Laserfiche WebLink
• 8.0 FRACTURE-CONTROLLED DRAINAGES <br />Based on mapping by Dunrud (2006, p. 16) in the Somerset area and the South of Divide mining <br />area, Dunrud believes that there is reasonably good, but certainly not conclusive, evidence that <br />some drainages are controlled by fractures and/or joints. The Dry Fork of Minnesota Creek and <br />some of its tributaries exhibit linear trends on satellite images and on high-altitude photographs that <br />indicate, or at least suggest, fracture control (Dunrud 1976, p. 14-15). These fractures may have <br />been caused in part by stresses generated by the West Elk Mountain intrusive bodies-particularly <br />Mt. Gunnison. <br />The conservative approach may be to assume that the drainage system is fracture controlled. <br />However, even if fractures control the present drainage system, they may not extend downward as <br />continuous joints of fractures to the E Seam located several hundreds of feet below. Even if the <br />fractures were present in the more brittle sandstone units, it would be very unlikely that these <br />fractures would occur in the softer siltstone and shale units. Even under the conservative approach <br />that the drainages in the South of Divide and Dry Fork mining areas are fracture controlled, it is <br />extremely unlikely that they extend downward to the E Seam through multiple shale, claystone, and <br />siltstone units. Using this conservative evaluation, it is now important to evaluate the potential <br />impact that subsidence may have on any pre-mining fractures. <br />Evaluation of subsidence due to downwarping of laterally constrained strata shows, as stated <br />previously,. that rock strata with different deformation and strength characteristics deform as <br />discrete units. For example, strata of shale and siltstone behave as units discrete from sandstone. <br />Above the fractured zone (Section 4.2) and within the continuous deformation zone (Section 4.3) <br />these units undergo continuous flexure (Figure 2, enlargement 2). Above the neutral surfaces, in <br />• zones of convex-upward curvature, the material is in tension and below them, and the material is in <br />compression. <br />Consequently, stresses change across neutral surfaces from tension to compression across each <br />successive rock unit that deforms as a plate. Fractures already present would thus tend to open <br />more in the zones of tension, but would close more in the zones of compression, which would close <br />these fractures more than they were prior to mining and subsidence. <br />After longwall mining is completed in the area and static conditions are attained, the zones of <br />tension and compression commonly cease, and any fractures present will likely resume the <br />premining condition. Therefore, the impacts on surface flow in the drainages of the South of Divide <br />and Dry Fork mining areas are likely to be minimal or non-existent under even the most <br />conservative assumptions. <br />• <br />Tetra Tech - 090717/P 17