UFC 3-270-04
15 March 2001
12.7. French Drains. French Drains are not recommended and should only be used if a drain-
age system with a perforated pipe cannot be used. These systems quickly clog if not properly
designed and constructed. They are expensive to repair if clogged and cannot be inspected.
If French Drains are required, a double-filter system (Figure 12.8.) should be used and is
essential to prevent the drain from clogging after relatively short periods of use. The criteria for
the double-filter system are as follows:
d15 percent passing size of filter material
<5
d85 percent passing size of protected material
d50 percent passing size of filter material
< 25
d50 percent passing size of protected material
d15 percent passing size of inner material
<5
d85 percent passing size of filter material
d50 percent passing size of inner material
< 25
d50 percent passing size of filter material
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