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Subsidence Evaluation For <br />608 South of Divide Mining Area Page 17 <br />• <br />been caused in part by stresses generated by the West Elk Mountain intrusive bodies- <br />particularly 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 <br />as continuous joints of fractures to the E Seam located several hundreds of feet below. Even if <br />the 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 South of Divide mining area aze fracture controlled, it is <br />extremely unlikely that they extend downwazd to the E Seam through multiple shale, claystone, <br />and siltstone units. Using this conservative evaluation, it is now important to evaluate the <br />potential 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 <br />(Section 4.3) these units undergo continuous flexure (Figure 2, enlazgement 2). Above the <br />neutral surfaces, in zones of convex-upwazd curvature, the material is intension and below them, <br />and the material is in 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 <br />close these fractures more than they were prior to mining and subsidence. <br />After longwall mining is completed in the azea and static conditions aze 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 drainages of the South of Divide <br />mining azea are likely to be minimal or non-existent under even the most conservative <br />assumptions. <br />9.0 WATER AND METHANE <br />Observations of the north and west flanks of Mt. Gunnison during azeal geologic mapping and an <br />October 1996 field trip revealed numerous talus and rock glacier deposits that occur in the <br />valleys and lower part of this intrusive body. Snowmelt and rain can easily infiltrate these <br />deposits, which may eventually enter any permeable rocks, faults, fractures, and joints near the <br />mountain. Coal beds and rocks in the deformed zone azound Mt. Gunnison may also contain <br />increased concentrations of methane where the coal is deformed and perhaps metamorphosed to a <br />higher rank by the intrusive body. Greater quantities of water and methane may therefore be <br />expected as coal is mined closer to Mt. Gunnison. <br /> <br />831-032.620 Wdght Water Engineers, lnc. <br />