2014-02-13_REVISION - M1977493 (2)

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Colorado Division of Water Resources Office of the State Engineer Dam Safety Branch 2754 Compass Drive #185 PO Box 456 p Grand Junction, CO 81506 Montrose, CO 81402 (970) 245 -8655 Office: (970) 249 -6622 woRKIIVC SMART SINGE �a89 Review Assistance Memorandum To: Eric Scott, Environmental Protection Specialist, DRMS Bill McCormick, P.E., Chief, Dam Safety Branch, DWR From: Garrett Jackson, P.E., Dam Safety Engineer, DWR, Grand Junction, CO Jason Ward, P.E., Dam Safety Engineer, DWR, Montrose, CO Date: February 13, 2014 Dam Name: Mayflower Tailing Storage Facility (aka 5 DAM) Subject: Review of engineering reports The purpose of this memorandum is to document comments provided by the Dam Safety Branch (DSB) of the Colorado State Engineer's Office (SEO) for the review of engineering reports prepared by URS Corporation on behalf of Climax Molybdenum Company (Climax) and submitted to the Colorado Division of Reclamation, Mining and Safety (DRMS). The Colorado DSB has agreed to provide technical assistance to DRMS for review of the seepage and stability analyses and operation and maintenance reports submitted by Climax for the Mayflower Tailing Storage Facility (aka 5 Dam) to satisfy DRMS requirements. Mill tailing impoundments are exempt from the State of Colorado's Rules and Regulations for Dam Safety and Dam Construction (Rules, 2007). However, many of the engineering principles and safety guidelines for water impoundment structures can be applied to tailings impoundments. The review comments provided herein are provided from the perspective of the state of the practice in embankment dam engineering. It is not appropriate to directly apply the Rules to the design of a tailings dam and the review comments and recommendations provided by the DSB do not constitute contingencies or stipulations of permitting requirements. Mill tailing impoundments are regulated by DRMS. Seepage and Stability Analyses Review Comments Following are comments documenting Garrett Jackson's review of the Mayflower Tailing Storage Facility 5 Dam Operating Condition Seepage and Stability Analyses Final Report, dated September 2013. Recommendations are provided. Section 1 — Introduction 1. Page 1 -2: The paragraph in the center of the page describes the existing dam drainage system composed of subdrains, drain trenches, and a foundation blanket drain. The subdrains are 8- and 12 -inch diameter corrugated metal pipes bedded in gravels and sands. CMP drains in dams have caused significant problems and resulted in some dam failures when they deteriorate and collapse or cease to function as designed. These problems can be especially severe when the seepage water is corrosive, as in many mining operations. It is recommended that the CMP drain pipes be inspected frequently by video camera, if possible.