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constructed up side slopes as liner placement progresses. As <br />tailings placement and consolidation occurs the flow into the <br />drainage layer will continue primarily through flow in areas <br />where the tailings thickness is minimal such as the valley <br />sides. Downward movement of water will be restricted by the <br />low permeability of the consolidated tailings. <br />The tailings underdrain system has been designed and <br />sized to collect water from the overlying tailings and route <br />the flow through a lined ditch system to the lined collection <br />pond. Assuming a tailings permeability of 5 x ]0-6 cm/sec at <br />the end of Year 1, a unit hydraulic gradient, xnd a zone of <br />influence equal to the entire tailings area, approximately 540 <br />gpm will be collected by the tailings underdrain system. <br />Calculations show that the main collector pipe flows will be <br />approximately 310 gpm from the area above the splitter <br />embankment to the lower disposal area and 540 gpm from the <br />lower disposal area to the collection pond. Based upon a <br />permeability of the drainage layer at the base of'the disposal <br />• area of 5 x 10-4 cm/sec, a drain spacing of 40 ft has been <br />selected. At this spacing, the buildup of a phreatic surface <br />or head on the liner between drain pipes will be limited to <br />less than two ft or the thickness of the drainage layer. <br />Calculations are provided in Appendix G of this report. <br />D.6.5.4 Embankment Design <br />The main and splitter embankments will be constructed over the <br />liner and solution recovery system. They have been designed as <br />loosely compacted earthfill structures constructed with various types <br />of native materials which are available at the disposal site. The <br />embankments are shown in plan view on Figures C-3 and C-4 and in <br />cross-sectional view on Figures C-5 and C-7. <br />L J <br />D-34 <br />