The filter may be too fine grained to convey enough water, to
provide a good working surface, or to pass the water freely without loss of
fines to a subdrain pipe. For this condition, a second filter layer is
placed on the first filter layer; the first filter layer is then considered
the soil to be protected, and the second filter layer is designed. The
finest filter soil is often at the base, with coarser layers above. This is
referred to as reversed or inverted filters.
Concrete sand (ASTM C33, Specifications for Concrete Aggregates)
suffices as a filter against the majority of fine-grained soils or silty or
clayey sands. For non-plastic silt, varved silt, or clay with sand or silt
lenses, use asphalt sand (ASTM D1073, Specifications for Fine Aggregates for
Bituminous Paving Mixtures) but always check the criteria in Figure 4.
Locally available natural materials are usually more economical than
processed materials, and should be used where they meet filter criteria.
The fine filter layer can be replaced with plastic filter cloths under the
following conditions (after Reference 3, Performance of Plastic Filter
Cloths as a Replacement for Granular Materials, by Calhoun, et al.):
(a) Non-woven filter cloths, or woven filter cloths with less than
4% open area should not be used where silt is present in sandy soils. A
cloth with an equivalent opening size (EOS) equal to the No. 30 sieve and an
open area of 36% will retain sands containing silt.
(b) When stones are to be dropped directly on the cloth, or where
uplift pressure from artesian water may be encountered, the minimum tensile
strengths (ASTM D1682, Tests for Breaking Load and Elongation of Textile
Fabrics) in the strongest and weakest directions should be not less than 350
and 200 lbs. respectively. Elongation at failure should not exceed 35%.
The minimum burst strength should be 520 psi (ASTM D751, Testing Coated
Fabrics). Where the cloths are used in applications not requiring high
strength or abrasion resistance, the strength requirements may be relaxed.
(c) Cloths made of polypropylene, polyvinyl chloride and
polyethylene fibers do not deteriorate under most conditions, but they are
affected by sunlight, and should be protected from the sun. Materials
should be durable against ground pollutants and insect attack, and
penetration by burrowing animals.
(d) Where filter cloths are used to wrap collection pipes or in
similar applications, backfill should consist of clean sands or gravels
graded such that the D+85, is greater than the EOS of the cloth. When
trenches are lined with filter cloth, the collection pipe should be
separated from the cloth by at least six inches of granular material.
(e) Cloths should be made of monofilament yarns, and the absorption
of the cloth should not exceed 1% to reduce possibility of fibers swelling
and changing EOS and percent of open area.
For further guidance on types and properties of filter fabrics see
Reference 4, Construction and Geotechnical Engineering Using Synthetic
Fabrics, by Koerner and Welsh.
7.1-274
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