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 Page 33 depth from 8.4 to 11.2 feet, are projected in this area. Maximum changes in slope (tilt), ranging from 1.3 to 6.4 percent, are also projected for this area. The maximum horizontal tensile and compressive strain is projected to range from 0.7 to 4.2 percent. Subsidence depressions and slope changes will be less above the gate road pillars than above solid coal barriers, because they are projected to yield during mining by as much as 4 feet (Table 2). No subsidence depressions or changes in stream gradient were observed in Deep Creek, located about 1,050 feet above mined B -seam longwall panel 17 during the annual traverse in July 2004 (Dunrud 2004a). There was no observable change in stream gradient or in stream flow. The depression and change in gradient were apparently sufficiently gradual, so as to not be perceived by Dunrud during his traverse along the trail by the stream 11.3.1 Potential for Hydraulic Connection Between Mine Workings and Surface Near the southwest corner of E -seam longwall panel E2, the Dry Fork channel encounters a short reach where the E -seam overburden is less than 400 feet, with a minimum of 375 feet. A prudent concern is whether mining induced subsidence could establish a hydraulic connection between the Dry Fork stream channel and the mine workings. To address this scenario, the maximum projected height of fracturing and the maximum depth of surface cracks were considered. As discussed in Section 5.2, the effective height of fracturing in the South of Divide and Dry Fork mining areas is estimated to range from 9t to 18t, or a maximum fracture height of 252 feet for a mining height of 14 feet. However, Peng (1992) states that the upper one -third of the fractured • zone has only minor fractures with little potential for water conductivity. Therefore, the height of the fractured zone capable of transmitting water would be two - thirds of the 18t, or 168 feet. The maximum height of the caved zone is projected to be 5t, or 70 feet, for the South of Divide and Dry Fork mining areas. When added to the effective fracture zone height of 168 feet, the combined heights of the caved and fracture zones capable of transmitting water is projected to be a maximum of 238 feet. As discussed in Section 5.3.2, the maximum crack depth in the South of Divide and Dry Fork mining areas is estimated to be 15 feet in terrains with slopes less than 30 percent, with depths up to 35 feet occurring locally in steep topography. For the Dry Fork channel near the western edge of panel E2, the maximum projected crack depth is 15 feet. Consequently, the combined maximum height of the caved and fractured zones and the maximum crack depth is 253 feet. Even at the minimum overburden thickness of 375 feet, an estimated 122 feet of unfractured bedrock will remain intact. In addition, the presence of soft shales and claystones in the E -seam overburden will increase the�probability that the strata will warp rather than fracture during the subsidence process. 11.4 Springs, Water - Bearing Zones, and Ground Water Wells MCC's Spring and Stock Pond Location Map for the South of Divide mining area shows one decreed spring (Spring 21), three springs found flowing at every site visit (in 1975, 1977, 1979, and 1980), thirty -three intermittent springs, and two groundwater wells that are currently monitored. The same map indicated seven perennial springs in the Dry Fork mining area. Only a few springs in the West Elk Mine area indicate a source from a local bedrock water - bearing 831 - 032.810 Wright Water Engineers, Inc.