2014-06-19_PERMIT FILE - C1981019A

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July 23, 2013 EXECUTIVE SUMMARY Page iv In a continuation of highwall mining at the Colowyo Mine with the ADDCAR system, Colowyo Coal Company (Colowyo) plans to highwall mine beneath the South Taylor Pit low wall and beneath portions of the final highwall of the West Pit. In support of that effort, Colowyo contracted Agapito Associates, Inc. (AAI), to evaluate the geotechnical design requirements and related operational considerations for the proposed mining. AAI personnel have previously provided highwall mining design criteria to Colowyo, both with AAI and under previous employment with NSA Engineering, Inc. Those design criteria have been applied to the extensive highwall mining that has been performed on the property in the past decade. To date, highwall mining has been successfully accomplished in the E2, X, Dl, D2, and C5 seams. As experience has been gained both at Colowyo and at other western highwall mining operations, the design guidelines have evolved. With this experience as a backdrop, this report addresses highwall mining beneath the South Taylor Pit low wall, and the final highwall in a portion of the West Pit. End wall areas in the South Taylor Pit may also be mined. The current study areas in relation to previous study areas are shown in Figure 1. In the South Taylor Pit, the combined G7 and G8 seams (hereafter referred to as the G78 Seam) are planned to be mined in the southwest portion of the pit. The G78 splits, with parting thickness between the G7 (stratigraphically higher) and G8 increasing to the northeast. It is anticipated that the seams will be mined together until mining the parting becomes untenable due to coal quality concerns. As the G7 is the thicker of the two seams, it will be mined where the parting thickness becomes excessive in the northeast portion of the pit, leaving the G8 Seam in the floor. After mining of the G7 is completed, the pit will be backfilled, and the combined D2 and E seams will be mined in the southwest portion of the pit. Where the D2 /E parting thickness becomes excessive, only the E Seam will be mined in the northeast portion of the pit. It is anticipated that the thicker portions of the combined seams will be mined in two passes (ramping up to the top portion of the seam, backing out, and mining the remainder of the seam, with no material left between the passes). In the West Pit, highwall mining is planned to commence in the F Seam. The pit will be backfilled to reach the successively higher E, D2, and C seams. Given the extensive experience with highwall mining on the property, AAI was asked to develop guidelines for the current study areas using that experience as a basis. Specifically, the following issues and concerns regarding highwall mining applications in the South Taylor and West Pit are addressed in this report: • What specific geotechnical/geological constraints exist within the South Taylor and West Pit areas that may limit highwall mining applications? For each seam to be mined, what minimum web pillar dimension (as a function of depth, mining height, and material properties) will limit the risk that highwall stability is affected by mining operations? • Are interburden thicknesses such that web/barrier pillars should be columnized, or can independent pillar designs be developed where multiple seams will be mined? Agapito Associates, Inc.