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%: <br />c <br />s <br />": <br />,. <br />i <br />i <br /> <br />CTL/Thompson <br />In the case of excess spoil disposal facilities, underdrain systems are <br />designed so that all ground water which seeps into the spoil and all water <br />which infiltrates the spoil is removed from the spoil before the spoil <br />becomes saturated. It is important, therefore, to estimate the quantity <br />of water which a subdrain can accommodate. The concepts and equations <br />proposed by Wilkins (1956) are useful in this respect. Wilkins proposed <br />the equation <br />Vv = Cpambin (Eq. 4.5) <br />where, <br />Vv = average velocity of water through the voids (equal <br />to discharge velocity divided by porosity), <br />C = a composite shape factor, <br />~ = the viscosity cf water, <br />m = the hydraulic mean radius of the rack voids (for a <br />given volume of particles, it is equal to the volume <br />of voids divided by total surface area of the particles, <br />or the void ratio divided by the surface area per unit <br />volume of solids) <br />i = hydraulic gradient, and <br />a,b,n = empirical constants <br />Based upon laboratory studies, Wilkins found that Eq. 4.5 can be rewritten <br />for clean gravels as: <br />Vv = 32.9 m0.5i0.59 (Eq. 4.6) <br />Wilkins checked this relation for rock passing the 8-inch sieve and <br />retained on the 7-inch sieve and found close agreement with minus 3/4-inch <br />rock and minus 3-inch rock. Leps (1973) expressed the equation in the <br />form <br />V = Wm0.5io.54 (Eq. 4.1) <br />v <br />where, <br />W = an empirical constant (C) for the rockfill material and <br />all other variables are as defined earlier. (According <br />to Wilkins, W varies from about 33 for crushed gravel to <br />about 46 for polished marbles in inch-second units). <br />-137- <br />