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Subsidence Evaluation For <br />Exhibit 60 The Apache Rocks Antl The Box Canyon Mining Areas Page 20 <br />• fractures have been caused in part by stresses generated by the West Elk Mountain intrusive <br />bodies-particularly Mt. Gunnison. Section 2.04.6 (Geology Description) includes additional <br />discussion and references relating to the nature and continuity of fractures. <br />The conservative approach may be to assume that the drainage system is fracture controlled. But <br />even if fractures control the present drainage system, they may not extend downwazd as <br />continuous joints of fractures to the B-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 <br />approach that the drainages in the Box Canyon permit revision area aze fracture controlled, it is <br />extremely unlikely that they extend downwazd to the B-Seam through multiple shale 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 that rock <br />strata with different deformation and strength characteristics deform as discrete units. For <br />example, strata of shale and siltstone behave as units discrete from sandstone. Above the <br />fractured zone (Section 4.2) and within the continuous deformation zone (Section 4.3) these units <br />undergo continuous flexure (Figure 1, enlargement 2). Above the neutral surfaces, in zones of <br />convex-upward curvature, the material is in tension and below them, the material is in <br />compression. <br />Consequently, stresses change across neutral surfaces from tension to compression with 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 and close more in the zones of compression than would be the case <br />for these fractures prior to mining and subsidence. The Box Canyon and unnamed drainage west <br />of Box Canyon channels are above the maximum estimated height of the fracture zone (i.e., 20t <br />or 240 feet fora 12-foot coal-extraction thickness). <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 pre- <br />mining condition. Therefore, the impacts on surface flow in the drainage of~ the Box Canyon <br />permit revision area are likely to be minimal or non-existent under even the most conservative <br />assumptions. <br />8.0 WATER AND METHANE <br />Observations of the north and west flanks of Mt. Gunnison during an October 1996 field trip <br />revealed numerous talus and rock glacier deposits that occur in the valleys and lower part of this <br />intrusive body. Snow melt and rain can easily infiltrate these deposits, which may eventually <br />• enter any permeable rocks, faults, fractures, and joints near the mountain. Coal beds and rocks in <br />831-032.181 Wright Water Engineers, Inc. <br />