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July 23, 2013 <br />1.0 INTRODUCTION <br />Page 1 <br />In a continuation of highwall mining at the Colowyo Mine with the ADDCAR system, <br />Colowyo Coal Company (Colowyo) plans to highwall mine beneath the South Taylor Pit low <br />wall and beneath portions of the final highwall of the West Pit. In support of that effort, <br />Colowyo contracted Agapito Associates, Inc. (AAI), to evaluate the geotechnical design <br />requirements and related operational considerations for the proposed mining. AAI personnel <br />have previously provided highwall mining design criteria to Colowyo, both with AAI and under <br />previous employment with NSA Engineering, Inc. (NSA). These efforts included the E2, D2, <br />and C5 seams in the East Pit (NSA 2003), the X Seam in the West Pit (NSA 2004a), the B Seam <br />in the East Pit (NSA 2004b), the D1 /D2 and E2 seams in the northwest portion of the West Pit <br />(AAI 2005a), and the E2 and F12 seams in the southwest and southeast portions of the West Pit <br />(AAI 2005b). In addition, modifications to design criteria for the combined D1 /D2 Seams in the <br />East Pit (AAI 2004) and the C5 and B seams of the East Pit (AAI 2005c) have been provided, as <br />well as a review of an extension of E2 Seam mining in southern portions of the West Pit (AAI <br />2007). <br />To date, highwall mining has been successfully accomplished in the E2, X, D1, D2, and <br />C5 seams. However, about the same time that mining was completed in the C5 Seam in the East <br />Pit, evidence of subsidence was noted above the West Pit X Seam highwall mining area <br />(AAI 2005d). After an analysis of available information related to this event, modifications to <br />the design criteria for the northwest portion of the West Pit were provided (AAI 2005e). That <br />report also noted face instabilities above the C5 Seam in the East Pit that may have been related <br />to subsidence. <br />With this experience as a backdrop, this report addresses highwall mining beneath the <br />South Taylor Pit low wall, and the final highwall in a portion of the West Pit. End wall areas in <br />the South Taylor Pit may also be mined. The current study areas in relation to previous study <br />areas are shown in Figure 1. <br />In the South Taylor Pit, plans call for the combined G7 and G8 seams (hereafter referred <br />to as the G78 Seam) to be mined in the southwest portion of the pit. The G78 splits, with parting <br />thickness between the G7 (stratigraphically higher) and G8 increasing to the northeast. Colowyo <br />personnel have indicated that the seams can be mined together if the parting is less than about <br />3 feet (ft) thick; beyond that thickness, ash in the mined product becomes excessive. As the G7 <br />is the thicker of the two seams, it will be mined where the parting thickness becomes excessive <br />in the northeast portion of the pit, leaving the G8 Seam in the floor. After mining of the G7 is <br />completed, the pit will be backfilled, and the combined D2 and E seams will be mined in the <br />southwest portion of the pit. Where the D2 /E parting thickness becomes excessive, only the E <br />Seam will be mined in the northeast portion of the pit. It is anticipated that the thicker portions <br />of the combined seams will be mined in two passes (ramping up to the top portion of the seam, <br />backing out, and mining the remainder of the seam, with no material left between the passes). <br />In the West Pit, highwall mining is planned to commence in the F Seam. The pit will be <br />backfilled to reach the successively higher E, D2, and C seams. <br />Agapito Associates, Inc. <br />