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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. Under the longwall mining methods 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 therefore in compression. This has an important bearing <br />on the hydrologic consequences of longwall mining. Any groundwater or surface water in <br />contact with a given subsidence crack is prevented from traveling downward beyond the <br />neutral surface of the deformed plate. Annual field observations from 1996 to spring 2018 <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. 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 to no lateral <br />support in at least one direction, which causes the associated rocks and unconsolidated <br />materials to deform as unconstrained beams, plates, or cantilevers as the longwall mining <br />faces move beneath them. This lack of lateral constraint allows subsidence cracks to <br />commonly extend completely through sandstones and other brittle units, and groundwater <br />or surface water present near or within these cracks will likely flow through and exit into <br />existing surface drainages. The relatively few cliffs and over -steepened slopes in the SOD <br />and Dry Fork mining areas tend to provide the lateral constraint needed to produce a more <br />uniform down -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 20 <br />November 2016 <br />