2010-12-17_REVISION - C1981019 (127)

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96 Mine water management i� Horizontal drains Horizontal drains will be installed as in the Option 1 dewatering system to depressurize specific zones in the pit slopes that are not adequately depressurized by vertical production wells. Drains are unlikely to be required until the floor of the mine extends below the water table, unless thick perched groundwater zones are encountered in the southern portion of the mine footprint. Piezometers will also be installed at strategic locations in the pit slopes to monitor the degree of slope depressurization that is achieved by the dewatering system. Vertical drains Vertical drains may be used as described in Option 1. However, these drains should be drilled out and sealed prior to placing spoil over them because they will provide a source of upward seepage into the spoil material. Piezometers placed at strategic locations beneath the pit floor should be monitored during dewatering operations to determine if vertical drains are needed. Spoils drains The revised mine plan calls for placement of the mine spoils in the upgradient side of the pit as the working face advances north (downdip). Analyses by CNI indicate that the stability of the mine spoils decrease as the water level within the spoils rises. The height of the water within the spoils is a function of the infiltration from the surface, flux from the • southern headwall, and the hydraulic conductivity of the spoils. Groundwater accumulation in the spoils can be managed using drainage trenches. The trenches would be installed at the bottom of the active pit after the coal is removed and prior to placement of the spoils. The drainage trenches would be approximately 1,000 ft apart and contain a perforated pipe to collect water from the overlying spoils. To facilitate drainage and increase the effective capture area, the trenches would be backfilled with gravel or drain rock. The trenches would be extended as the working face moves downdip. Water from the trenches could be piped or diverted to sumps in the pit. A conceptual plan of the proposed drainage trenches is provide in Figure 7.5. The performance of the drainage trenches should be monitored with a series of piezometers in the spoils. 7.3.3 Summary of dewatering estimates Table 7.6 summarizes the empirically derived average dewatering rate estimates for the dewatering system under the Option 2 mine plan. The estimated production rates are applicable once the mine advances far enough to the north such that the mine floor is about 200 ft below the water table. Production rates will be significantly less during the initial mining on the south side of the footprint, during which there will likely still be seepage into the pit walls similar to that which occurs at the existing Colowyo Mine. 2572 -R2 Colowyo Coal Company Water Management Consultants