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Fall 2009 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br /> <br />6.0 CONCLUSIONS <br />1. The conceptual model presented in Exhibit 60 and in Appendix A of this report has been <br />verified over the past fourteen years of annual field observations in the West Elk Mine <br />area. This basic concept comprises longwall extraction and uniform downwarping of the <br />overburden material as laterally constrained multiple beams (in two dimensions) or plates <br />(in three dimensions). This concept applies in the continuous deformation and near <br />surface zone as discussed in Exhibit 60 and Appendix A. <br />Under this concept of uniform longwall extraction and related uniform downwarping of <br />the overburden rocks and unconsolidated material as laterally constrained plates, cracks in <br />zones under tensile stress narrow with depth, and close at the neutral surface. Below the <br />neutral surface, the materials are in compression. <br />This concept has an important bearing on the hydrologic consequences of longwall <br />mining. Any groundwater or surface water in contact with a given crack area is prevented <br />from traveling downward beyond the neutral surface of the deformed plate. Annual field <br />observations over the last fourteen years (1996 to the spring of 2009) verify this <br />conceptual model in bedrock and surficial material (colluvimn, alluvium, mudflow, and <br />debris flow deposits) where the overburden is laterally constrained. <br />2. However, this concept of uniform downwarping under lateral constraint does not apply to <br />steep slopes and cliff areas where there is low to no lateral constraint. The rocks and <br />unconsolidated material in cliff areas deform as unconstrained beams, plates, or <br />cantilevers as the longwall mining faces move beneath them. Cracks in these areas <br />commonly extend completely through sandstones and other brittle units. Any <br />groundwater or surface water present near or within cracks in these areas will likely flow <br />into existing surface drainages. A more uniform downwarping under lateral constraint is <br />expected in the SOD mining area because significantly fewer cliffs and over-steepened <br />slopes occur in that area than in the Box Canyon and Apache Rocks mining areas. <br />831-032.791 <br />April 2010 <br />Wright Water Engineers, Inc. <br />Page 73