2010-07-27_PERMIT FILE - M2009076 (23)

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The Tailings Impoundment is to be built up in a series of benches, arranged in six different zones: Barrier Bench and Bench #1- Bench #5. Each of these Benches are to be constructed using strict procedures as described in Exhibit C, 6.3.3(2)(h). In these construction specifications, a beach width (LB, from embankment crest to ponded water) shall be maintained to achieve a ratio of LB /h > 5, where LB is beach width and h is incremental embankment lift height (typically, h = 10 feet). Considering the permeability relationships between the coarse and slimes fractions (reported by TRI in the geoteclmical analysis attached in the appendix of Exhibit C), this beach length ratio will provide for adequate control of the phreatic surface. This is acceptable, according to Vick's Planning, Design, and Analysis of Tailings Danis, because this is the lowest allowable limit for this upstream method of impoundment over an impermeable liner foundation. Embankment slope stability will be preserved and the phreatic surface will not break out in the embankment face. A schematic representation of the beach length/embankment height ratio as illustrated by Vick: L A-k, - tc L, 1H h - Figure 8.3 Influence of beach width on phrealic surface for homogeneous, anisotropic up- stream embankment on an impermeable foundation. Vick, Steven G. Planning Design, and Analysis of Tailings Dams. Vancouver, B.C. Canada: BiTech Publishers Ltd., 1990, p. 177. • A freeboard reserve of 12 inches shall be maintained from the ponded process water elevation and the top of the embankment crest elevation. In the event of a 100 year/24-hour storm, it can be expected that 3.8" of precipitation will become captured and co-mingled with the ponded process water (run-off has been managed with a diversion ditch network, see SWMP). Tlus will raise the ponded water elevation. Having 12 inches of freeboard will more than adequately provide for the necessary storage volume; preventing overflow and embankment erosion. To consider the Tailings Impoundment Water Balance and water storage values at each Bench, refer to the spreadsheet analysis below: Tailings Impoundment Water Balance Bench Inflow/Outflow Area Amount Flow Totals (s . ft.) (in./yr.) (gpm) (gpin) Barrier now Total 31. Process Water from Mill - - - - 31.11 Direct Precipitation (avg.) 16,410 15.72 0.31 Stormwater Run-on 0 0.00 0.00 Out ow Total -31. Available Decant Water Recycle - - - - -24.66 Pan Evaporation (avg.) 16,410 38.00 -0.74 Seepage (2.05e-4 gpm/sq. ft.) - leachate collected & recycled 16,410 - - -3.36 Void Space Entrainment - - - - -2.66 Direct Discharge - - - - 0.00 Total Available Reevele Water -28. Page 8 of 31