Excerpted from:
DEPARTMENT OF THE ARMY U.S. ARMY CORPS OF ENGINEERS
CEGS-02620 (August 1997)


In order to minimize the migration of fines into the coarser material while maintaining adequate permeability, the following criteria should be met:

All soils (except clays without a sand or silt fraction):

(15 percent size of drainage or filter material)/(85 percent size of material to be drained) = 5 (max)

(50 percent size of drainage or filter material)/(50 percent size of material to be drained) = 25 (max)

(15 percent size of drainage or filter material)/(85 percent size of material to be drained) = 5 (max)

(15 percent size of drainage or filter material = 0.4 (max)

All Soils

(15 percent size of drainage or filter material)/(15 percent size of material to be drained) = 5 (min)

for slotted/perforated subdrain pipes:
(50 percent size of drainage or filter material)/(slot width) = 1.2 (min)

(50 percent size of drainage or filter material)/(hole diameter) = 1.0 (min)

Additional information can be found in USACOE TM 5-820-2.

Thank you

you might consider installing a barrier to seepage...possibly a membrane cutoff placed by a trencher being careful to assure a water tight seal at the interface of the underside of the spillway structure with the cutoff membrane.