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Fall 2008 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br />• 6.0 CONCLUSIONS <br />1. The conceptual model presented in Exhibit 60 has been verified over the past thirteen <br />years of annual field observations in the West Elk Mine area. This basic concept <br />comprises longwall extraction and uniform downwarping of the overburden material as <br />laterally constrained multiple beams (in two dimensions) or plates (in three dimensions). <br />This concept applies in the continuous deformation and near surface zone as discussed in <br />Exhibit 60. <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 thirteen years (1996 to the fall of 2008) verify this conceptual <br />model in bedrock and surficial material (colluvium, alluvium, mudflow, and debris flow <br />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 them. Cracks in these areas commonly <br />extend completely through sandstones and other brittle units. Any groundwater or <br />surface water present near or within cracks in these areas will likely flow into existing <br />surface drainages. A more uniform downwarping under lateral constraint is expected in <br />the SOD mining area because significantly fewer cliffs and over-steepened slopes occur <br />in that area than in the Box Canyon and Apache Rocks mining areas. <br />i <br />831-032.790 Wright Water Engineers, Inc. Page 65 <br />December 2008