2021-08-09_PERMIT FILE - C1981019A (5)

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RULE 4 PERFORMANCE STANDARDS Rule 4 Performance Standards 4-35 Revision Date: 7/20/21 Revision No.: MR-235 by dragline, and 55% by truck/shovel. Since all mining at Colowyo was conducted by truck/shovel methods through 1979, the life-of-mine swell factor has continued to increase. Example: 363 feet overburden, 47 feet coal, (these conditions are found along the western edge of the pit in 1988), 20% swell factor 363 feet + 20% swell = final thickness = 1.06 363 feet + 47 feet initial thickness Example: 356 feet overburden, 49 feet coal (these conditions are found along the western edge of the pit in (1988), 20% swell factor 356 feet + 20% swell = final thickness = 1.05 363 feet + 49 feet initial thickness The original permit application utilized at 17% swell factor to project the anticipated postmining topography. During the initial permit review process the anticipated swell factor was subsequently revised to 23% to allow for sufficient pit development. At that time, excess swell was anticipated to raise the elevation of the postmining topography by about 5.3 feet, compared to the premining topography. As explained above, in 1984, as part of the 1983 Annual Report, Colowyo further refined its estimates based on measurements to date, and currently uses an overall 20% swell factor for estimating purposes. As indicated in the Annual Reports, the stripping accomplished by draglines can vary from 40% to 45% and from 55% to 60% for truck/shovel. Swell factor will continue to be monitored and the postmining topography adjusted, if necessary. Any adjustments will be minor, will be done gradually and will not affect the reclamation plan or postmining land use. Particularly, drainage channel gradients will not be changed; an entire drainage channel elevation could possibly be revised, but the gradient would remain as designed. If a change would be necessary, the dump plan elevations would be revised as appropriate. The mining plan, as described in Section 2.05.3, was a soundly designed and engineered open pit mining plan, which maximized coal conservation and recovery while minimizing adverse environmental impacts. Because of the multi-seam mining configuration used by Colowyo, an exemption from the 180 day or four spoil ridge limitation was and still is necessary. The mining plan was designed as a continuously moving open pit operation with the mine advancing approximately parallel to the dip of the numerous coal seams. The mining operation progressed from a southward direction with shovels/trucks/ proceeding along the entire length of the mining area uncovering the upper coal seams and the draglines uncovering the lower coal seams. With the numerous benches used in an open pit operation, the mine area was opened for some time, and backfill and grading operations are occurring now in the West Pit. As the mining operations remove coal seams (In the southward progression), the mining area must be left open until such time as the lower-most coal seam can be recovered. With the mining configuration, the time differences between mining the upper-most seam versus the lower-most