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COLORADO <br />Division of Reclamation, <br />Mining and Safety <br />Department of Natural Resources <br />1313 Sherman Street, Room 215 <br />Denver, CO 80203 <br />November 4, 2014 <br />Chris Gilbreath <br />Tri -State Generation and Transmission Association, Inc. <br />P.O. Box 33695 <br />Denver, CO 80233 -0695 <br />Re: Colowyo Mine (Permit No. C- 1981 -019), Fill Monitoring Reports (First, Second, and Third <br />Quarters 2014) <br />Dear Mr. Gilbreath: <br />After Division review of the first, second, and third quarter West Pit Fill monitoring reports for the year <br />2014, we fmd the reports in compliance with the following rules: <br />• 4.09. 1 (1 1)(a) — The fill was inspected for stability by a registered engineer during the placement and <br />compaction of fill materials. <br />• 4.09. 1 (1 1)(b) — The qualified registered engineer provided to the Division a certified report within <br />two weeks after the inspection that the fill is being constructed as specified in the design approved <br />by the Division. The West Taylor Fill was inspected on March 19 (first quarter), June 24 (second <br />quarter) and September 17, (third quarter) 2014 and the Quarterly Fill Monitoring Report was <br />received by the Division on April 17 (first quarter), July 1I (second quarter) and October 7 (third <br />quarter) 2014. No appearances of instability, structural weakness, and other hazardous conditions <br />were noted in any of the inspection reports. <br />• 4.09. 1 (1 1)(c) — Rule 4.09(11)(c) is no longer applicable during this stage of fill construction. <br />At this time, there are no issues or stability concerns with the West Pit Fill. However, there is an issue with <br />the monitoring of this fill that should be addressed. During the first quarter inspection report, it was noted <br />that the largest displacement of 0.167 feet occurred at the 7250' (elevation) West prism. When inspecting <br />the prism, it was observed that this prism was damaged. Colowyo stated that the large displacement was <br />most likely due to this prism being damaged while all other prisms are, on average, deviating less than 0.093 <br />feet. The 7250' West prism was replaced for second quarter readings, and, according to Colowyo, <br />confirmed that the damaged prism caused the large displacement. When comparing first and second quarter <br />readings at the 7250' West prism, the difference in readings is 0.028 feet. This displacement appears to be <br />about average with general displacement readings; the Division agrees with Colowyo's assessment that the <br />damaged prism was the reason behind the large displacement reading from the first quarter. <br />In the third quarter West Pit Fill report, it was noted that the 7250' West prism was missing, and therefore <br />no displacement readings were recorded for this elevation. Currently, it appears that Colowyo is only <br />reporting the single largest displacement at any point on the pit fill along with the average deviation. With <br />no available data for the 7250' West prism, Colowyo instead reported the readings at the 6950' East <br />°F cO`O� <br />o <br />1313 Sherman Street, Room 215, Denver, CO 80203 P 303.866.3567 F 303.832.8106 http: / /mining.state.co.us <br />John W. Hickenloo er Governor I Mike Kin g, Executive Director I Virginia Brannon Director * * <br />1 q6 <br />