2018-02-05_PERMIT FILE - C1981019

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October 19, 2017 Page 2 thick marine sandstone that represents the marine facies (beach) of the delta front. The high- quality, low -sulfur coal seams present in the Project Area occur within the Fairfield coal group of the Williams Fork Formation, which conformably overlies the Trout Creek Sandstone Member of the Iles Formation. A total of 13 coal seams occur within the Project Area. In descending order (the order in which they would be mined) they are X3, X4, D1, D2, D12, F6, FA, FB, G7, G8, G9, GA, and GB. These coal seams have been categorized into five composite units: X34, D1 -D2, F -AB, G7 -G9, and G -AB. The X34 unit occurs within the top portions of the ridges at the Collom Pit and averages approximately 9 ft thick. The D1 -D2 unit averages approximately 11 ft thick. The F -AB and G7 -G9 units are thick seams that constitute a large percentage of the coal in the Project Area. The G -AB unit resides at the base of the proposed mining sequence, with variable thickness (Kennecott Energy Company [KEC] 2005). 1.2 Collom Pit Mine Plan Colowyo intends to transition mining operations from the currently active South Taylor Pit to the Collom Pit beginning in approximately 2018. The mineable seams and host strata within the Collom Pit have variable dips of 4 to 14 percent (%), with the proposed maximum pit depth of approximately 750 ft. Subsurface strata in the Collom area are saturated approximately 250 to 300 ft below ground surface. The preliminary box cut will be made on the north side, and mining will advance updip to the south. Following box -cut development, the pit highwall will be advanced upslope at a rate dependent on the required production level at the time the cut is taken. The general layout of the mine is presented in Figure 1. The Collom Pit will be mined using a truck -and -shovel combination in the upper benches and dragline in the lower benches. Steeper overall slopes will be maintained in the lower dragline benches, and flatter overall slopes will be adopted in the upper truck/shovel benches. Given the anticipated presence of saturated strata mostly in the northern half and lower benches of the mine, Colowyo plans to employ extensive dewatering techniques to reduce the ground pore pressure for increased stability of the pit slopes during operations. Towards that end, several studies have been performed to design and assess the efficacy of an optimal dewatering program for the Collom Pit. The current study was initiated to evaluate the applicability of the past study and the soundness of its recommendations, beyond which this study aimed to develop a new dewatering program that was integrated with the mine plan from the Collom Pit. Agapito Associates, Inc.