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Mayflower Mill - Tailings Pond No. 4 <br />Seepage Control Plan <br />Page 2 <br />Three tailings depositional techniques were evaluated during design of the <br />Mayflower Mill Tailings Pond No. 4 structure; upstream, downstream, and <br />/modified downstream. The modified downstream method was selected for <br />tailings disposal operations to achieve maximum disposal capacity at the <br />�' �• pond site and to achieve adequate retention time for treatment of mill <br />waste waters before discharge. <br />The downstream tailings deposition method is implemented by first separating <br />the tailings slurry into coarse ( +200 mesh) and fine fractions ( -200 mesh) <br />employing a cyclone separator. Cyclone separation of the tailings slurry <br />into coarse and fine fractions is accomplished due to differences in the <br />specific gravity of the tailings particles. Upon separation the coarse <br />fraction is deposited downstream from centerline of the pond starter dike, <br />and used to construct the pond embankment. The fine fraction is deposited <br />upstream from centerline of the starter dike and forms a slime bench to <br />the interior of the tailings structure. Minimum freeboard of the embank- <br />ment is maintained by discharging ponded water through a decant system <br />installed at the rear of the pond to permit maximum retention time. Tail- <br />ings deposition continues in the described manner around the perimeter of <br />the pond starter dike, and upon completion of one depositional lift is <br />repeated by moving the cyclone as needed. This method of deposition creates <br />a fingering type of zone between slimes and coarse - grained tailings placed <br />during subsequent deposition cycles (see Figure 1 ). <br />One of the most important considerations in maintaining overall stability <br />of the tailings pond structure is control of the hydrologic pressures in <br />the pond embankment or dam. For this reason a blanket toe drain was in- <br />corporated into the pond design. The objective of the rain system i s to <br />permit free water entering the embankment from the interior ponding area <br />to drain and maintain phreatic levels in the berm at a minimum. If adequate <br />drainage is not achieved by the toe drain system, phreatic levels may rise, <br />resulting in piping and erosion of the embankment toe. <br />The toe drain incorporated into Tailings Pond No. 4 was constructed with <br />in -place river gravels, overlain by a pervious filter cloth to keep the <br />gravels from plugging. The filter cloth used was a monofilament poly- <br />propylene meeting design specifications of the pond engineers, F. M. Fox <br />and Associates. <br />Mill tailings deposition on Tailings Pond No. 4 commenced in August of 1976, <br />and a maximum embankment height of 40 feet was been attained to date. As <br />th- ..n. elevation has risen, h dros - - •, . nk- <br />ment has also increase., resulting in an iazu. e.. the volume of water <br />en Bring the embankment and higher phreat___levels. At present the volume <br />of pi5`n3`water migrating through the embankment is exceeding the capacity <br />