PERMFILE109143 - Laserfiche WebLink

Home Browse Search

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