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

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• 17.0 SUMMARY AND CONCLUSIONS One of the more important conclusions of this report is that all subsidence parameters (in terms of the ratio of maximum vertical displacement/overburden depth compared with the ratio of mining panel width/overburden depth) determined from the West Elk Mine subsidence monitoring networks fall within the range of those observed during annual observations by the author. The 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 plot favorably on 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: 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 Dry Fork above the western part of 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 longwall panels E2, E3, and E9, are 500 to 600 feet farther away from the reservoir footprint, using even using the most conservative 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 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 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 overburden thickness in the Deep Creek area portion of the Dry Fork mining area is 800 to 1,400 feet and should be affected in the same manner as the previously mined area of Deep Creek. Therefore, any effects on the alluvium due to mining are even less likely to occur. However, in order to compare field results with predicted results in this important stream valley environment (as described in Section 11.3), detailed • subsidence monitoring is recommended above the western limits of longwall panels E2 and E3-an area of minimum overburden and maximum predicted subsidence effects for the South of Divide mining area. Tetra Tech - 090717/P 35