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

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Status
Published
Publication Date
14-Jun-2021
Current Stage
6060 - International Standard published
Start Date
15-Jun-2021
Completion Date
15-Jun-2021
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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
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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
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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 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, 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 2
verification 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 / d
u u 60 10

d , d , d , diameters of particles for which 85 %, 60 %, 50 %, 30 % or 10 % of all soil particles are

85 60 50

smaller (e.g. d = 200 µm means that 85 % of the soil particles are smaller than 200 µm)

d or d
30 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
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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 soil
m 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 terms
BOD 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,

2 © ISO 2021 – All rights reserved
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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.

© ISO 2021 – All rights reserved 3
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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
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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
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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.
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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 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, 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 2
verification 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 / d
u u 60 10

d , d , d , diameters of particles for which 85 %, 60 %, 50 %, 30 % or 10 % of all soil particles are

85 60 50

d or d smaller (e.g. d = 200 µm means that 85 % of the soil particles are smaller than 200 µm)

30 10 85
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

Ε constant, used for the verification of the permeability criteria
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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 soil
m 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 terms
BOD 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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ISO/TR 18228-3:2021(E)

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).

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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.
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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)
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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.
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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
...

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