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<br />Mr Erich J Bower <br />Climax Molybdenum Company <br />August 10, 2000 <br />Page 3 <br />structure such as a tailing impoundment because ol'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 wits fisted tN,ith grout <br />from approximately station 28 + 48.5 (Tower 5N) upstream to station 25 25 to plan vickv, 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 plugL, <br />in,aallation. <br />DISCUSSION <br />DMG has a concern that the Dam No. 2 decant line penetration through Tailing Dam Nu 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 rrom the tower. This source has been eliminated by installation ol'the grout 1)10?; <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 1,111conti-Olled seepage along <br />outlet conduits and other structures penetrating a dam is the use of diaphragm 1 ilter sand collar:. <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 Darn 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 till. 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 mmerial 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 ver} long seepage <br />path for a potential water source <br />Based on our current obseR ations and our experience with the dam, we judge that the risk for <br />seepage or piping along the decant structure is low and is currentiv not a threat to dam stability <br />Climax should continue to monitor Tailing Dane 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 arva <br />around decant Tower 5N Changes in seepage quantity, clarity, or changes in the phrealic <br />surface elevation should be addressed if and when they occur. <br />a <br />H <br />Z <br />cW <br />G <br />U <br />a <br />d <br />J:WRAAS'FRWRCJF:T5''44a7 r4do7 11 f-?,-