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 44 17.0 SUMMARY AND CONCLUSIONS The measured subsidence parameters over the B -seam and E -seam longwall panels at the West Elk Mine has fallen within the range of predicted subsidence parameters developed from the original subsidence data collected over the Northwest B -seam longwall panels. In addition, the annual subsidence field studies have observed subsidence effects at the expected locations and consistent with the projections in the Exhibit 60 series. The subsidence parameters also fall within the range of those measured and calculated in four different coal mining areas by Dunrud in Colorado, Utah, and Wyoming ( Dunrud 1987). The subsidence parameters also are consistent with the appropriate National Coal Board graphs (Figures 4 and 5). Results of subsidence measurement and analysis in the West Elk subsidence monitoring area also are in general agreement with the computer modeling programs developed in the Eastern United States. These favorable comparative results calibrated by West Elk subsidence measurements, therefore, give added assurance that the subsidence parameters projected for the South of Divide and Dry Fork mining areas are realistic and correct. Specific conclusions are as follows: 1. Maximum vertical displacement (subsidence), tilt, and horizontal strain predicted for the South of Divide and Dry Fork mining areas are likely to be conservative values. Based . on annual observations of subsidence effects in the Apache Rocks mining area, maximum vertical displacement, tilt, and strain values may be less than those projected in Section 5.3 and Tables 2 and 3. However, it is recommended that subsidence measurements be made in the Dry Fork drainage above the western part of E -seam longwall panels E2 and E3, in order to compare predicted results (Table 3) with field measurements. 2. Mining of the nine planned longwall mining panels in the South of Divide and Dry Fork mining areas will not impact Minnesota Reservoir. The reservoir is completely out of the area of mining influence. All mine workings, including E -seam longwall panels E2, E3, and E9, are 500 to 600 feet farther away from the reservoir footprint, using even a 45- degree angle of draw. 3. No cracks are expected to occur in the alluvium in any of the drainages of the Dry Fork of Minnesota Creek, Lick Creek, or Deep Creek. As discussed in Section 5.3.2 of this report, no cracks have been observed during annual field visits by Dunrud in the alluvium of drainages, such as Deep Creek and Sylvester Gulch, when longwall mining occurred below. The alluvium in Deep Creek and Sylvester Gulch above B -seam longwall panels 15 and 16, which ranges from an estimated 25 to 150 feet thick beneath the stream channel, was subjected to maximum tilt and strain, because it was within the area of influence of the eastern boundaries of these panels. The alluvium in Dry Fork and its tributaries, in the main fork (north fork) of Lick Creek and the upper reaches of Deep Creek is estimated to have a similar thickness range in lower Deep Creek and Sylvester Gulch. Although the overburden thickness range of 375 831 - 032.810 Wright Water Engineers, Inc.