ISO/TR 20432:2007
(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
ISO/TR 20432:2007 provides guidelines for the determination of the long-term strength of geosynthetics for soil reinforcement. ISO/TR 20432:2007 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.
Lignes directrices pour la détermination de la résistance à long terme des géosynthétiques pour le renforcement du sol
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TECHNICAL ISO/TR
REPORT 20432
First edition
2007-12-01
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/TR 20432:2007(E)
©
ISO 2007
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ISO/TR 20432:2007(E)
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ISO/TR 20432:2007(E)
Contents Page
Foreword. iv
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 Introduction.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 Introduction.5
5.2 Extrapolation.6
5.3 Time-temperature superposition methods .6
5.4 Isochronous curves.7
5.5 Weathering, chemical and biological effects.8
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 Introduction.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 .12
7.5 Strain shifting and the stepped isothermal method.13
7.6 Extrapolation and definition of reduction factor or lifetime.15
7.7 Residual strength.15
7.8 Reporting of results.15
7.9 Procedure in the absence of sufficient data .15
8 Installation damage .16
8.1 General.16
8.2 Data recommended.16
8.3 Calculation of reduction factor.17
8.4 Procedure in the absence of direct data .17
9 Weathering, chemical and biological degradation.19
9.1 Introduction.19
9.2 Data recommended for assessment.19
9.3 Weathering.19
9.4 Chemical degradation .20
9.5 Biological degradation.28
10 Determination of long-term strength .28
10.1 Factor of safety f .28
s
10.2 Design for residual strength.29
11 Reporting.29
Bibliography .30
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ISO/TR 20432:2007(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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
In exceptional circumstances, 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), it may decide by a
simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely
informative in nature and does not have to be reviewed until the data it provides are considered to be no
longer valid or useful.
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.
ISO/TR 20432 was prepared by Technical Committee ISO/TC 221, Geosynthetics.
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TECHNICAL REPORT ISO/TR 20432:2007(E)
Guidelines for the determination of the long-term strength of
geosynthetics for soil reinforcement
1 Scope
This Technical Report provides guidelines for the determination of the long-term strength of geosynthetics for
soil reinforcement.
This Technical Report 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 Technical Report 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 Technical Report 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
Technical Report does not cover uncertainty in the design of the reinforced soil structure, nor the human or
economic consequences of failure.
Any prediction is not a complete assurance of durability.
2 Normative references
The following referenced documents are indispensable for the application 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, Geosynthetics — 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 and the following apply.
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
3.1.2
long-term strain
total strain predicted in the geosynthetic during the service lifetime as a result of the applied load
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ISO/TR 20432:2007(E)
3.1.3
reduction factor
factor (W 1) by which the tensile strength is divided to take into account particular service conditions in order to
derive the long-term strength
NOTE In Europe, the term 'partial factor' is used.
3.1.4
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 This should be assured by the manufacturer’s own quality assurance scheme or by independent assessment.
3.1.5
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
3.1.6
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
RF reduction factor to allow for chemical and biological effects
CH
RF reduction factor to allow for the effect of sustained static load
CR
RF reduction factor to allow for the effect of mechanical damage
ID
RF reduction factor to allow for weathering
W
SIM stepped isothermal method
TTS time-temperature shifting
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ISO/TR 20432:2007(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
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
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
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ISO/TR 20432:2007(E)
T long-term strength per width (including factor of safety)
D
T residual strength
DR
θ temperature of accelerated creep test
j
θ 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 (log 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 Introduction
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 is typically
D
50 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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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 Technical Report does not cover the effects of temperatures below 0 °C (see Clause 1).
5 Determination of long-term (creep) strain
5.1 Introduction
The design specification may set a limit on the total strain over the 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 log 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/TR 20432:2007(E)
Key
1 Laboratory creep test 5 New time = 0 for post construction creep
2 Load ramp period on wall 6 Wall construction time
3 Load ramp period in creep test X Time
4 Loading and creep of reinforcement in wall Y Strain
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 log 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 log 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 log 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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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 [2]
in the Bibliography for additional details on creep strain characterization.
Key
X Strain
Y Load
Figure 2 — Isochronous diagram
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ISO/TR 20432:2007(E)
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 [3] in the Bibliography for additional details on this issue. Thus,
no further adjustment is generally required beyond the effect of temperature.
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 statistical
char char
value generated from the mean strength of production material less two standard deviations sometimes
referred to as the minimum average roll value (MARV), unless otherwise defined.
6.2 Reduction factors
T can then be divided by the following four reduction factors, each of which represents a loss of strength
char
determined in accordance with this Technical Report, to arrive at the long-term strength T :
D
⎯ RF is a reduction factor to allow for the effect of sustained static load at the service temperature;
CR
NOTE The effect of dynamic loads is not included.
⎯ RF is a reduction factor to allow for the effect of mechanical damage;
ID
⎯ RF is a reduction factor to allow for weathering during exposure prior to installation or of permanently
W
exposed material;
⎯ RF is a reduction factor to allow for reductions in strength due to chemical and biological effects at the
CH
design temperature (see 4.4).
In addition to the reduction factors, a factor of safety, f , takes into account the statistical variation in the
s
reduction factors calculated (see 6.1). It does not consider the uncertainties related to the soil structure and
the calculation of loads.
6.3 Modes of degradation
Degradation of strength can be divided into three Modes according to the manner in which they take place
with time:
⎯ Mode 1: Immediate reduction in strength, insignificant further reduction with time;
⎯ Mode 2: Gradual, though not necessarily constant, reduction in strength;
⎯ Mode 3: No reduction in strength for a long period; after a certain period, onset of rapid degradation.
For Mode 1, of which installation damage is an example, it is appropriate to reduce the tensile strength by an
appropriate time-independent reduction factor. For Mode 2, where there is a progressive reduction in strength,
the tensile strength will be reduced by a time-dependent reduction factor. For Mode 3, it is not appropriate to
apply a reduction factor to the tensile strength but rather to restrict the service lifetime.
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ISO/TR 20432:2007(E)
These Modes are depicted schematically in Figure 3.
Key
1 Mode 1
2 Mode 2
3 Mode 3
X Time
Y Retained strength
Figure 3 — Retained strength plotted against time for the three Modes of degradation
7 Creep rupture
7.1 Introduction
Creep rupture, or lifetime under sustained load, is determined by measuring times to rupture of up to at least
10 000 h. The results are extrapolated to predict longer lifetimes at lower loads and thereby the reduction
factor RF .
CR
This procedure may be supported by measurements at higher temperatures. Conventional TTS of results
obtained on multiple specimens at elevated temperatures provides an improved prediction of the long-term
behaviour at ambient temperature. In the SIM, the temperature of a single specimen is increased in steps. The
sections of creep strain curve measured at each temperature step are then combined to predict the long-term
creep strain and rupture lifetime.
It should be noted that a creep rupture diagram depicts applied load plotted against time to rupture and is not
a statement of the loss of strength under continuous load. It has been predicted on the basis of accelerated
tests that many geosynthetics exposed to sustained load do not in fact significantly diminish in strength until
close to the end of their predicted life. When the strength equals the applied load, the material ruptures (see
Figure 4). Sustained load is therefore a Mode 3 form of degradation.
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ISO/TR 20432:2007(E)
Key
1 Creep rupt
...
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