2014-01-16_REVISION - C1996083 (3)

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STATE OF COLORADO DIVISION OF RECLAMATION, MINING AND SAFETY Department of Natural Resources 1313 Sherman St., Room 215 Denver, Colorado 80203 Phone: (303) 866 -3567 FAX: (303) 832 -8106 January 16, 2014 William A. Bear Bowie Resources, LLC P.O. Box 483 Paonia, CO 81428 Re: Bowie No. 2 Mine (Permit No. C- 1996 -083) Technical Revision No. 85 (TR- 85) Gob Pile #2 Temporary Drying Area Dear Mr. Bear: COLORADO ID I V I S I O N O F RECLAMATION MINING SAFETY John W. Hickenlooper Governor Mike King Executive Director Loretta E. Pineda Director The Division has completed a preliminary adequacy review of the technical revision application (TR -85) submitted by Bowie Resources, LLC (BRL) for the Bowie No. 2 Mine. The revision proposes to establish a gob drying area on Gob Pile #2 during the winter months of 2013 -2014. We have the following comments and questions regarding the application. Rules 4.10.1 and 4.10.4 1. The first bullet of the assumptions section of the December 5, 2013 Buckhorn Geotech report gives material properties for compacted coal mine waste 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 December 5, 2013 analysis, ensuring that consistent, appropriate values are utilized from one study to the next. 2. The second bullet of the assumptions section of the December 5, 2013 Buckhorn Geotech report reads, "No phreatic surface was modeled in either the gob or native material which is consistent with our analyses of pile #2 to date." The Division's review of Buckhorn's earlier Gob Pile #2 analyses found that the 22- Sep -2008 study did consider a phreatic surface based on pore pressure measurements and also conducted a sensitivity analysis using an increased phreatic surface. In that study, block failure as well as circular failure was considered. Buckhorn's Summary of Instrumentation Monitoring for the 3rd Quarter 2013 includes data for a series of Vibrating Wire Piezometers (VWPs) that have been installed in Gob Office of Office of Mined Land Reclamation Denver • Grand Junction • Durango Active and Inactive Mines