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gp Mine water management <br />A conceptual dewatering system plan layout for the Collom Phase 1 pit is shown on <br />Figure 7.1. A conceptual dewatering section for Option 1 is shown in Figure 7.2. It is <br />anticipated that the dewatering system will include vertical wells and in -pit sumps. <br />Upstream cutoffs in drainages intercepted by the pit will be installed as necessary. <br />Horizontal drains will be used to dewater sections in the pit that do not adequately <br />respond to the dewatering wells. The most likely targets for horizontal drains are the <br />thicker sandstone units that are not adjacent to a major coal bed, such as the <br />F sandstone. <br />Dewatering wells <br />Vertical dewatering wells are included in the Phase 1 box cut as shown in Figure 7.1. <br />The wells typically would be constructed using 6 -inch diameter PVC casing and screen <br />and drilled to about 100 ft below the floor of the box cut. They would be screened <br />across the major coal seams and gravel packed across their entire depth. Each well is <br />expected to produce 10 to 15 gpm, with about two thirds of the production coming from <br />the F/G coal seams and about one third coming 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 would also be produced from units below the G seams. <br />Table 7.3 shows the schedule for production wells based on the conceptual plan in <br />Figure 7.1. <br />The dewatering well layout for Phase 2 mining would be similar to that for Phase 1. <br />C However, since some depressurization of the Phase 2 areas that are adjacent to <br />Phase 1 will occur during the Phase 1 mining, fewer wells may be needed. However, it <br />may be necessary to continue to dewater some of the Phase 1 spoil to prevent inflow of <br />water from the spoil into the Phase 2 pit. <br />Horizontal drains <br />Horizontal drains are planned to depressurize specific zones in the pit slopes that are <br />not adequately depressurized by vertical production wells. The likely targets for <br />horizontal drains are thick sandstone /siltstone /mudstone units that are not located close <br />to the major coal seams. For instance, the F sandstone, which responded slowly to the <br />pilot dewatering test, is vertically fractured and may be more effectively depressurized by <br />horizontal drains than vertical wells. <br />Piezometers (e.g., standpipes and /or vibrating wire transducers) should be installed at <br />strategic locations in the pit slopes to monitor the degree of slope depressurization that <br />is achieved by the dewatering system. These piezometers can be used to determine <br />areas where additional depressurization by horizontal drains may be needed prior to <br />mining the areas. <br />• <br />2572 -R2 <br />Colowyo Coal Company <br />Water Management Consultants <br />