080121-08MN006-Sabina Hackett River Project Proposal ... - NIRB

Project Descriptionoperated as a zero discharge facility. This deficit situation also presents the opportunity to usethe tailings impoundment as a discharge location for other intercepted site waters, such as pitdewatering, dump drainage or any other runoff that would preferable not be discharged to thedownstream water system.2.7.1.3 Tailings DamsThe TMF will be formed by rockfill dams. Non acid generating (NAG) rock will be used toconstruct the bulk of the tailings dams. The rock will be either open pit waste rock or quarriedrock. The face of the dam will be lined with an impermeable liner. To mitigate seepage beneaththe dam, the liner will be extended beneath the dam to lengthen the seepage path. Seepage cannot occur through frozen soils, and if the liner extends far enough downstream of the crest of thedam, the permafrost into which the liner is anchored will remain frozen. Thermal analysis will becarried out as part of final dam design studies, to optimize the seepage barrier.The liner material for the dam face and foundation seepage barrier may be a plastic, such as lowlinear density polyethylene, or a bituminous liner such as Coletanche. Plastic liners have theadvantage of having excellent strain resistance (i.e., long extension to failure), but are moredifficult to work with in cold temperatures. Bituminous liners are tough, long lasting materialsthat can be placed and welded in very cold temperatures.To protect the liner, bedding layers would be placed between the liner and the rockfill shell of thedam to provide a smooth surface upon which to place the liner. The bedding material would bedesigned to filter the tailings in the event of any punctures in the liner. To create this filter, botha bedding layer for the liner and a coarser transition zone layer will be needed between the coarserockfill and the liner. The liner would also be susceptible to damage from ice on the surface ofthe tailings pond, and an ice protection layer will be placed over the liner. Borrow material forbedding materials will be obtained from the esker deposits near the site, or if necessary, fromcrushed and screened rock.The downstream dam slopes will be about 2 horizontal to 1 vertical downstream, dictated bynormal closure requirements. The upstream slope will be about 3 horizontal to 1 vertical, toallow installation of the liner ice protection cover layer. A dam crest width of 20 m was selectedfor constructability.It should be noted that the design cross section discussed above is one that has been successfulfor similar operations under similar climatic conditions. A comprehensive geotechnicalinvestigation of the surrounding area will be completed to develop details of this design to sitespecific conditions.The tailings dams will be constructed in stages. The starter dam will provide for 3 years oftailings and water storage, plus design freeboard. The dam will be raised in subsequent years bydownstream construction.January 2008 Hackett River Project – Project Proposal Sabina Silver CorporationReport Version B.1 2–32 Rescan Environmental Services Ltd. (Proj. #833-2)