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<br />~• <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br />l~ <br />3-5 <br />upstream of the slurry wall centerline at the sump to 30 feet upstream at the <br />west end of the pipe. The pipe was surrounded by clean, crushed rock with a <br />minimum and maximum size of 3 inches and LO inches, respectively. The pipe <br />a~ gravel were surrounded by Mirafi filter cloth in a configuration as shown <br />in Figure 3. Nearer the sump, where more yater was encountered during <br />installation of the pipe, more gravel was used below the pipe invert to <br />facilitate construction and also to intercept more seepage. Depths of the <br />invert of the pipe below the ground surface range from 7 feet to 11 feet, near <br />the sump and at the periphery of the system, respectively. The two drainage <br />pipes empty into the sump by way of 6 inch hales cast into the concrete rings <br />of [he sump at an invert elevation of 8831.0 feet. ' <br />Two additional pipes were covered with filter cloth and connected to the main <br />collection trench drainage pipe at Stations 2+31 and 3+00. The pipe at <br />Station 2+31 collects seepage from the "Y" drain pipe system that drains the <br />vet area between the toe of the lower tailings dam and the slurry wall. The <br />"Y" system was designed by Homestake and constructed under the supervision of <br />Homestake personnel. The other pipe drains the surface seepage that fills the <br />channel at Station 3+10 east of the old pumphouse. All open ends of the <br />drainage pipe were plugged. The drainage pipes were flowing clear and freely <br />when construction was completed. <br />3.4 PUMP-BACK SYSTEM <br />The pump-back system was designed to pump the collected seepage back Snto the <br />lower tailings pond. A concrete slab, 11.8 Eeet by 14.0 feet (parallel to the <br />slurry wall) and 7 inches thick was poured on a 4 inch bed of clean, river <br />gravel to form the base For the pumphouse and pumps and a working platform <br />above the sump. The top of the concrete slab is approximately 0.1 feet above <br />the top of the sump and !s at elevation 8846.1 feet. <br />Two Byron Jackson 6-stage vertical turbine pumps with 10 hp, 1750 rpm motors <br />were installed to pump back the collected seepage to the pond. The pumps are <br />self-lubricated with water, and are equipped with 8 inch bowls and L-type <br />~Jl~ ~l~ 1E' ~ <br />