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Fall 2015 Subsidence and Geologic Field Observations <br /> South of Divide and Dry Fork Mining Areas <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 has been verified by annual field observations in the various West Elk Mine <br /> mining areas. With the use of longwall mining methods where the uniform downwarping <br /> of the overburden rocks and unconsolidated material act as laterally constrained plates, <br /> cracks in zones under tensile stress narrow with depth, and close at the neutral surface. <br /> Below the neutral surface, the materials are therefore in compression. This has an <br /> important bearing on the hydrologic consequences of longwall mining. Any groundwater <br /> or surface water in contact with a given subsidence crack is prevented from traveling <br /> downward beyond the neutral surface of the deformed plate. Annual field observations <br /> from 1996 to spring 2018, inclusive, verify this conceptual model in bedrock and surficial <br /> material (colluvium, alluvium, mudflow, and debris flow deposits) where the overburden <br /> is laterally constrained. <br /> 2. Typically, uniform downwarping occurs in association with longwall mining when there <br /> is lateral constraint. Where there are steep slopes and cliffs,there is little lateral support in <br /> at least one direction, which causes the associated rocks and unconsolidated materials to <br /> deform like unconstrained beams,plates, or cantilevers as the longwall mining faces move <br /> beneath them. This lack of lateral constraint allows subsidence cracks to commonly extend <br /> completely through sandstones and other brittle units, and groundwater or surface water <br /> present near or within these cracks will likely flow through and exit into existing surface <br /> drainages. The relatively few cliffs and over-steepened slopes in the Southern Panels <br /> Mining Area tend to provide the lateral constraint needed to produce a more uniform down- <br /> warping with fewer significant subsidence cracks observable at the surface. <br /> 3. To date,there have been no observed or reported water losses associated with the longwall <br /> mining activities. <br /> 4. Continuous annual observations find substantial weathering of previously observed <br /> subsidence cracks with edges rounding, widths reducing, and depths filling with eroded <br /> 831-032.798 Wright Water Engineers, Inc. Page 29 <br /> November 2016 <br />