ISO/TR 13434:1998

TECHNICAL ISO/TR
REPORT 13434
First edition
1998-12-15
Geotextiles and geotextile-related
products — Guidelines on durability
Géotextiles et produits apparentés — Lignes directrices concernant
la durabilité
A
Reference number
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 main task of technical committees is to prepare International Standards, but in exceptional circumstances a
technical committee may propose the publication of a Technical Report of one of the following types:
 type 1, when the required support cannot be obtained for the publication of an International Standard, despite
repeated efforts;
 type 2, when the subject is still under technical development or where for any other reason there is the future
but not immediate possibility of an agreement on an International Standard;
 type 3, when a technical committee has collected data of a different kind from that which is normally published
as an International Standard (“state of the art”, for example).
Technical Reports of types 1 and 2 are subject to review within three years of publication, to decide whether they
can be transformed into International Standards. Technical Reports of type 3 do not necessarily have to be
reviewed until data they provide are considered to be no longer valid or useful.
Technical Reports are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.
Attention is drawn to the possibility that some of the elements of this Technical Report may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO/TR 13434, which is a Technical Report of type 3, was prepared by the European Committee for Standardization
(CEN) in collaboration with ISO Technical Committee TC 38, Textiles, Subcommittee SC 21, Geotextiles, in
accordance with the Agreement on technical cooperation between ISO and CEN (Vienna Agreement).
Throughout the text of this document, read “.this European prestandard.” to mean “.this Technical Report.”.
Annex A of this Technical Report is for information only.
Annex ZZ provides a list of corresponding International and European Standards for which equivalents are not given
in the text.
©  ISO 1998
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic
or mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case postale 56 • CH-1211 Genève 20 • Switzerland
Internet iso@iso.ch
Printed in Switzerland
ii
© ISO ISO/TR 13434:1998(E)
Foreword
This draft CEN Technical Report has been prepared by CEN/TC 189 "Geotextiles and
geotextile-related products" the secretariat of which is held by IBN. It is now submitted to the
CEN/BT for approval.
iii
© ISO ISO/TR 13434:1998(E)
1 Scope
This guide is intended to introduce the reader to the basic concepts of geotextiles durability
and its assessment. Consideration of the design parameters, the project conditions and the
geotextile properties leads to the definition of the appropriate tests to be performed for
assessing the durability of the geotextile .
Geotextiles and geotextile-related products (referred to below as geotextiles) are available in a
wide range of compositions appropriate to different applications and environments. The
synthetic polymers used consist mainly of polyamide, polyester, polyethylene and
polypropylene. These materials, when correctly processed and stabilised, are resistant to
chemical and microbiological attack encountered in normal soil environments and for normal
design lives. For such applications only a minimum number of screening or "index" tests are
necessary.
For applications in more severe environments such as soil treated with lime or cement, landfills
or industrial waste, or for applications with particularly long design lives, special tests including
"performance" tests with site-specific parameters may be required.
This guide does not cover products designed to survive for a limited time, such as erosion
control fabric based on natural fibres, nor does it cover geomembranes, nor geotextiles for
asphalt reinforcement. Creep and creep-rupture, which should be taken into consideration for
soil reinforcement applications, are described in outline but the use of the data in reinforced soil
design will be the subject of a separate document.
2 Normative references
ENV 1897 Geotextiles and geotextile related products - Determination of the
compressive creep properties
ENV 12224 Geotextiles and geotextile related products - Determination of the resistance
to weathering
ENV 12225 Geotextiles and geotextile related products - Method for determining the
microbiological resistance by a soil burial test
ENV 12226 Geotextiles and geotextile related products - General tests for evaluation
following durability testing
ENV 12447 Geotextiles and geotextile related products - Screening test method for
determining the resistance to hydrolysis
EN ISO 13431 Geotextiles and geotextile related products - Determination of tensile creep
and creep rupture behaviour
EN ISO 13437 Geotextiles and geotextile related products - Method for installing and
extracting samples in soil, and testing specimens in the laboratory
prENV ISO 13438 Geotextiles and geotextile related products - Screening test method for
determining the resistance to oxidation
ENV ISO 12960 Geotextiles and geotextile related products - Screening test method for
determining the resistance to liquids
ISO 10318 Geotextiles - Vocabulary
3 Definitions
3.1 Durability
When a geotextile is used in a civil engineering structure, it is intended to perform a particular
function for a minimum expected time, called the design life. Any application may require one
or more functions from the geotextile. The five functions defined in ISO 10318 are drainage,
filtration, protection, reinforcement and separation. Each function uses one or more properties
of the geotextile, such as tensile strength or water permeability. These are referred to as
functional properties.
Assessment of the durability of structures using geotextiles requires a study of the effects of
time on the functional properties. The physical structure of the geotextile, the nature of the
polymer used, the manufacturing process, the physical and chemical environment, the
conditions of storage and installation, and the different loads supported by the geotextile are all
parameters which govern the durability. The main task is to understand and assess the
evolution of the functional properties for the entire design life. This problem is quite complex
due to the combination and interaction of numerous parameters present in the soil environment,
and to the lack of well documented experience.
This guide is only intended to cover the durability of the materials; the durability of the
geotechnical structure as a whole should be treated separately.
© ISO ISO/TR 13434:1998(E)
The object of the durability assessment is to provide the design engineer with the necessary
information (generally defined in terms of material reduction or partial safety factors) so that the
expected design life can be achieved with confidence.
3.2 Available and Required Properties
Fig. 1: Typical available and required values of a functional property as a function of
time
It is first necessary to differentiate between the available and required values of a functional
property. Figure 1 is a schematic reresentation of the evolution of the available property of a
material as a function of time, as represented by the upper curve on the graph. The functional
property may be a mechanical property such as tensile strength or a hydraulic property such as
permeability. Along the time axis is indicated the events that happen between manufacture of
the product and the end of product life. The lower curve represents the changes in the required
property during these different and successive events. The shape illustrated applies to
mechanical strength but would not be very different for a hydraulic property. One can see that
after the loading phase, usually by the end of construction, the property required is considered
to be constant and equal to the level defined by the design. In some applications the required
level may change after a certain time, for example in the construction of a wall or increased
water flow in a drain, in which cases the effect of these changes on durability should be
assessed.
In the following sections the two curves are examined in more detail, using as an illustration the
tensile strength of a geotextile in a reinforcement application.
3.3 Required Property
During the first period of product life, a minimum strength is needed to resist handling and
transportation loads. Once on site, placing and compaction of the backfill may for a short time
require a strength higher than that required for the design life. After installation and as
construction progresses, the applied loads increase until they reach a peak .
The required tensile strength is estimated by means of empirical calculation methods. As
recognised by most codes, there are uncertainties in the intensities and effects of the applied
loads: weights, surcharges, earth pressure. To cover for these uncertainties, the calculated
loads are multiplied by a first series of partial safety factors (or load factors). The calculated
stresses are then multiplied by a second partial safety factor to cover the relative inadequacy of
the calculation model. This calculation defines the maximum design load deemed to be
constant throughout the entire design life. The design method and the definition of safety
factors are not the subject of the present document. Reference should be made to the
appropriate Eurocode.
3.4 Available Property
At any time, the tensile strength required by the design should be smaller than the available
strength. The evolution of the available product property with time is complex, and arises from
various mechanisms. These should be analysed in order to ensure sufficient available strength
at any time, in particular at the end of the anticipated design life.
A new product exhibits a 'short term' or 'initial' property as defined by a set measurement
standard. Depending on the level of quality control and quality assurance, a reduction factor
may be applied to cover variations in the initial property. During storage and installation, this
property may change due to weathering and mechanical damage.
The extent of the mechanical damage depends on the product, the nature of the materials in
contact with the geotextile, the equipment used and the care provided by the operator.
Installation reduction factors should be considered for each product and typical backfill, based
on systematic tests. A simple reduction factor equal to the ratio of the strengths of undamaged
and damaged material is convenient but may not describe adequately the effect of damage on
long-term strength. Surface scratches or cracks may not significantly alter the short-term
behaviour but could lead to a reduction in long-term rupture life. This is still a subject of
research. Results should be interpreted prudently.
After installation, the operating life of the structure starts. During the operating life the
geotextile is subjected to chemical, biological or physical actions due to the soil, its constituents,
and its air, water and organic content. The typical degradation mechanisms of the polymers
used to manufacture geotextiles will be reviewed in the next section. The reduction in strength
© ISO ISO/TR 13434:1998(E)
may be due to loss of mass, for example due to erosion of the surface, or to chemical
modification, and should be taken into account by specifying reduction factors.
In addition to the effect of the soil and its contents, the time to failure of the geotextile can also
diminish due to the level of the applied load: the greater the applied stress, the shorter the time
to failure. This is a particularly important phenomenon that will be described in 5.4.1. Thus
there is an interaction between the required property and the available property. There is no
absolute available property curve.
Obtaining the property curve is not an easy task. Temperature plays a major role in all
degradation mechanisms and in mechanical behaviour (creep and rupture). The results of
short-term accelerated tests, often using high temperatures as a means of acceleration and
lasting for one year or less, need to be related to long-term design life. This extrapolation
assumes that the degradation mechanisms are the same at both test and service temperatures
and over the entire design life of the structure.
Precautions should be taken to ensure that no transition, such as a change in the state of the
polymer or of the geotextile, occurs during the design life or in applying accelerated testing,
unless that transition is fully understood and taken into account in the extrapolation.
Failure implies that the geotextile can no longer perform the function for which it is being
applied. For example, if the function is filtration, a greater reduction in mechanical strength may
be acceptable than if the function is reinforcement.
Partial safety factors are required to describe the various degradation mechanisms (eg light
intensity, chemical concentration). These factors are listed in 7.9.
The testing techniques and the assessment methods for estimating the property curve will be
presented and discussed in later sections. As described in 7.1, index test methods are
intended to ensure a minimum level of durability and do not constitute a comprehensive
assessment procedure. Where this is needed it will be necessary to carry out further
performance tests more closely related to service conditions. These tests may also include
investigations on samples extracted from sites where the same product has been used for
several years in a similar environment. The procedure is described in prEN ISO 13437. As in
other fields of engineering, confidence in the durability of geotextiles can only be expected to
develop gradually as the technology matures and the results of long-term service experience
accumulate. Examples of experience to date are described in clause 6.
3.5 Design Life
The design life is specified on the time axis. It is set by the client and is decided at the design
stage. The codes generally propose several fixed durations according to whether the structure
is meant for short-term use (typically a few years and not exceeding 5 years), temporary use
(around 25 years) or permanent use (50 to >100 years). The nature of the structure and the
consequences of failure may influence this duration (example: 70 years for a wall, 100 years for
an abutment). Many geotextiles have a temporary function although the structure is
permanent, for example an embankment over a weak soil may require a geotextile
reinforcement until the embankment has settled. At the end of the anticipated design life, the
designer has to ensure a certain safety level (generally also indicated by codes), such that
failure is predicted to be well beyond the design life. The ratio of the predicted available
property to the predicted required property represents the total safety factor for that component.
This can also be expressed in terms of the time to reach failure if the geotextile were to be left
in service after the end of design life. These two representations of safety, the ratio of required
and available property at the design life, and the ratio of the predicted end of life to design life,
should be considered together because in combination they give a better idea of the real level
of safety that exists.
3.6 End of Life
End of life is the point on the time axis where the available property curve meets the required
property curve. At this point the product is predicted to fail. Residual service may remain either
if the expected loads are overestimated, or if they imply a combination of degradation
mechanisms that may not all have reached their maximum values. Whatever the case, beyond
that point on the graph the possibility of failure is high.
3.7 The durability study
The design and durability assessment of a structure using geotextile can be summarised as
follows:
The design consists of:
• defining the function of the geotextile
• making the inventory of loads and constraints imposed by the application
• defining the design life of the geotextile
• quantifying the required properties of the geotextile (eg strength, permeability)
• defining the quality and quantity of the geotextile material needed
© ISO ISO/TR 13434:1998(E)
• making sure that the estimated available properties at the end of the design life are
greater than the required properties; the factor of safety for the material being the
following ratio:
Available properties at the end of design life
factor of safety =
Required properties from loads and constraints
The factor of safety should be greater than unity.
The durability study consists of:
• listing significant environmental factors (see clause 5)
• defining the possible degradation phenomena with regard to the selected materials
and the environment
• estimating the available property as a function of time
•
supplying the designer with suitable reduction factors or available properties at the
end of their design life.
Details are given in clause 7.
4 Constituents of Geotextiles
4.1 General
The durability of a geotextile depends upon the basic polymer from which it is made, on any
additives compounded with it, on the polymer microstructure, fibre geometry and fabric layout.
The geotextile should be chemically and biologically resistant if it is to be suitable for long term
applications.
The polymers used to manufacture the geotextiles are generally thermoplastic materials which
may be amorphous or semi-crystalline. An amorphous polymer has a randomly coiled structure
which at the glass transition temperature T undergoes significant change: from a stiff, glassy,
g
brittle response to load below the glass transition temperature to a more ductile, rubbery
response above T . Most polymers used in geotextiles are semi-crystalline, that is they contain
g
small well-oriented, closely packed crystallites alternating with amorphous material. Since the
change in behaviour only affects the amorphous regions, the glass transition is less marked for
a semicrystalline polymer. At a higher temperature, however, the crystallites melt, which
produces an abrupt change in properties. In civil engineering applications polyesters are used
below their T while polypropylene and polyethylene are used above T . Any acceleration of
g g
laboratory tests crossing a transition such as T should be regarded with caution.
g
Any polymer, whether amorphous or semi-crystalline, consists of long chain molecules each
containing many identical chemical units. Each unit may be composed of one or more
monomers, the number of which determines the length of the polymeric chain and resulting
molecular weight. Molecular weight can affect physical properties such as the tensile strength
and modulus, impact strength and heat resistance as well as the durability properties. The
mechanical and physical properties of the plastics are also influenced by the bonds within and
between chains, chain branching, and the degree of crystallinity.
The orientation of polymers by mechanical drawing to form fibres and filaments results in higher
tensile properties and improved durability. As the molecules become more oriented, the fibres
become stronger. The crystallites are retained and the ratio of crystalline regions and
amorphous regions should be properly balanced to produce the physical properties necessary
for fibres used in geotextiles. The increased orientation and associated higher density leads to
higher environmental resistance.
Crystallinity has a strong effect on polymer properties, especially the mechanical properties,
because the tightly packed molecules within the crystallites results in dense regions with high
intermolecular cohesion and resistance to penetration by chemicals. An increase in the degree
of crystallinity leads directly to an increase in rigidity and yield or tensile strength, hardness and
softening point, and to a decrease in chemical permeability. Neighbouring crystallites may be
connected by single molecules running through the amorphous regions, which under tension
become taut and make a significant contribution to the mechanical behaviour. These 'tie'
molecules are, however, susceptible to chemical attack.
Durability may also be influenced by fibre thickness, and the volume to surface ratio. Some
means of degradation, such as oxidation and UV-exposure, are dependent on surface area,
while others such as diffusion and absorption are inversely related to thickness.
4.2 Individual Polymer Types
The polymers used in geotextiles are described below and three of their most important
physical properties are listed in Table 1. The most commonly used are polypropylene and
polyethylene.
Polypropylene (PP) is a thermoplastic long chain polymer. PP is normally used in the isotactic
stereoregular form in which propylene monomers are attached in head-to-tail fashion and the
methyl groups are aligned on the same side of the polymer backbone. PP has a semi-
crystalline structure which gives to it high stiffness, good tensile properties and resistance to
acids, alkalis and most solvents. It is possible for the tertiary carbon to react with free radicals,
so that stabilisers are added to prevent oxidation during manufacture and generally to improve
long term durability, including weathering.
© ISO ISO/TR 13434:1998(E)
Polyethylene (PE) is one of the simplest organic polymers. It is used in its low density form
(LDPE), which is known for its excellent pliability, ease of processing and good physical
properties, or as high density polyethylene (HDPE) which is more rigid and chemically resistant.
PE can be stabilised to increase its resistance to weathering. Certain grades can be
susceptible to environmental stress cracking.
Polyesters are a group of polymers. The type used most frequently in geotextiles is
polyethylene terephthalate (PET) which is a condensation polymer of a dibasic acid and a
dialcohol. Since it is used below its T , PET offers good mechanical properties, including a low
g
creep strain rate, and good chemical resistance to most acids and many solvents. The ester
group, the important polymeric link, can be hydrolysed very slowly in presence of water, and
more rapid attack occurs under highly alkaline conditions. As with other polymers PET is
sensitive to weathering.
Polyvinylchloride (PVC) is the most significant commercial member of the family of vinyl-based
resins. PVC is the most versatile of all plastics because its blending capability with plasticisers
and other additives allows it to take up a great variety of forms. Plasticisers are used in
quantities of up to 35% to create more flexible compounds, the choice of plasticiser being
dictated by the properties desired. Conversely, PVC absorbs certain organic liquids which have
a similar plasticising effect. PVC also tends to become brittle and darken when exposed to
ultraviolet light or heat-induced degradation.
Polyamides (PA) or nylons are melt processable thermoplastics that contain an amide group as
a recurring part of the chain. PA offers a combination of properties including high strength at
elevated temperatures, ductility, wear and abrasion resistance, low frictional properties, low
permeability by gases and hydrocarbons, and good chemical resistance. Its limitations include
a tendency to absorb moisture, with resulting changes in dimensional and mechanical
properties, and limited resistance to acids and weathering. The PA fibres used in geotextiles
have a T of 40-60 °C which reduces with moisture content.
g
Table 1
Typical physical properties for polymers used in geotextiles
HDPE PP PET PA PVC
Density, (g/cm ) 0.95 0.91 1.38 1.12 1.3 to
1.5
Melting temperature, (ºC) 130 165 260 220 to
Glass transition temperature, (ºC) -100 to -20 to 70 to 80 40 to 60 -25 to
-70 -12 100
4.3 Manufacturing Processes
Geotextiles and geotextile-related products are manufactured using several different processes.
In this section the main processing technologies for the manufacture of geotextiles, geogrids,
geonets, geocells and geocomposites will be described.
Geotextiles include nonwoven, woven and knitted products. All are made of polymers drawn
into fibres or yarns, which consist of a number of fibres. The different manufacturing processes
lead to geotextile products with a wide range of properties.
For the production of nonwoven geotextiles continuous filaments or staple fibres (cut fibres) are
used. Woven and knitted geotextiles are produced using different types of yarn such as spun
yarns, multifilament yarns, monofilaments and film tapes or split film yarns.
The types of fibres, multifilaments, monofilaments and tapes used in the manufacture of such
geotextiles are produced mainly by a melt spinning process. To produce fibres, multifilaments
and monofilaments the molten polymer is extruded through orifices of a die, cooled, drawn by
stretching and according to the end use:
1) laid on a screen to form a planar structure (continuous filament or spun bonded
nonwoven);
2) processed to staple fibres by crimping and staple cutting or
3) processed to multi- or monofilaments and winding the filaments after drawing and
annealing directly on to spools. In the case of multifilament production this technique is
known as spin drawing.
Spunbonded nonwovens are continuous filament nonwovens and are manufactured in a
continuous process starting with the polymer and proceeding through filament production,
geotextile formation and filament bonding in the same line, finishing with the roll of nonwoven.
Staple fibre nonwovens are manufactured in a two stage process: the first stage consists of
fibre production (extrusion and cutting) and the second stage consists of the formation of the
geotextile, bonding and production of the finished roll.
Woven geotextiles are also produced in a discontinuous process with at least two stages. The
first stage is the production of the yarn, monofilament or multifilament. The second stage is the
weaving either to flat wovens (or simply wovens) or knitted wovens (knits).
Film tapes and split yarns are normally only produced from polypropylene and polyethylene.
These products are made by extruding a film, cutting the film into individual tapes and
stretching them by a uniaxial drawing process. Coarse film tapes are too stiff for further
handling in beaming and weaving, and are therefore fibrillated after the drawing process and
before winding and twisting. These types of yarn are then called split film yarns.
© ISO ISO/TR 13434:1998(E)
The drawing process is very important in the production of the different types of polymeric
fibres, filaments and tapes. During this process the polymeric chains become aligned along the
filament or tape length and their crystallinity, mechanical properties and durability all increase.
The mechanical properties of the product depend upon the details of the manufacturing
process.
Bonding of nonwoven geotextiles formed from either continuous filaments or from staple fibres
is done mechanically by needle punching with felting needles, by thermal (cohesive) bonding
using heat with or without pressure (calendering), by chemical (adhesive) bonding, or by a
combination of these processes.
The physical structure and properties of the nonwoven products are linked to the bonding
system. For example heat bonded wovens and nonwovens (tape film wovens) are thin
products in which the fibres are oriented in a two-dimensional structure. Needle punched
nonwovens have a three-dimensional structure, the configuration of which may be fixed by a
final thermal bonding stage.
The structure of the fabric will contribute to the durability properties, for example thick fibres
and tapes are less susceptible to weathering. The stabilisation systems applied to improve the
properties are therefore adjusted to suit either a nonwoven geotextile of finer fibres, a woven
geotextile or a geogrid.
Extruded geogrids are manufactured from a polyolefin sheet containing holes that have been
punched or preformed during extrusion. The perforated sheet is then stretched either uniaxially
or biaxially under controlled conditions of load and temperature to achieve a high level of
molecular orientation.
Geonets are manufactured typically by an extrusion process in which a minimum of two sets of
strands (filaments) are overlaid to yield a three-dimensional product. The openings between
the strands permit in-plane flow of water or landfill leachate.
Geocomposites are composed of at least two different geotextiles or geotextile related products
joined together by a process such as bonding, gluing, welding, weaving or sewing.
Geocells are three-dimensional geosynthetics used for soil confinement in erosion control
applications. They are manufactured either by extrusion, HDPE strip welding or geotextile strip
welding.
A detailed description of current geotextile and geotextile related products, and of their
manufacturing processes, is given in Annex A, ref. 1.
Based on many years' experience of manufacturing and the development of quality assurance
procedures, geotextile products are made in such a way that good physical durability properties
are obtained.
4.4 Recycled Materials
It is common practice within the plastics industry to recycle the processed material (in-house
scrap polymer), since it can be considered as comparable to virgin material as long as it is used
in small percentages (less than 10%). Some producers manufacture their geotextiles using
100% post-consumer recycled polymer, for example reground PET bottles.
Recycled materials may originate from various stages of processing following their original
formulation, or from subsequent processes such as weaving. The materials may have been
used in service, whether in the form of textiles or as other products such as packaging. The
level of control over the quality of the material, and thus its durability, decreases with the
number of stages and processes it has gone through after leaving the original manufacturer's
plant.
For severe environments and for long-term applications it is advisable not to use post consumer
recycled polymer without proof of its long term durability. The composition of the polymer
should be assured.
4.5 Additives and Stabilisers
Additives play a major role in polymer stabilisation. Typical additives used in the production of
geotextiles are antioxidants and UV stabilisers.
Antioxidants prevent deterioration of the appearance and of the physical properties of polymers
caused by the oxidative degradation of polymer bonds. Oxidation is accelerated by the heat
generated during the manufacturing process. Thus some antioxidants are designed to work
during the manufacturing process (high temperatures), while others are intended to protect the
geosynthetic during its subsequent exposure to the environment (low temperatures).
Stabilisation is achieved by either providing alternative opportunities for termination reactions,
or by preventing the formation of free radicals and thus interrupting the chain of reaction. With
some stabilisers oxidation occurs over a short interval after a long incubation time, while with
others the reduction in properties is a gradual process. This can make the interpretation of
accelerated oxidation tests difficult.
UV stabilisers provide ultraviolet light stabilisation of polyolefins and other polymers by several
mechanisms such as reducing the rate of photo-oxidation, absorbing the light of the critical
wave length or by reduction in the kinetic chain length of the propagation stage of the photo-
oxidation mechanism. The kinetic chain length can be reduced by free radical trapping.
© ISO ISO/TR 13434:1998(E)
Typical light stabilisers are carbon black, hindered amine light stabilisers (HALS) and UV light
absorbers.
5 Environmental factors that may lead to degradation
5.1 The environment above ground
Ageing of geotextiles above ground is mainly initiated by the ultraviolet (UV) component of solar
radiation, heat and oxygen, with contributions from other climatic factors such as humidity, rain,
oxides of nitrogen and sulphur, ozone and deposits from polluted air.
The energy of ultraviolet radiation is sufficient to initiate rupture of the bonds within the polymer
leading to subsequent recombination with, for example, oxygen in the air, or initiating more
complex chain reactions. This is a general property of polymers and is not restricted to
geotextiles. Additives increase resistance to ultraviolet radiation in a variety of ways as
described in 4.6.
The resistance to ultraviolet radiation is affected both by the surface temperature of the sample
and by precipitation, for which reason accelerated weathering tests include control of
temperature and an intermittent spray cycle. Since natural weathering is both seasonal and
variable, artificial tests have the advantage not only of being able to increase the intensity of the
radiation, but also ensure that the radiation is constant, controlled and up to 24 hours a day.
The performance following accelerated testing is related to the duration of exposure on site as
described in 7.2.
In most applications geotextiles are exposed to UV light for only a limited time during storage,
transport and installation and are subsequently protected by a layer of soil. The need for either
short or long term resistance to weathering therefore depends on the application.
Exposure to UV has been shown to reduce the subsequent chemical resistance of thin textiles
but this has not been observed in geotextiles. In addition, atmospheric pollution and acid rain
may enhance UV degradation, particularly of PA, for longer exposures above ground. Attack
by birds has been observed during deliberate exposure of specimens during outdoor
weathering tests.
5.2 The environment below ground
5.2.1 Soils
Below ground the main factors affecting the durability of geosynthetics are as follows:
particle size distribution and angularity
acidity/alkalinity (pH) - humates, sodium or lime soils, lime hydration, concrete
metal ions present
presence of oxygen
moisture content
organic content
temperature
Adequate specification of the soil is thus essential for proper consideration of the durability of
the geotextile.
Soils as encountered in Western and Central Europe should be divided into topsoils (0.20 -
1.00m) and underlying sediments. Their nature depends primarily on the underlying rock and
on the local climate, including the mean temperature and the drainage conditions. Topsoil is a
mix of weathered sediments and humus produced by decaying organic material. The
conditions of decay can be aerobic, with oxygen present, or anaerobic.
Sediments are deposits of minerals and mostly lack organic material. They are generally
formed by the physical and chemical weathering of rocks. Silt, sand and gravel (particle size
0.002 to 60 mm) are formed by physical weathering, while clays (particle size < 0.002 mm) are
formed by chemical weathering. Fills and backfills originate from sediments, where particle size
and angularity is determined not only by the manner in which the sediment was formed but also
by any subsequent industrial processing such as crushing. Sediments can cause considerable
mechanical damage to geotextiles, in fact the exhumation of specimens after a number of years
often shows that this is the only form of degradation that can be identified with certainty. The
range of particle sizes of a soil is measured by sieving and is depicted by a graph of particle
size against percentage by weight. Mechanical damage increases with particle size, and with
the angularity of the particles. This is described further in 5.3.3.
The topsoil or sediments can be fully saturated, partially saturated or dry, or intermittently wet
and dry. In wetter climates the drainage is principally downwards, drawing soluble materials to
lower levels, while in drier climates moisture is removed by evaporation at the surface and the
resultant upward movement of the water draws these soluble fractions upwards and deposits
them at the surface. The water content of an unsaturated soil is described by the local relative
humidity.
The temperature of the soil is constant (to within ± 0.5ºC) only at a depth of 10 m or more. Its
value is then equal to the annual average atmospheric temperature at the surface. Daily and
seasonal variations occur with decreasing intensity as the distance from the surface increases.
For example, the daily variation in atmospheric temperature and solar radiation is felt to a depth
of half a metre (Annex A, ref. 2). Since higher temperatures increase the rates of ageing and
creep of polymers disproportionally, their effect on geotextile behaviour may need to be
considered for material installed close to the surface.
© ISO ISO/TR 13434:1998(E)
Chemical attack is most serious when the polymer chain backbone is broken, leading to a loss
of mechanical properties. This will generally occur by means of oxidation or hydrolysis,
depending on the type of polymer and on the acidity or alkalinity of the soil. Acidity and
alkalinity are expressed as pH, a scale with neutral soil having a pH of 7, lower values implying
acid soils and higher values alkaline soils.
In Europe topsoil generally has a pH of 5.5 - 7, but anaerobic peats or soils which have been
affected by acid rain may have a pH of approximately 4. Atmospheric carbon dioxide leads to
generally increased acidity at the surface. Limestone or chalk soils may have a pH of 8 - 8.5.
Geological deposits have a wide range of pH, as shown in Table 2, with values between 2 and
10 having been recorded.
Table 2: Some typical minerals and fills and their pH values
Minerals and fills Formula Maximum pH
Felspar
Albite
NaAlSi O 9 - 10
3 8
Anorthite
CaAl Si O 8
2 2 8
Orthoclase
KAlSi O 8 - 9
3 8
Sand
Quartz
SiO 7
Muscovite
KAl (OH,F) AlSi O 7 - 8
2 2 3 10
Clay
Kaolinite
Al (OH) Si O 5 - 7
4 8 4 10
Carbonate
Dolomite
CaMg(CO ) 9 - 10
3 2
Calcite
CaCO 8 - 9
The use of bentonite and other clays in civil engineering construction, such as diaphragm wall
construction, grouting processes, sealing layers in landfill and tunnelling, causes local areas of
high alkalinity with pH values of 8.5 to 10. Some geocomposites contain bentonite in dry form
which combines with local ground water to form a gel.
5.2.2 Chemical effects on the geotextile
Polyester and polyamide fibres are susceptible to hydrolysis, which in polyester fibres takes two
forms. The first, alkaline or external hydrolysis, occurs in alkaline soils above pH 10,
particularly in the presence of calcium, and takes the form of surface attack or etching. Caution
should be applied in the use of polyesters for long periods above pH 9. The second, internal
hydrolysis, occurs in aqueous solutions or humid soil at all values of pH. It takes place
throughout the cross-section of the fibre. The rate of hydrolysis is very slow, such that the
process has little effect at mean soil temperatures of 15 ºC or below in neutral soils, although it
can be accelerated in acids. The rate of internal hydrolysis in a partially saturated soil depends
upon the local relative humidity.
Polypropylene and polyethylene are susceptible to oxidation. This is accelerated by the catalytic
3+
effects of transition metal ions in a chemically activated state. Of these the ferric (Fe ) ion is
the most common but copper and manganese have also been shown to be important. On the
other hand, the tendency to oxidation decreases due to the reduced availability of oxygen in
soil.
All chemical reactions occur more rapidly at higher temperatures, as described by Arrhenius'
Law (see 7.3.5).
In the past 20 years there have been no reports of microbial attack on synthetic geotextiles
either in testing or in the ground. Only geotextiles containing vegetable fibres, most of which
are deliberately designed to degrade once natural vegetation has become established, are
likely to be affected. However, in topsoil micro-organisms such as bacteria and fungi might
attack geotextiles if they contain components that provide nutrition and if the micro-organisms
can penetrate the remaining polymer. The long chain molecules of thermoplastics used in
geotextiles are generally resistant to microbial attack. Also, low molecular components and
certain additives could be susceptible to biodegradation, but this can be countered by
biostabilisers. Micro-organisms could in theory produce degradation products that attack
geotextiles chemically. The soil burial test (ENV 12225) endeavours to provide a soil of
maximum biological activity to encourage any reaction that can occur.
Geotextiles in soil also come in contact with animals such as rodents and with the roots of
plants. Rodents can locally destroy a geotextile while roots can penetrate and clog it. To
simulate attack by rodents or penetration by roots no specific index tests are proposed.
5.3 Effects of Load and Mechanical Damage
5.3.1 Tensile Load: Creep and Stress-rupture
A major difference between polymers and metals is that at normal operating temperatures and
tensile loads, polymers extend with time, that is they creep. This was recognised early in the
development of geotextiles and led to an increasing number of testing programmes to provide
the information necessary for the design of reinforced soil structures.
At higher loads creep leads ultimately to stress-rupture, also known as creep-rupture or static
fatigue. The higher the applied load, the shorter the lifetime. Thus as mentioned in clause 3
the design load will itself limit the lifetime of the product.
© ISO ISO/TR 13434:1998(E)
Creep and stress-rupture should only be regarded as a relevant design criterion in slopes and
walls when the geotextile is expected to perform a reinforcing function in the long-term, or in
reinforcement over a soft foundation.
At the microscopic level, when a load is applied to the polymer it may cause the long chain
molecules to stretch or rearrange themselves. A particularly important part is played by the "tie"
molecules, which run from one crystallite to another, linking them together. In polyester
molecules the load can cause neighbouring links to change their orientation relative to one
another, resulting in the characteristic S-shaped stress-strain curve. These processes of
rearrangement continue under the c
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