15 March 2001
d15 percent passing size of aggregate in porous pipe
d85 percent passing size of filter material
12.5.3. Filter fabrics. The use of geotextile materials as filters is widely accepted in underdrains.
These materials may in some instances be used to replace one or more components of a graded
filter. Filter fabrics may be used to wrap the collector pipe (Figure 12.4.), thus permitting a
relatively fine backfill material to be used; or line the trench, (Figure 12.5.) allowing a relatively
course backfill material to be used (Figure 12.6.). Filter fabrics are rarely used to replace the
entire granular filter system. The only instance that a geotextile can completely replace a
granular system is with the soil being a clean granular material.
When a geotextile is used to wrap the collector pipe, the following criteria should be met:
d85 percent passing size of granular filter material (mm)
Apparent opening size (AOS) of geotextile (mm)
where: gradient ratio is the ratio of the hydraulic gradient through a soil-geotextile system to the
hydraulic gradient through the soil alone.
Gradient Ratio < 3