slope will be maintained. Since the coal lies along the base of the right triangle (45 ° ) created by the l:l slope, at least 60 meters of the coal seam would be left in place. This increases "lost" coal from the 200 -meter width lying directly under the alluvial valley floor to 320 meters, or a 60 percent in- crease in the amount of coal that would not be mined in this situation. Based then on the three percent value calculated in the initial analysis of'alluvial valley floors overlying coal lands (Table 3), this "pit wall stability" situation would increase the. amount of coal "lost" or "withdrawn" to about five percent. Additional coal may also be lost if the coal in an alluvial valley floor is covered by relatively thin overburden, since this coal may offset the higher extrac- tion costs of coal lying outside the alluvial valley floor and covered by thicker overburden. That is, the coal overlain by thin overburden in the alluvial valley is relatively cheap to mine and can be used to balance the higher expense of extracting .coal lying under thick overburden in the highlands. If this overburden factor enters into consideration when marketing coal from a leasehold and the "pit wall stability" situation is also encountered, the amount of coal "lost" could double to shout ten percent. Note again, that these calculations are hypothetical. Actual field situa- tions would have to be studied before firm estimates of the amount of coal lost could be made. In other extreme cases, mining in the entire drainage area could be prohibited unless it were deemed possible to maintain and reestablish critical geohydrologic characteristics such as effective aquicludes* and water quality. Such would be the case if reclamation could not reestablish the essential functions of an alluvial val- ley floor supported by a perched water table (thus lowering the water table), or if Aquicludes: Relatively impermeable rock capable of absorbing water slowly, but functions as an upper or lower boundary to an aquifer. Does not transmit ground_ water rapidly enough to supply a well or spring.