PERMFILE100540 - Laserfiche WebLink

Home Browse Search

West Elk Mine • • Heights of 9 to 11 times the coal extraction thickness (9 to l lt) aze reported where all the rocks consist of soft, plastic shales and claystones. The fractures also commonly close again under lateral vertical compression associated with static conditions, and become impermeable again. Within the South of Divide mining area, fracturing will likely become discontinuous with increasing height because of the alternating sequence of hazder and brittle and softer and yielding rocks. Due to the stratigraphic position of the E Seam, above the 170' to 250' thick Bowie Sandstone, the proportion of soft yielding strata as compared to the hard brittle strata in the fractured zone is higher than for the B Seam mining. The absence of the Bowie Sandstone in the fractured stratum and the high percentage of softer rocks is best illustrated in the Cross-Sections A-A' through F-F'. The height of the fracture zone, therefore, will likely be less, by possibly 10 to 20 percent, than the height predicted for the Apache Rocks and Box Canyon mining azeas because of the presence of more shale. Steeply dipping fractures near the top of the caved zone, therefore, will likely become less continuous with increasing height in the zone of fracturing. Also, with increasing height in this zone, and as lateral and vertical constraints increase, fracturing that could impact water-bearing zones will tend to occur more in zones of convex upwazd curvature, along separated bedding planes towazd the center of the panel, and along local cracks in zones of convex downward curvature (Figure 2, Exhibit 60B). Fracturing within the expected zone of fracture may cease completely where soft shales and claystones occur as alternating sequences with sandstones. Mr, Dunrud has concluded that the maximum height of fracturing above longwall panels in the B-Seam in the Apache Rocks mining area is estimated to range from about 15 to 20 times the extraction thickness (t) (for example, if t =12 feet, the maximum fracture height would be 240 feet at 20t) near the mid-range of 9 to 30 times coal extraction thickness. This estimate is viewed as conservative by Mr. Dunrud because rocks above the B Seam and below the Marine Sandstone, that underlies the D Seam, consist of about 150 to 200 feet of laminated sandstone and shale and sandy shale and sandstone. The maximum height of fracturing above longwall panels in the South of Divide mining area is estimated to range from about 10 to 20 times the extraction thickness. This is near the mid-range of 9 to 30 times coal extraction thickness as reported by Peng (1992, p. 7). This estimate may be conservative for rocks above the E Seam. Drainage, however, may cease after mining is complete and any water-bearing zones present may be restored. This is particularly likely in the upper part of the fractured zone in shale sequences between sandstone layers, once subsidence is completed and the separated beds re-compress and close in response to overburden load (see Exhibit 60B, Figure 2). Evidence of restored water levels has been measured and reported in some wells in the West Elk Mine subsidence monitoring area after mining and subsidence were complete. Continuous Deformation Zone and Neaz-Surface Zone These two zones aze discussed together because the ground surface is where nearly all measurements aze made that monitor subsidence processes active in the zone of continuous deformation. 1.05-1l1 RevrsedJiore 2005 PRIO