2018-02-05_PERMIT FILE - C1981019

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October 19, 2017 Page 16 Modeling results indicated that six perimeter wells (four on the lowwall and one each on the sidewalls) are likely to provide adequate depressurization to the box cut, in conjunction with four to five dewatering wells on the highwall that will be mined through as the highwall advances. Two alternate scenarios were simulated for the perimeter dewatering wells: first, with five wells immediately south of the box -cut highwall crest and second, with four wells two cuts south of the box -cut highwall crest. The recommended pumping well locations for both configurations are presented in Figures 4a and 4b. The coordinates, depths, and screened lengths of the wells for Scenario 1 and Scenario 2 are presented in Tables 5 and 6, respectively. Wells may be located within a 100 ft radius of the given locations for operational convenience. The screened depths and borehole lengths were chosen to provide a minimum of 15 ft of water between the pump intake and the bottom of the lowest dewatered layer and 25 ft between the bottom of the borehole and the bottom of the pump motor. (The screen is carried to within 5 ft of the bottom of the well.) The lowest layer dewatered was located more than 100 ft below the bottom of the GB coal seam. So long as the water table is drawn down more than 15 to 20 ft below the mining floor, floor heave is unlikely.) For both scenarios, dewatering can begin in all pumping well locations 3 years after the start of box -cut development (at the start of the 4th year). In Scenario 1, the five upslope dewatering wells will be lost during the tenth year of mining, whereas in the second scenario, the four upslope pumping wells will function for two additional years. For both scenarios, the lowwall and sidewall pumping well locations remain constant and will continue to function for the life of the mine. All the simulated pumping wells are assumed to be screened from the top of Layer 4 (top of E2 Seam) to the bottom of Layer 13 (top of I3 Seam) to maximize dewatering potential. The predictive analyses results for both the selected dewatering scenarios follow. The pre -mining and pre -pumping water table is graphically presented in Figure 5. The effects of dewatering on the Collom Pit for Scenario 1, along a selected north -south vertical section through dewatering well #2 are presented in Figures 6a—k. This represents the optimal lag of 3 years between the start of mining and the onset of dewatering. Results along all transverse and longitudinal sections crossing the pit were reviewed; however, for brevity, only results along this section are presented. Box -cut development progress and corresponding water level at the end of 3 years of mining is presented in Figure 6a, before pumping has been initiated. The pit bottom and pit slopes at this stage are above the zone of saturation and are unlikely to be impacted by the prevailing water head in the strata underlying the box cut. Figures 6b, 6c, and 6d display the progress of the box -cut development at the end of years 5, 7 and 9, along with the concurrent lowering of the water table and pressure heads due to the dewatering that was initiated at the end of the 3`d year of mining. Figure 6e presents the pit configuration at the end of the 10th year of mining, after which the upslope pumping wells (#6, 7, 8, 9, and 10) are assumed to be destroyed during stripping. Figures 6f, 6g, 6h, 6i, 6j, and 6k represent snapshots of the pit geometry and the state of the corresponding dewatering at the end of years 15, 20, 25, 30, 35, and 38, respectively. In all the analyzed periodic intervals of mining, pre- and post-box cut, the water table has been drawn below thepit floor. Even after the loss of the row of five pumping wells upslope of the highwall in the 10th year of mining, the functioning pumping wells in the lowwall and sidewall are likely to be adequate. The dewatering effort is also aided by the fact that pit benches and the pit floor gain elevation as the pit advances to the south. Agapito Associates, Inc.