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[[Tr • . <br />Mr. Erich J. Bower <br />Climax Molybdenum Company <br />August 10, 2000 <br />Page 3 <br />structure such as a tailing impoundment because of the potential for piping and uncontrolled <br />seepage into the open conduit. <br />In June 2000, Climax plugged a portion of the decant line. The decant line was filled with grout <br />from approximately station 28 + 48.5 (Tower SN) upstream to station 25 + 25. [n plan view, the <br />plug extends from about the toe of Tailing Dam No. 3 upstream a distance of over 300 feet. No <br />seepage has been observed from within or around the decant conduit following the plug <br />installation. <br />DISCUSSION <br />DMG has a concern that the Dam No. 2 decant line penetration through Tailing Dam No. 3 could <br />provide a pathway for uncontrolled seepage and possible piping. Seepage resulting in piping <br />would be the result of t~vo sources. The first would be seepage entering the decant pipe and then <br />discharging from the tower. This source has been eliminated by installation of the grout plug. <br />The second source would be seepage that would flow along the decant pipe exterior. <br />Currently, state of the practice in water retention dams for preventing uncontrolled seepage along <br />outlet conduits and other structures penetrating a dam is the use of diaphragm filter sand collars. <br />A filter sand collar is used to control seepage and to filter fine particulate, preventing piping <br />from occurring. <br />Limited historic records are available on the construction of the Dam No. 2 decant system. <br />However, based on the available records, supported by conversations with field staff present <br />during construction, the decant conduit was bedded in and surrounded by compacted sand. This <br />sand was eitherjetted or vibrated in place and then covered with compacted fill. The sand will <br />filter seepage along the decant conduit as a diaphragm sand filter collar would. No seepage has <br />been noted to date. <br />In addition, fine tailing was deposited above decant structure. This material is known to have a <br />low pemreability and will act as a barrier to seepage flows entering the sand around the conduit. <br />Finally, the decant pond is located over 2,000 feet from Dam No. 3 creating a very long seepage <br />path for a potential water source. <br />Based on our current observations and our experience with the dam, we judge that the risk for <br />seepage or piping along the decant structure is low and is currently not a threat to dam stability. <br />Climax should continue to monitor Tailing Dam No. 3 on a regular basis. Monitoring should <br />include measuring the existing piezometers, monitoring flow from existing toe drains and <br />conducting regular visual inspections of the dam. Special attention should betaken in the area <br />around decant Tower SN. Changes in seepage quantity, clarity, or changes in the phreatic <br />surface elevation should be addressed if and when they occur. <br />J \N4MASTERWROJEOTSdM0Ule4 doc 08116'00(3 31 PM) <br />