ISO/TS 20432:2022
(Main)Guidelines for the determination of the long-term strength of geosynthetics for soil reinforcement
Guidelines for the determination of the long-term strength of geosynthetics for soil reinforcement
This document provides guidelines for the determination of the long-term strength of geosynthetics for soil reinforcement. This document describes a method of deriving reduction factors for geosynthetic soil-reinforcement materials to account for creep and creep rupture, installation damage and weathering, and chemical and biological degradation. It is intended to provide a link between the test data and the codes for construction with reinforced soil. The geosynthetics covered in this document include those whose primary purpose is reinforcement, such as geogrids, woven geotextiles and strips, where the reinforcing component is made from polyester (polyethylene terephthalate), polypropylene, high density polyethylene, polyvinyl alcohol, aramids and polyamides 6 and 6,6. This document does not cover the strength of joints or welds between geosynthetics, nor whether these might be more or less durable than the basic material. Nor does it apply to geomembranes, for example, in landfills. It does not cover the effects of dynamic loading. It does not consider any change in mechanical properties due to soil temperatures below 0 °C, nor the effect of frozen soil. The document does not cover uncertainty in the design of the reinforced soil structure, nor the human or economic consequences of failure.
Lignes directrices pour la détermination de la résistance à long terme des géosynthétiques pour le renforcement du sol
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Standards Content (Sample)
TECHNICAL ISO/TS
SPECIFICATION 20432
First edition
2022-12
Guidelines for the determination
of the long-term strength of
geosynthetics for soil reinforcement
Lignes directrices pour la détermination de la résistance à long terme
des géosynthétiques pour le renforcement du sol
Reference number
ISO/TS 20432:2022(E)
© ISO 2022
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ISO/TS 20432:2022(E)
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ISO/TS 20432:2022(E)
Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions, abbreviated terms and symbols . 1
3.1 Terms and definitions . 1
3.2 Abbreviated terms . 2
3.3 Symbols . 3
4 Design procedure . 4
4.1 General . 4
4.2 Design lifetime . 4
4.3 Causes of degradation . 5
4.4 Design temperature . 5
5 Determination of long-term creep strain . 5
5.1 General . 5
5.2 Extrapolation . 6
5.3 Time-temperature superposition methods . 6
5.4 Isochronous curves . 7
5.5 Weathering, chemical and biological effects . 7
6 Determination of long-term strength . 8
6.1 Tensile strength . 8
6.2 Reduction factors . 8
6.3 Modes of degradation . 8
7 Creep rupture . 9
7.1 General . 9
7.2 Measurement of creep rupture: conventional method . 10
7.3 Curve fitting (conventional method) . 11
7.4 Curve fitting for time-temperature block shifting of rupture curves .13
7.5 Strain shifting and the stepped isothermal method . 14
7.6 Extrapolation and definition of reduction factor or lifetime . 15
7.7 Residual strength . 16
7.8 Reporting of results . 16
7.9 Procedure in the absence of sufficient data . 16
8 Installation damage . .17
8.1 General . 17
8.2 Data recommended . 17
8.3 Calculation of reduction factor . 18
8.4 Procedure in the absence of direct data . 18
8.4.1 General . 18
8.4.2 Interpolation from measurements with different soils. 18
8.4.3 Interpolation between products of the same product line . 19
8.4.4 Laboratory damage tests . 19
9 Weathering, chemical and biological degradation .19
9.1 General . 19
9.2 Data recommended for assessment . 20
9.3 Weathering . 20
9.4 Chemical degradation . 21
9.4.1 Causes of chemical degradation. 21
9.4.2 Evidence from service experience . 21
9.4.3 Accelerated chemical degradation tests .22
9.4.4 Oxidation of polyolefins . 26
9.4.5 Hydrolysis of polyesters . 27
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ISO/TS 20432:2022(E)
9.4.6 Procedure in the absence of sufficient data .29
9.5 Biological degradation .29
10 Determination of long-term strength .29
10.1 Factor of safety f .29
s
10.2 Design for residual strength .30
11 Reporting .30
Bibliography .31
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ISO/TS 20432:2022(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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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
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www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 221, Geosynthetics.
This first edition of ISO/TS 20432 cancels and replaces ISO/TR 20432:2007, which has been technically
revised. It also incorporates the Technical Corrigendum ISO/TR 20432:2007/Cor 1:2008.
The main changes are as follows:
— Subclause 7.4 has been modified to further detail and clarify the fitting of linear regression curves
to time-temperature block shifted creep-rupture test results.
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.
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TECHNICAL SPECIFICATION ISO/TS 20432:2022(E)
Guidelines for the determination of the long-term strength
of geosynthetics for soil reinforcement
1 Scope
This document provides guidelines for the determination of the long-term strength of geosynthetics for
soil reinforcement.
This document describes a method of deriving reduction factors for geosynthetic soil-reinforcement
materials to account for creep and creep rupture, installation damage and weathering, and chemical
and biological degradation. It is intended to provide a link between the test data and the codes for
construction with reinforced soil.
The geosynthetics covered in this document include those whose primary purpose is reinforcement,
such as geogrids, woven geotextiles and strips, where the reinforcing component is made from polyester
(polyethylene terephthalate), polypropylene, high density polyethylene, polyvinyl alcohol, aramids
and polyamides 6 and 6,6. This document does not cover the strength of joints or welds between
geosynthetics, nor whether these might be more or less durable than the basic material. Nor does it
apply to geomembranes, for example, in landfills. It does not cover the effects of dynamic loading. It does
not consider any change in mechanical properties due to soil temperatures below 0 °C, nor the effect of
frozen soil. The document does not cover uncertainty in the design of the reinforced soil structure, nor
the human or economic consequences of failure.
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, abbreviated terms and symbols
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 terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1.1
long-term strength
load which, if applied continuously to the geosynthetic during the service lifetime, is predicted to lead
to rupture at the end of that lifetime
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ISO/TS 20432:2022(E)
3.1.2
reduction factor
factor (≥ 1) by which the tensile strength is divided to take into account particular service conditions in
order to derive the long-term strength
Note 1 to entry: In Europe, the term 'partial factor' is used.
3.1.3
characteristic strength
95 % (two-sided) lower confidence limit for the tensile strength of the geosynthetic, approximately
equal to the mean strength less two standard deviations
Note 1 to entry: This should be assured by the manufacturer’s own quality assurance scheme or by independent
assessment.
3.1.4
block shifting
procedure by which a set of data relating applied load to the logarithm of time to rupture, all measured
at a single temperature, are shifted along the log time axis by a single factor to coincide with a second
set measured at a second temperature
3.1.5
product line
series of products manufactured using the same polymer, in which the polymer for all products in the
line comes from the same source, the manufacturing process is the same for all products in the line, and
the only difference is in the product mass per area or number of fibres contained in each reinforcement
element
3.2 Abbreviated terms
CEG carboxyl end group
DSC differential scanning calorimetry
HALS hindered amine light stabilizers
HDPE high density polyethylene
HPOIT high pressure oxidation induction time
LCL lower confidence limit
MARV minimum average roll value
OIT oxidation induction time
PA polyamide
PET polyethylene terephthalate
PP polypropylene
PTFE polytetrafluorethylene
PVA polyvinyl alcohol
SIM stepped isothermal method
TTS time-temperature shifting
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ISO/TS 20432:2022(E)
3.3 Symbols
A time-temperature shift factor
i
b gradient of Arrhenius graph
a
d mean granular size of fill
50
d granular size of fill for 90 % pass (10 % retention)
90
f factor of safety
s
G, H parameters used in the validation of temperature shift linearity (see 7.4)
m gradient of line fitted to creep rupture points (log time against load); inverse of gradient of
conventional plot of load against log time.
M number averaged molecular weight
n
n number of creep rupture or Arrhenius points
P applied load
R ratio representing the uncertainty due to extrapolation
1
R ratio representing the uncertainty in strength derived from Arrhenius testing
2
f reduction factor to allow for chemical and biological effects
R,CH
f reduction factor to allow for the effect of sustained static load
R,CR
f reduction factor to allow for the effect of mechanical damage
R,ID
f reduction factor to allow for weathering
R,W
S sum of squares of difference of log (time to rupture) and straight line fit
sq
S , S , S sums of squares as defined in derivation of regression lines in 9.4.3
xx xy yy
σ standard deviation used in calculation of LCL
0
t time, expressed in hours
t time to 90 % retained strength
90
t design life
D
t degradation time during oxidation
deg
t induction time during oxidation
ind
t LCL of time to a defined retained strength at the service temperature
LCL
t longest observed time to creep rupture, expressed in hours
max
t Student’s t for n – 2 degrees of freedom and a stated probability
n–2
t time to rupture, expressed in hours
R
t time to a defined retained strength at the service temperature
s
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ISO/TS 20432:2022(E)
T load per width
T batch tensile strength (per width)
B
T characteristic strength (per width) (see 6.1)
char
T unfactored long-term strength (see 9.4.3)
x
T long-term strength per width (including factor of safety)
D
T residual strength
DR
θ temperature of accelerated creep test
j
θ absolute temperature
k
T LCL of T due to chemical degradation
LCL char
θ service temperature
s
x abscissa: on a creep rupture graph the logarithm of time, in hours
x
mean value of x
x abscissa of an individual creep rupture point
i
x predicted time to rupture
p
y ordinate: on a creep rupture graph, applied load expressed as a percentage of tensile strength,
or a function of applied load
y value of y at 1 h (lg t = 0)
0
y
mean value of y
y ordinate of an individual creep rupture point
i
y value of y at time 0, derived from the line fitted to creep rupture points
0
4 Design procedure
4.1 General
The design of reinforced soil structures generally requires consideration of the following two issues:
a) the maximum strain in the reinforcement during the design lifetime;
b) the minimum strength of the reinforcement that could lead to rupture during the design lifetime.
In civil engineering design, these two issues are referred to as the serviceability and ultimate limit
state respectively. Both factors depend on time and can be degraded by the environment to which the
reinforcement is exposed.
4.2 Design lifetime
A design lifetime, t , is defined for the reinforced soil structure. For civil engineering structures this
D
is typically 50 years to 100 years. These durations are too long for direct measurements to be made in
advance of construction. Reduction factors have therefore to be determined by extrapolation of short-
term data aided, where necessary, by tests at elevated temperatures to accelerate the processes of
creep or degradation.
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ISO/TS 20432:2022(E)
4.3 Causes of degradation
Strain and strength may be changed due to the effects of the following:
— mechanical damage caused during installation;
— sustained static (or dynamic) load;
— elevated temperature;
— weathering while the material is exposed to light;
— chemical effects of natural or contaminated soil.
4.4 Design temperature
The design temperature should have been defined for the application in hand. In the absence of a defined
temperature or of site specific in-soil temperature data, the design temperature should be taken as the
temperature which is halfway between the average yearly air temperature and the normal daily air
temperature for the hottest month at the site. If this information is not available, 20 °C should be used
as the default value.
Many geosynthetic tests are performed at a standard temperature of (20 ± 2) °C. If the design
temperature differs, appropriate adjustments should be made to the measured properties.
This document does not cover the effects of temperatures below 0 °C (see Clause 1).
5 Determination of long-term creep strain
5.1 General
The design specification may set a limit on the total strain over the service lifetime of the geosynthetic,
or on the strain generated between the end of construction and the service lifetime. In the second case,
the time at “end of construction” should be defined, as shown in Figure 1. When plotted against lg t,
even a one-year construction period should have negligible influence on the creep strain curve beyond
10 years.
Levels of creep strain encountered in the primary creep regime (creep rate decreasing with time) are
thought not to adversely affect strength properties of geosynthetic reinforcement materials.
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ISO/TS 20432:2022(E)
Key
X time 3 load ramp period in creep test
Y strain 4 loading and creep of reinforcement in wall
1 laboratory creep test 5 new time = 0 for post construction creep
2 load ramp period on wall 6 wall construction time
Figure 1 — Conceptual illustration for comparing the creep measured in walls to laboratory
creep data
5.2 Extrapolation
Creep strain should be measured according to ISO 13431 and plotted as strain against the lg t. It
may then be extrapolated to the design lifetime. Extrapolation may be by graphical or curve-fitting
procedures, in which the formulae applied should be as simple as is necessary to provide a reasonable
fit to the data, for example, power laws. The use of polynomial functions is discouraged since they can
lead to unrealistic values when extrapolated.
5.3 Time-temperature superposition methods
Time-temperature superposition methods may be used to assist with extending the creep curves.
Creep curves are measured under the same load at different temperatures, with intervals generally
not exceeding 10 °C, and plotted on the same diagram as strain against lg t. The lowest temperature is
taken as the reference temperature. The creep curves at the higher temperatures are then shifted along
the time axis until they form one continuous “master” curve, i.e. the predicted long-term creep curve
for the reference temperature. The shift factors, i.e. the amounts (in units equivalent to lg t) by which
each curve is shifted, should be plotted against temperature where they should form a straight line or
smooth curve. The cautions given in 7.6 should be noted.
Experience has shown the strains on loading are variable. Since the increase in strain with time is
small, this variability can lead to wide variability in time-temperature shifting (TTS). The stepped
isothermal method (SIM) described in 7.5 avoids this problem by using a single specimen, increasing
the temperature in steps, and then shifting the sections of creep curve measured at the various
temperatures to form one continuous master curve.
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ISO/TS 20432:2022(E)
If a more accurate measure of initial strain is required, five replicates are recommended at each load.
Some of these can be of short duration (e.g. 1 000 s). At a series of loads, fewer replicates at each load will
suffice if the data are pooled using regression techniques. One approach is to use regression analysis
to develop an isochronous load versus strain curve at 0,1 h. The creep curve should then be shifted
vertically to pass through the mean strain measured after 0,1 h.
If the lowest test temperature is below the design temperature, the shift factor corresponding to the
design temperature should be read off the plot of shift factor against temperature. The time-scale of the
master curve should then be adjusted by this factor.
5.4 Isochronous curves
From the creep curve corresponding to each load, read off the strains for specified durations, typically
1 h, 10 h, 100 h, etc., and including the design lifetime. Set up a diagram of load against strain. For each
duration, plot the points of load against strain for the corresponding durations (see Figure 2). These are
called isochronous curves. Where a maximum strain is permitted over the design lifetime, or between
the end of construction (e.g. 100 h) and the design lifetime, it is possible to read off the corresponding
loads from these curves. Where the strain is measured from zero, note that in geosynthetics strains are
measured from a set preload (defined in ISO 10319 and ISO 13431 as 1 % of the tensile strength) and
that some woven and particularly non-woven materials may exhibit considerable irreversible strains
below this initial loading. See Reference [2] for additional details on creep strain characterization.
Key
X strain
Y load
Figure 2 — Isochronous diagram
5.5 Weathering, chemical and biological effects
Creep strain is generally insensitive to limited weathering, chemical and biological effects. In addition,
creep strains are in general not affected by installation damage, unless the damage is severe, or unless
the load level applied is very near the creep limit of the undamaged material. In most cases, the load
level applied is well below the creep limit of the material. See Reference [3] for additional details on this
issue. Thus, no further adjustment is generally required beyond the effect of temperature.
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ISO/TS 20432:2022(E)
Note, however, that artificially contaminated soils may contain chemicals, such as organic fuels and
solvents, which can affect the creep of geosynthetics. If necessary, perform a short-term creep test
according to ISO 13431 on a sample of geosynthetic that is immersed in the chemical or has just been
removed from it. If the creep strain is significantly different, do not use this geosynthetic in this soil.
6 Determination of long-term strength
6.1 Tensile strength
The characteristic strength, T , is taken as the basis for the long-term strength. T is typically a
char char
statistical value generated from the mean strength of production material less two standard deviations
s
...
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