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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 5N) upstream to station 25 + 25 In 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 two 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 either jetted 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 permeability 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 be taken in the area <br />around decant Tower 5N Changes in seepage quantity, clarity, or changes in the plireatic <br />surface elevation should be addressed if and when they occur. <br />J 1WPMAS'FR?PPCJECT5.d1100 e1 Ao Cbi10000 Tl p., <br />