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94 Mine water management <br />C, <br />0 7.3.2 Conceptual dewatering design <br />The hydrogeological factors for dewatering design discussed under Option 1 in <br />Section 7.2.2 also apply to Option 2. Numerical modeling of pit wall depressurization for <br />Option 2 was not conducted. However, it is anticipated that the depressurization <br />conditions on the advancing wall (north wall) will be similar to those predicted for the <br />highwall under Option 1 as described in Section 6. As in Option 1, it is considered that <br />depressurization to a depth of 100 to 150 ft below the floor of the pit is necessary to <br />prevent excessive floor heave, as shown in Figure 7.4. <br />Based on discussions with CNI, stability of the spoil pile on the south wall of the pit may <br />be an issue if water levels are allowed to build up in the spoil. Therefore, it is assumed <br />that the dewatering system will need to maintain fully drained conditions in the spoil for <br />at least 2,000 ft from the active spoil face. <br />Dewatering wells <br />The mine plan under Option 2 starts on the south side of the footprint area where <br />groundwater conditions are perched. Unlike Option 1, significant groundwater will not be <br />encountered in the initial box cut. Therefore, dewatering wells are not needed to <br />depressurize slopes in advance of mining during the initial two years. As the mine <br />advances to the north, it will encounter saturated conditions in the F/G sequence. For <br />purposes of developing a preliminary dewatering plan, it is planned that wells will be <br />installed in the advancing wall once the piezometric level in the HG sequence is about <br />100 ft above the Gb seam. As shown in Figure 7.3, this condition is expected to occur <br />first on the southeast side of the footprint based on the existing water level data. <br />Vertical dewatering wells are included (as shown in Figures 7.3 and 7.4) as the mine <br />advances north. The wells typically would be constructed using 6 -inch diameter PVC <br />casing and screen and drilled to about 100 to 150 ft below the floor of the box cut. They <br />would be screened across the major coal seams and gravel packed across their entire <br />depth. Each well is expected to produce 5 to 10 gpm until the mine advances far <br />enough north to where the piezometric heads in the F/G sequence are about 200 ft <br />above the Gb seam (i.e., about to the fourth row of dewatering wells to the north as <br />shown on Figure 7.3). After that, it is estimated that each well will produce about 10 to <br />15 gpm. As discussed in Section 7.2.2, about two thirds of the production is expected to <br />come from the F/G coal seams and about one third from the D and E coal seams. Only <br />minor amounts of water are anticipated to come from perched groundwater zones above <br />the D seams. Minor flows will also be produced in units below the G seams. Table 7.5 <br />shows the schedule for production wells based on the conceptual dewatering plan in <br />Figure 7.3 and the conceptual dewatering section in Figure 7.4. <br />2572 -R2 <br />Colowyo Coal Company <br />Water Management Consultants <br />