2014-03-25_REVISION - C1996083 (3)

Home Browse Search

Susan Burgmaier -2- March 20, 2014 modeled at the top of the compacted waste, then it appears that storage of uncompacted waste at this location cannot be approved. In a second stability analysis, Buckhorn evaluated a circular failure plane, using the same model conditions. The failure surface occurred within the uncompacted coal mine waste (U -CMW), and no information was provided about the effects of the U- CMW on the stability of the pile as a whole. The Division suggests an alternate approach would be useful, i.e. the volume of U -CMW could be replaced by a distributed static load of comparable magnitude so that the lower shear strength of the U -CMW does not mask an evaluation of the stability of the pile as a whole. The Division's question regarding elevated pore pressures, and any effect this may have on the stability of Gob Pile #2 with the addition of the proposed drying area, has not been satisfactorily addressed. Please submit a thorough stability analysis of the proposed configuration, with all inputs and assumptions clearly identified, establishing that the required minimum FoS of 1.5 will be maintained at all times. Certain earlier Buckhorn analyses (Feb -2006, Feb -2007, and Mar -2007) analyzed Total Stress as well as Effective Stress. Later reports eliminated the Total Stress model under the assumption that moisture was draining from the compacted waste and pore pressures were reducing with time. As demonstrated by quarterly measurements at vibrating wire piezometers VWP -06 and VWP -08, that assumption may not be valid. Please incorporate Total Stress analyses, if appropriate, or include a detailed explanation of why this approach is not applicable to the current evaluation of Gob Pile #2. BOW2: Please see HBET's response in attached document, section 3.2. b) A response to this question was provided by Buckhorn, but it may be more appropriately addressed within the permit text. It seems apparent, based on the photo of revised page 14a, that VWP -06 is located well within the proposed footprint of the storage area. BRL's intention should be clearly stated: either damage to VWP -06 will be avoided (may require a vertical extension of the pipe), or damage to this instrument is likely or expected. If the latter is true, then plans and commitments for its replacement should be made at this time. There are only two functioning piezometers remaining in the upper reaches of Gob Pile No. 2. Given the slow rate at which the pile is dewatering, it is imperative that collection of pore pressure data not be interrupted. Please clarify whether damage to VWP -06 is anticipated, and whether it will be protected or replaced when gob is placed in the temporary drying area BOW2: BRL will do their best to avoid damage to VWP -06, however, if it is damaged it would be replaced as soon as possible. Please see revised page 19. 3. Buckhorn responded that BRL's surface operations staff will maintain the 25 -foot buffer, and that no changes to the design are proposed. That may be the case. However, the stability analysis assumes a 25 -foot setback. If there is a chance