filter (several times greater than the adjacent soil). It requires the D15 size of the filter to be at least 4 or 5
times the D15 of the soil. This criterion should provide adequate permeability for structural bedding
layers, but be insufficient for the groundwater drains.
Inadequate or No Filtering
Proper Filter Design
Several organizations have developed further restrictive criteria for filters. For instance, the
Bureau of Reclamation allows no filter aggregates larger than 3.0 inches, and the Corps of Engineers
specifies that,
D50 (filter) / D50 (soil)
<= 25
(8)
In other words, the D50 of the filter cannot be greater than 25 times the D50 of the soil. This is
intended to insure that the gradation curves of the filter and soil are generally parallel.
For perforated or slotted pipe, the D85 of the filter must be greater than the hole width or slot
diameter.
50
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