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Interoffice Memorandum <br /> <br />To: Jim Stark <br />From: Zach Trujillo <br />Subject: West Elk – TR 133 <br />Date: 7/17/2014 <br /> <br />Jim – <br />I have reviewed Mountain Coal Company’s (MCC) Technical Revision 133 is proposing a redesign of the Refuse Pile <br />Expansion East (RPEE) waste rock pile. <br />Barr Engineering Co. has submitted seepage and slope stability modeling and assessment for the modified buttress and <br />refuse placement on RPEE. Barr’s modeling results concluded an overall static factor of safety (FoS) range of 1.54 <br />through 1.73 for their three dimensional analysis. It should also be noted that Barr included a two dimensional slope <br />stability analysis which resulted in a FoS range of 1.18 through 1.33. As per rule 4.10.1 and 4.10.4(2), coal mine waste <br />banks shall have a minimum static FoS of 1.5 which has been met by Barr’s three dimensional analysis but not in their <br />two dimensional analysis. It can be seen that the computed three dimensional FoS are higher than the two dimensional <br />FoS. Most likely, this result is due to the added effect of the shear resistance along the flanks of the potential failure <br />surface which is not accounted for in a two dimensional analysis. Barr states that because of this added parameter, the <br />slope geometry is more accurately represented by the three dimensional analysis. <br />Seismic activity was also taken into consideration in the design of the RPEE. Regarding seismic activity, the Division has <br />no rule to a minimum FoS for coal mine waste banks. Barr’s model reported all FoS above 1.0 while under horizontal <br />seismic acceleration. <br />It is my opinion that Barr’s analysis most accurately reflects the final construction of the RPEE. Although Barr’s two <br />dimensional analysis resulted in some FoS being below the Division’s minimum criteria of 1.5, I agree with Barr in that <br />the three dimensional analysis more accurately represents final construction of the proposed redesign of RPEE. All FoS <br />results for the three dimensional analysis for the proposed RPEE meet the Division’s minimum criteria as per the Rules <br />as well as general engineering practices. I have no comments regarding MCC’s TR-133. <br />If you have any questions, feel free to contact me at any time. <br />Sincerely, <br />Zach <br />