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Susan Burgmaier C -1996 -083 TR -85 FAR- Geotechnical 09 -Jan -2014 Page 2 of 3 CMWDA #2 and are read on a quarterly basis. VWP -06 and -08 are located in the general vicinity of the proposed Drying Area, and continue to show somewhat elevated pore pressures, in the range of 14 to 16 psi. These values stand in sharp contrast to the 1 to 3 psi reported for VWP -05, -09 and -10 which are installed near the toe of the pile. a) Given the fact that pore pressures in the upper portion of CMWDA #2 have not yet diminished, please expand the stability analysis to address the effects (if any) of the proposed Drying Area on the pile as a whole, using the approved outslope configuration and the current slope of the compacted CMW that will form the base of the Drying Area. Based on previous Buckhorn studies, this may require consideration of a phreatic surface and block -type failure surfaces using total stress analyses and un- drained strength parameters. b) Currently, there are only two functioning VWPs in the upper levels of CMWDA #2: VWP -06 and -08. Please address any impacts to these VWPs that may occur as a result of constructing the Drying Area, e.g. will either installation be destroyed, or made inaccessible. If any impacts are expected, please provide recommendations regarding any proposed mitigation, such as installation of new VWPs. 2. Buckhorn's first Assumptions bullet gives material properties for compacted CMW that differ from those presented in the recent 15- Oct -2013 Buckhorn analysis submitted for TR -84. Specifically, the average moist unit weight is given as 109 pcf in this study, whereas the October report used 104.6 pcf. While the material does include inherent variability, as established by the regular compaction tests that are conducted, it would be helpful to use consistent values in all stability analyses, with any sensitivity to this parameter being considered as necessary. Please review the properties for the various materials used in the analysis, ensuring that consistent, appropriate values are utilized from one study to the next. Buckhorn's Model #1, a 1.5 -foot pile at 1.5h:ly, results in a Factor of Safety (FoS) of essentially 1.0, which implies that slippage of material down the slope is imminent at all times. While this slope configuration may be acceptable on the sides of the Drying Area, the Division questions whether this is appropriate for the front face of the pile, where a commitment has been made to maintain a 25 -foot buffer. Continual sloughing from the face of the Drying Area pile may produce a maintenance challenge. 4. In the Results Discussion section, Buckhorn refers to the long term FoS of 1.5 required by Rule 4.09.1. That rule is specific to disposal of excess spoil. For Coal Mine Waste Banks, Rule 4.10.4(2) applies, which requires a minimum static safety factor of 1.5 (not limited to the "long term" condition). Please revise this paragraph to reference Rule 4.10.4(2) rather than 4.09.1. (The Division concurs that the Drying Area itself need not be held to a FoS of 1.5; what is proposed with TR -85 represents a significant improvement in the handling of wet CMW over methods used historically at the site.)