ISO/TR 18228-3:2021
(Amendment)Design using geosynthetics — Part 3: Filtration
Design using geosynthetics — Part 3: Filtration
This document provides general considerations to support the design guidance to geotechnical and civil engineers involved in the design of structures in which a geotextile is used as a filter. The key potential failure mechanisms are described, and guidance is proposed to select engineering properties.
Design pour géosynthétiques — Partie 3: Filtration
General Information
Standards Content (sample)
TECHNICAL ISO/TR
REPORT 18228-3
First edition
2021-06
Design using geosynthetics —
Part 3:
Filtration
Design pour géosynthétiques —
Partie 3: Filtration
Reference number
ISO/TR 18228-3:2021(E)
ISO 2021
---------------------- Page: 1 ----------------------
ISO/TR 18228-3:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021
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
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/TR 18228-3:2021(E)
Contents Page
Foreword ........................................................................................................................................................................................................................................iv
Introduction ................................................................................................................................................................................................................................vi
1 Scope ................................................................................................................................................................................................................................. 1
2 Normative references ...................................................................................................................................................................................... 1
3 Terms, definitions, symbols and abbreviated terms ....................................................................................................... 1
3.1 Terms and definitions ....................................................................................................................................................................... 1
3.2 Symbols ......................................................................................................................................................................................................... 1
3.3 Abbreviated terms ............................................................................................................................................................................... 2
4 Concepts and fundamental principles ........................................................................................................................................... 2
4.1 General ........................................................................................................................................................................................................... 2
4.2 The filtration function of geotextiles.................................................................................................................................... 3
4.3 Filter selection fundamentals..................................................................................................................................................... 4
5 Typical applications .......................................................................................................................................................................................... 4
5.1 General considerations .................................................................................................................................................................... 4
5.2 Soil filtration ............................................................................................................................................................................................. 5
5.3 Slurry filtration ....................................................................................................................................................................................... 5
6 Materials ....................................................................................................................................................................................................................... 5
7 Functional properties relevant to design ................................................................................................................................... 8
7.1 Characteristic opening size .......................................................................................................................................................... 8
7.2 Velocity index and permittivity (permeability) .......................................................................................................... 9
7.3 Resistance to water penetration .............................................................................................................................................. 9
7.4 Number of constrictions ................................................................................................................................................................. 9
7.5 Percent open area ................................................................................................................................................................................. 9
8 Principles of design ........................................................................................................................................................................................10
8.1 General considerations .................................................................................................................................................................10
8.2 Designing geotextiles for soil filtration ...........................................................................................................................13
8.2.1 General...................................................................................................................................................................................13
8.2.2 Retention criteria of the soil skeleton ........................................................................................................13
8.2.3 Non-retention criteria of fines in suspension ......................................................................................14
8.2.4 Permeability criteria ........................................................................................................................................... .......14
8.3 Designing geotextiles for slurry and suspended solids filtration .............................................................14
9 Testing the soil/geotextile filtration compatibility .......................................................................................................15
9.1 General ........................................................................................................................................................................................................15
9.2 Soil/geotextile compatibility ...................................................................................................................................................15
9.2.1 Gradient ratio ...................................................................................................................................................................15
9.2.2 Hydraulic conductivity ratio ...............................................................................................................................16
9.2.3 Retention performance of geotextiles exposed to turbulent flow conditions .........16
9.3 Evaluation of slurry/geotextile behaviour ...................................................................................................................16
9.4 Biological or chemical clogging potential .....................................................................................................................16
9.5 Impact of abrasion on the filtration properties of geotextiles ....................................................................17
9.5.1 Abrasion resistance – Index property ........................................................................................................17
9.5.2 Abrasion resistance – Rotary drum method .........................................................................................17
10 Examples of material specifications and design guidance ....................................................................................17
Bibliography .............................................................................................................................................................................................................................19
© ISO 2021 – All rights reserved iii---------------------- Page: 3 ----------------------
ISO/TR 18228-3:2021(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.
A list of all parts in the ISO/TR 18228 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.iv © ISO 2021 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/TR 18228-3:2021(E)
© ISO 2021 – All rights reserved v
---------------------- Page: 5 ----------------------
ISO/TR 18228-3:2021(E)
Introduction
The ISO/TR 18228 series provides guidance for designs using geosynthetics for soils and below ground
structures in contact with natural soils, fills and asphalt. The series contains parts which cover designs
using geosynthetics, including guidance for characterization of the materials to be used and other
factors affecting the design and performance of the systems which are particular to each part, with
ISO/TR 18228-1 providing general guidance relevant to the subsequent parts of the series.
The series is generally written in a limit state format and guidelines are provided in terms of partial
material factors and load factors for various applications and design lives, where appropriate.
This document includes information relating to the filtration function. Details of design methodology
adopted in a number of regions are provided. The characteristics of the geosynthetics and the test
methods normally used to quantify the properties of the geosynthetics are described. Some regional
specific rules and regulations that normally apply to designs using geosynthetics in these regions are
also provided.vi © ISO 2021 – All rights reserved
---------------------- Page: 6 ----------------------
TECHNICAL REPORT ISO/TR 18228-3:2021(E)
Design using geosynthetics —
Part 3:
Filtration
1 Scope
This document provides general considerations to support the design guidance to geotechnical and
civil engineers involved in the design of structures in which a geotextile is used as a filter. The key
potential failure mechanisms are described, and guidance is proposed to select engineering properties.
2 Normative referencesThe 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 definitions3 Terms, definitions, symbols and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 10318-1 and the following
apply.ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp— IEC Electropedia: available at http:// www .electropedia .org/
3.2 Symbols
B, B , B factors function of the application, soil properties and hydraulic conditions, used for the
1 2verification of the retention criteria of the soil skeleton
C constant, used for the verification of the non-retention criteria of fines in suspension
C coefficient of uniformity of the soil, defined as C = d / du u 60 10
d , d , d , diameters of particles for which 85 %, 60 %, 50 %, 30 % or 10 % of all soil particles are
85 60 50smaller (e.g. d = 200 µm means that 85 % of the soil particles are smaller than 200 µm)
d or d30 10
d indicative diameter of the soil, for retention criteria
d indicative diameter of the soil, for non-retention criteria of fines in suspension
ΔH water head used to measure the indicative velocity in the laboratory test, i.e. ΔH = 0,05 m
© ISO 2021 – All rights reserved 1---------------------- Page: 7 ----------------------
ISO/TR 18228-3:2021(E)
Ε constant, used for the verification of the permeability criteria
i hydraulic gradient prevailing immediately upstream of the geotextile (0 to 0,025 m away
from the geotextile as in ASTM D5101)i hydraulic gradient prevailing in the soil, in the vicinity of the geotextile filter
k permeability of the soilm number of constrictions
V is the indicative velocity of the water passing through the filter, which is the flow rate
divided by the total area of passage (apparent area) at a water head of ΔH = 0,05 m
permittivity, represents 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
3.3 Abbreviated termsBOD biological oxygen demand
COD chemical oxygen demand
COS characteristic opening size of a geotextile
PI plasticity index
POA percent open area
PVD prefabricated vertical drain
UV ultra-violet
4 Concepts and fundamental principles
4.1 General
Soils are porous media containing 20 % to 40 % voids in between soil particles, which are typically
filled with gas such as air, liquid such as water, or both. Displacement of water within the voids of the
soil generates a dragging force on each of the grains of the soil. When the grains are supported, e.g. by
other grains, they stay in place and the water moves without disturbing the soil structure. However,
if the grains of soil are not in contact with a solid that can offer a resisting force, internal erosion can
develop: soil particles are dragged by the water until they reach an obstacle, or until they exit the soil
structure.Soil structures often include configurations where the water has to flow away from the soil, e.g. into a
drain. To prevent internal erosion, a filter media can be installed, in order to offer a resisting force to
the largest particles of the soil, called the soil skeleton, and to prevent it from being dragged away by
the water. However, this filter media should not restrain the flow of water, in particular to avoid pore
pressure build-up, which could also adversely affect the stability of the soil structure.
In geosynthetics, the filtration function is to stabilize the soil by maintaining in place the soil skeleton
in contact with the geotextile surface and restrain uncontrolled passage of soil, while allowing the
passage of water or other fluids and some of the finest particles transported in suspension across the
filter. The geotextile filter can be thought of as a catalyst to create a natural granular filter in the thin
soil layer in contact.Filters may be installed at the interface with a water-transport media, e.g. soil with high permeability,
drainage pipe, edge drain, drainboard, PVDs. They may also be installed between soil and rip-rap,
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ISO/TR 18228-3:2021(E)
gabions or product with the same function, for example for coastal protection, banks protections, in
dams.When the geotextiles are not covered with soil and laid alone, e.g. as silt fences, etc., the filtration
behaviour is different: there is no soil skeleton to stabilize, the geotextile is intended to trap or screen
all moving particles.The performance of a filter may be qualified by its ability to fulfil the two contradictory functions
required for filtration:— prevent damages caused by the transport of an excessive quantity of particles from one side of the
filter to the other (piping), such as internal erosion of the soil being filtered which could modify in its
engineering properties, formation of cavities on the upstream side of the filter. Such damages also
include excessive contamination of the structure located downstream by particles piped through
the filter, e.g. blocking of a drainage pipe;— minimize restriction to the flow of water passing through the filter, to avoid pore pressure build-up
on the upstream side of the filter.4.2 The filtration function of geotextiles
Long term performance of geotextile filters can be endangered by the blocking of the surface (blinding)
or of the pores (clogging) of a soil or geotextile filter. These mechanisms can be caused by the
accumulation of fine particles and development of a “cake” on the surface of the geotextile (blinding),
or inside the geotextile structure (clogging), resulting in a drastic reduction of the permittivity of the
filter.In some cases, reduction of the capacity of the geotextile filter to let water pass across its plane might
also be caused by the precipitation of chemicals, e.g. iron ochre, calcium, or the development of a
biological activity. Evidences of clogging caused by the presence of air pockets trapped into, or in the
vicinity of, the filter have also been observed.These mechanisms suggest that the best performance for a filter is obtained when using a material
which openings are small enough to stabilize the largest particles in contact (soil skeleton) but large
enough to let pass the finest particles in suspension and to avoid internal erosion of the soil, and piping.
For both piping and clogging mechanisms, the parameter controlling the performance of a filter is
the size of the voids through which particles of the soil are likely to travel at the surface or inside the
geotextile. The filtration characteristics of a geotextile filter, such as opening size or permeability, are
therefore crucial to its design. These properties need to be selected with consideration to the properties
of the soil to be filtered and hydraulic conditions prevailing on a particular site.
Other parameters characterizing the structure of a geotextile have also been investigated, such as the
pore size distribution (determined using ASTM D6767) or the number of constriction (determined
using ASTM D7138). However, although there is a consensus regarding the fact these parameters
can influence the filtration performance of a filter, they are still being investigated by the research
community. Design guidance was still not available at the time this document was prepared.
Depending on their structure, some geotextiles can be compressed under load. Consequently, their
permeability might decrease when compared to the property measured without load. This phenomenon
was investigated. Some reliability issues were identified with the testing techniques and the impact
of normal load found to be difficult to quantify. There is no consensus at the time this document was
prepared regarding the need, nor the value, that should be used as a safety factor to be applied on the
values of permeability without load to address this issue.In some cases, account also needs to be taken of potential loading mechanisms, presence of iron ochre,
potential biological activity, and mineralogy of the soil, which can all affect the long-term performance
of the filter. For extreme situations, it might be necessary to use alternatives to geotextiles.
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ISO/TR 18228-3:2021(E)
4.3 Filter selection fundamentals
A geotextile filter should be selected considering the following parameters:
— filtration performance: ability to retain the soil skeleton and to let the water pass perpendicularly
to its plane, depending on the soil properties, i.e. grain size distribution, cohesion and permeability
of the adjacent soils, and taking into account the type of pore water flow (turbulent, laminar);
— suspended solids concentration of the water to be filtered, property of the suspended particles
when applicable;— survivability, or capacity to resist the stress caused by installation and subsequent construction
works;— capacity to resist mechanical stresses encountered during its service, when applicable;
— durability, which includes resistance to the chemical environment in which it is installed; resistance
to UV oxidation during construction or in service when applicable; and long-term durability;
— penetration of roots, anchorage of an overlying structure, or any other alien material likely to affect
its continuity or its properties;— location of the geotextile filter within the soil structure.
Most applications involve the seepage of water in a single direction, typically perpendicular to the
plane of the geotextile filter. However, some applications involve bi-directional flow, cyclic flows, which
involve significantly different hydraulic and/or mechanical stresses which are likely to affect the
filtration performance of a geotextile.Mechanical stress prevailing on the soil/geotextile interface might vary from one application to
another. While many drainage and filtration applications involve static, constant loads, others such as
bank protection can experience dynamic loading, which can affect the stability of the filter. For many
roadway applications, the repetitive passage of trucks on a road generates dynamic stresses which are
likely to affect the stability of the soil structure in the vicinity of the filter.
Geotextile filters are typically used to drain water but may also be used with leachate or other liquids
from adjacent soils, waste, or other solid porous media, such as in landfill leachate collection systems.
They are occasionally used to filter gases in soils, e.g. gas collection layers in landfills, or gas drainage
layers installed beneath building basements.5 Typical applications
5.1 General considerations
There are two key types of application of geotextile filters, which involve different filtration
mechanisms, thus different approaches to design:— filtration of soils, where the geotextile is in intimate contact with the soil. The water then moves
within the soil matrix, and might drag the finest particles away depending on the equilibrium of
each particle in contact at the geotextile surface;— filtration of slurries or suspended particles, where each particle reaching the geotextile (or soil
accumulated on the geotextile) is suspended in water.Designing using one or the other approach might lead to the recommendation of different properties for
the geotextile filter. Consequently, the long-term performance of the geotextile filter is first determined
by the correct identification of the filtration mechanism actually involved. One impact might be the
recommendation of preventive measures to ensure that a geotextile designed for soil filtration is not
exposed to slurry filtration (i.e. include a requirement to backfill immediately after installation).
4 © ISO 2021 – All rights reserved---------------------- Page: 10 ----------------------
ISO/TR 18228-3:2021(E)
5.2 Soil filtration
Soil filtration typically involves the filtration/retention of compacted soils exposed to a flow of water,
where an intimate contact between the soil and the geotextile is assumed. Examples of such situations
include:— drainage systems (buildings, agriculture, etc.);
— dams;
— PVDs;
— roadways;
— rail track bed;
— waterways/canals;
— coastal protection;
— landfills – leachate collection systems.
Soil filtration can also involve the separation/retention of soils exposed to the flow of gases, for example:
— protection against the intrusion in buildings of radon and other subsurface gases;
— gases collection layers in landfills.5.3 Slurry filtration
Typical applications where the geotextile is designed to separate water from solid particles, where
there is little or no particle-to-particle contact, include:— filtration of slurries and dewatering applications;
— geocontainers;
— silt fences.
6 Materials
Geotextiles intended to perform as a filter for permanent applications are usually manufactured with
polypropylene, polyester or polyethylene fibres. They may be woven, non-woven or knitted. Typical
ranges of properties are given in this Clause, however, there are products on the market offering
characteristics beyond the proposed limits, intended for use in specific applications. Manufacturers
should be contacted for further details on their products.Woven geotextiles may be either slit-films (Figure 1), monofilaments, multi-filaments or a combination
thereof (Figure 2). They offer opening sizes varying between 0,05 mm and 2,0 mm, POA from 0,5 %
and 40 %, and velocity index from 0,001 m/s to 1 m/s. Their construction may include multifilament
polyester yarns, polypropylene or polyethylene tapes or strands.© ISO 2021 – All rights reserved 5
---------------------- Page: 11 ----------------------
ISO/TR 18228-3:2021(E)
[31]
SOURCE: Reproduced with permission from Kaytech .
Figure 1 — Structure of a woven slit film geotextile
[29]
SOURCE: Reproduced with permission from CTT Group / Sageos .
Figure 2 — Structure of a woven geotextile combining a tape and a monofilament
Non-woven geotextiles may be continuous filaments or staple fibres, with opening sizes varying
between 0,05 mm and 0,5 mm, numbers of constrictions from 5 to 50, and velocity index from 0,005
m/s to 0,5 m/s. They are typically polypropylene or polyester fibres. They can be needle-punched
(Figure 3) or heat-bonded (Figure 4).6 © ISO 2021 – All rights reserved
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ISO/TR 18228-3:2021(E)
[31]
SOURCE: Reproduced with permission from Kaytech .
Figure 3 — Structure of a non-woven, needle-punched geotextile
[31]
SOURCE: Reproduced with permission from Kaytech .
Figure 4 — Structure of a non-woven, heat-bonded geotextile
Knitted geotextiles (Figure 5) are typically constructed using multifilament polyester yarns, to form a
structure with opening size varying between 0,15 mm and 0,5 mm and velocity index from 0,05 m/s to
1 m/s.© ISO 2021 – All rights reserved 7
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ISO/TR 18228-3:2021(E)
[30]
SOURCE: Reproduced with permission from Geofabrics .
Figure 5 — Structure of a knitted geotextile
Woven and non-woven geotextiles are offered in rolls with widths varying typically between 1 m and
6 m or more. Knitted geotextiles are typically manufactured in a seam-free, tubular fashion, and are
typically used as filters to envelope drainage pipes, up to 1,2 m in diameter, used in subsurface drainage
applications.Some manufacturers use lubricants in their manufacturing process. Traces of these lubricants might
be found in the product. These lubricants can be hydrophobic or hydrophilic, which might affect the
resistance to water penetration of the geotextile. These traces are typically washed out over time, but
might affect positively or negatively the behaviour of geotextiles immediately after their installation.
However, geotextiles are usually deep enough into the soil to keep a permanent moisture content, and
this issue is ty...
TECHNICAL ISO/TR
REPORT 18228-3
First edition
Design using geosynthetics —
Part 3:
Filtration
Design pour géosynthétiques —
Partie 3: Filtration
PROOF/ÉPREUVE
Reference number
ISO/TR 18228-3:2021(E)
ISO 2021
---------------------- Page: 1 ----------------------
ISO/TR 18228-3:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021
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
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii PROOF/ÉPREUVE © ISO 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/TR 18228-3:2021(E)
Contents Page
Foreword ........................................................................................................................................................................................................................................iv
Introduction ................................................................................................................................................................................................................................vi
1 Scope ................................................................................................................................................................................................................................. 1
2 Normative references ...................................................................................................................................................................................... 1
3 Terms, definitions, symbols and abbreviated terms ....................................................................................................... 1
3.1 Terms and definitions ....................................................................................................................................................................... 1
3.2 Symbols ......................................................................................................................................................................................................... 1
3.3 Abbreviated terms ............................................................................................................................................................................... 2
4 Concepts and fundamental principles ........................................................................................................................................... 2
4.1 General ........................................................................................................................................................................................................... 2
4.2 The filtration function of geotextiles.................................................................................................................................... 3
4.3 Filter selection fundamentals..................................................................................................................................................... 4
5 Typical applications .......................................................................................................................................................................................... 4
5.1 General considerations .................................................................................................................................................................... 4
5.2 Soil filtration ............................................................................................................................................................................................. 5
5.3 Slurry filtration ....................................................................................................................................................................................... 5
6 Materials ....................................................................................................................................................................................................................... 5
7 Functional properties relevant to design ................................................................................................................................... 8
7.1 Characteristic opening size .......................................................................................................................................................... 8
7.2 Velocity index and permittivity (permeability) .......................................................................................................... 9
7.3 Resistance to water penetration .............................................................................................................................................. 9
7.4 Number of constrictions ................................................................................................................................................................. 9
7.5 Percent open area ................................................................................................................................................................................. 9
8 Principles of design ........................................................................................................................................................................................10
8.1 General considerations .................................................................................................................................................................10
8.2 Designing geotextiles for soil filtration ...........................................................................................................................13
8.2.1 General...................................................................................................................................................................................13
8.2.2 Retention criteria of the soil skeleton ........................................................................................................13
8.2.3 Non-retention criteria of fines in suspension ......................................................................................14
8.2.4 Permeability criteria ........................................................................................................................................... .......14
8.3 Designing geotextiles for slurry and suspended solids filtration .............................................................14
9 Testing the soil/geotextile filtration compatibility .......................................................................................................15
9.1 General ........................................................................................................................................................................................................15
9.2 Soil/geotextile compatibility ...................................................................................................................................................15
9.2.1 Gradient ratio ...................................................................................................................................................................15
9.2.2 Hydraulic conductivity ratio ...............................................................................................................................16
9.2.3 Retention performance of geotextiles exposed to turbulent flow conditions .........16
9.3 Evaluation of slurry/geotextile behavior ......................................................................................................................16
9.4 Biological or chemical clogging potential .....................................................................................................................16
9.5 Impact of abrasion on the filtration properties of geotextiles ....................................................................17
9.5.1 Abrasion resistance – Index property ........................................................................................................17
9.5.2 Abrasion resistance – Rotary drum method .........................................................................................17
10 Examples of material specifications and design guidance ....................................................................................17
Bibliography .............................................................................................................................................................................................................................19
© ISO 2021 – All rights reserved PROOF/ÉPREUVE iii---------------------- Page: 3 ----------------------
ISO/TR 18228-3:2021(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.
A list of all parts in the ISO/TR 18228 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.iv PROOF/ÉPREUVE © ISO 2021 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/TR 18228-3:2021(E)
© ISO 2021 – All rights reserved PROOF/ÉPREUVE v
---------------------- Page: 5 ----------------------
ISO/TR 18228-3:2021(E)
Introduction
The ISO/TR 18228 series provides guidance for designs using geosynthetics for soils and below ground
structures in contact with natural soils, fills and asphalt. The series contains parts which cover designs
using geosynthetics, including guidance for characterization of the materials to be used and other
factors affecting the design and performance of the systems which are particular to each part, with
ISO/TR 18228-1 providing general guidance relevant to the subsequent parts of the series.
The series is generally written in a limit state format and guidelines are provided in terms of partial
material factors and load factors for various applications and design lives, where appropriate.
This document includes information relating to the filtration function. Details of design methodology
adopted in a number of regions are provided. The characteristics of the geosynthetics and the test
methods normally used to quantify the properties of the geosynthetics are described. Some regional
specific rules and regulations that normally apply to designs using geosynthetics in these regions are
also provided.vi PROOF/ÉPREUVE © ISO 2021 – All rights reserved
---------------------- Page: 6 ----------------------
TECHNICAL REPORT ISO/TR 18228-3:2021(E)
Design using geosynthetics —
Part 3:
Filtration
1 Scope
This document provides general considerations to support the design guidance to geotechnical and
civil engineers involved in the design of structures in which a geotextile is used as a filter. The key
potential failure mechanisms are described, and guidance is proposed to select engineering properties.
2 Normative referencesThe 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 definitions3 Terms, definitions, symbols and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 10318-1 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp— IEC Electropedia: available at http:// www .electropedia .org/
3.2 Symbols
B, B , B factors function of the application, soil properties and hydraulic conditions, used for the
1 2verification of the retention criteria of the soil skeleton
C constant, used for the verification of the non-retention criteria of fines in suspension
C coefficient of uniformity of the soil, defined as C = d / du u 60 10
d , d , d , diameters of particles for which 85 %, 60 %, 50 %, 30 % or 10 % of all soil particles are
85 60 50d or d smaller (e.g. d = 200 µm means that 85 % of the soil particles are smaller than 200 µm)
30 10 85d indicative diameter of the soil, for retention criteria
d indicative diameter of the soil, for non-retention criteria of fines in suspension
ΔH water head used to measure the indicative velocity in the laboratory test, i.e. ΔH = 0,05 m
Ε constant, used for the verification of the permeability criteria© ISO 2021 – All rights reserved PROOF/ÉPREUVE 1
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i hydraulic gradient prevailing immediately upstream of the geotextile (0 to 0,025 m away
from the geotextile as in ASTM D5101)i hydraulic gradient prevailing in the soil, in the vicinity of the geotextile filter
k permeability of the soilm number of constrictions
V is the indicative velocity of the water passing through the filter, which is the flow rate
divided by the total area of passage (apparent area) at a water head of ΔH = 0,05 m
permittivity, represents 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
3.3 Abbreviated termsBOD biological oxygen demand
COD chemical oxygen demand
COS characteristic opening size of a geotextile
PI plasticity index
POA percent open area
PVD prefabricated vertical drain
UV ultra-violet
4 Concepts and fundamental principles
4.1 General
Soils are porous media containing 20 % to 40 % voids in between soil particles, which are typically
filled with gas such as air, liquid such as water, or both. Displacement of water within the voids of the
soil generates a dragging force on each of the grains of the soil. When the grains are supported, e.g. by
other grains, they stay in place and the water moves without disturbing the soil structure. However,
if the grains of soil are not in contact with a solid that can offer a resisting force, internal erosion can
develop: soil particles are dragged by the water until they reach an obstacle, or until they exit the soil
structure.Soil structures often include configurations where the water has to flow away from the soil, e.g. into a
drain. To prevent internal erosion, a filter media can be installed, in order to offer a resisting force to
the largest particles of the soil, called the soil skeleton, and to prevent it from being dragged away by
the water. However, this filter media should not restrain the flow of water, in particular to avoid pore
pressure build-up, which could also adversely affect the stability of the soil structure.
In geosynthetics, the filtration function is to stabilize the soil by maintaining in place the soil skeleton
in contact with the geotextile surface and restrain uncontrolled passage of soil, while allowing the
passage of water or other fluids and some of the finest particles transported in suspension across the
filter. The geotextile filter can be thought of as a catalyst to create a natural granular filter in the thin
soil layer in contact.Filters may be installed at the interface with a water-transport media, e.g. soil with high permeability,
drainage pipe, edge drain, drainboard, PVDs. They may also be installed between soil and rip-rap, gabions
or product with the same function, for example for coastal protection, banks protections, in dams.
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When the geotextiles are not covered with soil and laid alone, e.g. as silt fences, etc., the filtration
behaviour is different: there is no soil skeleton to stabilize, the geotextile is intended to trap or screen
all moving particles.The performance of a filter may be qualified by its ability to fulfil the two contradictory functions
required for filtration:— prevent damages caused by the transport of an excessive quantity of particles from one side of the
filter to the other (piping), such as internal erosion of the soil being filtered which could modify in its
engineering properties, formation of cavities on the upstream side of the filter. Such damages also
include excessive contamination of the structure located downstream by particles piped through
the filter, e.g. blocking of a drainage pipe;— minimize restriction to the flow of water passing through the filter, to avoid pore pressure build-up
on the upstream side of the filter.4.2 The filtration function of geotextiles
Long term performance of geotextile filters can be endangered by the blocking of the surface (blinding)
or of the pores (clogging) of a soil or geotextile filter. These mechanisms can be caused by the
accumulation of fine particles and development of a “cake” on the surface of the geotextile (blinding), or
inside the geotextile structure (clogging), resulting in a drastic reduction of the permittivity of the filter.
In some cases, reduction of the capacity of the geotextile filter to let water pass across its plane might
also be caused by the precipitation of chemicals, e.g. iron ochre, calcium, or the development of a
biological activity. Evidences of clogging caused by the presence of air pockets trapped into, or in the
vicinity of, the filter have also been observed.These mechanisms suggest that the best performance for a filter is obtained when using a material
which openings are small enough to stabilize the largest particles in contact (soil skeleton) but large
enough to let pass the finest particles in suspension and to avoid internal erosion of the soil, and piping.
For both piping and clogging mechanisms, the parameter controlling the performance of a filter is
the size of the voids through which particles of the soil are likely to travel at the surface or inside the
geotextile. The filtration characteristics of a geotextile filter, such as opening size or permeability, are
therefore crucial to its design. These properties need to be selected with consideration to the properties
of the soil to be filtered and hydraulic conditions prevailing on a particular site.
Other parameters characterizing the structure of a geotextile have also been investigated, such as the
pore size distribution (determined using ASTM D6767) or the number of constriction (determined
using ASTM D7138). However, although there is a consensus regarding the fact these parameters
can influence the filtration performance of a filter, they are still being investigated by the research
community. Design guidance was still not available at the time this document was prepared.
Depending on their structure, some geotextiles can be compressed under load. Consequently, their
permeability might decrease when compared to the property measured without load. This phenomenon
was investigated. Some reliability issues were identified with the testing techniques and the impact
of normal load found to be difficult to quantify. There is no consensus at the time this document was
prepared regarding the need, nor the value, that should be used as a safety factor to be applied on the
values of permeability without load to address this issue.In some cases, account also needs to be taken of potential loading mechanisms, presence of iron ochre,
potential biological activity, and mineralogy of the soil, which can all affect the long-term performance
of the filter. For extreme situations, it might be necessary to use alternatives to geotextiles.
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4.3 Filter selection fundamentals
A geotextile filter should be selected considering the following parameters:
— filtration performance: ability to retain the soil skeleton and to let the water pass perpendicularly
to its plane, depending on the soil properties, i.e. grain size distribution, cohesion and permeability
of the adjacent soils, and taking into account the type of pore water flow (turbulent, laminar);
— suspended solids concentration of the water to be filtered, property of the suspended particles
when applicable;— survivability, or capacity to resist the stress caused by installation and subsequent construction
works;— capacity to resist mechanical stresses encountered during its service, when applicable;
— durability, which includes resistance to the chemical environment in which it is installed; resistance
to UV oxidation during construction or in service when applicable; and long-term durability;
— penetration of roots, anchorage of an overlying structure, or any other alien material likely to affect
its continuity or its properties;— location of the geotextile filter within the soil structure.
Most applications involve the seepage of water in a single direction, typically perpendicular to the
plane of the geotextile filter. However, some applications involve bi-directional flow, cyclic flows, which
involve significantly different hydraulic and/or mechanical stresses which are likely to affect the
filtration performance of a geotextile.Mechanical stress prevailing on the soil/geotextile interface might vary from one application to
another. While many drainage and filtration applications involve static, constant loads, others such as
bank protection can experience dynamic loading, which can affect the stability of the filter. For many
roadway applications, the repetitive passage of trucks on a road generates dynamic stresses which are
likely to affect the stability of the soil structure in the vicinity of the filter.
Geotextile filters are typically used to drain water but may also be used with leachate or other liquids
from adjacent soils, waste, or other solid porous media, such as in landfill leachate collection systems.
They are occasionally used to filter gases in soils, e.g. gas collection layers in landfills, or gas drainage
layers installed beneath building basements.5 Typical applications
5.1 General considerations
There are two key types of application of geotextile filters, which involve different filtration
mechanisms, thus different approaches to design:— filtration of soils, where the geotextile is in intimate contact with the soil. The water then moves
within the soil matrix, and might drag the finest particles away depending on the equilibrium of
each particle in contact at the geotextile surface;— filtration of slurries or suspended particles, where each particle reaching the geotextile (or soil
accumulated on the geotextile) is suspended in water.Designing using one or the other approach might lead to the recommendation of different properties for
the geotextile filter. Consequently, the long-term performance of the geotextile filter is first determined
by the correct identification of the filtration mechanism actually involved. One impact might be the
recommendation of preventive measures to ensure that a geotextile designed for soil filtration is not
exposed to slurry filtration (i.e. include a requirement to backfill immediately after installation).
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5.2 Soil filtration
Soil filtration typically involves the filtration/retention of compacted soils exposed to a flow of water,
where an intimate contact between the soil and the geotextile is assumed. Examples of such situations
include:— drainage systems (buildings, agriculture, etc.);
— dams;
— PVDs;
— roadways;
— rail track bed;
— waterways/canals;
— coastal protection;
— landfills – leachate collection systems.
Soil filtration can also involve the separation/retention of soils exposed to the flow of gases, for example:
— protection against the intrusion in buildings of radon and other subsurface gases;
— gases collection layers in landfills.5.3 Slurry filtration
Typical applications where the geotextile is designed to separate water from solid particles, where
there is little or no particle-to-particle contact, include:— filtration of slurries and dewatering applications;
— geocontainers;
— silt fences.
6 Materials
Geotextiles intended to perform as a filter for permanent applications are usually manufactured with
polypropylene, polyester or polyethylene fibres. They may be woven, non-woven or knitted. Typical
ranges of properties are given in this Clause, however, there are products on the market offering
characteristics beyond the proposed limits, intended for use in specific applications. Manufacturers
should be contacted for further details on their products.Woven geotextiles may be either slit-films (Figure 1), monofilaments, multi-filaments or a combination
thereof (Figure 2). They offer opening sizes varying between 0,05 mm and 2,0 mm, POA from 0,5 %
and 40 %, and velocity index from 0,001 m/s to 1 m/s. Their construction may include multifilament
polyester yarns, polypropylene or polyethylene tapes or strands.© ISO 2021 – All rights reserved PROOF/ÉPREUVE 5
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[31]
SOURCE: Reproduced with permission from Kaytech .
Figure 1 — Structure of a woven slit film geotextile
[29]
SOURCE: Reproduced with permission from CTT Group / Sageos .
Figure 2 — Structure of a woven geotextile combining a tape and a monofilament
Non-woven geotextiles may be continuous filaments or staple fibres, with opening sizes varying
between 0,05 mm and 0,5 mm, numbers of constrictions from 5 to 50, and velocity index from 0,005
m/s to 0,5 m/s. They are typically polypropylene or polyester fibres. They can be needle-punched
(Figure 3) or heat-bonded (Figure 4)6 PROOF/ÉPREUVE © ISO 2021 – All rights reserved
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[31]
SOURCE: Reproduced with permission from Kaytech .
Figure 3 — Structure of a non-woven, needle-punched geotextile
[31]
SOURCE: Reproduced with permission from Kaytech .
Figure 4 — Structure of a non-woven, heat-bonded geotextile
Knitted geotextiles (Figure 5) are typically constructed using multifilament polyester yarns, to form
a structure with opening size varying between 0,15 mm and 0,5 mm and velocity index from 0,05 m/s
to 1 m/s.© ISO 2021 – All rights reserved PROOF/ÉPREUVE 7
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[30]
SOURCE: Reproduced with permission from Geofabrics .
Figure 5 — Structure of a knitted geotextile
Woven and non-woven geotextiles are offered in rolls with widths varying typically between 1 m and
6 m or more. Knitted geotextiles are typically manufactured in a seam-free, tubular fashion, and are
typically used as filters to envelope drainage pipes, up to 1,2 m in diameter, used in subsurface drainage
applications.Some manufacturers use lubricants in their manufacturing process. Traces of these lubricants might
be found in the product. These lubricants can be hydrophobic or hydrophilic, which might affect the
resistance to water penetration of the geotextile. These traces are typically washed out over time, but
might affect positively or negatively the behaviour of geotextiles...
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