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  <br />   <br />   <br />  <br />  <br /> <br />  <br /> 715 Horizon Drive, Suite 340 1536 Cole Blvd., Suite 310, Bldg. 4   <br /> Grand Junction, CO 81506 USA Lakewood, CO 80401  <br /> +1 970.242.4220 +1 303.271.3750  <br />www.agapito.com  <br />March 28, 2022 227-27 <br /> <br /> <br />Mr. Lee Sampson, P.E. <br />Senior Mining Engineer <br />Colowyo Coal Company <br />5731 State Hwy 13 <br />Meeker, CO 81641-9119 <br /> <br />Re: Updated Slope Stability Assessment for the Collom Out-of-Pit Dump and the Final <br />Reclaimed Surface <br /> <br />Dear Mr. Sampson: <br /> <br />This letter report summarizes the results of a slope stability assessment undertaken for the planned <br />finished slopes of the Collom out-of-pit dump area and the final reclaimed surface once mining <br />has finished in the area. The assessment area is shown in Figure 1 and is denoted by the green <br />boundary. The assessment examined the slopes along the two cross-section lines shown in Figure 1 <br />using the limit equilibrium analysis software SLOPE/W.1 <br /> <br />The assessment builds on previous work undertaken by Shannon and Wilson in 2009,2 which <br />assessed the stability of the slopes associated with the Collom temporary spoil fill area. This <br />assessment will therefore utilize the results of the original geotechnical investigation in the dump <br />area and consider the geotechnical assumptions used in the original assessment. <br /> <br />The updated temporary spoil fill dump geometry has been provded by the Colowyo Mine and is <br />indicated by the gray contours in Figure 1. Also provided in red in Figure 1, are the proposed <br />contours for the final reclaimed surface once mining is completed. <br /> <br />The report does not consider any underdrain2 or surface-water control requirements for either the <br />temporary dump or the final reclaimed surface. <br />SITE DESCRIPTION <br />The assessment area shown in Figure 1 is located north and next to the West Collom Box Cut. <br />Colowyo has indicated that the out-of-pit dump area for the Collom pit will be located on top of <br />areas where mining will not occur and topsoil has been removed prior to placing dump material. <br />The existing topography of the dump area dips to the northeast around 5 degrees (°), where it is <br />located on a flatter limb of a northwest–southeast trending asymmetric syncline. At its maximum <br />height, the dump pile is planned to be approximately 175 feet (ft) above the existing ground <br />surface. <br /> <br />1 GeoStudio (2022), “SLOPE/W Features,” available at https://www.geoslope.com/products/slopew/features. <br />2 Shannon & Wilson, INC (2009), “Geotechnical Study Collom Temporary Soil Fill, KCCC – Colowyo Mine,” Report <br />Revision PR-03. August 5th.