2011-02-14_PERMIT FILE - C1980007A (3)

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Subsidence Evaluation for the Exhibit 60E South of Divide and Dry Fork Mining Areas Page 45 • to 800 feet is less than that beneath the lower Deep Creek and Sylvester Gulch (800 to 1,050 feet), the alluvium in the South of Divide mining area contains more Wasatch clays than in Deep Creek and Sylvester Gulch The E -seam overburden thickness of the Deep Creek channel in the Dry Fork mining area is 900 to 1,300 feet, which is comparable to the B -seam overburden thickness of the Deep Creek channel over mined B -seam longwall panels 15, 16, and 17. Therefore, the E -seam mining under the Deep Creek channel should have the same minimal affect as occurred due to B -seam mining to the north. However, in order to compare field results with predicted results in this important stream valley environment (as described in Section 11.3), subsidence monitoring is recommended above the western limits of E -seam longwall panels E2 and E3 —an area of minimum overburden and maximum predicted subsidence effects for the South of Divide mining area. 4. Mining impacts on rockfalls were not observed during annual subsidence observations in the Apache Rocks and Box Canyon mining areas over 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. It is recommended that evidence of naturally occurring rockfalls, such as remnant boulders near roads near the base of steep slopes, be documented prior to mining. • 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, or in Lick Creek and its tributaries, or 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 streamflow continue to be monitored, particularly before, during, and after mining within the area of mining influence in the Dry Fork, Lick Creek, and Deep Creek drainages, if ground and weather conditions allow, 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 E -seam longwall panel E9, are not expected to be noticeably impacted by subsidence or seismic activity caused by longwall mining. Based on field observations during the past ten years, the major fording 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 these areas be periodically monitored prior to mining, during mining, and after mining to verify predictions. 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 831 - 032.810 Wright Water Engineers, Inc.