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Susan Burgmaier <br />C -1996 -083 TR -85 PAR No. 2 - Geotechnical <br />21- Feb -2014 <br />Page 2 of 4 <br />configuration and the current slope of the compacted CMW that will form the base of <br />the Drying Area. Based on previous Buckhorn studies, this may require consideration of <br />a phreatic surface and block -type failure surfaces using total stress analyses and un- <br />drained strength parameters. <br />MLT 02/21: Buckhorn provided two new stability analyses in the 04- Feb -2014 response. <br />The first evaluated the stability of CMWDA #2 with an assumed phreatic surface at the <br />top surface of the compacted coal mine waste. The computed FoS (block failure) for <br />this analysis was 1.319, which is less than the 1.5 required by Rule 4.10.4(2). A <br />minimum FoS of 1.5 must be maintained in the pile at all times. If the pore pressures <br />reported for VWP -06 and -08 dictate that the phreatic surface be modeled at the top of <br />the compacted waste, then it appears that storage of uncompacted waste at this <br />location cannot be approved. <br />In a second stability analysis, Buckhorn evaluated a circular failure plane, using the same <br />model conditions. The failure surface occurred within the uncompacted coal mine <br />waste (U -CMW), and no information was provided about the effects of the U -CMW on <br />the stability of the pile as a whole. The Division suggests an alternate approach would <br />be useful, i.e. the volume of U -CMW could be replaced by a distributed static load of <br />comparable magnitude so that the lower shear strength of the U -CMW does not mask <br />an evaluation of the stability of the pile as a whole. <br />The Division's question regarding elevated pore pressures, and any effect this may have <br />on the stability of CMWDA #2 with the addition of the proposed drying area, has not <br />been satisfactorily addressed. Please submit a thorough stability analysis of the <br />proposed configuration, with all inputs and assumptions clearly identified, establishing <br />that the required minimum FoS of 1.5 will be maintained at all times. <br />Certain earlier Buckhorn analyses (Feb -2006, Feb -2007, and Mar -2007) analyzed Total <br />Stress as well as Effective Stress. Later reports eliminated the Total Stress model under <br />the assumption that moisture was draining from the compacted waste and pore <br />pressures were reducing with time. As demonstrated by quarterly measurements at <br />vibrating wire piezometers VWP -06 and VWP -08, that assumption may not be valid. <br />Please incorporate Total Stress analyses, if appropriate, or include a detailed <br />explanation of why this approach is not applicable to the current evaluation of <br />CMWDA #2. <br />b) MLT 01/09: Currently, there are only two functioning VWPs in the upper levels of <br />CMWDA #2: VWP -06 and -08. Please address any impacts to these VWPs that may <br />occur as a result of constructing the Drying Area, e.g. will either installation be <br />destroyed, or made inaccessible. If any impacts are expected, please provide <br />recommendations regarding any proposed mitigation, such as installation of new VWPs. <br />MLT 02/21: A response to this question was provided by Buckhorn, but it may be more <br />appropriately addressed within the permit text. It seems apparent, based on the photo <br />of revised page 14a, that VWP -06 is located well within the proposed footprint of the <br />