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The tailings leave the flotation plant at approximately 30% pulp density via a 2" double walled BDPE pipeline that <br />gravity feeds down to the Tailings Impoundment. The concrete leachate barrier provides for the first stage starter <br />dyke for the tailings. Directly up-gradient of the concrete leachate barrier is a small "bowl" that will be lined with a <br />composite liner, more detail will follow on the liner system. This first stage has been named the Barrier Bench and <br />can be seen in the drawing of Exhibit E5. <br />The tailings pipeline will cross along the top of concrete leachate barrier. Valved spigots are to be spaced at 20' <br />intervals along the pipeline facing towards the impoundment. These spigots may be variably pitched downward at <br />an angle 20°-45° below horizontal. The spigot discharge rate and velocity can be controlled by adjusting the valve. <br />Deposition rates and classification density can be controlled by adjusting the spigot discharge angle. <br />Using this upstream method, an embankment begins to rise as the coarse fraction segregates and is deposited most <br />closely to the point of discharge. The slimes fraction flows on past this rising embankment and forms a beach that <br />slopes gradually downward away from the point of discharge. Process water will begin to pond at the toe of the <br />beach slope as the tailings begin to settle, drain, and consolidate. This water is recycled back to the mill utilizing <br />floating pumps. Care must be taken to avoid allowing ponding water to encroach too closely to the embankment <br />face, creating a high phreatic surface condition. <br />A beach width (LB, from embankment crest to ponded water) shall be maintained to achieve a ratio of LB /h >_ 5, <br />where LB is beach width and his incremental embankment lift height. (typically, h = 10 feet). Considering the <br />permeability relationships between the coarse and slimes fractions (reported by TRI in the geotechnical analysis <br />attached in the appendix), this beach length ratio will provide for adequate control of the phreatic surface. This is <br />acceptable, according to Vick's Planning, Design, and Analysis of Tailings Dams, because this is the lowest <br />allowable limit for this upstream method of impoundment over an impermeable liner foundation. Embankment <br />slope stability will be preserved and the phreatic surface will not break out in the embankment face. <br />A schematic representation of the beach length/embankment height ratio as illustrated by Vick: <br /> <br /> <br />?. v, - I C L, 1H = y <br /> <br />k <br />Figure 8.3 Influence of beach width on phreatic surface for homogeneous, anisotropic up- <br />stream embankment on an impermeable foundation. <br />Vick, Steven G. Planning, Design. and Analysis of Tailings Dams. Vancouver, B.C. Canada: <br />BiTech Publishers Ltd., 1990, p. 177. <br />To consider the Tailings Impoundment Water Balance and water storage values at each Bencl-L refer to the <br />spreadsheet analysis below: <br />Page 17 of 37