United States Department of Agriculture
Soil of
Conservation Service
P.O. Box 2890
Washington, D.C.
2001 3
December 19, 1991
DESIGN NOTE
NUMBER 24
GUIDE FOR THE USE OF GEOTEXTILES
1. INTRODUCTION.
Geotextiles are made to be versatile for solving problems associated with soil
and water conservation work. They can be very effective in improving drainage,
protecting against
piping or erosion and providing reinforcement or
separation of fill materials. Proper design is necessary to ensure adequate
function or service. They are manufactured from materials that deteriorate with
time depending on the use environment, installation period and/or method of
pre-installation storage. Careful evaluation of the location and intended
function is necessary to determine if geotextiles are appropriate and of the
type that should be used.
Properties that must be evaluated include strength,
apparent opening size (AOS) for filtering, net open area, and durability
(length of life) under storage, construction and installed conditions. The
actual life of geotextiles is not known and their use in inaccessible areas
must be considered carefully.
Geotextiles have a great variety of engineering properties
and physical characteristics. The designer must recognize the testing methods
and the test results that are significant to the intended application. Tests
are used to evaluate the acceptability of geotextiles for a particular use, and
it is important to verify the physical properties of the specified geotextile.
The use of geotextiles in SCS engineering practices is
generally defined by four functional applications; drainage, filtration,
reinforcement and separation.
a. Drainage. The ability of a geotextile to
convey fluid within the plane of the fabric. The thicker nonwoven geotextiles
have this capacity, while woven and heat bonded nonwoven geotextiles do not.
b. Filtration. The ability of a geotextile to
allow fluid flow thru the fabric plane but prevent the movement of soil
particles. Permeability of the fabric is an important design consideration, as
is the size and distribution of the openings in the geotextile.
c. Reinforcement. The ability of a geotextile to
distribute the loading imposed on soil, develop tensile strength and bridge
over voids, cracks, or gaps.
d. Separation. The ability of a geotextile to
keep two different materials apart during installation and subsequent use that
would otherwise tend to mix and compromise the intended integrity.
(210-VI-DN-24, November 1991)
2. SPECIFIED
USES. The following uses have been identified and specifications developed to
meet each as indicated in Table 1, Requirements
for Woven Geotextiles and Table 2, Requirements
for
Nonwoven Geotextiles.
a. Slope
protection. Utilizes both the filtration and the separation functions. In
some soft soil conditions, the reinforcement function may also be used.
Important performance properties included adequate permeability to permit
subgrade drainage; proper size and distribution of openings for soil retention,
and sufficient strength to withstand installation stresses. Installation
stresses are directly related to the height of rock drop allowed and the use of
sand or gravel to protect the geotextile and cushion the rock drop. A rough
fabric surface to promote bond with the base soils and resistance to sliding by
the rock is also a factor to be considered in assessing the adequacy for use in
slope protection. Nonwoven needle punched geotextiles are superior in this
feature.
b. Subsurface Drainage. The functions involved in
subsurface drains are filtration and separation. Adequate permeability and size
and distribution of opening size are important properties. Strength is mostly a
factor in the installation of the geotextile. Flexibility and the ability of
the fabric to fit closely to the surface irregularities is important.
c. Road Stabilization. The functions involved in this
use are separation, reinforcement, and, to a lesser degree, filtration. Some
permeability is required, but the key factors are the ability to take tensile
loads with uniform deformation. Backfill or covering procedures are also
important for the geotextile to function as intended.
3. PHYSICAL
PROPERTIES.
a. Tensile Strength - This property is measured by a grab test method. It determines the
effective strength of the fabric when subjected to forces that tend to pull the
fabric apart. (ASTM-D4595)
b. Elongation - The stretching or elongation of a geotextile when measured at the
point of failure or rupture of the fabric during the tensile strength test.
(ASTM-D4595)
c. Bursting Strength - The force or pressure required to rupture fabric by an expanding
diaphragm over which the fabric is clamped. (ASTM-D3786)
d. Puncture - The force required to penetrate or rupture a fabric with a small
rounded projectile. (ASTM-D4833) (210-VI-DN-24, November 1991)
e. Apparent Opening Size (AOS) 095 - The approximate size of the largest particle that would effectively
pass through the geotextile. (ASTM-D4751; Formerly EOS, Og5)
f.
Percent Open
Area (POA) - For a woven geotextiles, the summation of the open
area of an observed unit area of fabric divided by the total area, expressed in
percent. (CWO-02215-86 Corp of Engineers)
g. Permittivity - Permeability or hydraulic conductivity of a
geotextile divided by the geotextile thickness. The volumetric flow rate of
water per unit cross sectional area per unit head under laminar flow
conditions, normal to the plane of the fabric. (ASTM-D4491)
h. Transmissivity - The volumetric flow rate of water per unit width of a geotextile
specimen per unit gradient in a direction parallel to the plane of the
specimen. (ASTM-D4716)
i.
Ultraviolet
Light Resistance - A measure of the potential for the deterioration of
tensile strength in the fabric due to exposure to ultraviolet light and water.
(ASTM-D4355)
j.
Minimum
Average Roll Value
- A value in testing that is two
standard deviations less than the mean value. This means that the results of
approximately 98% of the tests taken would be equal to or greater than the
minimum average roll value.
4. DESIGN CONSIDERATIONS. For filtration, the designer must
specify geotextile properties that will allow retention of the soil being
protected while allowing sufficient flow through the geotextile, and prevent
clogging. Woven geotextile requires more critical evaluation and analysis than
nonwovens in most applications and the following guidelines are recommended for
determining requirements.
a. Woven Geotextiles. The AOS is critical when the
geotextile fabric serves as a filter, or if seepage gradients are significant.
It is also critical if the geotextile fabric protects underlying soils from
erosion, and flow or splash characteristics could affect the underlying base
soil. The AOS test provides a means of evaluation the retention characteristics
of a geotextile while POA provides a measure of flow through the geotextile and
adequate resistance to any reduction in permeability over time (clogging). POA
is used 0nlv for woven geotextiles. Since the POA data is obtained from laboratory evaluations without soil covering the
geotextile, it is an index test and doesn't provide a direct measure of field
performance. The base soil gradation must be known. If the base soil has particles larger than the #4 sieve, the gradation of the base soils must first be
regarded. (See SCS Soil Mechanics Note-1). The following recommendations then
apply to the regraded soil. (210-VI-DN-24, November 1991)
(1) Soils with more than 85%
passing the #200 sieve. Do not use a woven geotextile adjacent to these
soils. A layer of fine sand must be used between the base soil and the
geotextile if a woven geotextile is to be used. The AOS of the geotextile is
then determined considering the sand used as the base soil.
(2) Soils with 51-85% finer than
the #200 sieve. The AOS should be no larger than the openings in the U.S.
Standard Sieve #70, and no smaller than the openings in the U.S. Standard Sieve
#lo0 for proper soil retention.
For added clogging
protection, the POA should be 4% or greater.
For permeability, k of
the geotextile > k (soil).
(3) Soils with 15 to 50% finer than the
#200 sieve. The
AOS of the geotextile should be less than or equal to 1.0 times the d85 of
the base soil. The POA
should be 4% or
greater.
For permeability, k of the geotextile > 10k (soil).
(4) Soils with less than 15% finer than
the #200 sieve.
The AOS of the geotextile should be less -*than
or equal to 2.0 times the d85 of the base soil. The POA should be 6% or greater. The POA should
be as large as possible and may range up to 30% for available geotextiles that
meet the above criteria for this category of base soils.
For permeability, k of the geotextile > 10k (soil).
(5) In general, to reduce
the possibility of clogging, the geotextile should not be specified with an AOS
smaller than the U.S. Standard Sieve #l00. If the criteria in 2, 3, or 4 above results in an AOS smaller
than the #l00 sieve, laboratory tests will be necessary for evaluating the
clogging potential for a specific base soil and geotextile combination.
The AOS for each category should be as large as
possible without exceeding the criteria listed.
b. Nonwoven Geotextiles. AOS is not a controlled property
with nonwoven geotextiles. These geotextiles have a wide range of size
openings. (210-VI-DN-24, November 1991)
(1) The maximum size opening should be
no larger than a U.S. Standard Sieve #40. (As determined by methods in
ASTM-D4751.)
(2) In general, nonwoven geotextiles
retain more soil fines than do woven geotextiles. The structure of the
mechanically bonded needle-punched fabric helps to decrease the internal fabric
clogging potential.
(3) The nonwoven geotextiles have very
good permeability characteristics and should be strongly considered where
seepage flows are a concern.
(4) Nonwoven fabrics have a rougher surface than wovens. Therefore, the bond
between the soil and the fabrics offer more resistance to sliding along the
plane of contact.
c. Ultra Violet Light Protection. If the geotextile will be
subjected to any long periods of exposure, a more severe test for ultra violent
light should be specified in Section 7. The specified value of 150 hours (Table
1 or 2) is adequate for normal construction installation where the geotextile
will be covered as specified. A est period
of 500 hours should be specified where significant longer duration exposure is
anticipated.
5. CONSTRUCTION CONSIDERATIONS.
a. Satisfactory performance of the
selected geotextile is greatly dependent on the installation procedures and
field preparation of the surface to be protected. When geotextile fabrics are
used adjacent to fill or backfill, the fill soil placement is critical in
preventing conditions subject to plugging of the fabric. The following
techniques help minimize the movement of soils particles toward the fabric
surface and provide more area for flow through the fabric.
(1) Prepare soil surfaces adjacent to
fabrics so that all flow channels or voids larger than the openings in the
fabric are eliminated.
(2) Utilize soil compaction and
placement techniques to ensure that intimate contact between the fabric and the
soil is maintained.
(3) Provide a surface area as large as possible for the
filter (i.e., it is better to place the geotextile around the periphery of the
drain trench with gravel and pipe inside than to place the fabric around the
pipe where the surface area is smallest). (210-VI-DN-24, November 1991)
b. Other construction considerations
specific to the function or type of application are:
(1) Slope Protection - Class I and I1 (as
indicated in Tables 1 and 2, Material Specification m).
a. The method of placement of rock or
other material on the geotextile may have to be specified. Placement should be
accomplished by equipment capable of controlling the drop. Pushing or rolling
rock over the geotextile should not be allowed. The maximum drop is 3 feet for protected or unprotected geotextile. Where
conditions require a larger drop, the strength of the geotextile and/or
thickness of cushioning materials need to be increased.
Class I - Unprotected -
limit the height for
dropping stone onto bare geotextile to 3 feet.
Class I1 - Protected - require the use of 6 inch sand or soil cushion for bedding the stone on
the geotextile, and limit the height of drop to 3 feet.
b. To prevent movement of surface soil
where groundwater and seepage pressures are a factor, the geotextile must be in
intimate contact with the soil. This is especially true on sloping surfaces
where flow may occur beneath the fabric. A sand
layer bedding material may have to be specified to insure this contact in some
cases. Gravel placed on the geotextile will hold it in place and minimize voids
under the riprap. Careful grading of this gravel placed over the geotextile is
not important and pit-run gravel is often adequate if it is free draining. Embedment of the geotextile in a
trench to form a cutoff at regular intervals down the slope will also help
prevent rilling beneath the fabric. Cutoffs are to be more closely spaced in
highly erodible soils and spaced wider apart in more stable soils.
c. When a geotextile is used as a
filter material replacement for the purpose of preventing particle migration,
it is recommended that laps of adjacent fabric require machine sewing or other
positive joining method. The method of overlap should be specified on the
drawings or in the construction details.
(2) Subsurface Drainage - Class I11 (as
indicated in Tables 1 and 2, Material Specification 592).
a. Tables 1 & 2 are intended for normal operating
conditions where material will not be dropped more (210-VI-DN-24, November
1991) than five (5) feet onto the geotextile, where trench depths from
the normal ground surface will be no deeper than ten (101 feet and sharp, angular aggregates are not used.
b. The materials listed in Tables 1 & 2 are for average conditions. When the materials are
used in trenches deeper than ten (10) feet or with sharp, angular aggregates,
heavier geotextiles are recommended; tensile strength should be increased to
150 lbs. minimum. burst strength should be increased to 300 psi minimum.
c. To prevent movement of surface soil
where groundwater and seepage pressures are a factor, the geotextile must be in
intimate contact with the subgrade soil. Voids between the geotextiles and the
base soil need to be minimized to prevent the collecting of fines behind the
fabric and subsequent clogging. The geotextile should be pulled flat during
installation to eliminate wrinkles and folds that create voids.
d. If flow in the plane of the
geotextile is a concern in the drain installation, the type and thickness of
the fabric becomes an important criterion. A heavier weight nonwoven needle
punched fabric should be used.
(3) Road Stabilization - Class IV (as indicated in Tables 1 and 2, Material
Specification 592).
a. Tables 1 and 2 are intended for
light to medium loading in both weight and frequency of traffic.
b. (If the subgrade is soft and it is
determined in design that the potential for rutting is high, the minimum
overlap should be increased to the following: Recommended Geotextile Overlap
(210-VI-DN-24, November 1991)
6. GENERAL DISCUSSION OF TABLES 1 and 2.
a. The
materials covered under Table 1 are woven monofilaments that are generally
black in color and don't change noticeably from one class to the next. Heavier
monofilaments will produce stronger fabric, but if the AOS and POA also change,
a heavier fabric could have the same strength as a lighter fabric. In woven
geotextiles the AOS and POA must be considered along with the fabric mass per
unit area, thus making field classification difficult.
b. The nonwoven materials covered
under Table 2 are restricted to needle punched geotextiles with the exception
of Class IV. These needle-punched fabrics can generally be recognized in the
field by weight. The Class I geotextiles are approximate 8 oz./yd; Class 11, 6
oz / yd, and Class 111, 9
5 oz./yd. Class IV allows the use of heat bonded geotextiles
that are generally not previously needle punched. These materials weigh about
the same as the Class I11
needle punched.
c. A class of geotextile should be
selected based on the intended use with appropriate considerations for
construction and installation methods. For the most part, once a class of
geotextiles is selected, it is intended that materials from either Table 1 or 2
may be used. Where specific desired characteristics are provided by one type of
geotextile vs. the other, the appropriate class and type may be specified.
GEOTEXTILE REFERENCES:
1. Specifiers Guide (Annual Issue),
Geotechnical Fabrics Report, Industrial Fabrics Association International, 345
Cedar Bldg., Suite 450, St. Paul, MN 55101.
2. Geotextile Engineering Manual, Federal Highway administration Geotechnical and Material Branch, HHO-33, Room 3203, 400 7th Street S.W., Washington, D.C. 20590.
3. Construction Fabric General
Specifications Guidelines, Task Force 25, ASSHTO-AGC-ARTBA Joint Committee.
4. Koerner, Robert M. (1986), Designing with Geosvnthetics, Prentice-Hall,
Englewood Cliffs, NJ.
5. Design and Practice Manual, (1987)
Polyfelt Inc., Technical Department, P.O. Box 727, Evergreen, AL 36401
(210-VI-DN-24, November 1991)
