TM 5-818-8/AFJMAN 32-1030
a standard 2 by 2 inch slide cover, so that it can
one of the following conditions exists:
be put into a slide projector and projected onto a
(1) The soil is very widely graded, having a
coefficient of uniformity C greater than 20.
screen. Any method to hold the geotextile section
U
and maintain it perpendicular to the projected
(2) The soil is gap graded. (Soils lacking a
light can be used.
range of grain sizes within their maximum and
(2) Slide projector. The slide projector should
minimum grain sizes are called "gap graded" or
be placed level to eliminate any distortion of the
"skip graded" soils.) Should these conditions exist
geotextile openings. After placing the slide in the
in combination with risk of extremely high repair
projector and focusing on a sheet of paper approxi-
costs if failure of the filtration system occurs the
mately 8 to 10 feet away, the opening outlines can
gradient ratio test may be required.
be traced.
(3) Representative area. Draw a rectangle of
tion in permeability or permittivity of a geotextile
about 0.5 to 1 square foot area on the "projection
due to blocking of the pores by either soil particles
screen" sheet of paper to obtain a representative
or biological or chemical deposits. Some clogging
area to test; then trace the outline of all openings
takes place with all geotextiles in contact with
inside the designated rectangle.
soil. Therefore, permeability test results can only
(4) Finding the area. After removing the
be used as a guide for geotextile suitability. For
sheet, find the area of the rectangle, using a
woven geotextiles, if the POA is sufficiently large,
planimeter. If necessary, the given area may be
the geotextiles will be resistant to clogging. The
divided to accommodate the planimeter.
POA has proved to be a useful measure of clogging
(5) Total area of openings. Find the total area
resistance for woven textiles but is limited to
of openings inside rectangle, measuring the area of
woven geotextiles having distinct, easily measured
each with a planimeter.
openings. For geotextiles which cannot be evalu-
(6) Compute percent. Compute POA by the
ated by POA, soil- geotextile permeameters have
equation:
been developed for measuring soil-geotextile per-
meability and clogging. As a measure of the
Total Area Occupied by Openings
degree to which the presence of geotextile affects
POA=
x 100
the permeability of the soil- geotextile system, the
Total Area of Test Rectangle
gradient ratio test can be used (ASTM D 5101).
c. Flow Reversals. Piping criteria are based on
The gradient ratio is defined as the ratio of the
granular drain criteria for preventing drain mate-
hydraulic gradient across the geotextile and the 1
rial from entering openings in drain pipes. If flow
inch of soil immediately above the geotextile to
through the geotextile drain installation will be
the hydraulic gradient between 1 and 3 inches
reversing and/or under high gradients (especially
above the geotextile.
if reversals are very quick and involve large
3-5.
Permeability
changes in head), tests, modeling prototype condi-
a. Transverse Permeability. After installation,
tions, should be performed to determine geotextile
geotextiles used in filtration and drainage applica-
requirements.
tions must have a flow capacity adequate to
d. Clogging. There is limited evidence (Giroud
prevent significant hydrostatic pressure buildup in
1982) that degree of uniformity and density of
the soil being drained. This flow capacity must be
granular soils (in addition to the D8 5 size) influ-
maintained for the range of flow conditions for
ence the ability of geotextiles to retain the drained
that particular installation. For soils, the indicator
soil. For very uniform soils (uniformity coefficient
of flow capacity is the coefficient of permeability
2 to 4), the maximum AOS may not be as critical
as expressed in Darcy's Law (TM 5-820-2/
as for more well graded soils (uniformity coeffi-
AFSMAN 32-1016 ). The proper application of
cient greater than 5). A gradient ratio test with
Darcy's Law requires that geotextile thickness be
observation of material passing the geotextile may
considered. Since the ease of flow through a
be necessary to determine the adequacy of the
geotextile regardless of its thickness is. the prop-
material. In normal soil- geotextile filter systems,
erty of primary interest, Darcy's Law can be
detrimental clogging only occurs when there is
modified to define the term permittivity, Ψ, with
migration of fine soil particles through the soil
-1
units of sec. , as follows:
matrix to the geotextile surface or into the geotex-
tile. For most natural soils, minimal internal
migration will take place. However, internal mi-
(eq 3-1)
gration may take place under sufficient gradient if
3-2
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