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• 5.4 Underdrains <br />The refuse pile will be built as a side hill fill. As such, <br />there is no water course within the disposal area and the <br />surface runoff occurs as an overland flow. Previous sub- <br />, surface investigations indicate groundwater at depths greater <br />than some 35 feet below the ground surface. Therefore, the <br />• site is expected to be relatively dry. <br />Previous investigations (6, 7) indicate that the permeability <br />of compacted refuse material range from 1.6 x 10 -3 to 3 x 10 <br />-8 cm/sec., with a mean value of 1.2 X 10 -4 cm/sec. The <br />maximum in-place surface area of the refuse pile at elevation <br />~ • <br />6695' is approximately 200,000 sq. ft. Considering the mean <br />permeability, an infiltration rate of approximately 360 gpm <br />is obtained. This estimate is based on the assumption that <br />water will be available continuously for infiltration. <br />Calculating the permeability of the rock underdrain material <br />i <br />to be 10 cm/sec, and using a 4' wide x 2' deep section for <br />underdrain at a slope of 7%, an underdrain capacity of 84 gpm <br />is obtained. This results in a requirement of 4-5 under- <br />drains. It should be recognized that these computations <br />provide a guide for design and that some modifications as to <br />the need for fewer or more underdrains should be evaluated <br />. from time to-time based upon actual field observations. <br />24 <br />