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Colowyo Coal Company, L.P. <br />Attn.: Mr. Juan Garcia <br />March 10, 2015 <br />Page 6 of 7 <br />SHANNON 6 WILSON, INC. <br />If we consider only the siltstone and sandstone, they comprise about 54 percent of the spoil with <br />a weighted average UCS for these two rock types of 5,940 psi. Considering all three rock types <br />comprising 77 percent of the anticipated spoil material, the weighted average UCS is 5,484 psi. <br />Both of these combinations result in a weighted average UCS higher than the average UCS for <br />sandstone. <br />This evaluation of hardness and the massive bedding of the rock types is the basis for our <br />opinion that a drain area of 64 sf corresponding to predominantly sandstone material is <br />appropriate. The valley drain size dimensions indicated on Figure 1 result in a drain area of 64 <br />sf. Additional details regarding drains in our previous report remain valid. <br />CLOSURE <br />Our services were provided in accordance with our Master Consulting Contract, M0024. The <br />opinions and recommendations presented in this report were prepared in accordance with <br />generally accepted professional geotechnical engineering principles and practice in this area at <br />the time we prepared this report. We make no other warranty, either express or implied. <br />If you have any questions or require further information, please contact me at 303-825-3800. <br />Sincerely, <br />SHANNON & WILSON, INC. <br />a X23— 0- . <br />iLifxlAL��C� <br />Mark G. Gray, P.E. <br />Senior Associate <br />MGG:GRF/jjs <br />Reviewed by: <br />Gregory R Fischer, P. E. <br />Senior Vice President <br />23-1-01403-002 <br />