Laserfiche WebLink
Spring 2013 Subsidence and Geologic Field Observations <br />South of Divide and Dry Fork Mining Areas (E -Seam) <br />5.0 CONCLUSIONS <br />1. The conceptual B- and E -seam mining model presented in the Exhibit 60 series of the <br />mining permit and in Appendix A of this report has been verified over the past eighteen <br />years of annual field observations in the West Elk Mine area. Under this concept of <br />uniform longwall extraction and related uniform downwarping of the overburden rocks and <br />unconsolidated material as laterally constrained plates, cracks in zones under tensile stress <br />narrow with depth, and close at the neutral surface. Below the neutral surface, the materials <br />are in compression. This has an important bearing on the hydrologic consequences of <br />longwall mining. Any groundwater or surface water in contact with a given crack area is <br />prevented from traveling downward beyond the neutral surface of the deformed plate. <br />Annual field observations over the last eighteen years (1996 to spring 2013 inclusive) <br />verify this conceptual model in bedrock and surficial material (colluvium, alluvium, <br />mudflow, and debris flow deposits) where the overburden is laterally constrained. <br />2. Uniform downwarping under lateral constraint does not apply to steep slopes and cliff areas <br />where there is little to no lateral constraint. The rocks and unconsolidated material in cliff <br />areas deform as unconstrained beams, plates, or cantilevers as the longwall mining faces <br />move beneath them. Cracks in these areas commonly extend completely through <br />sandstones and other brittle units. However, any groundwater or surface water present near <br />or within cracks in these areas will likely flow through and exit into existing surface <br />drainages. <br />Observations in the SOD and Dry Fork mining area find a more uniform downwarping <br />under lateral constraint due to significantly fewer cliffs and over - steepened slopes. <br />3. The maximum cumulative horizontal tensile strain in the SOD and Dry Fork mining areas <br />is expected to be less than or equal to measured values obtained from the Apache Rocks <br />mining area (see detailed discussion in Spring 2011 Subsidence Report). <br />831 - 032.795 Wright Water Engineers, Inc. Page 33 <br />August 2013 <br />