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STATE OF COLORADO <br />DIVISION OF RECLAMATION, MINING AND SAFETY <br />Department of Natural Resources <br />COLORADO <br />1313 Sherman St., Room 215 D l v t s 1 o N o f <br />Denver, Colorado 80203 RECLAMATION <br />Phone: (303) 866-3567 MINING <br />FAX: (303) 832-8106 <br />SAFETY <br />MEMORANDUM Bill Ritter, Jr. <br />Governor <br />Mike King <br />Executive Director <br />To: Michael Cunningham, Environmental Protection Specialist Loretta Pineda <br />Director <br />From: Tim Cazier, P.E., Environmental Protection Specialist <br />Date: November 19, 2010 <br />Re: Slope Stability Report Review for Derr Pit TR-01, Permit No. M-2008-017 <br />The Division of Reclamation, Mining and Safety engineering staff (DRMS) have reviewed the October 2010 Slope <br />Stability Report for the Derr Pit, Weld County, Colorado prepared by J&T Consulting for Broken Arrow Investments, <br />LLC. This reviewed focused on the stability analysis related to the change from dry mining with a slurry wall in place to <br />wet mining (prior to construction of the proposed slurry wall). DRMS does not necessarily concur with all assumptions <br />and determinations included in the report, with respect to the wet mining, as listed below: <br />1. The Design Analysis and Criteria section of the subject report specifies an HS 20 highway load to simulate heavy <br />equipment loading related to maintenance and construction activities. No mention is made of loads applied to <br />simulate mining equipment, such as a dragline or other equipment necessary to excavate materials to the proposed <br />85-foot depth during wet mining analyzed in model WELL4AWE. Please indicate what type of equipment will <br />be used for wet mining and justify the appropriate distributed load(s) and location during wet mining. The loads <br />should consider the weight and moments of equipment and excavated material when lifting a full load from under <br />the water at the maximum proposed pit depth. <br />2. The internal friction angle and dry density are lower than what is typical for an untested GP material. However, <br />these lower values will tend to result in a lower calculated Factor of Safety and are thereby deemed conservative <br />and adequate for this analysis. <br />DRMS is unable to assess the proposed setbacks in the Technical Revision without some idea of the loads imposed by <br />active mining equipment. The operator needs to provide appropriate loads and position as discussed in Item 1 in order to <br />complete the subject review. <br />Office of Office of <br />Mined Land Reclamation Denver • Grand Junction • Durango Active and Inactive Mines