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Brock Bowles <br />19- Oct -2012 <br />C- 1996 -083 TR -75 <br />Page 2 of 2 <br />A Stability Analysis of the as -built conditions, conducted by Buckhorn, has been provided with <br />the application for TR -75 in the form of a two -page letter. The letter indicates that Stover's <br />office provided them with the new topographic data, which Buckhorn then used to confirm the <br />factor of safety for the current conditions. According to the letter, two model parameters for the <br />as -built stability analysis differed from those used in the original design: the height of the pile <br />(165 feet, as opposed to 150 feet), and elimination of the phreatic surface. <br />Buckhorn has determined the as -built Factor of Safety to be 1.686. This exceeds the minimum <br />static safety factor of 1.5 required by Rule 4.10.4(2). In their analysis, the failure surface is <br />located just above the bottom bench, as shown in Figure 1. <br />Adequacy Comments <br />Rule 4.10.4 Coal Mine Waste Banks — Construction Requirements <br />Coal Mine Waste Disposal Area No. 4, as constructed, differs in certain aspects from the <br />approved design. The TR -75 application recognizes that the height of the pile was increased <br />from 150 feet to 165 feet, and this change has been incorporated into Buckhorn's stability <br />analysis of the as -built configuration. It is not apparent, however, that the new analysis closely <br />considered the topographic differences between Buckhorn's May 2009 design and what has <br />actually been constructed. <br />The Division made manual measurements of the as -built topography (as provided on Volume IX <br />Figure 2 of the TR -75 application), and calculated the slopes at numerous places across the face <br />of the pile. As indicated above, the approved design assumed a slope of 2.5H: IV for the <br />lowermost 50 feet of the pile, below the first bench. For this area, the Division found the slopes <br />to range between 2.13 and 2.75, with an average of 2.36H: IV. For the area above the bench, the <br />design assumed an overall slope of 3H: IV. As shown on the attached illustration, the overall <br />slope (blue ink) appears to range from 2.55 to 3. 10, giving an arithmetic average overall slope of <br />2.85H:IV. <br />Of greater potential concern than the overall slopes described above are the shorter zones of <br />much steeper slopes. Shorter segments of slopes were also spot- checked by the Division; these <br />are shown in red ink on the attached drawing. All locations between 2.01H:1V and 2.50H:1V <br />are highlighted in yellow. Green -blue identifies those locations where the slopes appear to be <br />2.00H:1 V or steeper. The Division does not know whether these discrepancies are sufficient to <br />affect the factor of safety for CMWDA4, but their existence should be acknowledged and <br />considered in the as -built stability analysis. Please revise the stability analysis for TR -75 to <br />specifically address the slope configurations that have been constructed for CMWDA4. <br />Attachment — 1 <br />cc: Sandy Brown, DRMS <br />