TM 5-818-8/AFJMAN 32-1030
CHAPTER 3
FILTRATION AND DRAINAGE
retention), flow capacity, and clogging potential.
3-1 Water Control
These properties are indirectly measured by the
Control of water is critical to the performance of
apparent opening size (AOS) (ASTM D 4751),
buildings, pavements, embankments, retaining
permittivity (ASTM D 4491), and gradient ratio
walls, and other structures. Drains are used to
test (ASTM D 5101). The geotextile must also have
relieve hydrostatic pressure against underground
the strength and durability to survive construction
and retaining walls, slabs, and underground tanks
and long-term conditions for the design life of the
and to prevent loss of soil strength and stability in
drain. Additionally, construction methods have a
slopes, embankments, and beneath pavements. A
critical influence on geotextile drain performance.
properly functioning drain must retain the sur-
rounding soil while readily accepting water from
3-4. Piping Resistance
the soil and removing it from the area. These
general requirements apply to granular and geo-
of a geotextile to retain solid particles and is
textile filters. While granular drains have a long
related to the sizes and complexity of the openings
performance history, geotextile use in drains is
or pores in the geotextile. For both woven and
relatively recent and performance data are limited
nonwoven geotextiles, the critical parameter is the
to approximately 25 years. Where not exposed to
AOS. Table 3-1 gives the relation of AOS to the
sunlight or abrasive contact with rocks moving in
gradation of the soil passing the number 200 sieve
response to moving surface loads or wave action,
for use in selecting geotextiles.
long-term performance of properly selected geotex-
tiles has been good. Since long-term experience is
Table 3-1. Geotextile Filter Design Criteria.
limited, geotextiles should not be used as a substi-
Protected Soil
Permeability
tute for granular filters within or on the upstream
(Percent Passing
Woven Nonwoven2
face of earth dams or within any inaccessible
Piping1
No. 200 Sieve)
portion of the dam embankment. Geotextiles have
POA 3 > 10% k G> 5k S
AOS (mm) <0.6
Less than 5%
been used in toe drains of embankments where
(mm)
(Greater than #30
they are easily accessible if maintenance is re-
US Standard
quired and where malfunction can be detected.
Sieve)
Caution is advised in using geotextiles to wrap
POA > 4% k G> 5k S
AOS (mm) < 0.6
5 to 50%
permanent piezometers and relief wells where they
(mm)
form part of the safety system of a water retaining
(Greater than #30
US Standard
structure. Geotextiles have been used to prevent
Sieve)
infiltration of fine-grained materials into piezo-
AOS (mm) < 0.297 POA > 4% kG> 5kS
50 to 85%
meter screens but long-term performance has not
(mm)
been measured.
(Greater than #50
US Standard
3-2. Granular Drain Performance
Sieve)
Greater than 85% AOS (mm) < 0.297
kG > 5k S
To assure proper performance in granular drains,
(mm)
the designer requires drain materials to meet
(Greater than #50
grain-size requirements based on grain size of the
US Standard
surrounding soil. The two principal granular filter
Sieve)
criteria, piping and permeability, have been devel-
1
When the protected soil contains appreciable quantities of
oped empirically through project experience and
material retained on the No. 4 sieve use only the soil passing
laboratory testing. The piping and permeability
the No. 4 sieve in selecting the AOS of the geotextile.
criteria are contained in TF 5-820-2/ AFJMAN
2
k, is the permeability of the nonwoven geotextile and k S is
32-1016, Chap. 2.
the permeability of the protected soil.
3
POA = Percent Open Area.
3-3. Geotextile Characteristics Influencing Fil-
b. Percent Open Area Determination Procedure
ter Functions
for Woven Geotextiles.
The primary geotextile characteristics influencing
(1) Installation of geotextile. A small section
filter functions are opening size (as related to soil
of the geotextile to be tested should be installed in
3-1
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