ASTM D4491/D4491M-22

Standard Test Methods for Water Permeability of Geotextiles by Permittivity

SIGNIFICANCE AND USE
5.1 These test methods are considered satisfactory for acceptance testing of commercial shipments of geotextiles since the methods have been used extensively in the trade for acceptance testing.  
5.1.1 In case of a dispute arising from differences in reported test results when using these test methods for acceptance testing of commercial shipments, the purchaser and the supplier should conduct comparative tests to determine if there is a statistical bias between their laboratories. Competent statistical assistance is recommended for the investigation of bias. As a minimum, the two parties should take a group of test specimens that are as homogeneous as possible and that are from a lot of material of the type in question. The test specimens should then be randomly assigned in numbers to each laboratory for testing. The average results from the two laboratories should be compared using Student's t-test for unpaired data and an acceptable probability level chosen by the two parties before the start of testing. If a bias is found, either its cause must be found and corrected, or the purchaser and the supplier must agree to interpret future test results in light of the known bias.  
5.1.2 When the dispute involves test results produced with either the Method B falling head test or the Method C air flow test, the Method A constant head test performed with a 50 mm head should be used as the referee method.  
5.1.3 When the dispute involves Method C, the actual water temperature used for the water flow tests must be recorded and the viscosity of water at the test temperature must be used in the conversion from the air flow to water flow as described in Section 16, without the application of the temperature correction.  
5.1.4 Permittivity is an indicator of the quantity of water that can pass through a geotextile in an isolated condition.  
5.1.5 As there are many applications and environmental conditions under which a geotextile may be used, care should be taken when...
SCOPE
1.1 These test methods cover procedures for determining the hydraulic conductivity (water permeability) of geotextiles in terms of permittivity under standard testing conditions, in the uncompressed state. Included are three procedures: the constant head and falling head methods using a water flow apparatus, and the air flow method using an air flow apparatus.  
1.2 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system are not necessarily exact equivalents; therefore, to ensure conformance with the standard, each system shall be used independently of the other, and values from the two systems shall not be combined.  
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 30-Apr-2022
• ICS: 59.080.70 - Geotextiles
• Technical Committee: D35 - Geosynthetics
• Drafting Committee: D35.03 - Permeability and Filtration

Relations

• Refers: ASTM D4439-24 - Standard Terminology for Geosynthetics
• Effective Date: 01-Feb-2024
• Refers: ASTM D5199-12(2019) - Standard Test Method for Measuring the Nominal Thickness of Geosynthetics
• Effective Date: 15-Jun-2019
• Refers: ASTM D4439-18 - Standard Terminology for Geosynthetics
• Effective Date: 15-Apr-2018
• Refers: ASTM D4439-17 - Standard Terminology for Geosynthetics
• Effective Date: 01-Aug-2017
• Refers: ASTM D123-17 - Standard Terminology Relating to Textiles
• Effective Date: 01-Mar-2017
• Refers: ASTM D123-15b - Standard Terminology Relating to Textiles
• Effective Date: 15-Sep-2015
• Refers: ASTM D123-15a - Standard Terminology Relating to Textiles
• Effective Date: 01-Sep-2015
• Refers: ASTM D4439-15a - Standard Terminology for Geosynthetics
• Effective Date: 01-Sep-2015
• Refers: ASTM D4439-15 - Standard Terminology for Geosynthetics
• Effective Date: 01-Jul-2015
• Refers: ASTM D123-15 - Standard Terminology Relating to Textiles
• Effective Date: 01-Apr-2015
• Refers: ASTM D653-14 - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Aug-2014
• Refers: ASTM D4439-14 - Standard Terminology for Geosynthetics
• Effective Date: 01-Mar-2014
• Refers: ASTM D123-13a - Standard Terminology Relating to Textiles
• Effective Date: 15-Jun-2013
• Refers: ASTM D123-13ae1 - Standard Terminology Relating to Textiles
• Effective Date: 15-Jun-2013
• Refers: ASTM D123-13 - Standard Terminology Relating to Textiles
• Effective Date: 15-May-2013

Overview

ASTM D4491/D4491M-22: Standard Test Methods for Water Permeability of Geotextiles by Permittivity provides procedures for determining the hydraulic conductivity (water permeability) of geotextiles through measurement of permittivity under standard, uncompressed conditions. These test methods are crucial for the acceptance testing of commercial shipments of geotextiles, ensuring product consistency and performance across a wide range of environmental applications.

The standard describes three test procedures: constant head, falling head (both using water flow), and an air flow method. These tests are instrumental for characterizing the ability of geotextiles to transmit water, supporting their selection and use in geotechnical engineering projects such as drainage, filtration, and soil stabilization.

Key Topics

• Permittivity Measurement: Focuses on determining the rate of water flow per unit of cross-sectional area per unit head through a geotextile under laminar flow conditions. Permittivity is a key indicator of geotextile performance.
• Constant Head Test (Method A): Maintains a steady 50 mm head of water on the geotextile, serving as the referee test for resolving result disputes and for products with high flow rates.
• Falling Head Test (Method B): Suitable for lower permeability geotextiles, measures water flow as the head decreases over time.
• Air Flow Test (Method C): Measures airflow through the geotextile and converts this value to water flow, useful for quality control and comparative testing.
• Sampling and Specimen Preparation: Includes guidance for representative sampling from commercial rolls and specimen preparation procedures to ensure accurate, reproducible results.
• Data Integrity: The standard outlines dispute resolution in case of differences in reported results, recommending comparative laboratory testing and statistical analysis to determine and resolve bias.
• Safety and Compliance: Emphasizes adherence to proper safety, health, and environmental practices during testing.

Applications

The ASTM D4491/D4491M-22 standard is widely used in:

• Construction and Civil Engineering: Assessing geotextiles for use in drainage systems, filtration, embankment stabilization, retaining walls, and erosion control.
• Quality Control: Acceptance testing of geotextile shipments to ensure products meet specified permeability requirements, supporting procurement decisions and regulatory compliance.
• Research and Development: Characterizing new geotextile materials during product development or for performance benchmarking.
• Dispute Resolution: Providing impartial methods for resolving discrepancies between suppliers and purchasers, enhancing transparency in supplier-customer relationships.

Testing geotextiles by permittivity is especially valuable because it provides a direct, standardized comparison between different materials, regardless of thickness or structure. This ensures that performance in controlled lab settings can be reliably translated to field expectations, with clear documentation supporting specification compliance and selection.

Related Standards

• ASTM D123: Terminology Relating to Textiles
• ASTM D653: Terminology Relating to Soil, Rock, and Contained Fluids
• ASTM D4439: Terminology for Geosynthetics
• ASTM D5199: Test Method for Measuring the Nominal Thickness of Geosynthetics
• ASTM E691: Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method

These related ASTM standards assist in ensuring measurement consistency, clear terminology, and interoperability of test data across various applications in materials testing and civil engineering.

---

Keywords: water permeability, permittivity, geotextiles, ASTM D4491, hydraulic conductivity, constant head test, falling head test, air flow test, geotechnical engineering, quality control

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D4491/D4491M − 22
Standard Test Methods for
Water Permeability of Geotextiles by Permittivity
This standard is issued under the fixed designation D4491/D4491M; the number immediately following the designation indicates the
year of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last
reapproval. A superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope E691Practice for Conducting an Interlaboratory Study to
Determine the Precision of a Test Method
1.1 These test methods cover procedures for determining
2.2 ASTM Adjuncts:
the hydraulic conductivity (water permeability) of geotextiles
in terms of permittivity under standard testing conditions, in Detailed Drawings and Materials List for Construction, 10
Drawings
the uncompressed state. Included are three procedures: the
constant head and falling head methods using a water flow
3. Terminology
apparatus, and the air flow method using an air flow apparatus.
3.1 Definitions:
1.2 The values stated in either SI units or inch-pound units
3.1.1 geotextile, n—a permeable geosynthetic comprised
are to be regarded separately as standard. The values stated in
solely of textiles.
each system are not necessarily exact equivalents; therefore, to
ensure conformance with the standard, each system shall be
3.1.2 permeability, n—the rate of flow of a liquid under a
used independently of the other, and values from the two
differential pressure through a material.
systems shall not be combined.
3.1.2.1 Discussion—The nominal thickness is used as it is
difficult to evaluate the pressure on the geotextile during the
1.3 This standard does not purport to address all of the
test, thereby making it difficult to determine the thickness of
safety concerns, if any, associated with its use. It is the
the fabric under these test conditions.
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter-
3.1.3 permeability, n—of geotextiles, hydraulic conductiv-
mine the applicability of regulatory limitations prior to use.
ity.
1.4 This international standard was developed in accor-
3.1.4 permittivity, (ψ), (T−1), n—of geotextiles, the volu-
dance with internationally recognized principles on standard-
metric flow rate of water per unit cross-sectional area per unit
ization established in the Decision on Principles for the
head under laminar flow conditions, in the normal direction
Development of International Standards, Guides and Recom-
through a geotextile.
mendations issued by the World Trade Organization Technical
3.1.5 For the definitions of other terms relating to
Barriers to Trade (TBT) Committee.
geotextiles, refer toTerminology D4439. For the definitions of
2. Referenced Documents
textile terms, refer to Terminology D123. For the definition of
coefficient of permeability, refer to Terminology D653.
2.1 ASTM Standards:
D123Terminology Relating to Textiles
4. Summary of Test Method
D653Terminology Relating to Soil, Rock, and Contained
Fluids
4.1 Water Flow Test Methods—These test methods describe
D4439Terminology for Geosynthetics
proceduresfordeterminingthepermittivityofgeotextilesusing
D5199Test Method for Measuring the Nominal Thickness
constant head or falling head test procedures with a water flow
of Geosynthetics
apparatus:
4.1.1 MethodA– Constant Head Test—Ahead of 50 mm of
These test methods are under the jurisdiction of ASTM Committee D35 on
water is maintained on the geotextile throughout the test. The
Geosynthetics and are the direct responsibility of Subcommittee D35.03 on
quantity of flow is measured versus time. The constant head
Permeability and Filtration.
test is used when the flow rate of water through the geotextile
Current edition approved May 1, 2022. Published June 2022. Originally
approved in 1985. Last previous edition approved in 2021 as D4491/D4491M–21. is so large that it is difficult to obtain readings of head change
DOI: 10.1520/D4491_D4491M-22.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on Detailed drawings and a materials list for construction are available from
the ASTM website. ASTM International Headquarters. Order Adjunct No. ADJD4491.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D4491/D4491M − 22
versustimeinthefallingheadtest.Theconstantheadtestisthe be taken when attempting to apply the results of these test
referee method for this standard. methods to the field performance of a geotextile.
NOTE 1—Data has shown agreement between the falling and constant
5.2 Sincetherearegeotextilesofvariousthicknessesinuse,
head methods of determining permittivity of geotextiles. Selection of the
evaluation in terms of their Darcy coefficient of permeabilities
test method, that is, constant or falling head, is left to the technician
can be misleading. In many instances, it is more significant to
performing the test.
evaluate the quantity of water that would pass through a
4.1.2 Method B – Falling Head Test—Acolumn of water is
geotextile under a given head over a particular cross-sectional
allowed to flow through the geotextile and readings of head
area; this is expressed as permittivity.
changes versus time are taken. The flow rate of water through
5.3 If the permeability of an individual geotextile is of
thegeotextilemustbeslowenoughtoobtainaccuratereadings.
importance, a nominal coefficient of permeability, as related to
4.1.3 Method C – Air Flow Test—A geotextile specimen is
geotechnical engineering, may be computed. By multiplying
subjected to increasing air flow while the flow rate and
permittivity times the nominal thickness of the geotextile, as
differential pressure are measured. Two flow rate data points
determined by Test Method D5199, the nominal coefficient of
are obtained at pressures of 250 and 500 Pa, which are used to
permeability is obtained.
determinethecharacteristicflowequationofthespecimen.The
waterpermittivityat50mmwaterheadisthencalculatedusing
NOTE 2—The nominal thickness is used as it is difficult to evaluate the
the conversion algorithm described in this standard.
pressure on the geotextile during the test, thereby making it difficult to
determine the thickness of the fabric under these test conditions.
5. Significance and Use
6. Apparatus
5.1 These test methods are considered satisfactory for ac-
ceptance testing of commercial shipments of geotextiles since
6.1 Water Flow Apparatus—The apparatus for performing
the methods have been used extensively in the trade for
the water flow tests shall conform to one of the following
acceptance testing.
arrangements:
5.1.1 In case of a dispute arising from differences in
6.1.1 The apparatus must be capable of maintaining a
reported test results when using these test methods for accep-
constant head of water on the geotextile being tested, or
tance testing of commercial shipments, the purchaser and the
6.1.2 Theapparatusmustbecapableofbeingusedasfalling
suppliershouldconductcomparativeteststodetermineifthere
head apparatus.
is a statistical bias between their laboratories. Competent
6.1.3 Thelocationofthemanometerformeasuringthehead
statistical assistance is recommended for the investigation of
loss in either constant head or falling head methods shall be
bias.Asaminimum,thetwopartiesshouldtakeagroupoftest
locateddirectlybeneaththespecimen.Forthedeviceshownin
specimens that are as homogeneous as possible and that are
Fig. 1, this may be accomplished by drilling and tapping a
from a lot of material of the type in question. The test
small (3 mm) diameter hole in the top plate of the bottom
specimens should then be randomly assigned in numbers to
reservoir tank directly beneath the specimen, and attaching the
each laboratory for testing. The average results from the two
manometer to this plate.
laboratories should be compared using Student’s t-test for
unpaireddataandanacceptableprobabilitylevelchosenbythe
6.2 In addition, the apparatus must not be the controlling
two parties before the start of testing. If a bias is found, either
agent for flow during the test. It will be necessary to establish
itscausemustbefoundandcorrected,orthepurchaserandthe
a calibration curve of volumetric flow rate versus head for the
suppliermustagreetointerpretfuturetestresultsinlightofthe
apparatus alone in order to establish compliance with this
known bias.
requirement (see 11.7).
5.1.2 When the dispute involves test results produced with
6.3 Refer to Fig. 1 for a schematic drawing of a device that
either the Method B falling head test or the Method C air flow
conforms to all of the above requirements.The device consists
test, the MethodAconstant head test performed with a 50 mm
of an upper and lower unit, which fasten together. The
head should be used as the referee method.
geotextile specimen is positioned in the bottom of the upper
5.1.3 WhenthedisputeinvolvesMethodC,theactualwater
unit. There is a standpipe for measuring the constant head
temperature used for the water flow tests must be recorded and
value. The rotating discharge pipe allows adjustment of the
the viscosity of water at the test temperature must be used in
head of water at the bottom of the specimen. See ADJD4491.
the conversion from the air flow to water flow as described in
Section 16, without the application of the temperature correc- 6.4 Air Flow Apparatus—The apparatus for performing the
tion. air flow tests shall conform to the following specifications:
5.1.4 Permittivity is an indicator of the quantity of water
6.4.1 Clean Gas Pressure Source, with regulation (filtered
that can pass through a geotextile in an isolated condition.
air).
5.1.5 As there are many applications and environmental
6.4.2 Pressure Sensor—Pressure measurements must be ob-
conditions under which a geotextile may be used, care should
tained with a digital pressure transducer accurate to 65Pato
1000Pa, and 61% above 1000Pa.
6.4.2.1 The head (upstream) pressure manometer tap must
Supporting data have been filed atASTM International Headquarters and may
be installed immediately upstream, within 10 mm of the test
beobtainedbyrequestingResearchReportRR:D35-1007.ContactASTMCustomer
Service at [e-mail]. specimen surface.
D4491/D4491M − 22
FIG. 1 Constant and Falling Head Permeability Apparatus
6.4.2.2 The tail (downstream) pressure sensor must be 6.4.3.3 The filter holder should be checked for leaks by
installed25mmormorefromthegeotextiletestspecimen,and placinganimpermeablemembraneintheholderandincreasing
within the 25 mm diameter section. the pressure to the maximum capacity of the pressure sensor
6.4.3 Closed Specimen Holder: and holding it for a period of 1 min. The flow rate measured
6.4.3.1 Specimen holder for the test specimens that fully during this period must be zero, indicating a leak-free seal.
confines the perimeter of the specimen to prevent any lateral 6.4.4 Metal Punch, used to cut a suitable size geotextile
pressure losses. from the test sheet to fit the test specimen holder.
6.4.3.2 The specimen flow area shall be 25 mm to 50 mm 6.4.5 Flow Rate Measurement Sensors—The apparatus
diameter. Smaller diameter devices are not acceptable. should be equipped with a digital flow meter to measure the
D4491/D4491M − 22
flow rates at the two desired pressures. The flow meter must 8.3 The test water must be maintained at the standard
haveanaccuracyofatleast 60.5lpmor 62%ofthemeasured atmosphere for geosynthetic testing, which is 21 6 2°C.
value, whichever is larger.
9. Specimen Preparation
6.4.6 Flow Section—The geometry of the air flow apparatus
at the section where the test specimen is located shall have a
9.1 To obtain a representative value of permittivity, take
uniform pipe diameter equal to the flow area, both upstream
four specimens from each full-width laboratory sample as
and downstream of the test specimen, for a minimum distance
described below.
of two diameters.
9.2 Referring to Fig. 2, select four specimens,A, B, C, and
D, as follows:
7. Sampling
9.2.1 Select four specimens equally spaced across the width
7.1 LotSample—Asalotsampleforacceptancetesting,take
of the sample. For woven geotextiles, take the test specimens
at random the number of rolls of geotextile directed in an
alongadiagonallineextendingfromthelowerleft-handcorner
applicable material specification or other agreement between
to the upper right-hand corner of the laboratory sample. None
the purchaser and the supplier. Consider rolls of geotextile to
of the test specimens shall be closer to the corner of the
be the primary sampling units. If the specification requires
laboratory sample than 200 mm [8 in.].
samplingduringmanufacture,selecttherollsforthelotsample
9.2.2 For the Method C air flow tests, take ten test speci-
at uniformly spaced time intervals throughout the production
mensequallyspacedacrossthewidthofthesample.Forwoven
period.
geotextiles, take the ten test specimens equally spaced along a
diagonal line extending from the lower left-hand corner to the
NOTE 3—An adequate specification or other agreement between the
purchaser and the supplier requires taking into account the variability
upper right-hand corner of the laboratory sample. None of the
between rolls of geotextile and between specimens from a swatch from a
specimens shall be closer to the edge of the laboratory sample
roll of geotextile so as to provide a sampling plan with a meaningful
than 200 mm [8 in.].
producer’s risk, consumer’s risk, acceptable quality level, and limiting
9.2.2.1 Five test specimens may be obtained in lieu of ten if
quality level.
theairflowdevicehasaflowareaof50mmdiameter,provided
7.2 Laboratory Sample—Take for the laboratory sample a
thetestresultcomplieswiththeMethodAresultinaccordance
full roll-width sample extending a minimum of 1 m along the
with 16.7.
selvage from each sample roll such that the requirements of
9.2.3 Cut specimens shall fit the testing apparatus, for
Section 9 can be met.Take a sample that will exclude material
example,73mm[2.87in.]indiameterforthedeviceillustrated
from the outer wrap of the roll or the inner wrap around the
in Fig. 1.
core unless the sample is taken at the production site, at which
NOTE 5—If the illustrated device is used for the water flow tests, the
point inner and outer wrap material may be used.
specimens are attached to the specimen ring by contact cement.
8. Test Water Preparation – Water Flow Tests
9.3 Condition the water flow test specimens by soaking in a
closed container of de-aired water, at room conditions, for a
8.1 To provide reproducible test results, the test water used
minimumperiodof2h.Theminimumspecimendiameteristo
for the water flow tests shall be de-aired to bring the dissolved
be 50 mm [2 in.].
oxygen content down to a maximum of six parts per million.
The dissolved oxygen content may be determined by either
10. Apparatus and Operator Process Control
commerciallyavailablechemicalkitsorbyadissolvedoxygen
meter. 10.1 Option1–WaterFlowApparatusMethodsAandBvia
No. 200 Sieve Mesh:
NOTE 4—The de-airing system may be either a commercially available
10.1.1 Prepare one specimen of a No. 200 standard U.S.
system or one consisting of a vacuum pump capable of removing a
sieve mesh to fit the test apparatus.
minimumof
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D4491/D4491M − 21 D4491/D4491M − 22
Standard Test Methods for
Water Permeability of Geotextiles by Permittivity
This standard is issued under the fixed designation D4491/D4491M; the number immediately following the designation indicates the
year of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last
reapproval. A superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope
1.1 These test methods cover procedures for determining the hydraulic conductivity (water permeability) of geotextiles in terms
of permittivity under standard testing conditions, in the uncompressed state. Included are three procedures: the constant head and
falling head methods using a water flow apparatus, and the air flow method using an air flow apparatus.
1.2 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each
system are not necessarily exact equivalents; therefore, to ensure conformance with the standard, each system shall be used
independently of the other, and values from the two systems shall not be combined.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D123 Terminology Relating to Textiles
D653 Terminology Relating to Soil, Rock, and Contained Fluids
D4439 Terminology for Geosynthetics
D5199 Test Method for Measuring the Nominal Thickness of Geosynthetics
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
2.2 ASTM Adjuncts:
Detailed Drawings and Materials List for Construction, 10 Drawings
3. Terminology
3.1 Definitions:
3.1.1 geotextile, n—a permeable geosynthetic comprised solely of textiles.
These test methods are under the jurisdiction of ASTM Committee D35 on Geosynthetics and are the direct responsibility of Subcommittee D35.03 on Permeability and
Filtration.
Current edition approved May 1, 2021May 1, 2022. Published May 2021June 2022. Originally approved in 1985. Last previous edition approved in 20202021 as
D4491/D4491M – 20.D4491/D4491M – 21. DOI: 10.1520/D4491_D4491M-21.10.1520/D4491_D4491M-22.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Detailed drawings and a materials list for construction are available from ASTM International Headquarters. Order Adjunct No. ADJD4491.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D4491/D4491M − 22
3.1.2 permeability, n—the rate of flow of a liquid under a differential pressure through a material.
3.1.2.1 Discussion—
The nominal thickness is used as it is difficult to evaluate the pressure on the geotextile during the test, thereby making it difficult
to determine the thickness of the fabric under these test conditions.
3.1.3 permeability, n—of geotextiles, hydraulic conductivity.
3.1.4 permittivity, (ψ), (T−1), n—of geotextiles, the volumetric flow rate of water per unit cross-sectional area per unit head under
laminar flow conditions, in the normal direction through a geotextile.
3.1.5 For the definitions of other terms relating to geotextiles, refer to Terminology D4439. For the definitions of textile terms,
refer to Terminology D123. For the definition of coefficient of permeability, refer to Terminology D653.
4. Summary of Test Method
4.1 Water Flow Test Methods—These test methods describe procedures for determining the permittivity of geotextiles using
constant head or falling head test procedures with a water flow apparatus:
4.1.1 Method A – Constant Head Test—A head of 50 mm of water is maintained on the geotextile throughout the test. The quantity
of flow is measured versus time. The constant head test is used when the flow rate of water through the geotextile is so large that
it is difficult to obtain readings of head change versus time in the falling head test. The constant head test is the referee method
for this standard.
NOTE 1—Data has shown agreement between the falling and constant head methods of determining permittivity of geotextiles. Selection of the test
method, that is, constant or falling head, is left to the technician performing the test.
4.1.2 Method B – Falling Head Test—A column of water is allowed to flow through the geotextile and readings of head changes
versus time are taken. The flow rate of water through the geotextile must be slow enough to obtain accurate readings.
4.1.3 Method C – Air Flow Test—A geotextile specimen is subjected to increasing air flow while the flow rate and differential
pressure are measured. Two flow rate data points are obtained at pressures of 250 and 500 Pa, which are used to determine the
characteristic flow equation of the specimen. The water permittivity at 50 mm water head is then calculated using the conversion
algorithm described in this standard.
5. Significance and Use
5.1 These test methods are considered satisfactory for acceptance testing of commercial shipments of geotextiles since the
methods have been used extensively in the trade for acceptance testing.
5.1.1 In case of a dispute arising from differences in reported test results when using these test methods for acceptance testing of
commercial shipments, the purchaser and the supplier should conduct comparative tests to determine if there is a statistical bias
between their laboratories. Competent statistical assistance is recommended for the investigation of bias. As a minimum, the two
parties should take a group of test specimens that are as homogeneous as possible and that are from a lot of material of the type
in question. The test specimens should then be randomly assigned in numbers to each laboratory for testing. The average results
from the two laboratories should be compared using Student’s t-test for unpaired data and an acceptable probability level chosen
by the two parties before the start of testing. If a bias is found, either its cause must be found and corrected, or the purchaser and
the supplier must agree to interpret future test results in light of the known bias.
5.1.2 When the dispute involves test results produced with either the Method B falling head test or the Method C air flow test,
the Method A constant head test performed with a 50 mm head should be used as the referee method.
5.1.3 When the dispute involves Method C, the actual water temperature used for the water flow tests must be recorded and the
Supporting data have been filed at ASTM International Headquarters and may be obtained by requesting Research Report RR:D35-1007. Contact ASTM Customer
Service at [e-mail].
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viscosity of water at the test temperature must be used in the conversion from the air flow to water flow as described in Section
16, without the application of the temperature correction.
5.1.4 Permittivity is an indicator of the quantity of water that can pass through a geotextile in an isolated condition.
5.1.5 As there are many applications and environmental conditions under which a geotextile may be used, care should be taken
when attempting to apply the results of these test methods to the field performance of a geotextile.
5.2 Since there are geotextiles of various thicknesses in use, evaluation in terms of their Darcy coefficient of permeabilities can
be misleading. In many instances, it is more significant to evaluate the quantity of water that would pass through a geotextile under
a given head over a particular cross-sectional area; this is expressed as permittivity.
5.3 If the permeability of an individual geotextile is of importance, a nominal coefficient of permeability, as related to geotechnical
engineering, may be computed. By multiplying permittivity times the nominal thickness of the geotextile, as determined by Test
Method D5199, the nominal coefficient of permeability is obtained.
NOTE 2—The nominal thickness is used as it is difficult to evaluate the pressure on the geotextile during the test, thereby making it difficult to determine
the thickness of the fabric under these test conditions.
6. Apparatus
6.1 Water Flow Apparatus—The apparatus for performing the water flow tests shall conform to one of the following arrangements:
6.1.1 The apparatus must be capable of maintaining a constant head of water on the geotextile being tested, or
6.1.2 The apparatus must be capable of being used as falling head apparatus.
6.1.3 The location of the manometer for measuring the head loss in either constant head or falling head methods shall be located
directly beneath the specimen. For the device shown in Fig. 1, this may be accomplished by drilling and tapping a small (3 mm)
diameter hole in the top plate of the bottom reservoir tank directly beneath the specimen, and attaching the manometer to this plate.
6.2 In addition, the apparatus must not be the controlling agent for flow during the test. It will be necessary to establish a
calibration curve of volumetric flow rate versus head for the apparatus alone in order to establish compliance with this requirement
(see 11.7).
6.3 Refer to Fig. 1 for a schematic drawing of a device that conforms to all of the above requirements. The device consists of an
upper and lower unit, which fasten together. The geotextile specimen is positioned in the bottom of the upper unit. There is a
standpipe for measuring the constant head value. The rotating discharge pipe allows adjustment of the head of water at the bottom
of the specimen. See ADJD4491.
6.4 Air Flow Apparatus—The apparatus for performing the air flow tests shall conform to the following specifications:
6.4.1 Clean Gas Pressure Source, with regulation (filtered air).
6.4.2 Pressure Sensor—Pressure measurements must be obtained with a digital pressure transducer accurate to 65 Pa to 1000 Pa,
and 61 % above 1000 Pa.
6.4.2.1 The head (upstream) pressure manometer tap must be installed immediately upstream, within 10 mm of the test specimen
surface.
6.4.2.2 The tail (downstream) pressure sensor must be installed 25 mm or more from the geotextile test specimen, and within the
25 mm diameter section.
6.4.3 Closed Specimen Holder:
6.4.3.1 Specimen holder for the test specimens that fully confines the perimeter of the specimen to prevent any lateral pressure
losses.
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FIG. 1 Constant and Falling Head Permeability Apparatus
6.4.3.2 The specimen flow area shall be 25 mm to 50 mm diameter. Smaller diameter devices are not acceptable.
6.4.3.3 The filter holder should be checked for leaks by placing an impermeable membrane in the holder and increasing the
pressure to the maximum capacity of the pressure sensor and holding it for a period of 1 min. The flow rate measured during this
period must be zero, indicating a leak-free seal.
6.4.4 Metal Punch, used to cut a suitable size geotextile from the test sheet to fit the test specimen holder.
D4491/D4491M − 22
6.4.5 Flow Rate Measurement Sensors—The apparatus should be equipped with a digital flow meter to measure the flow rates at
the two desired pressures. The flow meter must have an accuracy of at least 60.5 lpm or 1.0 %62 % of the measured value,
whichever is larger.
6.4.6 Flow Section—The geometry of the air flow apparatus at the section where the test specimen is located shall have a uniform
pipe diameter equal to the flow area, both upstream and downstream of the test specimen, for a minimum distance of two diameters.
7. Sampling
7.1 Lot Sample—As a lot sample for acceptance testing, take at random the number of rolls of geotextile directed in an applicable
material specification or other agreement between the purchaser and the supplier. Consider rolls of geotextile to be the primary
sampling units. If the specification requires sampling during manufacture, select the rolls for the lot sample at uniformly spaced
time intervals throughout the production period.
NOTE 3—An adequate specification or other agreement between the purchaser and the supplier requires taking into account the variability between rolls
of geotextile and between specimens from a swatch from a roll of geotextile so as to provide a sampling plan with a meaningful producer’s risk,
consumer’s risk, acceptable quality level, and limiting quality level.
7.2 Laboratory Sample—Take for the laboratory sample a full roll-width sample extending a minimum of 1 m along the selvage
from each sample roll such that the requirements of Section 9 can be met. Take a sample that will exclude material from the outer
wrap of the roll or the inner wrap around the core unless the sample is taken at the production site, at which point inner and outer
wrap material may be used.
8. Test Water Preparation – Water Flow Tests
8.1 To provide reproducible test results, the test water used for the water flow tests shall be de-aired to bring the dissolved oxygen
content down to a maximum of six parts per million. The dissolved oxygen content may be determined by either commercially
available chemical kits or by a dissolved oxygen meter.
NOTE 4—The de-airing system may be either a commercially available system or one consisting of a vacuum pump capable of removing a minimum of
150 L/min of air in connection with a non-collapsible storage tank with a large enough storage capacity for the test series, or at least one specimen at
a time.
8.2 Allow the de-aired water to stand in a closed storage tank under a slight vacuum until room temperature is attained.
8.3 The test water must be maintained at the standard atmosphere for geosynthetic testing, which is 21 6 2 °C.
9. Specimen Preparation
9.1 To obtain a representative value of permittivity, take four specimens from each full-width laboratory sample as described
below.
9.2 Referring to Fig. 2, select four specimens, A, B, C, and D, as follows:
FIG. 2 Specimen Locations for Woven Geotextile Water Flow Tests
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9.2.1 Select four specimens equally spaced across the width of the sample. For woven geotextiles, take the test specimens along
a diagonal line extending from the lower left-hand corner to the upper right-hand corner of the laboratory sample. None of the test
specimens shall be closer to the corner of the laboratory sample than 200 mm [8 in.].
9.2.2 For the Method C air flow tests, take ten test specimens equally spaced across the width of the sample. For woven
geotextiles, take the ten test specimens equally spaced along a diagonal line extending from the lower left-hand corner to the upper
right-hand corner of the laboratory sample. None of the specimens shall be closer to the edge of the laboratory sample than 200
mm [8 in.].
9.2.2.1 Five test specimens may be obtained in lieu of ten if the air flow device has a flow area of 50 mm diameter, provided the
test result complies with the Method A result in accordance with 16.7.
9.2.3 Cut specimens shall fit the testing apparatus, for example, 73 mm [2.87 in.] in diameter for the device illustrated in Fig. 1.
NOTE 5—If the illustrated device is used for the water flow tests, the specimens are attached to the specimen ring by contact cement.
9.3 Condition the water
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