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below the invert of the channel a distance of approximately 2 ft. <br /> and be backfilled and compacted. <br /> Underdrain <br /> An underdrain system for the waste pile is required by the <br /> regulations. The purpose of this drain is to intercept water which <br /> infiltrates through the waste pile and thus may become <br /> contaminated. Rain falling directly on the pile and snowmelt are <br /> the primary sources of water reaching the pile; groundwater is a <br /> minor source. Because of the small area involved and the near <br /> absence of groundwater flow in this vicinity, the minimum size <br /> underdrain as specified in the MLRB regulations is used. The <br /> underdrain will extend from one end of the waste pile to the other <br /> (900 ft. ) and will begin at the toe of the pile and extend upslope <br /> for 10 feet. A thickness of 4 ft is required unless bedrock is <br /> encountered during excavation. See figure 1. No more than 10% of <br /> the rock material can be less than 12 inches and no single rock <br /> shall be larger than 30 inches. Flow emanating from the drain will <br /> enter the lower intercept channel and subsequently be diverted to <br /> the east pond for settling. <br /> 14 <br />