Laserfiche WebLink
West Elk Mine <br />j• • Heights of 9 to 11. tunes the coal extraction thickness (9 to 1 lt) are reported where all the rocks <br />consist of soft, plastic shales and claystones. The fractures also commonly close again under <br />lateral vertical compression associated with static conditions, and become impermeable again. <br />Within the South of Divide mining area, fracturing will likely become discontinuous with increasing <br />height because of the alternating sequence of harder and brittle and softer and yielding rocks- Due' <br />to the stratigraphic position of the E Seam, above the 170' to 250' thick Bowie Sandstone, the <br />proportion of soft yielding strata as compared to the hard brittle strata in the fractured zone is higher <br />than for the B Seam mining. The absence of the Bowie Sandstone in the fractured stratum and the <br />high percentage of softer rocks is best illustrated in the Cross-Sections A-A' through F-F'. The <br />height of the fracture zone, therefore, will likely be less, by possibly 10 to 20 percent, than the <br />height predicted for the Apache Rocks and Box Canyon mining areas because of the presence of <br />more shale. Steeply dipping fractures near the top of the caved zone, therefore, will likely become <br />less continuous with increasing height in the zone of fracturing. <br />The maximum height of fracturing above longwall panels in the South of Divide and Dry Fork <br />muting areas are estimated to range from about 10 to 20 times the extraction thickness. This is near <br />the rnid-range of 9 to 30 tunes coal extraction thickness as reported by Peng (1992, p. 7). This <br />estimate may be conservative for rocks above the E Seam. <br />Also, with increasing height in this zone, and as lateral and vertical constraints increase, <br />fracturing that could impact water-bearing zones will tend to occur more in zones of convex <br />upward curvature, along separated bedding planes toward the center of the panel, and along local <br />cracks in zones of convex downward curvature (Figure 2, Exhibit 60B). Fracturing within the <br />expected zone of fracture may cease completely where soft shales and claystones occur as <br />alternating sequences with sandstones. <br />Mr. Dunrud has concluded that the maximum height of fracturing above longwall panels in the B- <br />Seam in the Apache Rocks mining area is estimated to range from about 15 to 20 times the <br />extraction thickness (t) (for example, if t = 12 feet, the maximum fracture height would be 240 feet <br />at 200 near the inid-range of 9 to 30 times coal extraction thickness. This estimate is viewed as <br />conservative by n-. Dunrad because rocks above the B Seam and below the Marine Sandstone, that <br />underlies the D Seam, consist of about 150 to 200 feet of laminated sandstone and shale and sandy <br />shale and sandstone. <br />Drainage, however, may cease after mining is complete and any water-bearing zones present <br />may be restored. This is particularly likely in the upper part of the fractured zone in shale <br />sequences between sandstone layers, once subsidence is completed and the separated beds re- <br />compress and close in response to overburden load (see Exhibit 60B, Figure 2). Evidence of <br />restored water levels has been measured and reported in at least one well (SOM 38-H-1) in the <br />West Elk Mine subsidence monitoring area after mining and subsidence were complete. <br />E?6F EfFbb?EF? ?e F1rFF8ation Zone and Aleat -Surfirce Zaire <br />These two zones are discussed together because the ground surface is where nearly all <br />measurements are made that monitor subsidence processes active in the zone of continuous <br />deforination. The near-surface zone, which typically consists of weathered bedrock, colluviums, <br />and soil ranging in depth from a few feet to a few tens of feet, may deform differently than the <br />2.05-125 Revised June 2005 PRIG, Rev. March 2006; A•]a), 2006 PRIG, Nov. 2006TR107,April 2007TRIO8.•Sep. 2007 PR12, Feb. 2008 PR-12