ISO/TS 18198:2023
(Main)Determination of long-term flow of geosynthetic drains
Determination of long-term flow of geosynthetic drains
This document specifies methods of deriving reduction factors for geosynthetic drainage materials to account for intrusion of filter geotextiles, compression creep, and chemical and biological degradation. It is intended to provide a link between the test data and the codes for design with geosynthetic drains. The geosynthetics covered include those whose primary purpose is planar drainage, such as geonets, cuspated cores only, or cuspated cores combined with laminated filter geotextiles, and drainage liners, where the drainage core is made from polypropylene and high-density polyethylene. The majority of geosynthetic drains are geocomposites with geotextiles laminated to a drainage core and it is important, where possible, to consider the drainage behaviour of the geocomposite as a whole rather than the behaviour of the component parts in isolation. This document does not cover the strength of overlaps or joints between geosynthetic drains nor whether these might be more or less durable than the basic material. It does not apply to geomembranes, for example, in landfills. It does not cover the effects of dynamic loading nor any change in mechanical properties due to soil temperatures below 0 °C, or the effects of frozen soil. This document does not cover uncertainty in the design of the drainage structures, nor the human or economic consequences of failure. Design guidance for geosynthetic drains is found in ISO/TR 18228-4.
Détermination de l'écoulement à long terme des drains géosynthétiques
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TECHNICAL ISO/TS
SPECIFICATION 18198
First edition
2023-03
Determination of long-term flow of
geosynthetic drains
Reference number
ISO/TS 18198:2023(E)
© ISO 2023
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ISO/TS 18198:2023(E)
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© ISO 2023
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
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Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
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ISO/TS 18198:2023(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Test equipment and procedures for determination of short-term in- plane water
flow . 1
4.1 Measurement of maximum hydraulic transmissivity and flow rate . 1
4.2 Test equipment . 2
4.2.1 Unidirectional flow . 2
4.2.2 Index and performance tests . . 5
4.3 Normal compressive loading and seating time . 5
4.4 Number of test specimens per sample per test . 6
4.5 Hydraulic gradient . 7
5 Determination of long-term flow performance . 7
5.1 General . 7
5.2 Reduction factors (R ) . 8
F
5.3 Reduction factor for intrusion (R and R ) . 9
F,in F,GI
5.4 Reduction factor for creep (R ) . 10
F,cr
5.4.1 General . 10
5.4.2 Time-temperature superposition methods . 11
5.5 Reduction factors for chemical clogging (R ) and biological clogging (R ) .12
F,CC F,BC
5.6 Additional considerations . 13
5.6.1 Design life .13
5.6.2 Design temperature .13
5.6.3 Installation damage . 14
5.6.4 Durability of the polymers . 14
6 Alternative procedures to determine Q .14
a
6.1 General . 14
6.2 Long-term reduction of water flow capacity due to compressive creep by the BAM
(Germany) method . 14
6.3 Thickness-dependent short-term flow testing using SIM . 17
6.4 Time dependent loading followed by flow capacity measurements . 19
Bibliography .20
iii
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ISO/TS 18198:2023(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 221, Geosynthetics.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
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ISO/TS 18198:2023(E)
Introduction
The most commonly used drainage geosynthetics are the geocomposites which are produced by
laminating one or two geotextiles, with a filter function, onto a drainage core. Examples are included in
Figure 1.
a) Geonet core b) Geomat core c) Cuspated core d) Perforated tube core
Figure 1 — Examples of drainage cores
The components generally have the following characteristics under operating conditions:
— filtering component:
— adequate permeability to gases and liquids in the direction perpendicular to the filter plane;
— retention capacity of the soil particles;
— drainage core:
— adequate permeability to gases and liquids in the direction planar to the drainage structure;
— adequate compressive strength and creep resistance for the loads to be applied.
The geocomposites are often defined by the drainage cores: geomats (GMA), geonets (GNT), geospacers
(GSP), multi-linear drains.
v
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TECHNICAL SPECIFICATION ISO/TS 18198:2023(E)
Determination of long-term flow of geosynthetic drains
1 Scope
This document specifies methods of deriving reduction factors for geosynthetic drainage materials to
account for intrusion of filter geotextiles, compression creep, and chemical and biological degradation.
It is intended to provide a link between the test data and the codes for design with geosynthetic drains.
The geosynthetics covered include those whose primary purpose is planar drainage, such as geonets,
cuspated cores only, or cuspated cores combined with laminated filter geotextiles, and drainage liners,
where the drainage core is made from polypropylene and high-density polyethylene. The majority
of geosynthetic drains are geocomposites with geotextiles laminated to a drainage core and it is
important, where possible, to consider the drainage behaviour of the geocomposite as a whole rather
than the behaviour of the component parts in isolation.
This document does not cover the strength of overlaps or joints between geosynthetic drains nor
whether these might be more or less durable than the basic material. It does not apply to geomembranes,
for example, in landfills. It does not cover the effects of dynamic loading nor any change in mechanical
properties due to soil temperatures below 0 °C, or the effects of frozen soil. This document does not
cover uncertainty in the design of the drainage structures, nor the human or economic consequences of
failure. Design guidance for geosynthetic drains is found in ISO/TR 18228-4.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 10318-1, Geosynthetics — Part 1: Terms and definitions
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 10318-1 apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 Test equipment and procedures for determination of short-term in- plane
water flow
4.1 Measurement of maximum hydraulic transmissivity and flow rate
The primary function of geosynthetic drains is to convey or transmit fluid within the flow direction(s)
of a drainage layer. The discharge capacity can be given in terms of:
— Specific flow rate, which is the discharge per unit width in the geosynthetic drain, under a specified
hydraulic gradient, as per Formula (1):
Qq= / B (1)
1
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ISO/TS 18198:2023(E)
Some users of flow tests desire to index the discharge rate per unit width to the applied hydraulic
energy or hydraulic gradient at which flow is measured. In this case:
— Hydraulic transmissivity, which is the discharge per unit width of the geocomposite and per unit of
hydraulic gradient, as per Formula (2):
θ =()qB / /i (2)
The concepts of transmissivity and flow capacity were developed specifically to avoid consideration
of the thickness as it is often difficult to specifically define the thickness of a geosynthetic drain in
application.
Transmissivity is equal to flow rate only at a gradient of 1. Note also that the numerical value of
transmissivity can be very different than the numerical value of the specific flow rate at small hydraulic
gradients (e.g. at i = 0,1 transmissivity is 10 times the specific flow rate).
The discharge capacity test for a geosynthetic drain is performed in accordance with ISO 12958-1,
ISO 12958-2 or ASTM D4716.
4.2 Test equipment
4.2.1 Unidirectional flow
The apparatus for these test methods are relatively simplistic in their design and ability to measure a
discharge capacity or flow rate per unit width or transmissivity (Figure 2). By maintaining a constant
head during the test, at a given normal stress, boundary conditions, and seating time, the flow rate Q of
the geosynthetic drain can be determined using Formula (3):
q
Q =⋅ R (3)
σ ,,it ,bT
B
Where
Q is the numerical value of the in-plane water flow capacity per unit width at a defined stress
σ,i,t,b
σ, gradient i, seating time under load prior to flow measurement t and boundary conditions
b, [l/(m·s)]
q is the numerical value of the discharge capacity for a geosynthetic drain of width B meas-
ured in the test (l/s);
B is the numerical value of width of flow (m)
R is the numerical value of the correction factor converting to a test temperature of 20 °C.
T
2
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ISO/TS 18198:2023(E)
Key
1 water supply 7 water reservoir
2 water collection 8 normal compressive load
3 upstream water head manometers / piezometers 9 overflow weirs
4 specimen 10 effective flow length (≥ 300 mm)
5 material used as boundary (e.g. soil) 11 water head of discharge
6 loading platen 12 downstream water head (≤ 100 mm)
Figure 2 — Example of test apparatus in horizontal test configuration
3
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ISO/TS 18198:2023(E)
Key
1 manometers
2 normal compressive loading ram
3 test box
i H/L = Hydraulic gradient
β width of flow (m)
3
Q rate of flow (m /sec-m)
Figure 3 — Example of test equipment for water flow capacity of planar drainage geosynthetic
The test equipment shown in Figure 3 has the ability of constructing specific design cross-sections
within the apparatus and then applying the required load(s) to the product, for which the geosynthetic
drain shall perform in the proposed design. The normal load is applied vertically across the entire
sample cross-section, typically by a pneumatic bladder or loading piston. The hydraulic gradient for the
test is set by adjusting the hydraulic head H prior to the start of the test.
While sharing a number of common technical features for measuring flow, a specific flow capacity test
as performed may differ slightly in testing details and prescribed procedural approaches. Depending
on which manner a test is performed, the resulting data may be either of “index” type suitable for
use in manufacturing quality control (see ISO 12958-1 or ASTM D4716), or of a “performance” type
suitable for use in design and performance verification (see ISO 12958-2 or ASTM D4716). ISO 12958-1
prescribes flow measurements at predetermined hydraulic gradients and normal compressive stresses,
as well as standardized superstratum and substratum (closed cell foam materials or rigid boundaries).
The procedures in ISO 12958-2 and ASTM D4716, instead, invite the user of the test to determine all
of the test parameters specific to the designed drainage application. For performance testing, as an
example, the following test parameters or variables should be specified so as to represent anticipated
4
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ISO/TS 18198:2023(E)
or site-specific conditions such as: compressive load, hydraulic gradient, temperature, site-specific
superstratum, site-specific substratum, and seating time prior to flow measurement.
4.2.2 Index and performance tests
Manufacturers typically quantify the relative capacity of their geosynthetic drainage products via index
testing and document this performance on marketing documents and material data sheets. However,
manufacturer’s testing often reflects the flow rates of the product tested between two rigid plates, or
standardized closed cell foam pads, under a specific load and gradient, and with a limited seating time
(i.e. 15 min, 1 h, etc.). Thus, the manufacturer’s data typically represents the short-term flow capacity
for the product and serves to confirm production quality control.
To approach an understanding of the performance-related flow capacity of the geosynthetic drain it
shall be tested using test parameters representative of field service conditions. Testing to provide an
estimate of performance flow is described in ISO 12958-2 and ASTM D4716.
A performance flow test should allow materials above and below the geosynthetic drain to intrude
into the void space of the product under compressive load to simulate real project conditions. Sand,
for example, placed above the drain and loaded to design conditions will cause the upper filter
geotextile to elongate and intrude into the space between each parallel strand of a geonet or the cusps
of the cuspated core. The degree of intrusion has a direct relationship to the structural properties and
bonding/no bonding and type of bonding (to the core) of the geotextile specified for the drain core and
the amount of normal load applied to the design cross-section. Please note that the design engineer will
need to supply the testing laboratory with a sufficient volume of representative soil sample to perform
the number of required tests. The compaction requirements for the soil shall also be specified in order
to set-up the test section to reflect the design conditions. If a geosynthetic clay liner is adjacent to the
geosynthetic drain, the degree of saturation shall be specified along with the load at hydration, prior to
placement of the normal load on the design cross-section test sample.
The ISO 12958-1 test procedure defines closed-cell foam materials meeting specific compressibility
characteristics to simulate these field conditions. These “soft” superstratum and substratum materials
assist in replicating test conditions and also avoiding contamination of test water with site-specific
soils. Use of these standardized superstratum and substratum also enable manufacturers to publish
like-data on their drainage products for product comparison while providing their own estimate
of performance flow. ISO 12958-2 and ASTM D4716 allow use of site representative soils and other
materials to more closely replicate field conditions.
4.3 Normal compressive loading and seating time
For performance testing, the normal stress used during testing should be equal to the maximum
overburden pressure the material may experience during its service life. The practice of specifying a
test pressure higher than anticipated field pressure is conservative when following the most common
design procedures. Any uncertainties associated with long-term flow performance under load may be
accounted through a factor of safety rather than a higher than expected normal pressure.
Most geosynthetic drains are constructed of polymeric material, which can deform under load and over
time. This gradual deformation of the polymeric structure under a fixed load is known as “creep.” The
rate of ductile movement occurs rapidly initially (primary creep) and decreases overtime (secondary
creep).
5
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ISO/TS 18198:2023(E)
Key
X log time (h)
Y percent retained thickness
Figure 4 — Example of compressive creep curve for a drainage core
As compressive creep proceeds the thickness of the geosynthetic drain reduces; thereby reducing
the porosity or available cavity through which the liquid can move through the product. The amount
of thickness reduction is dependent on the compressive load placed on the product and the physical
composition and structure of the geosynthetic drain, and time. Figure 4 illustrates the typical behaviour
of a geosynthetic drain core, for which often (but not always), the majority of the creep will occur over
the first 100 h after the design load is applied.
The requirement of the 100 h seating time can be a difficult burden for the geosynthetic testing
community, requiring dedicated test apparatus for over four days prior to performing the test.
Those requiring the 100 h seating time may allocate a sufficient testing period prior to the start of
the construction project in order to obtain the conformance test results. The testing period for flow
capacity verification might be significant for large projects where numerous conformance tests may be
required.
4.4 Number of test specimens per sample per test
The ISO 12958-1 test requires flow measurements of three test specimens in both the machine and
cross-machine direction be measured. ASTM D4716 requires the testing of two specimens per sample
in the engineered flow direction to take account of manufacturing variation. Due to how a product is
manufactured, this is an important requirement to obtain an acceptable representation for the reported
test result. Processing parameters and manufacturing settings can have significant impact on drainage
product flow rate capacity and how consistent this capacity is throughout the product structure. The
test requirement for two or three specimens per sample attempts to average the variability in thickness
and porosity of the product. While less of a concern for cuspated cores and prefabricated vertical drains,
this variability should be captured in flow testing of all geosynthetic drains.
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ISO/TS 18198:2023(E)
4.5 Hydraulic gradient
The flow rate of a geosynthetic drain is proportional to the hydraulic gradient i which is defined as in
Formula (4):
i = δ / L (4)
h
where
δ is the numerical value of the hydraulic head loss along the distance L for the fluid flow in the
h
geosynthetic (m);
L is the numerical value of the distance between two points along the average direction of flow
in the geosynthetic (m).
Performance oriented flow capacity tests of geosynthetic drains should be performed using a hydraulic
gradient equal to or slightly smaller than sin β where β is equal to the slope angle of the geosynthetic
drain with the horizontal. It is not conservative to test at higher gradients. Note that transmissivity
or specific flow rate is a non-linear function of gradient because the flow regime with water in a
geosynthetic drain is typically turbulent.
When performing an in-plane flow capacity test using a hydraulic gradient of 0,1 or less, it may be
challenging to ensure the accuracy of the hydraulic gradient using open pipe manometers . These lower
gradients are best established with the aid of accurate digital pressure gauges that enable confirmation
of gradients as low as 0,01.
An alternative approach to measuring flow rates at very low gradients involves measuring flows at
several higher gradients (for example gradients at 0,1, 0,25, 0,5 and 1) so as to develop an empirical
relationship between flow rate and hydraulic gradient. The relationship between flow rate and gradient,
at given applied pressure, boundary conditions and seating time, is usually as per Formula (5):
n
Qa=⋅i (5)
where:
2
Q is the numerical value of the flow rate (m /sec);
2
a is the numerical value of the constant equal to the flow rate at unit gradient (m /sec);
i is the numerical value of the gradient (dimensionless); and
n is the numerical value of the constant (dimensionless).
Formula (5) has been verified by performing a number of tests on various materials under different
test conditions. Constants a and n depend on type of geosynthetic drain, boundary conditions, normal
stress, and test duration.
NOTE The use of this equation for very low gradients can be used with caution as other phenomenon can
interfere with the actual field performance of the product, such as the contact angle between water and the
polymer(s) used to manufacture the geosynthetic drain.
5 Determination of long-term flow performance
5.1 General
A design service lifetime, t , is defined for the drainage structure. For civil engineering structures such
D
as roads and containment facilities, this service life is typically 50 years to 100 years. Some applications
7
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ISO/TS 18198:2023(E)
may require shorter service lives, such as sports fields (20 years to 50 years) or mining heap leach pad
(5 years to 50 years). Additionally, some applications may have service lives lasting 1 year to 5 years.
All of these durations are too long for direct measurements to be made in advance of construction.
There are, therefore, various considerations which may be modelled as reduction factors to short-term
flow measurements which account for the time dependent changes in product performance likely to
occur during service life.
Assessing the long-term flow performance of a geosynthetic drain consists in the comparison of the
required flow rate q , determined by the application, and the allowable flow rate for the particular
reqd
product used in a particular service environment for a defined service life Q . A factor of safety F can
a S
be determined as per Formula (6):
F = Q / q (6)
S a reqd
The application of a series of reduction factors applied to the initial flow property of the geosynthetic
drain incorporates the consideration of potential causes of reduction of the water flow. In this way an
effort is made to ensure that the geosynthetic drain will always have a flow capacity equal to or in
excess of the requirements associated to a particular application.
Because there are a variety of geosynthetic drain product types having a variety of different structures
and compositions, the application of any specific reduction factor for the determination of long-term
flow may be minimal in assigned magnitude, or inappropriate for the given application. For example,
the time-dependent compressive creep resistance of a single nonwoven geotextile is negligible as
ductile compression under compressive load generally happens readily and with little or no time-
dependent resistance. Therefore, a reduction factor associated with time dependent creep may not be
appropriately applied to a relatively long (e.g. 100 h) flow test.
On the other hand, the determination of long-term flow performance of a prefabricated vertical drain
in a relatively short service life in a swamp wetland dewatering application would be wise to consider
potential reductions of flow caused by biological clogging as applied to short terms flows measured in
the crimped configuration.
ISO/TR 18228-4 provides precise guidance on how to establish q .
reqd
5.2 Reduction factors (R )
F
According to ISO/TR 18228-4, the allowable flow rate is given by Formula (7):
Q = Q / R (7)
a L F
where:
RR = ⋅⋅ RR ⋅⋅ RR (8)
FF,inF,cr-QF,ccF,bcF,L
Q is the numerical value of the available long-term flow rate for the geosynthetic drain (l/s/m
a
2
or m /s). The long-term planar flow rate that will not result in failure of the geosynthetic
drain during the required design life, calculated on a flow rate per unit of drain width basis;
Q is the numerical value of the short-term flow rate of the geosynthetic drain from hydraulic
L
flow tests using site-specific test conditions, i.e. compressive stress, hydraulic gradient,
2
seating time and boundary conditions l/s/m or m /s);
R is the numerical value of a combined reduction factor to account for potential long-term
F
reduction of flow due to creep-induced thickness reduction, intrusion of adjacent materials
(examp
...
© ISO 2019 – All rights reserved
ISO/TS 18198:2022(E)
Date: 2022-12-15
ISO TC 221/SC ##/WG 5
Secretariat: BSI
Determination of long-term flow of geosynthetic drains
WD/CD/DIS/FDIS stage
Warning for WDs and CDs
This document is not an ISO International Standard. It is distributed for review and comment. It is subject to
change without notice and may not be referred to as an International Standard.
Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of
which they are aware and to provide supporting documentation.
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© ISO 2019
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ISO/TS 18198:2022(E)
© ISO 2022
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part
of this publication may be reproduced or utilized otherwise in any form or by any means, electronic or
mechanical, including photocopying, or posting on the internet or an intranet, without prior written
permission. Permission can be requested from either ISO at the address below or ISO’s member body
in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.orgwww.iso.org
Published in Switzerland
© ISO #### – All rights reserved
iv © ISO 2022 – All rights reserved
---------------------- Page: 3 ----------------------
ISO/TS 18198:2022(E)
Contents
Foreword . vi
Introduction. vii
1 Scope . 1
2 Normative references . 2
3 Terms and definitions . 2
4 Test equipment and procedures for determination of short-term in- plane water
flow . 3
4.1 Measurement of maximum hydraulic transmissivity and flow rate . 3
4.2 Test equipment . 4
4.2.1 Unidirectional flow . 4
4.2.2 Index and performance tests . 7
4.3 Normal compressive loading and seating time . 7
4.4 Number of test specimens per sample per test . 9
4.5 Hydraulic gradient . 10
5 Determination of long-term flow performance . 11
5.1 General . 11
5.2 Reduction factors (RF) . 12
5.3 Reduction factor for intrusion (R and R ) . 14
F,in F,GI
5.4 Reduction factor for creep (R ) . 15
F,cr
5.4.1 General . 15
5.4.2 Time-temperature superposition methods . 17
5.5 Reduction factors for chemical clogging (R ) and biological clogging (R ) . 19
F,CC F,BC
5.6 Additional considerations . 20
5.6.1 Design life . 20
5.6.2 Design temperature . 20
5.6.3 Installation damage . 21
5.6.4 Durability of the polymers . 21
6 Alternative procedures to determine Q . 21
a
6.1 General . 21
6.2 Long-term reduction of water flow capacity due to compressive creep by the BAM
(Germany) method . 21
6.3 Thickness-dependent short-term flow testing using SIM . 26
6.4 Time dependent loading followed by flow capacity measurements . 28
Bibliography . 29
© ISO 2019 – All rights reserved © ISO 2022 – All v
rights reserved
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ISO/TS 18198:2022(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO
collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of any
patent rights identified during the development of the document will be in the Introduction and/or on
the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the World
Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee[or Project Committee] ISO/TC [or ISO/PC]###,
[name of committee], Subcommittee SC ##, [name of subcommittee]. ISO/TC 221, Geosynthetics.
This second/third/… edition cancels and replaces the first/second/… edition (ISO #####:####), which
has been technically revised.
The main changes compared to the previous edition are as follows:
— xxx xxxxxxx xxx xxxx
A list of all parts in the ISO ##### series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
© ISO #### – All rights reserved
vi © ISO 2022 – All rights reserved
---------------------- Page: 5 ----------------------
ISO/TS 18198:2022(E)
Introduction
The most commonly used drainage geosynthetics are the geocomposites which are produced by
laminating one or two geotextiles, with a filter function, onto a drainage core. Examples are included in
Figure 1.
a) Geonet core b) Geomat core c) Cuspated core d) Perforated tube core
Figure 1 — Examples of drainage cores
The components generally have the following characteristics under operating conditions:
— filtering component:
— adequate permeability to gases and liquids in the direction perpendicular to the filter plane;
— retention capacity of the soil particles;
— drainage core:
— adequate permeability to gases and liquids in the direction planar to the drainage structure;
— adequate compressive strength and creep resistance for the loads to be applied.
The geocomposites are often defined by the drainage cores: geomats (GMA), geonets (GNT), geospacers
(GSP), multi-linear drains.
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TECHNICAL SPECIFICATION ISO/TS 18198:2022(E)
Determination of long-term flow of geosynthetic drains
1 Scope
This document specifies methods of deriving reduction factors for geosynthetic drainage materials to
account for intrusion of filter geotextiles, compression creep, and chemical and biological degradation. It
is intended to provide a link between the test data and the codes for design with geosynthetic drains.
1.1 Introduction
The geosynthetics covered include those whose primary purpose is planar drainage, such as geonets,
cuspated cores only, or cuspated cores combined with laminated filter geotextiles, and drainage liners,
where the drainage core is made from polypropylene and high-density polyethylene. The majority of
geosynthetic drains are geocomposites with geotextiles laminated to a drainage core and it is important,
where possible, to consider the drainage behaviour of the geocomposite as a whole rather than the
behaviour of the component parts in isolation.
This document does not cover the strength of overlaps or joints between geosynthetic drains nor whether
these might be more or less durable than the basic material. It does not apply to geomembranes, for
example, in landfills. It does not cover the effects of dynamic loading nor any change in mechanical
properties due to soil temperatures below 0 °C, or the effects of frozen soil. This document does not cover
uncertainty in the design of the drainage structures, nor the human or economic consequences of failure.
Design guidance for geosynthetic drains is found in ISO/TR 18228-4e-4.
1.2 Materials
The most commonly used drainage geosynthetics are the geocomposites which are produced by
laminating one or two geotextiles, with a filter function, onto a drainage core. Examples are included in
Figure 1.
Geonet Core Geomat Core Cuspated Core Perforated Tube Core
Figure 1 —Examples of Drainage Cores
The components generally have the following characteristics under operating conditions:
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ISO/TS 18198:2022(E)
• filtering component:
− adequate permeability to gases and liquids in the direction perpendicular to the filter plane;
− retention capacity of the soil particles;
• drainage core:
− adequate permeability to gases and liquids in the direction planar to the drainage structure;
− adequate compressive strength and creep resistance for the loads to be applied.
The geocomposites are often defined by the drainage cores: geomats (GMA), geonets (GNT), geospacers
(GSP), multi-linear drains
272 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 10318-1, Geosynthetics — Part 1: Terms and definitions
283 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 10318 and the following-1
apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp
Field Code Changed
— IEC Electropedia: available at https://www.electropedia.org/
3.1
geotextile (GTX)
planar, permeable, polymeric (synthetic or natural) textile material, which may be nonwoven, knitted or
woven, used in contact with soil and/or other materials in geotechnical and civil engineering applications
3.2
nonwoven geotextile (GTX N)
geotextile made of directionally or randomly orientated fibres, filaments or other elements, mechanically
and/or thermally and/or chemically bonded
3.3
knitted geotextile (GTX K)
geotextile produced by interlooping one or more yarns, filaments or other elements
3.4
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ISO/TS 18198:2022(E)
woven geotextile (GTX W)
Woven geotextile (GTX W): geotextile produced by interlacing, usually at right angles, two or more sets
of yarns, filaments, tapes or other elements
3.5
geotextile related product (GTD)
planar, permeable, polymeric (synthetic or natural) material used in contact with soil and or other
materials in geotechnical and civil engineering applications, which does not comply with the definition of
a geotextile
3.6
geomat (GMA)
a three-dimensional permeable structure made of polymeric monofilaments, and/or other elements
(synthetic or natural), mechanically and/or thermally and/or chemically and/or otherwise bonded
3.7
geonet (GNT)
geosynthetic consisting of parallel sets of ribs overlying and integrally connected with similar sets at
various angles
3.8
geospacer (GSP)
a three-dimensional polymeric structure designed to create an air space in soil and/or other materials in
geotechnical and civil engineering applications. A typical geospacer is a geocuspate typically formed from
an extruded geomembrane and formed into a cuspated sheet.
3.9
geocomposite (GCO)
manufactured, assembled material using at least one geosynthetic product among the components.
Additional definitions include:
3.10
Multilinear geocomposite drains
manufactured product composed of a series of parallel single drainage conduits regularly spaced across
its width sandwiched between two or more geosynthetics. Typical linear drains include perforated pipes
or strips of other drain cores.
614 Test equipment and procedures for determination of short-term in- plane
water flow
61.14.1 Measurement of maximum hydraulic transmissivity and flow rate
The primary function of geosynthetic drains is to convey or transmit fluid within the flow direction(s) of
a drainage layer. The discharge capacity can be given in terms of:
• — Specific flow rate =, which is the discharge per unit width in the geosynthetic drain, under
a specified hydraulic gradient:
𝑄𝑄 =𝑞𝑞 / 𝐵𝐵 , as per Formula (1)):
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ISO/TS 18198:2022(E)
Qq= / B (1)
Some users of flow tests desire to index the discharge rate per unit width to the applied hydraulic energy
or hydraulic gradient at which flow is measured. In this case:
— Hydraulic transmissivity =, which is the discharge per unit width of the geocomposite and per unit of
hydraulic gradient. , as per Formula (2):
θ= q / B / i
( )
𝜃𝜃 = (𝑞𝑞 / 𝐵𝐵)/𝑖𝑖
(2)
The concepts of transmissivity and flow capacity were developed specifically to avoid consideration of
the thickness as it is often difficult to specifically define the thickness of a geosynthetic drain in
application.
Transmissivity is equal to flow rate only at a gradient of 1. Note also that the numerical value of
transmissivity can be very different than the numerical value of the specific flow rate at small hydraulic
gradients (e.g. at i = 0.,1 transmissivity is 10 times the specific flow rate).
The discharge capacity test for a geosynthetic drain is performed in accordance with ISO 12958-1,
ISO 12958-2 or ASTM D4716.
61.24.2 Test equipment
61.2.14.2.1 Unidirectional flow
The apparatus for these test methods are relatively simplistic in their design and ability to measure a
discharge capacity or flow rate per unit width or transmissivity (Figure 2). By maintaining a constant
head during the test, at a given normal stress, boundary conditions, and seating time, the flow rate (Q) of
the geosynthetic drain can be determined using Formula (3):
q
Q ⋅R (3)
σ ,,i t ,b T
B
Where
𝑞𝑞
𝑄𝑄 = ∙𝑅𝑅 (3)
T
𝜎𝜎,𝑖𝑖,𝑡𝑡,𝑏𝑏
𝐵𝐵
Where
Q is the numerical value of the in-plane water flow capacity per unit width at a defined stress
σ,gi,t,b
“σ”,σ, gradient “i”,, seating time under load prior to flow measurement “t” and boundary
conditions “b”, in liters per metre second, [l/(m·s)]
Qq is the numerical value of the discharge capacity for a geosynthetic drain of width B measured
in the test (l/s);
B is the numerical value of width of flow (m)
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ISO/TS 18198:2022(E)
R is the numerical value of the correction factor converting to a test temperature of 20 °C.
T
Key
1 water supply 7 water reservoir.
2 water collection 8 normal compressive load
3 upstream water head manometers / piezometers 9 overflow weirs
4 specimen 10 effective flow length (≥ 300 mm)
5 material used as boundary (e.g. soil) 11 water head of discharge.
6 loading platen 12 downstream water head (≤ 100 mm)
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ISO/TS 18198:2022(E)
Figure 2 — Example of test apparatus in horizontal test configuration
Key
1 Manometersmanometers
Deleted Cells
2 Normalnormal compressive loading RAMram
Deleted Cells
3 Test Boxtest box
i H/L = Hydraulic gradient
β Widthwidth of flow (m)
3
Q Raterate of flow (m /sec-m)
Figure 3 — Example of test equipment for water flow capacity of planar drainage geosynthetic
(ASTM D4716, ISO 12958)
The test equipment shown in Figure 3 has the ability of constructing specific design cross-sections within
the apparatus and then applying the required load(s) to the product, for which the geosynthetic drain
shall perform in the proposed design. The normal load is applied vertically across the entire sample cross-
section, typically by a pneumatic bladder or loading piston. The hydraulic gradient for the test is set by
adjusting the hydraulic head (H) prior to the start of the test.
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ISO/TS 18198:2022(E)
While sharing a number of common technical features for measuring flow, a specific flow capacity test as
performed may differ slightly in testing details and prescribed procedural approaches. Depending on
which manner a test is performed, the resulting data may be either of “index” type suitable for use in
manufacturing quality control (see ISO 12958-1 or ASTM D4716), or of a “performance” type suitable for
use in design and performance verification (see ISO 12958-2 or ASTM D4716). ISO 12958-1 prescribes
flow measurements at predetermined hydraulic gradients and normal compressive stresses, as well as
standardized superstratum and substratum (closed cell foam materials or rigid boundaries). The
procedures in ISO 12958-2 and ASTM D4716, instead, invite the user of the test to determine all of the
test parameters specific to the designed drainage application. For performance testing, as an example,
the following test parameters or variables should be specified so as to represent anticipated or site-
specific conditions such as: compressive load, hydraulic gradient, temperature, site-specific
superstratum, site-specific substratum, and seating time prior to flow measurement.
61.2.24.2.2 Index and performance tests
Manufacturers typically quantify the relative capacity of their geosynthetic drainage products via index
testing and document this performance on marketing documents and material data sheets. However,
manufacturer’s testing often reflects the flow rates of the product tested between two rigid plates, or
standardized closed cell foam pads, under a specific load and gradient, and with a limited seating time
(i.e. 15 minutes min, 1 hour h, etc.). Thus, the manufacturer’s data typically represents the short-term
flow capacity for the product and serves to confirm production quality control.
To approach an understanding of the performance-related flow capacity of the geosynthetic drain it shall
be tested using test parameters representative of field service conditions. Testing to provide an estimate
of performance flow is described in ISO 12958-2 and ASTM D4716.
A performance flow test should allow materials above and below the geosynthetic drain to intrude into
the void space of the product under compressive load to simulate real project conditions. Sand, for
example, placed above the drain and loaded to design conditions will cause the upper filter geotextile to
elongate and intrude into the space between each parallel strand of a geonet or the cusps of the cuspated
core. The degree of intrusion has a direct relationship to the structural properties and bonding/no
bonding and type of bonding (to the core) of the geotextile specified for the drain core and the amount of
normal load applied to the design cross-section. Please note that the design engineer will need to supply
the testing laboratory with a sufficient volume of representative soil sample to perform the number of
required tests. The compaction requirements for the soil shall also be specified in order to set-up the test
section to reflect the design conditions. If a geosynthetic clay liner is adjacent to the geosynthetic drain,
the degree of saturation shall be specified along with the load at hydration, prior to placement of the
normal load on the design cross-section test sample.
The ISO 12958-1 test procedure defines closed-cell foam materials meeting specific compressibility
characteristics to simulate these field conditions. These “soft” superstratum and substratum materials
assist in replicating test conditions and also avoiding contamination of test water with site-specific soils.
Use of these standardized superstratum and substratum also enable manufacturers to publish like-data
on their drainage products for product comparison while providing their own estimate of performance
flow. ISO 12958-2 and ASTM D4716 allow use of site representative soils and other materials to more
closely replicate field conditions.
61.34.3 Normal compressive loading and seating time
For performance testing, the normal stress used during testing should be equal to the maximum
overburden pressure the material may experience during its service life. The practice of specifying a test
pressure higher than anticipated field pressure is conservative when following the most common design
procedures. Any uncertainties associated with long-term flow performance under load may be accounted
through a factor of safety rather than a higher than expected normal pressure.
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ISO/TS 18198:2022(E)
Most geosynthetic drains are constructed of polymeric material, which can deform under load and over
time. This gradual deformation of the polymeric structure under a fixed load is known as “creep.” The
rate of ductile movement occurs rapidly initially (primary creep) and decreases overtime (secondary
creep).
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ISO/TS 18198:2022(E)
Key
X LOG TIME (HR)
Y PERCENT RETAINED THICKNESS
X log time (h)
Y percent retained thickness
Figure 4: — Example of compressive creep curve for a drainage core.
As compressive creep proceeds the thickness of the geosynthetic drain reduces; thereby reducing the
porosity or available cavity through which the liquid can move through the product. The amount of
thickness reduction is dependent on the compressive load placed on the product and the physical
composition and structure of the geosynthetic drain, and time. Figure 4 illustrates the typical behaviour
of a geosynthetic drain core, for which often (but not always), the majority of the creep will occur over
the first 100 hours h after the design load is applied.
NOTE: The requirement of the 100-hour h seating time can be a difficult burden for the geosynthetic
testing community, requiring dedicated test apparatus for over four days prior to performing the test.
Those requiring the 100-hour h seating time may allocate a sufficient testing period prior to the start of
the construction project in order to obtain the conformance test results. The testing period for flow
capacity verification might be significant for large projects where numerous conformance tests may be
required.
61.44.4 Number of test specimens per sample per test
The ISO 12958-1 test requires flow measurements of three test specimens in both the machine and cross-
machine direction be measured. ASTM D4716 requires the testing of two specimens per sample in the
engineered flow direction to take account of manufacturing variation. Due to how a product is
manufactured, this is an important requirement to obtain an acceptable representation for the reported
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ISO/TS 18198:2022(E)
test result. Processing parameters and manufacturing settings can have significant impact on drainage
product flow rate capacity and how consistent this capacity is throughout the product structure. The test
requirement for two or three specimens per sample attempts to average the variability in thickness and
porosity of the product. While less of a concern for cuspated cores and prefabricated vertical drains, this
variability should be captured in flow testing of all geosynthetic drains.
61.54.5 Hydraulic gradient
The flow rate of a geosynthetic drain is proportional to the hydraulic gradient i which is defined as: in
Formula (4):
i = δ / L (4)
h
where
δh is the numerical value of the hydraulic head loss along the distance L for the fluid flow in the
geosynthetic (m);
L is the numerical value of the distance between two points along the average direction
of flow in the geosynthetic (m).
δ is the numerical value of the hydraulic head loss along the distance L for the fluid flow in the
h
geosynthetic (m);
L is the numerical value of the distance between two points along the average direction of flow in
the geosynthetic (m).
Performance oriented flow capacity tests of geosynthetic drains should be performed using a hydraulic
gradient equal to or slightly smaller than sin β where β is equal to the slope angle of the geosynthetic
drain with the horizontal. It is not conservative to test at higher gradients. Note that transmissivity or
specific flow rate is a non-linear function of gradient because the flow regime with water in a geosynthetic
drain is typically turbulent.
When performing aan in-plane flow capacity test using a hydraulic gradient of 0,1 or less, it may be
challenging to ensure the accuracy of the hydraulic gradient using open pipe manometers . These lower
gradients are best established with the aid of accurate digital pressure gauges that enable confirmation
of gradients as low as 0,01.
An alternative approach to measuring flow rates at very low gradients involves measuring flows at
several higher gradients (for example gradients at 0,1, 0,25, 0,5, and 1) so as to develop an empirical
relationship between flow rate and hydraulic gradient. The relationship between flow rate and gradient,
at given applied pressure, boundary
...
TECHNICAL ISO/TS
SPECIFICATION 18198
First edition
Determination of long-term flow of
geosynthetic drains
PROOF/ÉPREUVE
Reference number
ISO/TS 18198:2022(E)
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ISO/TS 18198:2022(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2022
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
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Phone: +41 22 749 01 11
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Website: www.iso.org
Published in Switzerland
ii
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ISO/TS 18198:2022(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Test equipment and procedures for determination of short-term in- plane water
flow . 1
4.1 Measurement of maximum hydraulic transmissivity and flow rate . 1
4.2 Test equipment . 2
4.2.1 Unidirectional flow . 2
4.2.2 Index and performance tests . . 5
4.3 Normal compressive loading and seating time . 5
4.4 Number of test specimens per sample per test . 6
4.5 Hydraulic gradient . 7
5 Determination of long-term flow performance . 7
5.1 General . 7
5.2 Reduction factors (R ) . 8
F
5.3 Reduction factor for intrusion (R and R ) . 9
F,in F,GI
5.4 Reduction factor for creep (R ) . 10
F,cr
5.4.1 General . 10
5.4.2 Time-temperature superposition methods . 11
5.5 Reduction factors for chemical clogging (R ) and biological clogging (R ) .12
F,CC F,BC
5.6 Additional considerations . 13
5.6.1 Design life .13
5.6.2 Design temperature .13
5.6.3 Installation damage . 14
5.6.4 Durability of the polymers . 14
6 Alternative procedures to determine Q .14
a
6.1 General . 14
6.2 Long-term reduction of water flow capacity due to compressive creep by the BAM
(Germany) method . 14
6.3 Thickness-dependent short-term flow testing using SIM . 17
6.4 Time dependent loading followed by flow capacity measurements . 19
Bibliography .20
iii
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ISO/TS 18198:2022(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 221, Geosynthetics.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
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ISO/TS 18198:2022(E)
Introduction
The most commonly used drainage geosynthetics are the geocomposites which are produced by
laminating one or two geotextiles, with a filter function, onto a drainage core. Examples are included in
Figure 1.
a) Geonet core b) Geomat core c) Cuspated core d) Perforated tube core
Figure 1 — Examples of drainage cores
The components generally have the following characteristics under operating conditions:
— filtering component:
— adequate permeability to gases and liquids in the direction perpendicular to the filter plane;
— retention capacity of the soil particles;
— drainage core:
— adequate permeability to gases and liquids in the direction planar to the drainage structure;
— adequate compressive strength and creep resistance for the loads to be applied.
The geocomposites are often defined by the drainage cores: geomats (GMA), geonets (GNT), geospacers
(GSP), multi-linear drains.
v
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TECHNICAL SPECIFICATION ISO/TS 18198:2022(E)
Determination of long-term flow of geosynthetic drains
1 Scope
This document specifies methods of deriving reduction factors for geosynthetic drainage materials to
account for intrusion of filter geotextiles, compression creep, and chemical and biological degradation.
It is intended to provide a link between the test data and the codes for design with geosynthetic drains.
The geosynthetics covered include those whose primary purpose is planar drainage, such as geonets,
cuspated cores only, or cuspated cores combined with laminated filter geotextiles, and drainage liners,
where the drainage core is made from polypropylene and high-density polyethylene. The majority
of geosynthetic drains are geocomposites with geotextiles laminated to a drainage core and it is
important, where possible, to consider the drainage behaviour of the geocomposite as a whole rather
than the behaviour of the component parts in isolation.
This document does not cover the strength of overlaps or joints between geosynthetic drains nor
whether these might be more or less durable than the basic material. It does not apply to geomembranes,
for example, in landfills. It does not cover the effects of dynamic loading nor any change in mechanical
properties due to soil temperatures below 0 °C, or the effects of frozen soil. This document does not
cover uncertainty in the design of the drainage structures, nor the human or economic consequences of
failure. Design guidance for geosynthetic drains is found in ISO/TR 18228-4.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 10318-1, Geosynthetics — Part 1: Terms and definitions
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 10318-1 apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 Test equipment and procedures for determination of short-term in- plane
water flow
4.1 Measurement of maximum hydraulic transmissivity and flow rate
The primary function of geosynthetic drains is to convey or transmit fluid within the flow direction(s)
of a drainage layer. The discharge capacity can be given in terms of:
— Specific flow rate, which is the discharge per unit width in the geosynthetic drain, under a specified
hydraulic gradient, as per Formula (1):
Qq= / B (1)
1
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ISO/TS 18198:2022(E)
Some users of flow tests desire to index the discharge rate per unit width to the applied hydraulic
energy or hydraulic gradient at which flow is measured. In this case:
— Hydraulic transmissivity, which is the discharge per unit width of the geocomposite and per unit of
hydraulic gradient, as per Formula (2):
θ =()qB / /i (2)
The concepts of transmissivity and flow capacity were developed specifically to avoid consideration
of the thickness as it is often difficult to specifically define the thickness of a geosynthetic drain in
application.
Transmissivity is equal to flow rate only at a gradient of 1. Note also that the numerical value of
transmissivity can be very different than the numerical value of the specific flow rate at small hydraulic
gradients (e.g. at i = 0,1 transmissivity is 10 times the specific flow rate).
The discharge capacity test for a geosynthetic drain is performed in accordance with ISO 12958-1,
ISO 12958-2 or ASTM D4716.
4.2 Test equipment
4.2.1 Unidirectional flow
The apparatus for these test methods are relatively simplistic in their design and ability to measure a
discharge capacity or flow rate per unit width or transmissivity (Figure 2). By maintaining a constant
head during the test, at a given normal stress, boundary conditions, and seating time, the flow rate Q of
the geosynthetic drain can be determined using Formula (3):
q
Q =⋅ R (3)
σ ,,it ,bT
B
Where
Q is the numerical value of the in-plane water flow capacity per unit width at a defined stress
σ,i,t,b
σ, gradient i, seating time under load prior to flow measurement t and boundary conditions
b, [l/(m·s)]
q is the numerical value of the discharge capacity for a geosynthetic drain of width B meas-
ured in the test (l/s);
B is the numerical value of width of flow (m)
R is the numerical value of the correction factor converting to a test temperature of 20 °C.
T
2
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Key
1 water supply 7 water reservoir
2 water collection 8 normal compressive load
3 upstream water head manometers / piezometers 9 overflow weirs
4 specimen 10 effective flow length (≥ 300 mm)
5 material used as boundary (e.g. soil) 11 water head of discharge
6 loading platen 12 downstream water head (≤ 100 mm)
Figure 2 — Example of test apparatus in horizontal test configuration
3
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ISO/TS 18198:2022(E)
Key
1 manometers
2 normal compressive loading ram
3 test box
i H/L = Hydraulic gradient
β width of flow (m)
3
Q rate of flow (m /sec-m)
Figure 3 — Example of test equipment for water flow capacity of planar drainage geosynthetic
The test equipment shown in Figure 3 has the ability of constructing specific design cross-sections
within the apparatus and then applying the required load(s) to the product, for which the geosynthetic
drain shall perform in the proposed design. The normal load is applied vertically across the entire
sample cross-section, typically by a pneumatic bladder or loading piston. The hydraulic gradient for the
test is set by adjusting the hydraulic head H prior to the start of the test.
While sharing a number of common technical features for measuring flow, a specific flow capacity test
as performed may differ slightly in testing details and prescribed procedural approaches. Depending
on which manner a test is performed, the resulting data may be either of “index” type suitable for
use in manufacturing quality control (see ISO 12958-1 or ASTM D4716), or of a “performance” type
suitable for use in design and performance verification (see ISO 12958-2 or ASTM D4716). ISO 12958-1
prescribes flow measurements at predetermined hydraulic gradients and normal compressive stresses,
as well as standardized superstratum and substratum (closed cell foam materials or rigid boundaries).
The procedures in ISO 12958-2 and ASTM D4716, instead, invite the user of the test to determine all
of the test parameters specific to the designed drainage application. For performance testing, as an
example, the following test parameters or variables should be specified so as to represent anticipated
4
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or site-specific conditions such as: compressive load, hydraulic gradient, temperature, site-specific
superstratum, site-specific substratum, and seating time prior to flow measurement.
4.2.2 Index and performance tests
Manufacturers typically quantify the relative capacity of their geosynthetic drainage products via index
testing and document this performance on marketing documents and material data sheets. However,
manufacturer’s testing often reflects the flow rates of the product tested between two rigid plates, or
standardized closed cell foam pads, under a specific load and gradient, and with a limited seating time
(i.e. 15 min, 1 h, etc.). Thus, the manufacturer’s data typically represents the short-term flow capacity
for the product and serves to confirm production quality control.
To approach an understanding of the performance-related flow capacity of the geosynthetic drain it
shall be tested using test parameters representative of field service conditions. Testing to provide an
estimate of performance flow is described in ISO 12958-2 and ASTM D4716.
A performance flow test should allow materials above and below the geosynthetic drain to intrude
into the void space of the product under compressive load to simulate real project conditions. Sand,
for example, placed above the drain and loaded to design conditions will cause the upper filter
geotextile to elongate and intrude into the space between each parallel strand of a geonet or the cusps
of the cuspated core. The degree of intrusion has a direct relationship to the structural properties and
bonding/no bonding and type of bonding (to the core) of the geotextile specified for the drain core and
the amount of normal load applied to the design cross-section. Please note that the design engineer will
need to supply the testing laboratory with a sufficient volume of representative soil sample to perform
the number of required tests. The compaction requirements for the soil shall also be specified in order
to set-up the test section to reflect the design conditions. If a geosynthetic clay liner is adjacent to the
geosynthetic drain, the degree of saturation shall be specified along with the load at hydration, prior to
placement of the normal load on the design cross-section test sample.
The ISO 12958-1 test procedure defines closed-cell foam materials meeting specific compressibility
characteristics to simulate these field conditions. These “soft” superstratum and substratum materials
assist in replicating test conditions and also avoiding contamination of test water with site-specific
soils. Use of these standardized superstratum and substratum also enable manufacturers to publish
like-data on their drainage products for product comparison while providing their own estimate
of performance flow. ISO 12958-2 and ASTM D4716 allow use of site representative soils and other
materials to more closely replicate field conditions.
4.3 Normal compressive loading and seating time
For performance testing, the normal stress used during testing should be equal to the maximum
overburden pressure the material may experience during its service life. The practice of specifying a
test pressure higher than anticipated field pressure is conservative when following the most common
design procedures. Any uncertainties associated with long-term flow performance under load may be
accounted through a factor of safety rather than a higher than expected normal pressure.
Most geosynthetic drains are constructed of polymeric material, which can deform under load and over
time. This gradual deformation of the polymeric structure under a fixed load is known as “creep.” The
rate of ductile movement occurs rapidly initially (primary creep) and decreases overtime (secondary
creep).
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Key
X log time (h)
Y percent retained thickness
Figure 4 — Example of compressive creep curve for a drainage core
As compressive creep proceeds the thickness of the geosynthetic drain reduces; thereby reducing
the porosity or available cavity through which the liquid can move through the product. The amount
of thickness reduction is dependent on the compressive load placed on the product and the physical
composition and structure of the geosynthetic drain, and time. Figure 4 illustrates the typical behaviour
of a geosynthetic drain core, for which often (but not always), the majority of the creep will occur over
the first 100 h after the design load is applied.
The requirement of the 100 h seating time can be a difficult burden for the geosynthetic testing
community, requiring dedicated test apparatus for over four days prior to performing the test.
Those requiring the 100 h seating time may allocate a sufficient testing period prior to the start of
the construction project in order to obtain the conformance test results. The testing period for flow
capacity verification might be significant for large projects where numerous conformance tests may be
required.
4.4 Number of test specimens per sample per test
The ISO 12958-1 test requires flow measurements of three test specimens in both the machine and
cross-machine direction be measured. ASTM D4716 requires the testing of two specimens per sample
in the engineered flow direction to take account of manufacturing variation. Due to how a product is
manufactured, this is an important requirement to obtain an acceptable representation for the reported
test result. Processing parameters and manufacturing settings can have significant impact on drainage
product flow rate capacity and how consistent this capacity is throughout the product structure. The
test requirement for two or three specimens per sample attempts to average the variability in thickness
and porosity of the product. While less of a concern for cuspated cores and prefabricated vertical drains,
this variability should be captured in flow testing of all geosynthetic drains.
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4.5 Hydraulic gradient
The flow rate of a geosynthetic drain is proportional to the hydraulic gradient i which is defined as in
Formula (4):
i = δ / L (4)
h
where
δ is the numerical value of the hydraulic head loss along the distance L for the fluid flow in the
h
geosynthetic (m);
L is the numerical value of the distance between two points along the average direction of flow
in the geosynthetic (m).
Performance oriented flow capacity tests of geosynthetic drains should be performed using a hydraulic
gradient equal to or slightly smaller than sin β where β is equal to the slope angle of the geosynthetic
drain with the horizontal. It is not conservative to test at higher gradients. Note that transmissivity
or specific flow rate is a non-linear function of gradient because the flow regime with water in a
geosynthetic drain is typically turbulent.
When performing an in-plane flow capacity test using a hydraulic gradient of 0,1 or less, it may be
challenging to ensure the accuracy of the hydraulic gradient using open pipe manometers . These lower
gradients are best established with the aid of accurate digital pressure gauges that enable confirmation
of gradients as low as 0,01.
An alternative approach to measuring flow rates at very low gradients involves measuring flows at
several higher gradients (for example gradients at 0,1, 0,25, 0,5 and 1) so as to develop an empirical
relationship between flow rate and hydraulic gradient. The relationship between flow rate and gradient,
at given applied pressure, boundary conditions and seating time, is usually as per Formula (5):
n
Qa=⋅i (5)
where:
2
Q is the numerical value of the flow rate (m /sec);
2
a is the numerical value of the constant equal to the flow rate at unit gradient (m /sec);
i is the numerical value of the gradient (dimensionless); and
n is the numerical value of the constant (dimensionless).
Formula (5) has been verified by performing a number of tests on various materials under different
test conditions. Constants a and n depend on type of geosynthetic drain, boundary conditions, normal
stress, and test duration.
NOTE The use of this equation for very low gradients can be used with caution as other phenomenon can
interfere with the actual field performance of the product, such as the contact angle between water and the
polymer(s) used to manufacture the geosynthetic drain.
5 Determination of long-term flow performance
5.1 General
A design service lifetime, t , is defined for the drainage structure. For civil engineering structures such
D
as roads and containment facilities, this service life is typically 50 years to 100 years. Some applications
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may require shorter service lives, such as sports fields (20 years to 50 years) or mining heap leach pad
(5 years to 50 years). Additionally, some applications may have service lives lasting 1 year to 5 years.
All of these durations are too long for direct measurements to be made in advance of construction.
There are, therefore, various considerations which may be modelled as reduction factors to short-term
flow measurements which account for the time dependent changes in product performance likely to
occur during service life.
Assessing the long-term flow performance of a geosynthetic drain consists in the comparison of the
required flow rate q , determined by the application, and the allowable flow rate for the particular
reqd
product used in a particular service environment for a defined service life Q . A factor of safety F can
a S
be determined as per Formula (6):
F = Q / q (6)
S a reqd
The application of a series of reduction factors applied to the initial flow property of the geosynthetic
drain incorporates the consideration of potential causes of reduction of the water flow. In this way an
effort is made to ensure that the geosynthetic drain will always have a flow capacity equal to or in
excess of the requirements associated to a particular application.
Because there are a variety of geosynthetic drain product types having a variety of different structures
and compositions, the application of any specific reduction factor for the determination of long-term
flow may be minimal in assigned magnitude, or inappropriate for the given application. For example,
the time-dependent compressive creep resistance of a single nonwoven geotextile is negligible as
ductile compression under compressive load generally happens readily and with little or no time-
dependent resistance. Therefore, a reduction factor associated with time dependent creep may not be
appropriately applied to a relatively long (e.g. 100 h) flow test.
On the other hand, the determination of long-term flow performance of a prefabricated vertical drain
in a relatively short service life in a swamp wetland dewatering application would be wise to consider
potential reductions of flow caused by biological clogging as applied to short terms flows measured in
the crimped configuration.
ISO/TR 18228-4 provides precise guidance on how to establish q .
reqd
5.2 Reduction factors (R )
F
According to ISO/TR 18228-4, the allowable flow rate is given by Formula (7):
Q = Q / R (7)
a L F
where:
RR = ⋅⋅ RR ⋅⋅ RR (8)
FF,inF,cr-QF,ccF,bcF,L
Q is the numerical value of the available long-term flow rate for the geosynthetic drain (l/s/m
a
2
or m /s). The long-term planar flow rate that will not result in failure of the geosynthetic
drain during the required design life, calculated on a flow rate per unit of drain width basis;
Q is the numerical value of the short-term flow rate of the geosynthetic drain from hydraulic
L
flow tests using site-specific test conditions, i.e. compressive stress, hydraulic gradient,
2
seating time and boundary conditions l/s/m or m /s);
R is the numerical value of a combined redu
...
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