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• <br />6.0 CONCLUSIONS <br />Spring 2011 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <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 fifteen <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 <br />and unconsolidated material as laterally constrained plates, cracks in zones under tensile <br />stress narrow with depth, and close at the neutral surface. Below the neutral surface, the <br />materials are in compression. This has an important bearing on the hydrologic <br />consequences of longwall mining. Any groundwater or surface water in contact with a <br />given crack area is prevented from traveling downward beyond the neutral surface of the <br />deformed plate. Annual field observations over the last fifteen years (1996 to spring 2011 <br />inclusive) verify this conceptual model in bedrock and surficial material (colluvium, <br />alluvium, mudflow, and debris flow deposits) where the overburden is laterally <br />constrained. <br />2. Uniform downwarping under lateral constraint does not apply to steep slopes and cliff <br />areas where there is little to no lateral constraint. The rocks and unconsolidated material <br />in cliff areas deform as unconstrained beams, plates, or cantilevers as the longwall mining <br />faces move beneath them. Cracks in these areas commonly extend completely through <br />sandstones and other brittle units. Any groundwater or surface water present near or <br />within cracks in these areas will likely flow into existing surface drainages. <br />Observations in the SOD mining area find a more uniform downwarping under lateral <br />constraint due to significantly fewer cliffs and over - steepened slopes. <br />3. The maximum cumulative horizontal tensile strain measured at Apache Rocks during the <br />last nine field visits (April 2007 through June 2011) compares very closely to the <br />maximum amount determined in 2000, which was measured shortly after the cracks <br />occurred. The maximum range of horizontal tensile strain measured in 2000 was 0.47 <br />percent, compared with values of 0.45 to 0.48 percent measured and calculated for the <br />831 - 032.793 <br />September 2011 <br />Wright Water Engineers, Inc. Page 44 <br />