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INTEROFFICE MEMORANDUM <br />Date: 19 October 2012 <br />To: Brock Bowles, Lead Specialist <br />From: Marcia L. Talvitie, P.E. <br />Subject: Bowie No. 2 Mine (Permit No. C- 1996 -083) <br />TR -75 (Coal Mine Waste Disposal Area No. 4 As -Built Plans) <br />Adequacy Review — Engineering / Geotechnical <br />In response to your request, I have conducted a review of the TR -75 application submitted by <br />J.E. Stover and Associates (Stover) on behalf of Bowie Resources, LLC (BRL). Specifically, I <br />have focused on the engineering and geotechnical aspects of the application: Volume IX Text, <br />the Stability Analysis prepared by Buckhorn Geotech, Inc. ( Buckhorn), and Figures 1 and 2. 1 <br />have one adequacy comment at the end of this memo, related to Rule 4.10.4. <br />In 2012, BRL completed construction of Coal Mine Waste Disposal Area No. 4 (CMWDA4). <br />The project involved reworking and final placement of the material from the preparation plant <br />that had been stockpiled on the disposal area during the preceding winter. <br />Approved Design for Coal Mine Waste Disposal Area No. 4 <br />The approved design for CMWDA4 is found in Volume IX of the permit — the May 18, 2009 <br />Buckhorn Geotech report entitled, "Revised Stability Evaluation of Coal Mine Refuse Disposal <br />Area No. 4." This design was incorporated into the permit under TR -64 in December 2009. <br />Section 5.1 of the May 2009 report describes the model geometry used in Buckhorn's stability <br />evaluation; the bullets are repeated here: <br />➢ A slope of 2.5H: IV for the first 50 vertical feet of the stockpile (beginning at the toe) <br />➢ A 15 -foot wide bench at a vertical height of 50 feet <br />➢ An overall slope of 3H: IV above the first 50 vertical feet <br />➢ An ultimate waste height of 150 feet was modeled, as measured from the downgradient <br />toe to the top of the bank. <br />The results of Buckhorn's 2009 stability evaluations appear in Section 5.4 of the report, which <br />states, "Critical failure surfaces for the analyses were found to be shallow failures at the lower <br />portion of the refuse bank where slopes are steepest (2.5H: IV)." A series of diagrams in <br />Appendix D of the report illustrate the predicted failure surfaces. In its analysis of stability, <br />Buckhorn incorporated a phreatic surface within the pile, due to the presence of pore pressures <br />within the CMW. <br />As -Built Details for Coal Mine Waste Disposal Area No. 4 <br />Following final construction of CMWDA4, BRL commissioned an aerial flight from which <br />topographic contours were generated. The as -built topography, with a contour interval of 2 feet, <br />is depicted on Figures 1 and 2 of Volume IX, as revised for the TR -75 application. <br />