Excerpted from:<br>
DEPARTMENT OF THE ARMY U.S. ARMY CORPS OF ENGINEERS<br>
CEGS-02620 (August 1997)<br>
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In order to minimize the migration of fines into the coarser material while maintaining adequate permeability, the following criteria should be met:<br>
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All soils (except clays without a sand or silt fraction):<br>
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(15 percent size of drainage or filter material)/(85 percent size of material to be drained) = 5 (max)<br>
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(50 percent size of drainage or filter material)/(50 percent size of material to be drained) = 25 (max)<br>
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(15 percent size of drainage or filter material)/(85 percent size of material to be drained) = 5 (max)<br>
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(15 percent size of drainage or filter material = 0.4 (max)<br>
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All Soils<br>
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(15 percent size of drainage or filter material)/(15 percent size of material to be drained) = 5 (min)<br>
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for slotted/perforated subdrain pipes:<br>
(50 percent size of drainage or filter material)/(slot width) = 1.2 (min)<br>
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(50 percent size of drainage or filter material)/(hole diameter) = 1.0 (min)<br>
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Additional information can be found in USACOE TM 5-820-2.<br>