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SMITH WILLIAMS CONSULTANTS, INC. <br />4.5 Underdrains <br />Although there is no evidence of shallow groundwater seepage within the footprint of the Phase <br />5 VLF, an underdrain system composed of primary, secondary, and tertiary underdrains will be <br />constructed beneath the composite liner systems. Primary underdrains will collect and convey <br />any. observed flows from the secondary underdrains. The secondary underdrains will need to be <br />installed in topographic lows beneath the Phase 5 VLF area. Tertiary underdrains will need to be <br />installed during construction if seepage is observed outside of the topographic lows. General <br />layouts of the proposed underdrain system are shown on Drawings A220, A240, and A250. The <br />underdrain will route flow into an earthen underdrain sump located at the downstream toe of the <br />PSSA embankment. The underdrain sump is sized to contain 24 hours of underdrain flow at an <br />average flow rate of 25 gpm. <br />The primary underdrains will consist of a minimum 15-square-foot cross-sectional area of <br />underdrain fill (minus 12-inch fill) with a 6-inch PCPE pipe wrapped in a 12-oz/yd2 nonwoven <br />geotextile and. will be covered with a minimum of 6 inches of select structural fill. The primary <br />underdrain will be placed at a minimum grade of 1 percent and will extend beneath the Phase 5 <br />VLF area. <br />The secondary underdrains will consist of a minimum 8-square-foot cross-sectional area of <br />underdrain fill wrapped in a 12-oz/yd2 nonwoven geotextile and will be covered with a minimum <br />_ of 6 inches of select structural fill. The tertiary underdrains will consist of a minimum <br />6-square-foot cross-sectional area of underdrain fill wrapped in a 12-oz/yd2 nonwoven geotextile <br />and will be covered with a minimum of 6 inches of select structural fill. The grades of the <br />secondary and tertiary underdrains will vary to fit the final site-grading plan. <br />As part of the Phase 5 VLF design, calculations were conducted to size the flow capacity of the _ <br />underdrains under various hydraulic heads and to confirm the flow capacity of existing VLF <br />underdrains under 800 feet of ore depth. These calculations indicate that that underdrains will <br />function under 800 feet of ore. These calculations are presented in Appendix C. <br />4.6 Liner System <br />4.6.1 Ore Storage Area <br />The ore storage area composite liner system design includes a minimum 1-foot-thick low- <br />permeability SLF overlain by an 80-mil single-sided textured (texture face down) LLDPE <br />geomembrane, overlain by 2 feet of DCF. This is the same liner configuration approved and <br />used for Amendment No. 8. A textured LLDPE geomembrane was selected as it provides higher <br />S:\PROJECTS\1125 CCBV PHASE 5 VLFU 12 - DESIGMAREQUA VLF\VLF PHASE 5 FINAL REPORT V2.DOC 23 <br />