D85 (filter) / Hole diameter
> (1.0 to 1.2)
(9)
D85 (filter) / Slot width
> (1.2 to 1.4)
(10)
These and other criteria for graded filters are illustrated on Figure 24.
The above criteria also apply to the armor layer in relation to the filter layer. That is, the armor
layer must retain the filter layer as the filter retains the soil. In some cases, two filter layers may be
required to provide the necessary transition from the soil to the armor.
Synthetic filter fabrics, available in woven and non-woven varieties, can be used in place of
graded stone filters. Woven cloths, manufactured of high strength nylon or other synthetic fibers,
provide a uniform mesh with a constant opening size which can be matched to the soil characteristics.
Non-woven cloths, manufactured from masses of somewhat randomly oriented fibers bonded together by
chemicals, heat or pressure,, come in various standard thicknesses. Unlike woven cloths, however, they
lack uniform-sized openings, their principal advantage being lower cost.
Guidance on the selection of filter fabrics is contained in Plastic Filter Fabric [U. S. Army Corps
of Engineers (1977a)]. Selection is based on the equivalent opening size (EOS), which the Corps defines
as "the number of the U. S. Standard Sieve having openings closest in size to the filter fabric openings".
Material will first be retained on the sieve whose number is equal to the EOS. The EOS of commonly
used filter fabrics is given in Table 10. The appropriate filter fabric should be selected as follows:
For granular soils with less than 50 percent by weight fines
(minus No. 200 materials)
85%, Passing Size of Soil
=> 1
(11)
Opening Size of the EOS Sieve
For other soils, the EOS should not be less than 70 (0.0083 in.). Furthermore, to reduce problems with
clogging, no fabric should be used whose EOS is greater than 100 (openings smaller than the mesh of a
No. 100 sieve). Also, no filter fabric should be used alone if the underlying soil contains more than 85
percent of particles finer than the No. 200 sieve. In those cases, an intermediate sand layer may provide
the necessary transition layer from the in-situ soil to the filter fabric.
51
Previous Page
