2011-02-14_PERMIT FILE - C1980007A (3)

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Subsidence Evaluation for the Exhibit 60E South of Divide and Dry Fork Mining Areas ?age 22 8.0 FRACTURE- CONTROLLED DRAINAGES Based on mapping by Dunrud in the Somerset area and the South of Divide mining area, Dunrud believes that there is reasonably good, but certainly not conclusive, evidence that some drainages are controlled by fractures and/or joints. The Dry Fork of Minnesota Creek and some of its tributaries exhibit linear trends on satellite images and on high - altitude photographs that indicate, or at least suggest, fracture control ( Dunrud 1976, p. 14 -15). These fractures may have been caused in part by stresses generated by the West Elk Mountain intrusive bodies — particularly Mt. Gunnison. The conservative approach may be to assume that the drainage system is fracture controlled. However, even if fractures control the present drainage system, they may not extend downward as continuous joints of fractures to the E -seam located several hundreds of feet below. Even if the fractures were present in the more brittle sandstone units, it would be very unlikely that these fractures would occur in the softer siltstone and shale units. Even under the conservative approach that the drainages in the South of Divide and Dry Fork mining areas are fracture controlled, it is extremely unlikely that they extend downward to the E -seam through multiple shale, claystone, and siltstone units. Using this conservative evaluation, it is now important to evaluate the potential impact that subsidence may have on any pre - mining fractures. Evaluation of subsidence due to downwarping of laterally constrained strata shows, as stated previously, that rock strata with different deformation and strength characteristics deform as discrete units. For example, strata of shale and sihstone behave as units discrete from sandstone. Above the fractured zone (Section 4.2) and within the continuous deformation zone (Section 4.3) these units undergo continuous flexure (Figure 2, enlargement 2). Above the neutral surfaces, in zones of convex -upward curvature, the material is in tension and below them, and the material is in compression. Consequently, stresses change across neutral surfaces from tension to compression across each successive rock unit that deforms as a plate. Fractures already present would thus tend to open more in the zones of tension, but would close more in the zones of compression, which would close these fractures more than they were prior to mining and subsidence. After longwall mining is completed in the area and static conditions are attained, the zones of tension and compression commonly cease, and any fractures present will likely resume the pre - mining condition. Therefore, the impacts on surface flow in the drainages of the South of Divide and Dry Fork mining areas are likely to be minimal or non - existent under even the most conservative assumptions. 831 - 032.810 Wright Water Engineers, Inc.