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With the addition of gravel layers and geocomposite above and below the leach field <br />liners, there should be adequate protection for the liners. The gravel layers will also be <br />compacted to reduce shifting. Also, the pipes diameters and DR value (wall thickness) of <br />the HDPE has been analyzed to determine the collapse pressure compared to the actual <br />pressure of the rock to be placed above it and it was found to be adequate. See <br />Appendix 12 for this evaluation. <br />10. The Division has similar concerns about the protection of the upper drain pipe, which is <br />to be installed on top of the crushed fill, and then covered with a layer of several feet of <br />existing onsite waste rock. Please describe the steps in the installation of the pipe and <br />placement of waste rock. <br />A detailed narrative of the installation process has been added to Appendix 12. <br />11. Drawing G -10 shows the gathering pipes in relation to the topographic contours of the <br />liner. The text states that the gathering pipes are to be located at the bottom of the <br />leach field. While the diagram shows the "center" gathering pipe to be located along the <br />low point of the leach field, the "two" lateral" gathering pipes are almost 10 feet above <br />the invert elevation of the leach field (the contour interval being 2 feet). This, as well as <br />the fact that the gathering pipes themselves should be depicted in cross - section, would <br />seem to render cross - section Z -Z' slightly incorrect. Please explain or revise as needed. <br />The entire leach field configuration has been redone. All cross - sections have been <br />redone as well. Many additional detail diagrams depict the specific configuration of the <br />pipes and liners. Please see Maps G -10, G -11, and G -12. <br />12. Drawing G -10 shows the leach field liner configured such that there is a constriction near <br />the middle of the floor, creating a high area or "saddle" in the bottom of the liner. Is it <br />intended that there is a saddle near the middle of the leach field, and /or that the <br />gathering pipes must pass over this higher point? This depiction appears to conflict with <br />the cross - section T -T, which shows a uniformly level floor despite this high point. <br />Please explain or revise as needed. <br />The leach field has been redesigned and the "saddle" is no longer there. <br />13. The three perforated gathering pipes become three un- perforated drain pipes which <br />convey water from the leach field to the mine water pond. These conveyance pipes will <br />be near the invert elevation of the leach field to adequately drain it, cross through <br />approximately 200 feet of waste rock at a slight downward gradient, and then enter the <br />mine water pond atone foot above its invert elevation. The plan states that the method <br />of sealing the points of the drain pipe penetration of the leach field liner against leaks is <br />by use of bentonite. The Division has concerns that shifting or settling of the substrate <br />or of the loaded liners, or the erosional loss of bentonite may cause the seal to fail. <br />Please provide information about this proposed sealing method, including additional <br />diagrams as needed. <br />After meeting with a representative from Colorado Lining International, the method of <br />sealing the pipes to the liners will be through the use of Pipe Boot Seals. The detailed <br />diagram of the design is provided in Appendix 14. Also, a technician from Colorado <br />Lining International will be on -site to install the pipe boot seals if a qualified and <br />experienced person is not available to perform this work on site. <br />4 <br />