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I <br />J <br />I <br />I <br />I <br />16 <br />1 <br />I <br />ID <br />was lined with coarse river gravel across the crest and along the upstream and <br />downstream embankment slope to protect against erosion. The elevation of the <br />top of the waste rock in the spillway is approximately 8842 feet. As con- <br />structed, the spillway can accommodate a flow of approximately 80 cubic feet <br />per second with 0.5 feet of freeboard, greater than the peak rate of runoff <br />from the Probable Maximum Precipitation for the small drainage area upstream <br />of the slurry wall embankment. <br />The impoundment created by the embankment is capable of storing approximately <br />2.2 acre-feet of water, more than sufficient to store runoff from the 25 -year, <br />24-hour storm in the watershed between the embankment and the lower tailings <br />dam, plus a volume of seepage accumulated in a 24-hour period if the pump -back <br />system does not operate during that period. <br />3.3 SUMP AND DRAINAGE TRENCH <br />A concrete sump and drainage collection trench were constructed just upstream <br />of the slurry wall to collect seepage that would be intercepted by the new <br />slurry wall cutoff. The concrete sump was constructed of precast concrete <br />man -hole rings stacked upon one another and surrounded with mechanically <br />tamped soil. The top four joints were sealed with bituminous joint sealer. <br />The location of the center of the sump is approximately 26 feet upstream of <br />the slurry wall center line at Station 2+72. The inside bottom of the sump is <br />at elevation 8824.3 feet. The sump is 21.7 feet deep from the top inside lip <br />to the inside bottom. To facilitate seepage collection around the sump, clean <br />crushed rock was placed below the sump bottom and around the sump circumfer- <br />ence about 6 feet in all directions up to approximate elevation 8835 feet. <br />All mud and rock was removed from the concrete sump bottom before setting the <br />pumps. <br />Six -inch -diameter slotted polyethylene drainage pipe was laid below grade both <br />east and west of the sump, parallel to the slurry wall, and was connected to <br />the sump. The drainage pipe extended 100 feet east of the sump (Station 3+75) <br />and 150 feet west of the sump (Station 1+19). The pipe ranged from 26 feet <br />