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Interoffice Memorandum <br />To: Rob Zuber <br />cc: Dan Hernandez <br />From: Zach Trujillo <br />Subject: Colowyo —TR -98 Review of Geotechnical Analysis <br />Date: 1/10/2013 <br />•.. <br />After my review of Shannon & Wilson, Inc.'s (S &W) Addendum No. 4 as part of Colowyo's proposed TR -98, I have the <br />following comments: <br />S &W's slope stability analysis of the proposed temporary excess spoil fill included a total of eight different scenarios <br />with failures through only spoil; through spoil and clay; and through spoil, clay, and bedrock. The scenarios used average <br />values attained through test results and /or "low bound" values previously requested by the Division. Two scenarios of <br />the proposed temporary excess spoil fill submitted by S &W did not meet the minimum required factor of safety (FS) of <br />1.5 as per Rule 2.05.3(6). As labeled by S &W in Addendum No. 4, Table 1, Case 2a and Case 2c achieved FS of 1.4 and <br />1.3. Both cases were using values considered as "lower bound" values for a conservative result. Please have Colowyo <br />submit further reasoning on why these FS attained in Case 2a and Case 2c ensure long term stability in relation to the <br />Division's minimum requirements for slope stability of temporary excess spoil. Also, please have Colowyo submit an <br />additional slope stability analysis regarding the temporary excess spoil fill showing that with alternative conservative <br />values (in between to the average values and the "low bound" values used), the proposed excess spoil fill will meet <br />the Division's minimum requirements of a FS of 1.5. <br />Additionally, S &W's addendum No. 4 does not address the excess spoil overlying the previously mined area from <br />highwall mining. Please have Colowyo address the potential impact of the additional load from the proposed excess <br />spoil fill on the excavated sections from highwall mining and the integrity of the proposed excess spoil fill. <br />Attached are the results from my analysis of Colowyo's proposed excess spoil fill using the program Galena. As you may <br />notice, results between S &W's and mine vary. This is due to the difference in complexity of programs and assumptions. <br />My results through Galena should be viewed more as a mathematical check to S &W's analysis. It is my opinion that <br />S &W's analysis of the proposed temporary excess spoil fill more accurately reflects possible on site results from <br />Colowyo's TR -98 submittal. <br />This concludes my comments and concerns regarding Shannon & Wilson, Inc.'s (S &W) Addendum No. 4 as part of <br />Colowyo's proposed TR -98. If you have any questions, feel free to contact me. <br />Sincerely, <br />Zach Trujillo <br />