1989-10-10_REVISION - M1988112 (12)

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The East Dike must be constructed at the start of operations, however the North Dike will not be needed until approximately • Year 3.75. D.6.5.5 Collection Ditch and Pond Design The collection ditch at the toe of the main embankment will be an extension of the drainage and filter layer and will collect flows from the drainage/filter layer and convey these flows to the collection pond. The collection ditch will be double lined with a 60-mil HDPE upper liner and a 40-mil VLDPE lower liner. A layer of geonet will be placed between the liners. The collection ditch will be three feet in depth with a five foot bottom width. The ditch side slopes will be 3H:1V to allow liner placement. The maximum ditch capacity with one foot of freeboard is 690 cfs. The ditch wil'I follow the existing 3.3% gradient into the collection pond. Ditch details are shown on Figure C-il. • Based on the worst case drainage layer seepage calculations presented in Appendix E, the ditch design is adequate. The worst case seepage occurs at the initiation of tailings disposal when the tailings are just beginning to consolidate. The calculations assume that a 100 year 24 hour storm event occurs at the onset of disposal and most of the water flows through the drainage layer at a rapid rate. The tollection pond will collect flows from the drainage ditch. The pond will be double lined due to the high hydraulic head which can occur in the pond and the direct exposure to site climatic conditions. The lining will consist of a 60-mil HDPE upper liner and a 40-mil VLDPE lower liner. A layer of geonet will be placed between the liners to collect any leakage from the upper liner and convey the leakage to a secondary recovery sump located on the downgradient side of the collection pond. The collection pond is shown in plan view,on r~ LJ D-3t's