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Two rock subdrainage systems consisting of durable non -acid-forming and non- <br />toxic -forming rock will be constructed to intercept seepage from the Fire <br />Mountain Canal to assure the foundation of the coal mine waste pile does not <br />become saturated. The drains will be about 100 feet wide near where the <br />seepage emerges below the upper diversion ditch and then taper down to a two <br />foot wide by two feet deep drain for the West subdrainage system, and a three <br />feet wide by one feet deep drain for the East subdrainage system. A perforated <br />pipe will be installed in the rock drain. See Map 21-1 for details. <br />For the West subdrain, the majority of the rock drain will have 4 square feet of <br />3/4" screened or washed rock. The upper section of the rock drain will have up <br />to 200 square feet of screened or washed rock. The drain will have a slope of <br />2% to 8%. Darcy's Law can be used to calculate the flow though the drain. <br />Darcy's Law is Q = kA dh/dl <br />where: <br />Q = rate of water flow calculated by formula (0.16 to 64.0 cubic feet per minute) <br />k = hydraulic conductivity of clean gravel (2.0 to 200 ft/minute)' <br />A = column cross sectional area (4 SF) <br />dh/dl = hydraulic gradient, the change in head over the length of interest (0.02 to 0.08). <br />The estimated flow rate through the rock drain is 1.2 to 479 gallons per minute. <br />The seepage from the Fire Mountain Canal does not have a perceptible flow. <br />Therefore, even if the rock drain can only pass 1.2 gallons per minute, it is <br />adequate for the seepage from the Fire Mountain Canal. The perforated pipe in <br />the rock drain will significantly increase its flow capacity. <br />The rock drain including the upper wide and tapering segment of the drain will be <br />wrapped in geotextile fabric. Twelve inches of common fill will be placed over <br />the rock drain so as to protect against the entrance of surface water or leachate <br />from the coal processing waste. The perforated pipe in the rock drain will extend <br />the full length of the drain except for the final 20 feet which shall be a solid pipe. <br />The solid pipe will discharge into ditch D -J3. A screen shall be placed over the <br />outlet of the solid pipe to keep rodents out of the pipe. <br />Volume XI <br />TR -114 - 7 - 10/17 <br />