2008-03-21_PERMIT FILE - C1980007A (8)

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• 4. Mining impacts on rockfalls were not observed during annual subsidence observations in the Apache Rocks and Box Canyon mining areas over the last six years (1999 to 2004 inclusive), in areas where the estimated rockfall potential was moderate to high. The highest estimated rockfall potential is classified as moderate to high in the South of Divide mining area, and many of these are located in areas where no mining is currently planned. Consequently, the natural rockfall process is not expected to be significantly accelerated by longwall mining. Evidence of naturally occurring rockfalls, such as remnant boulders near roads near the base of steep slopes, should be documented prior to mining. It is suggested that signs stating, "Watch for Falling Rock" (similar to signs posted along major highways in high rockfall potential areas), be posted on the more well-traveled roads, such as the Dry Fork and Main Fork of Minnesota Creek roads, when (or before) mining begins in these areas. 5. As discussed in Sections 5.3.2, 11.0, and 15.3, no cracks are predicted to occur in the alluvium in the Dry Fork of Minnesota Creek and its tributaries, in Lick Creek and its tributaries, and in Deep Creek. This prediction is based on annual subsidence observations that no cracks occurred in alluvium during longwall mining in the Apache Rocks area. Therefore, water flow in these streams, and tributaries, and diversion ditches would is not expected to be impacted. However, it is suggested that stream flow be monitored before, and during, and after mining within the area of mining influence and also outside of the area of mining influence in the Dry Fork, Lick Creek, and Deep Creek drainages, in order to verify this prediction. 6. Based on ten years of annual observations in the Jumbo Mountain, Apache Rocks, and Box Canyon mining areas by Dunrud, the landslides that are located north and south of • Minnesota Reservoir and above the northern part of longwall panel E9, are not expected to be noticeable impacted by subsidence or seismic activity caused by longwall mining. Based on field observations during the past ten years, the major finding is that landslide movement occurs in response to moisture and ground saturation, and is not noticeably affected by subsidence or any mine-related seismic activity caused by longwall mining beneath or near the landslides. However as discussed in Section 11. 1, it is recommended that this area be periodically monitored prior to mining, during mining, and after mining to verify predictions, based on the historical evidence. 7. There is no historical record from annual observations in the West Elk Mine area regarding effects of mining on springs and local water-bearing zones, with sources in either surficial material or bedrock. It is therefore recommended that spring and water well monitoring be conducted before, during, and after mining in order to determine any mining effects. 8. Although no cracks are expected in the soft, pliable alluvium in Dry Fork, some cracks are expected to occur on the harder and more highly compacted Dry Fork access road, particularly in the area near the confluence of Deer Creek and Dry Fork. As discussed previously, the range of maximum vertical displacement, tilt, and horizontal strain is projected to be 8.4 to 11.2 feet, 1.3 to 6.4 percent, and 0.7 to 4.2 percent (respectively) in the North Fork stream valley and road above the solid coal boundaries at western limits of longwall panels E2 and E3. 9. The extension of mining into the Dry Fork mining area is expected to minimally impact the surface. The mining thickness is generally less (9 feet and 10 feet) than is the South of divide mining area and the minimum overburden is thicker (800 feet). The maximum vertical displacement ranges from 6.2 to 7.0 feet, tilt from 1.0 to 1.9 percent, and horizontal strain • from 0.6 to 1.0 percent. Thus impacts should be minimal. Tetra Tech - 090717/P 36