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Susan Bur.maier <br />-2- November 30, 2012 <br />4. DRMS: Coal Mine Waste Disposal Area No. 4, as constructed, differs in certain aspects <br />from the approved design. The TR -75 application recognizes that the height of the pile was <br />increased from 150 feet to 165 feet, and this change has been incorporated into <br />Buckhorn's stability analysis of the as -built configuration. It is not apparent, however, that <br />the new analysis closely considered the topographic differences between <br />Buckhorn's May 2009 design and what has actually been constructed. <br />The Division made manual measurements of the as -built topography (as provided on <br />Volume IX Figure 2 of the TR -75 application), and calculated the slopes at numerous places <br />across the face of the pile. As indicated above, the approved design assumed a slope of2. <br />5H: 1V for the lowermost 50 feet of the pile, below the first bench. For this area, the Division <br />found the slopes to range between 2.13 and 2. 75, with an average of 2. 36H: 1V. For the <br />area above the bench, the design assumed an overall slope of 3H: 1V. As shown on <br />the attached illustration, the overall slope (blue ink) appears to range from 2.55 to 3.10, <br />giving an arithmetic average overall slope of 2. 85H: 1V. <br />Of greater potential concern than the overall slopes described above are the shorter zones <br />of much steeper slopes. Shorter segments of slopes were also spot- checked by the <br />Division; these are shown in red ink on the attached drawing. <br />All locations between 2. 01H: 1V and 2. 50H: 1V are highlighted in yellow. Green -blue <br />identifies those locations where the slopes appear to be 2. OOH: 1V or steeper. The Division <br />does not know whether these discrepancies are sufficient to affect the factor of safety for <br />CMWDA4, but their existence should be acknowledged and considered in the as -built <br />stability analysis. Please revise the stability analysis for TR -75 to specifically address the <br />slope configurations that have been constructed for CMWDA4 the slope configurations that <br />have been constructed for CMWDA4. <br />Please fee free to contact me if you have any questions. <br />Sincerely, <br />rnrn-c <br />Tamme Bishop, P.E. <br />Project Engineer <br />Enclosures: <br />BOW -2: Please see the attached response from Buckhom Geotech. <br />Volume 111: Maps 15 -3, 18 and 21 -2 <br />Volume IX: Figure 2, Stability Analysis, Certification for Appendix B <br />