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West Elk Mine <br />r2. This area is located within USFS land beneath a perennial stream near the headwaters of <br />Huntington Canyon. The Skyline Mine, recently started a third panel beneath the stream at an <br />average minimum depth of approximately 600 feet. The longwall mining panel width is about <br />700 feet, which makes the depth roughly critical. The subsidence factor reportedly ranges from <br />about 0.5 to 0.8. Mining just recently began in the third panel beneath the perennial stream, so <br />all information should be considered preliminary. No impact on stream base flow has been <br />measured thus far, however, changes in the transverse stream channel profile and some other <br />hydrologic parameters have been observed (USFS, 1995). <br />The E Seam overburden thickness varies from approximately 400 feet to 1,100 feet over the three <br />west longwall panels (i.e. the 12NW through 13aNW longwall panels) from which the E Seam, as <br />well as the B Seam, will be mined. However, less than one percent of the mining area has an <br />overburden depth of less than 500 feet. Overburden thicknesses between 800 and 2,250 feet exist <br />over the four eastern B Seam panels (i.e., the 14SE through 17SE longwall panels). Minimum <br />overburden thickness above the NE longwall panels is 600 feet. Within the Box Canyon permit <br />revision area, the overburden thickness is between 500 and 2,200 feet. No portion of the proposed <br />mining area has an overburden depth of less than 500 feet. Minimum overburden thickness above <br />the NE longwall panels is 600 feet. These considerable overburden thicknesses greatly reduce the <br />potential of connection between subsidence -induced surface cracks and the mine workings. <br />The Utah experience indicates that when overburden thickness is at least 500 feet, there was no <br />disruption of streamflows. Most of the observed impacts occurred in areas where the overburden <br />thickness was less than 300 feet. Although the South of Divide mining area does have stream <br />reaches with overburden. thicknesses less than 500 feet, site-specific data was used to evaluate these <br />channels. <br />As indicated in Section 2.05.6 (6) (e) (i) (C&D), Depth of Surface Cracks, subsidence -induced <br />surface cracks, to the degree they develop, will generally extend no deeper than 25 to 35 feet. <br />Combining the maximum expected surface crack depth (50 feet) with the maximum predicted <br />caved and fracture zone height (280 feet), reveals that a minimum overburden depth of 330 <br />feet should be sufficient to ensure that surface water does not reach the mine workings. Map <br />14 and Map 19 show the B and E Seam overburden thicknesses for the West Elk Mine coal lease <br />area. To establish a connection between the near surface zone and the caved zone, the overburden <br />in the proposed permit revision area needs to be less than 330 feet thick under the worst conditions. <br />The overburden in the SOD mining area is 375 feet or more. Consequently, there will always be a <br />zone, which deforms but does not experience cracking from either the mine workings up or from the <br />surface down. When looking at a B Seam plan view of the Apache Rocks and Box Canyon permit <br />revision areas, it is clear that less than 6 percent of the total surface area has overburden thicknesses <br />of less than 470 feet and less than 3 percent has overburden thicknesses of less than 370 feet. <br />Within the vicinity of the IONE and l INE longwall panels, overburden ranges from 600 to 1,200 <br />feet; therefore, there is no area with less than 470 feet of overburden thickness. it should be noted <br />that these percentages drop to zero relative to the areas Influenced by a I _ i t a in the Box <br />Canyon permit revision area. <br />• For a connection to be established, a surface crack will actually need to occur. As previously <br />discussed, in 2.05, the probability of a surface crack at any given location over a pillar is only 0.2 <br />2.05-193 Revised JUM 2005 PRIG, Janiarn 2006, March 2006: Rev. May 2006 PRI Q Alov. 2006 TR107: Sep. 2007 PR12:Feb 2008 PR12 <br />