9 - Geotextile durabilitydownload.xuebalib.com/xuebalib.com.33681.pdfThe impact of geotextile degradation may be realized immediately, gradually, or suddenly after an extended period

Geotextile durability 9S.R. AllenTRI/Environmental Inc., Austin, TX, United States

9.1 Introduction to geotextile durability assessment

Geotextiles are formulated, manufactured, purchased, and applied with a predictionand expectation of delivered service life or design life. For purposes of this discussion,the design life of a geotextile is the required time during which the geotextile mustpossess all required properties under the specified conditions of use, such as theload, temperature, and exposure to environmental stresses (chemicals and UV energy).Often the term “service life” is used for the design requirement of the geotextile. In thiscase, the design life requires the geotextile to possess properties under the specifiedconditions of use to ensure the required performance.

Whereas the type of polymer and structure of a geotextile contribute to its selectionfor a given application, the long-term functionality of the product is not readilyapparent to the user and must be assessed by testing the finished product and strictquality control during production and subsequent installation. A meaningful assess-ment of durability involves a robust understanding of the geotextile’s required servicelife as well as the environment in which the geotextile will be used. Also useful is anunderstanding of the kinds of degradation that can occur. For example, resistance todegradation of geotextiles installed in the ground and covered may include:

• Time-dependent chemical attack that may weaken a geotextile owing to exposure to oxygen,water, or chemicals in soil

• Energy loading resulting from exposure to UV radiation or elevated temperatures causingaccelerated aging

• Mechanical loads from surcharges, earthquakes, or dynamic stresses that can causetime-dependent deformation, fatigue, and mechanical damage to a geotextile

• Geosynthetic clay liners that may weaken as a result of shear forces and chemical changes inthe clay and component geotextiles

• Geosynthetic drains that may compress under load and lose structural integrity and associ-ated functional flow capacity

In addition, many features of the material and its use have a role in a geosynthetic’sresistance to degradation:

• Polymer type (high-density polyethylene [HDPE], polyvinyl chloride [PVC], chlorosulfo-nated polyethylene, polyvinyl acetate, etc.)

• Product thickness and surface area to mass ratio• Formulation of a product’s resin with additives• Residual or manufactured stresses inherent in the material• Purity of raw materials and presence of impurities• Storage or raw materials before production and produced material after production

Geotextiles. http://dx.doi.org/10.1016/B978-0-08-100221-6.00009-7Copyright © 2016 Elsevier Ltd. All rights reserved.

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With many factors contributing to geotextile durability, the primary concern forsuccessful application is the required duration of service life.

The project service life of a geotextile may require full functionality for the durationof the application or structure. Examples of this include:

• Landfill barriers• Reinforced walls• Roadways• Dams

The geotextile may be needed for only a limited duration for other project featuresto become established; examples include:

• Erosion control mats preventing soil loss until vegetation is established• Prefabricated vertical drains providing dewatering until water evacuation and associated soil

consolidation occur• Geotextile tubes for dewatering dredged spoils until water is removed• Geocells filled with stone for a temporary access road for equipment delivery, to be removed

when equipment is delivered

The designer’s approach to geotextile durability is related to the geotextile’s func-tion within the designed application. Each function has an accompanying area ofconcern. Greenwood et al. (2011) organized the characteristic design features of ageotextile, expressed in Table 9.1. They provide a relevant international test procedureto measure the geotextile’s characteristics.

The design life of a project including the geotextile fully or partially depends on thedurability of the geotextile. It is important to acknowledge that whether a geotextile’sdesigned service life requirement is a few months or more than 100 years, it is possibleto formulate, manufacture, and correctly install geosynthetic products that will meetthis need.

The following sections provide details regarding durability challenges and degrada-tion mechanisms, durability measurements, and the acceleration of durabilityassessments.

9.2 Geotextile degradation modes

The impact of geotextile degradation may be realized immediately, gradually, orsuddenly after an extended period of time. These modes of degradation are depictedin Fig. 9.1 and the subsequent discussion.

The first mode is the most challenging to study because it is often difficult to predictthe timing of significant loss in geotextile strength. For example, although the antiox-idant is present in a stabilized polyolefin geotextile, its strength is anticipated to remainrelatively unchanged related to oxidation. However, once the antioxidant has beenused and is depleted, the strength may fall quickly. This is mode 1. Hydrolysis of apolyester product is relatively constant because water penetrates the polymer structure

178 Geotextiles

Table 9.1 Geotextile durability assessment tests (from CUR report 243with ASTM International standards included)

Requirement Geosynthetic characteristic

Test/ measurement procedure

Function

ReinforcementFiltration Separation Drainage Barrier Erosionprotection

Confinement

Physical features

Thickness ASTMD5199/D5994 - - - - - o -

Mass/unit area

ASTM D5261 - - - - X X o

Hydraulic design

Hydraulic property

Retention Opening size ASTM D6767ASTM D4751 ISO 12956

Permeability /flow rate

Permeability normal to the plane

ASTM D4491 ASTM D5493 ASTM D7701 ASTM D7880 ISO 11058 ISO 10776

Flow capacity

Flow capacity in the plane/ transmissivity

ASTM D4716 ASTM D6574 ASTM D6918 ASTM D498 ISO 12958 ISO 18325

Resistance to clogging

Clogging capacity

ASTM D1987 ASTM D5101 ASTM D5141 ASTM D5567

Resistance to compression

Compressive strength compressive creep

ASTM D6454 ASTM D6364 ASTM D7406 ASTM D7361 ISO 25619-1 ISO 25619-2

X X X - - X X

X X X - - X X

- - X - - - -

X - X - - - -

- - X - - o o

Long term flow capacity

Long term planar flow capacity

GRI GC8 - - X - - - -

Mechanical design

Mechanical property

Strength Tensileproperties

ASTM D5035 X X - - - X -ASTM D4632 X X - - - - -ASTM D4595 X X - X - X XASTM D4885 - - - - X - -ASTM D6637 - - - X - - -ASTM D7179 - - X - - - -ASTM D6818 - - - - - X -ASTM D882 - - - - X - -ASTM D6693 - - - - X - -ISO 527 - - - - X - -ISO 10319 X X X X X X

Burst strength

strength

ASTM D3787 X XXD5671MTSA

Tear ASTM D1004 ASTM D5884 ISO 34

- - - - X - -

ASTM D4533 X X - - - - -Puncture resistance

ASTM D4833 - - - - X - -ASTM D5494 - X - - X - -ASTM D5514 X X X X X X XASTM D6241 X X X - X X -ISO 12236 X X X - X X -ISO 13433 X X X - X - -

Long term strength/ strain

Creep and creep rupture

ASTM D5262 ASTM D6992 ISO 10334

- - - X - - -

Stability Friction strength/ shear resistance

ASTM D5321 ASTM D6243 ISO 12957-1ISO 12957-2

o o o X o o o

Geotextile durability 179

slowly and results in a loss in strength. This is mode 2. Installation damage is imme-diate and results in a rapid and irreversible loss of strength. However, there is no addi-tional degradation of strength damage related to installation. This is mode 3.

All modes of geotextile degradation are important to understand and measure forservice life predictions. Modes 1 and 2 are challenging in this context owing to thepracticality of realizing and documenting geotextile degradation trends within reason-able testing times. The use of acceleration measures in durability testing helps to meetthis need. For example, a UV weathering chamber can increase the frequency of lightexposure delivered to an exposed geotextile. Exposure of a geotextile to an aggressivechemical bath can increase the severity of an anticipated chemical exposure in

Robustness Resistance toinstallation damage

ASTM D5818 ISO 10722-1

Resistance to abrasion

ASTM D4886 ISO 13427 BAW Abrasion

Thermal sensitivity

Thermal stability

Temperature affects

Temperature resistance

ASTM D4594

Dimensional stability

Coefficient of Linear Thermal Expansion (CLTE)

ASTM D698

X X X X - X X

X X X X X X X

X X - X - - -

- - - - X - -

Low temp. brittleness

Low Temp. resistance

ASTM D746 ASTM D1790 ASTM D136

- - - - X - -

Durability DurabilityChemical resistance

Chem. resistance screening

ASTM D5322 X X X X X X XASTM D5747 - - - - X - -ASTM D6141 - - - - X(1) - -ASTM D5496 X X X X X X XASTM D6213 - - - X - - -ASTM D6388 - - X - - - -ASTM D6389 X X X X - X -ASTM D6766 - - - - X(1) - -ASTM D7409 X X - X - - -ISO 12447 X X X X X X XISO 12960 X X X X X X X

Oxidation resistance

Oxidation resistance

ASTM D5721 ISO 13438

X X X X X X X

Weathering Resistance to weathering

ASTM D4355 ASTM D5970 ASTM D7238

X X X X X X X

Cracking resistance

Env. stress crack resistance

ASTM D5397 o o o o X o o

Stability of antioxidants

Oxidative induction time

ASTM D 3895 ASTM D 5885

o (2) o (2) o (2) o (2) o (2) o (2) o (2)

X To be used- Not applicable O To be used in applicable applications only (1) Specific to geosynthetic clay liners (2) Specific to those geosynthetics having antioxidant additives in their formulation Note: There are other standard test procedures that are sometimes used in the determination of geosynthetic durability, this table lists

only some of the standards published by international standards organizations ASTM International and International Standards International (ISO)

Greenwood, et al., 2011. CUR building and infrastructure report 243 on the durability of geosynthetics. CURCommittee C 187.

180 Geotextiles

application to achieve chemical degradation more rapidly. Testing at elevated temper-atures also facilitates the acceleration of observed material degradation.

Whereas the measure of geotextile durability employs these acceleration techniquesto achieve practical testing regimes, their specific procedural approaches are vital forthe integrity and reliability of developed results. The next sections will review degra-dation mechanisms, their application to geotextile materials, and how they are docu-mented through testing.

9.2.1 Chemical resistance

“Chemical resistance” is a term widely used in the geotextiles industry but sometimesnot well understood. The reason for the misunderstanding is that chemical resistancemay refer to different and sometimes unrelated phenomenon that may affect a geotex-tile product’s performance. It is useful to separate the phenomena that affect the formu-lated polymer itself and those that affect the final product performance. Table 9.2presents some of the events that can occur when a geotextile product comes into con-tact with chemicals.

Table 9.2 Possible effects of chemicals on geosynthetic products

Reversible physical effects Irreversible physical effects Chemical reactions

Swelling Dissolution Oxidation/reduction

Softening/plasticization Extraction of additives Ozonolysis

Chemical induced relaxation Hydrolysis

Environmental stress cracking Other reactions

Mode 1

Mode 2

Mode 3

Time

Ret

aine

d st

reng

th

Figure 9.1 Modes of geotextile degradation.


The importance of knowing about reversible effects is because short-term tests areoften performed on geotextiles and short-term tests do not always correlate withlong-term problems. The best way to determine whether changes are permanent isto allow the chemicals to desorb and then test again. In this way, one would knowboth the temporary and permanent changes caused by chemical exposure.

Irreversible physical effects are permanent changes caused by exposure to chemi-cals. One serious effect is when a chemical dissolves the geotextile. This is rare butpossible in applications with high concentrations of chemicals in contact with geotex-tiles. The extraction of additives is also an irreversible effect but not always obviouswithout measuring those additives as a function of chemical exposure. For example,if antioxidants are extracted by chemicals from a polyolefin geotextile, there maynot be an obvious change in material properties; however, if this material subsequentlyexperiences stress or UV energy, it may degrade rapidly as in mode 1 described pre-viously. On the other hand, if chemical exposure results in extraction of the plasticizerin a PVC geomembrane, the material will be immediately harder, stronger, more brit-tle, and less flexible. Environmental stress cracking is slow crack growth through thethickness of a geotextile and may be accelerated by chemical exposure. The otherkinds of chemical degradation of geotextiles are well documented and include reac-tions with oxygen (oxidation), ozone (ozonolysis), and water (hydrolysis). Thesewill be discussed later.

A true understanding of the effects of exposure to chemicals is obtained only byevaluating all potential effects a chemical might have on a product over an extendedperiod of time. In this case, testing involving liquid or vapor immersion of the geotex-tile is used to assess chemical resistance. Baseline and exposed materials are testedafter specific exposure durations to observe the rate and degree of change in materialproperties.

9.2.1.1 Oxidation of geotextiles: polyolefins

It is well documented that polyolefins, which include polyethylene (PE) and polypro-pylene (PP), may degrade via reaction with oxygen. A geotextile’s oxidation resistancedepends strongly on the specific polymer, the stabilizer package, and the product’sphysical properties such as thickness and the surface area to mass ratio.

The oxidative degradation of PE and PP is initiated by the formation of free radi-cals. The formation is due to peroxides, oxygenated compounds (formed during theprocessing of the polymer), and catalyst residues in combination with oxygen. UVand high-energy radiation also produce free radicals and free radical initiators. Oncefree radicals are formed and oxygen is available, a chain reaction can start. Withineach propagation cycle of the chain reaction, hydro peroxides are formed. Decompo-sition of the hydro peroxides initiates new chain reactions. Oxidative degradation istherefore an auto-accelerated process (autoxidation is an autocatalytic reaction). Aftera certain induction period is experienced with no significant changes in productproperties, the degradation rate increases rapidly (mode 1), the molecular weight dis-tribution is shifted toward lower values, and the material becomes brittle and losesfinally all mechanical resistance. Depending on the amount of free radical sources

182 Geotextiles

and the sensitivity of the polymer to oxidation, the induction time might range from afew years to some decades.

Antioxidants are blended into the polymer resin during manufacturing to protect itfrom oxidation and are either compounds which trap free radicals and prevent reactionchain initiation or compounds which decompose hydro peroxides, therefore prevent-ing them from forming free radicals and new reaction chains. Typically, a combinationof both types of antioxidants is used in polyolefin geotextiles.

Each antioxidant has a temperature range in which it functions most effectively.There are antioxidants which have an effective temperature range at higher temperatureand are used as processing stabilizers (phosphites) and others which have their mosteffective temperature range at ambient temperature (hindered amine light stabilzer[HALS]) or over a wide range of temperature (hindered phenols) and which are usedas long-term stabilizers to provide protection during the low-temperature service time.

Typically a composition of a phosphite and a hindered phenol is used as an antiox-idant package for most geotextiles available on the market. In addition, HALS orcarbon black is added as the UV stabilizer component.

Oxidation stability at high temperature (above the melting point) can be measureddirectly using thermoanalytical methods. By thermoanalytical measurement, the timeinterval (oxidative induction time [OIT]) to the onset of exothermic oxidation of apolymer at a specified OIT testing temperature in a specified oxygen atmosphere isdetermined. It is neither possible to extrapolate from OIT values (measured athigh temperatures) the induction times at ambient temperature nor is it permissibleto classify the oxidation stability of different resins and products according to smalldifferences in their OIT values. However, the change in OIT values may be usedsuccessfully to monitor the change in the level of stabilization as a function of expo-sure. Especially for these stabilizer packages, the OIT value is roughly proportionalto the antioxidant concentration with some specific proportionality constant for agiven resin.

Hsuan and Koener (1998, 2005) recognized that for landfill applications, many ofthe antioxidant packages described previously result in a three-stage oxidation process,as shown subsequently.

Stage A: Depletion of antioxidantsAntioxidants are depleted environmental factors (consumption, extraction, evaporation, etc.).Changes in antioxidant content may be measured via OIT and high-pressure (HP)-OIT(Table 9.3). The mechanical properties are unchanged because the changes in molecularmass are small and inconsequential. The end of stage A is defined as the point at whichthe OIT and/or HP-OIT reaches a predetermined minimum level such as 0 or

Stage C: Mechanical degradationThe geotextile mechanical strength decreases and there is a significant reduction in molecularmass. The material is without protection and rapid oxygen consumption and associatedmolecular chain destruction proceed. Stage C ends when the mechanical strength or othermechanical property reaches a defined minimum such as 50% of baseline or unexposed value.

This concept was used, for example, to estimate the induction time of HDPE geo-membranes by Hsuan and Koerner (1998). OIT and its HP application via HP-OITtesting are employed in specifications for HDPE, linear low-density polyethylene,and PP geomembranes (see Geosynthetic Research Institute standard specifications).

Testing for geotextile oxidation resistance typically involves exposure of thegeotextile to an accelerated oxidation environment by means of an air oven. Afteroven exposure for a specified duration, retained OIT, HP-OIT, and mechanical featuressuch as tensile properties may be measured to determine at what stage oxidation isachieved, and the rate and degree of oxidation. During oven aging tests, the loss ofantioxidant additives is possible only through evaporation from the dry oven exposureenvironment. In most geotextile applications however, the material encounters liquidwhich may serve to leach the antioxidants from the material. Oven aging has also beencriticized based on the high temperatures used and related concerns that the mecha-nism of oxidation may not be the same as that experienced at application or close toroom temperature.

Leaching of antioxidants from a geotextile may be modeled by immersion in water(or chemicals). In this context, pressurized air and oxygen have been used to acceleratethe aging of geotextiles in autoclaves. The pressurized oxygen-rich environmentemploys a lower exposure temperature and serves to achieve observed oxidation inrelatively short times.

In all cases, employing different exposure temperatures and using Arrheniusmodeling can afford a prediction of oxidation stability of the geotextile product.The industry has learned a great deal regarding the oxidation protection of polyolefingeotextiles, and many resources are now available for designers to have confidence intheir ability to use stable and oxidation resistant materials.

Table 9.3 ASTM international oxidative induction timemeasurements

Test Standard High pressure

Standard ASTM D3895 ASTM D5885

Specimen mass (g) w2 w2

Test pressure (kPa) 35 3500

Test temperature (�C)/dwell 200 in N2, 1-min dwell,switch to O2

150 in N2, 1-min dwell,switch to O2

Comment: HALS, an important class of antioxidants referenced previously, is not detected by the OIT test. It is, however,detected in the HP-OIT test. Because HALS responds so strongly in the HP-OIT test, other stabilizers that may be presentcan be masked or hidden during the HP-OIT test. Thus, both tests are used.

184 Geotextiles

9.2.1.2 Stress cracking of polyolefins

Environmental stress crack resistance is covered in chapters Geotextile resinsand additives and Mechanical properties, behavior, and testing of geotextiles ofthis book.

9.2.1.3 Hydrolysis of geotextiles: polyesters

Compared with the complex mechanism of the oxidation of polyolefins, the hydrolysisof polyethylene terephthalate (PET) or polyesters is relatively straightforward. It hasbeen established that PET has two different mechanisms for hydrolysis, dependingupon the pH of the solution (2e6). In pH solutions of 9 and below, water is absorbedby the PET fibers and hydrolysis takes place throughout the entire cross-section ofthe PET product, leading to progressive rupture of the molecular chains and conse-quent loss of product strength. This degradation mechanism is termed “internalhydrolysis” and proceeds at a rate related to the number of carboxyl end groupspresent at the end of the polymer chains. A higher molecular weight of a PETgeotextile indicates a lower population of carbonyl end group (CEG) and thus alower tendency to experience hydrolysis. In addition, the high degree of orientationin some PET geosynthetics reduces the free volume of the amorphous phase wherereactions take place and thus increase the resistance to hydrolysis.

In higher-pH or alkaline environments, however, base-catalyzed hydrolysis occurson the outer surface of the fibers and the PET is hydrolyzed and eroded from the sur-face. The result is that the remaining material retains its original structure even thoughthe fibers are becoming increasingly smaller. As material is removed from the surface,the loss in strength is proportional to the reduction in cross-section. This externalhydrolysis occurs because the aggressive hydroxyl ions are too large to penetratethe PET and permeate the polymer. Therefore, only the accessible part of the fiberreacts with the hydroxyl ions.

The chemistry of alkaline or high-pH solutions should be considered whenperforming hydrolysis experiments or contemplating the long-term performance ofPET geotextiles. This was pointed out in a review article by Van Shoors (2007).The most important aspect is the reaction of any metalehydroxide solution withatmospheric carbon dioxide. Examples for NaOH and Ca(OH)2 are shown inEqs. [9.1] and [9.2]:

2NaOH þ CO2 ¼ Na2CO3 þ H2O [9.1]

Ca(OH)2 þ CO2 ¼ CaCO3 þ H2O [9.2]Eq. [9.1] shows that sodium hydroxide is converted to sodium carbonate. This is

important because sodium hydroxide is a strong base whereas sodium carbonate is arelatively weak base. The weak base would be far less reactive toward PET. Also,because they are both bases, the pH would not show a dramatic change during thisconversion. For example, the pH of a 0.1N solution of sodium hydroxide is 13.0and the pH of a 0.1N solution of sodium carbonate is 11.6. If 75% of the sodium


hydroxide were converted to carbonate, the pH would still be around 12.4. Therefore,conversion from a strong base to a weak base is difficult to detect by a simple pHmeasurement.

Eq. [9.2] shows the reaction between calcium hydroxide (hydrated lime) and carbondioxide. In this case, it is possible to detect this reaction because the product, calciumcarbonate, is insoluble in water. Therefore, a white precipitate is formed when carbondioxide is present. This chemistry makes hydrated lime a good carbon dioxide trap,and some have proposed this reaction as a way to reduce CO2 emissions into theatmosphere. These reactions need to be considered in the context of the long-termperformance of coated PET geotextiles. In some cases, such as fresh concrete, therewill be a high, yet rapidly declining pH. In other applications, such as lime-treatedsoil, there will be a more constant high concentration of calcium hydroxide.

Wan et al. (2013) measured pore water pH values in curing concrete before andafter atmospheric carbonation. They showed that the pH could drop from over 13 toaround 8 as a result of CO2 exposure. A Research Note from the Oregon Departmentof Transportation (2003) reported the pH of water in a drainage ditch near a freshlypoured foundation shaft. Measurements were made right after the pour and 31 h later.Results within a few feet of the shaft showed a maximum pH of 10.4 right after thepour and a pH of 8.2 at the same location 31 h later. A location several meters fromthe shaft peaked at 9.6 and was down to 7.3 after 31 h. This shows how quickly thepH can change from water dilution and atmospheric carbonation. The GeosyntheticsInstitute reported pH measurements taken at a geogrideblock face interface ofsegmented retaining walls (2002, 2005, 2006). The wall included three different man-ufacturers of blocks, and pH measurements were taken between the blocks and thegeogrid for over 5 years. Results showed that at the beginning, the pH of the threeblocks was 10.5, 10.0, and 9.2. After 2 years, values had dropped to 8.8, 8.7, and8.3. Results showed that within 2 years, the pH was less than 9.0 for all three of thetested materials. Incidentally, only the pH was measured. It is not known how muchof the solution was calcium hydroxide and how much was calcium carbonate. The sec-ond paper (2006) reported the results of pH measurement taken on 25 different retain-ing walls in seven different states in the United States. The ages of the walls were from0.5 to 8 years and measurements were made only a single time. The results showed thattwo walls were 9.0 or greater (9.4 and 9.0), two walls were between pH 8.0 and 9.0(8.2 and 8.0), and the other 21 walls had pH values less than 8.0. Again, it was shownthat the pH near cured concrete is not high.

The significance of all of these studies is that high pH is not encountered often andwhen it is, when atmospheric CO2 is present, the chemistry changes rapidly and be-comes much less aggressive. Except in the case of hydrated lime modified sols, wherethe pH is targeted to be 12.4, a highly alkaline environment is not likely for other civilengineering applications. Thus although PET geotextiles are susceptible to hydrolysis,the degradation acceleration related to high pH values is rarely observed in application.

Still, the hydrolysis resistance of a PET geotextile is often measured to ensure min-imum stability. Accelerated aging tests are performed by immersing PET geotextiles,typically in fiber or strap form, in hot water and measuring the retained strength after aspecified duration of exposure. It has also been practiced in specifications for PET

186 Geotextiles

geotextiles that a minimum number average molecular weight and a maximum CEGserve to promote a minimum level of hydrolysis resistance.

9.2.2 Weathering of geotextiles

All geosynthetics are susceptible to degradation when exposed to solar radiation, heat,water, and oxygen. The spectrum of nearly all solar electromagnetic radiation strikingthe earth’s atmosphere spans a range of 100 nm to about 1 mm (1,000,000 nm).Whereas the light we see is in the wavelength range between 380 and 780 nm, themost damaging to geosynthetics is the shorter wavelengths in which radiation energyis highest and corresponds to UV radiation, which is included in the full solar spec-trum but is invisible to the naked eye. UV radiation is further defined as UV-B, withwavelengths less than 320 nm, and UV-A, with wavelengths between 320 and400 nm. As expected, UV exposure is more intense, with more energy, in southerntropical climates near the equator. However, in polar regions the extremes of longsummer and winter days couple with depletion of the ozone layer to representchallenging UV exposure conditions for geotextiles. Figs. 9.1e9.3 show a worldmap of UV radiation with darker regions corresponding to higher levels. Generally,UV-related degradation of geotextiles is most intense at decreasing wavelengthswhich correspond to increasing photon energy. The most sensitive region for mosttypes of polymers falls into the range of 325e360 nm.

Geotextiles are normally exposed to weathering for a relatively short but varyingtime during construction work. The duration of exposure of geotextiles to UV energycan vary from the filtration, separation, or drainage of geotextiles that must be imme-diately covered at installation to perform their function to the permanently exposedgeomembrane that lines the face of a dam or water reservoir.

100

50

0

A B

t0 t85 t75 t65 t55

T55T65T75

T85

Aging time (log scale)

A = antioxidant depletion timeB = induction timeC = 50% property degradation

time (the “half-life”)

Pro

perty

reta

ined

(%)

C

Figure 9.2 Stage oxidation process from Koerner and Hsuan.Greenwood, et al., 2011. CUR building and infrastructure report 243 on the durability ofgeosynthetics. CUR Committee C 187. GRI White Paper #6, Koerner et al, February 8, 2011.Geomembrane lifetime Prediction: Unexposed and Exposed Conditions.


180° 160°W 140°W 120°W 100°W 80°W 60°W 40°W 20°W 0°10°E 30°E 50°E 70°E 90°E 110°E 130°E 150°E 170°E

180° 160°W 140°W 120°W 100°W 80°W 60°W 40°W 20°W 0°10°E 30°E 50°E 70°E 90°E 110°E 130°E 150°E 170°E

90°

80°N

70°N

60°N

50°N

40°N

30°N

20°N

10°N

0°

10°S

20°S

30°S

40°S

50°S

60°S

70°S

80°S

80°N

70°N

60°N

50°N

40°N

30°N

20°N

10°N

0°

10°S

20°S

30°S

40°S

50°S

60°S

70°S

80°S

90°

Figure 9.3 Monthly averaged annual ambient erythemally weighted UVR, 1997e2003.Lucas, R., et al., 2006. Environmental burden of disease series, no. 13: solar ultraviolet radiation, global burden of disease from solar ultravioletradiation. In: Pr€uss-€Ust€un, A., Zeeb, H., Mathers, C., Repacholi M., (Eds.). World Health Organization. Public Health and the Environment Geneva.

188Geotextiles

The properties of unstabilized geotextiles are such that just 1 week of outdoor expo-sure can seriously damage the material. The mechanism of degradation is complex andvaries depending on the base polymer. Polyolefins are affected by photooxidation initi-ated by the generation of free radicals that in turn start a chain oxidation resulting instrength loss. PET is affected by molecular chain breakage leading to a loss in strengthand generation of carboxyl groups. PVC is affected by bond breakage initiating theelimination of HCl and the formation of polyenes. UV can also affect plasticizersand related flexibility. In all cases, UV radiation exposure directly or indirectly resultsin the breakage of the main polymer chain, which in turn leads to a reduction instrength. Other impacts may include discoloration of some polyolefins owing todestruction of some phenolic antioxidants.

Geotextiles are measured for their resistance to UV exposure by either naturalweathering conducted outside or via the use of accelerated weatherometers servingto expose geotextile samples to aggressive cycles of UV light, moisture, and temper-ature. The principle advantage of natural weathering is the use of actual sunlightremoving the need to interpret measured material changes as a function of exposureto artificial light, or the need to correlate results to actual field conditions. Equatorialmount with mirrors for acceleration (EMMAQUA) is the most widely used outdooraccelerated weathering test methods. EMMAQUA concentrates natural sunlight viareflective, coated mirrors onto a specimen target area with an intensity of approxi-mately eight suns. The device, shown in Fig. 9.4, tracks the sun and exposes specimensto the full spectrum of sunlight, which makes it one of the most realistic acceleratedtests available. Other outdoor procedures are established that provide protocols formounting samples without the sophistication of mirror concentration or sun tracking.

Figure 9.4 Equatorial mount with mirrors for acceleration (EMMAQUA) testing.www.directindustry.com.


http://www.directindustry.com

However, weaknesses of the outdoor test approach are related to the length of timenecessary to see changes in highly stabilized materials and the acknowledgment thattemperature, which is important to the rate of degradation and the resulting materialchanges, is typically not well controlled. Even natural light exposures performed usingsome minimum temperature control do not always control temperature tightly, insteadjust ensuring that samples do not burn. Overheating is often a concern when mirrorsare used to capture and concentrate light on an exposed geotextile sample to acceleratethe duration of UV exposure.

Accelerated UV resistance testing of geotextiles in UV weatherometers has thedistinct advantages of realizing test results faster and accomplishing a controlled testwith consistent test conditions. In most test procedures, the geotextile is exposed tohigh-intensity UV radiation, effectively increasing the number of photons per unittime bombarding the material, but doing so also under prescribed conditions ofelevated temperature and periodic wetedry cycling.

In the geotextiles industry, generally two different types of artificial light are used inaccelerated weathering equipment. The most commonly used worldwide is fluorescentUV exposure equipment (ASTM D7238, ISO 4892-3) (Fig. 9.5), which employs fluo-rescent lamps that deliver artificial light representing a portion of natural daylight withwavelengths between 300 and 400 nm and a peak at 340 nm. The concentration oflight at the most damaging wavelengths and the economy of operation contribute tothe popularity of fluorescent UV weatherometers.

Figure 9.5 Fluorescent UV weatherometer.

190 Geotextiles

Unlike fluorescent weatherometers, Xenon arc exposure equipment (ASTMD4355, ISO 4892-2) (Fig. 9.6) uses charged Xenon gas to emit a spectrum of artificiallight closely matching that of natural daylight. The irradiance is regulated to controlsample temperature, which typically runs hotter than would UV fluorescent exposurebecause the Xenon arc provides all light including light in the visible and infrared UVregions.

There is an important relationship between Xenon arc and natural weatheringexposures and geotextiles that are manufactured with color. Whereas most geotex-tiles are manufactured with different shades of black and white, some are colored,such as artificial turf drains for landfill covers or tan geomembranes for desert waterreservoirs. Pigmented geotextiles with color will absorb a unique range of UVwavelengths from the natural spectrum that may not be provided by the fluorescentUV weatherometer alone. For example, orange safety fences, blue swimming poolliners, and architectural tenting fabrics are often tested with Xenon arc artificiallight sources because they provide a fuller representation of natural sunlight. It isimportant to consider this when testing the UV resistance of pigmented or coloredgeotextiles.

Predicting geotextile durability performance in the field using accelerated testingresults has proven problematic in practice. This is primarily because the standard accel-erated tests most often used would need to last multiple years, much longer than aproduct acquisition and installation cycle. Still, the usefulness accelerated testing to

Figure 9.6 Xenon-arc weatherometer.


predict field performance is recognized and there are three approaches to establish ageotextile’s UV durability in the field to accelerated test results:

1. Research testing. Weathering devices may be controlled to achieve varying specimen expo-sure temperatures and UV intensities. Employing multiple temperatures and levels of UVintensity, one may use an Arrhenius modeling approach to develop test results suitable forextrapolation to longer times.

2. World UV intensity maps. Using long-term exposure results resulting in geotextile failure,one may compare the UV energy absorbed during artificial weathering with UV energyexposure intensities across the world to match predicted exposure to a predicted degradationtrend.

3. Recreating field failure. If one is fortunate (or unfortunate) enough to secure a UV-failedgeotextile and is aware of the corresponding time to failure, a baseline or unexposed versionof that same sample with the same formulation may be used to repeat the failure in a weath-erometer. This resulting relationship may be used to predict UV-related service life in futureapplications.

9.3 Tensile creep and creep rupture

When geotextiles are used to reinforce soil, they act to carry some of the load, whichthey would not be able to do if supported by the soil alone. For example, a geotextile-reinforced embankment may be constructed with slopes steeper than would be possiblewithout reinforcement. In this structure, the load on the geotextile is considered con-stant. In some cases a geotextile may be prestressed to support a wall, bridge abutment,or other vertical structure. In this case, there generally is no further strain on the geo-textile, but rather the load of the reinforcement may decrease with time. This is calledstress relaxation. Geotextile reinforcements used in soil walls under operationalloading are expected to undergo both creep and stress relaxation, simultaneouslyresulting in some extension and reduction in load (Allen and Bathurst, 2002).

Although a geotextile reinforcement product may demonstrate short-term ultimatetensile strength (UTS) in a rapid loading tensile test performed to rupture, it will alsodemonstrate rupture after a long duration at a lesser load. The former is known aselastic failure and the latter is known as creep rupture. The higher the load is, theshorter will be the service duration or time to rupture. In addition, when load is appliedto geotextiles, they may extend or stretch for as long as that load is applied. If the loadis removed, part of the extension will be permanent and the geotextile will not return toits original length. This is known as viscoelastic behavior, or more commonly in thegeosynthetics industry, creep. The amount of creep strain experienced by a geotextiledepends on:

• the geotextile polymer type and formulation• the load applied• the temperature of application• geosynthetic geometry• the method of the manufacturer

192 Geotextiles

If a load-bearing geotextile ruptures, it may cause catastrophic failure of the soilstructure. In addition, if the geotextile reinforcement creeps without rupturing, itmay cause the soil structure to deform, shift, or sag. For a given soil structure, the limitof allowable deformation or strain of the geotextile is called the strain-limited creep.The total strain of a geotextile reinforcement includes the elastic strain experiencedduring loading as well as the creep strain experienced during time under load.

The effect of long-term tensile load/stress on a geotextile reinforcement is gener-ally measured using standard procedures (ASTM D5262, ASTM D6992, ISO 13431)in which a test specimen is placed under constant load for a time, either defined by astandardized test duration such as 10,000 h or until observed by specimen behaviorsuch as rupture or achievement of a maximum allowable strain (Fig. 9.7). Such testsare usually performed over a range of tensile loads, all usually expressed as a percent-age of the measured UTS. Specimens are tested in the same direction in which theload will be supported by the geotextile, using the same clamps as used to measureUTS. Load is applied and the elongation is measured via linear variable differentialtransformers, potential displacement devices, noncontact laser measurement, digitalimage analysis, and, for very slow creep over very long durations, manually read, andeconomical, dial indicators. All standardized geotextile creep and creep-rupture testprocedures prescribe in-air unconfined tests because soil loading and geotextileconfinement cannot easily be simulated in the test laboratory. Tests performed inair are reproducible and highly controlled, and because they result in more observedstrain than would be observed in a confined creep test, they are consideredconservative.

Figure 9.7 Example of creep test.


In general, there are up to three stages of creep observed in polymeric materials.These include primary, secondary or steady-state, and tertiary creep (Fig. 9.8). Primarycreep strains are typically linear when plotted against a log time scale and increase at adecreasing rate on an arithmetic time scale. Secondary creep strains are typically linearwhen plotted against an arithmetic time scale. Tertiary creep is the rupture phase ofcreep and is characterized by a rapidly increasing creep rate with time. Geotextilestructure tends to dominate primary creep (at least for nonwoven and somewhat forwoven geotextiles, and not at all for geogrids), and the polymer characteristics tendto dominate secondary and tertiary creep mechanisms (Allen, 1991).

Creep strain data are typically presented as curves relating strain to time. Theinitial portion of the curves represents elastic strain, whereas the major portion ofthe curve represents creep. The elastic portion of the curve can be significantlyaffected by the rate of load application and the sensitivity of strain measurement.This is especially common in polyester reinforcements in which the initial strain ishighly sensitive to the method of loading and clamp arrangement. To ensure accuratevertical location of creep curves on a strainetime plot, very short term creep tests,called ramp-and-hold tests, are performed, each lasting an hour or less. The strainsmeasured at the end of three or more ramp-and-hold test periods are averaged. Usingthese test results, the long-term creep data are then shifted up or down on the strainaxis, enabling the long-term creep test plot to pass through this average strain at thesame set time. This is demonstrated in Fig. 9.9(a) and (b), in which replicates oframp-and-hold tests were performed at 5%, 10%, and 20% of UTS. The resultingshort-term creep curves were then averaged, enabling extrapolation to strain at1000 h.

Results of conventional creep and creep-rupture testing are often presented in plotsof creep strain versus log time, with accompanying tabular data. The strain is

20

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00 10

Time

Stra

in

Primary

Secondary

Tertiary

Rupture

20 30 40

Figure 9.8 Stages of observed creep behavior in polymeric materials.

194 Geotextiles

6.0Individual ramp & hold curvesbefore strain normalization

Average ramp & hold curvesafter strain normalization

Log linear trend line

1000 hours

R&H @ 5% UTS

R&H @10% UTS

R&H @20% UTS

5.0

4.0

3.0

2.0

1.0

0.0

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7.0

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0.0–4 –3 –2 –1 0

5%UTS

10%UTS

20%UTS

–4 –3 –2 –1 0Log time (h)Log time (h)

% s

train

% s

train

1 2 3 4

(a) (b)

Figure 9.9 (a) Ramp-and-hold tests at various percents of UTS, (b) Strains are averaged then extrapolated to 1000 hours.

Geotextile

durability195

calculated as the amount of creep strain divided by the original gauge length. It is goodpractice to minimize sample to sample variability by selecting the same ribs in geo-grids, or fiber distribution area along the roll width in geotextiles with which toperform testing. Time is typically plotted in log hours, as shown in Fig. 9.10.

Because creep strains are measured over much shorter durations than expectedservice lives, predictions are necessary to determine a long-term service life strain.This is accomplished by extending creep curves numerically or by timeetemperatureshifting. It is generally advised, and often regulated, to avoid extrapolation of creepcurves more than a factor of 10.

9.3.1 Accelerated creep and creep-rupture testing

Whereas most conventional creep and creep-rupture tests are performed at a referencetemperature of 20e23�C, the use of elevated test temperatures is common to achieveaccelerated creep behavior and subsequent extrapolation exercises to achievelong-term strain or rupture predictions. Temperature acceleration of creep curvesuses the established method of timeetemperature superposition of the creep of geotex-tiles without direct use of Arrhenius’ formula. A procedure employing acceleratedtemperatures follows.

• A series of separate creep tests is performed under the same load but at different test temper-atures. When elevated temperature is used to obtain accelerated creep data, it is recommen-ded that minimum increments of 10�C be used to select temperatures for elevatedtemperature creep testing. The highest temperature tested, however, should always be belowany transitions for the polymer in question. If one uses test temperatures below 70e75�C forPP, HDPE, and PET geotextiles, significant polymer transitions will be avoided.

• Creep results are plotted on the same diagram as strain versus log time.

80

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60

50

40

30

200 1 2 3 4

Log time (h)

Creep rupture curve

Rup

ture

stre

ngth

(% U

TS)

5 6

75 years 114 years

7

Figure 9.10 Example creep-rupture curve.

196 Geotextiles

• With the creep curve generated at the lowest temperature as a reference, shift the creep curvesgenerated at higher temperatures along the time axis until they overlap.

• Conceptual individual creep curves generated at different temperatures and all at 40% ofUTS are shown in Fig. 9.11(a). The master predicted long-term creep curve for the referencetemperature is shown in Fig. 9.11(b). Notice the x axis log AT(t) for accelerated time.

The design of a geotextile soil structure often includes a limit on the acceptable totalstrain during the service life. One may also apply accelerated creep testing to a specificstrain event. Fig. 9.12(a)e(d) shows creep curves generated at different temperatureswith each grouping performed at a different percentage of UTS. Each grouping is thenshifted as before, but the master curve is then extrapolated to the time required toachieve 10% strain.

One may then plot the times to 10% strain for each of percent UTS on a logelogscale to extrapolate to a design service life, as shown in Fig. 9.12(e).

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2

01.E+00 1.E+02 1.E+04

Time (log scale)

Stress = constant 40% UTSS

train

, %

1.E+06 1.E+08 1.E+10

50°C

40°C

30°C20°C

(a)

0 2 4 6 8 10

50°C40°C30°C20°C

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0

Log AT(t)

Stress = constant 40% UTS

Stra

in, %

(b)

Figure 9.11 (a) Creep strains measured at various temperatures at 40% UTS and(b) extrapolated creep strains to generate “master curve” at 20�C reference temperature.


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01.E+00 1.E+02 1.E+04

Time (log scale)1.E+06 1.E+08 1.E+10

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(a)Stress = constant 30% UTS

50°C

Reference temperature = 20°C

Mastercurve

40°C

30°C20°C

(b) 14

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Time (log scale)


Mastercurve


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Time (log scale)

Stra

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50°C

40°C

30°C20°C


Mastercurve

Stress = constant 50% UTS(c)

Figure 9.12 (a) Creep strains measured at various temperatures at 30% UTS and resultingmaster curve at 20�C reference temperature, (b) creep strains measured at various temperaturesat 40% UTS and resulting master curve at 20�C reference temperature, (c) creep strainsmeasured at various temperatures at 50% UTS and resulting master curve at 20�C referencetemperature, (d) creep strains measured at various temperatures at 60% UTS and resultingmaster curve at 20�C reference temperature, and (e) strain-limited “master” curve showingtimes to achieve 10% strain at various stress levels.

198 Geotextiles

9.3.1.1 A special case of time temperature shifting:the stepped isothermal method

The stepped isothermal method (SIM) is a special case of timeetemperature super-position testing. SIM employs a single test specimen serving to avoid specimen-to-specimen variability. The method is standardized in ASTM D6992 and consistsof a series of timed isothermal creep tests performed at a sequence of increasing tem-peratures. These are analogous to individual tests performed at elevated temperaturespreviously described. As in a normal creep test, the load is held constant and the creepstrain is measured for the duration of the test. The number, magnitude, and duration ofthe temperature steps are designed to produce a master curve of creep representing avery long-term period under load.

SIM tests are performed in environmental chambers capable of affecting tempera-ture changes quickly and accurately, typically in less than 1 min and at an accuracy of

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1.E+00 1.E+02 1.E+04 1.E+06 1.E+08 1.E+10Time (log scale)


Mastercurve


50°C

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20°C

10080

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30

101.E+04 1.E+05 1.E+06 1.E+07

Accelerated time (log scale)

10% strain limited strength byclassical time–temperature

superposition

Stre

ss, %

UTS

(log

sca

le)

1.E+08 1.E+09 1.E+10

114 years


(e)

Figure 9.12 Continued.


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00 50,000

50°C40°C30°C20°C

100,000Time, t

150,000 200,000

Stress = constant 40% UTSS

train

, %

(a)

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Log t6 8 10

Stra

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50°C40°C30°C20°C


tʹ0

95,000

45,000

(c)

Figure 9.13 (a) Raw data from SIM test at 40% UTS showing strain response at varioustemperature steps, (b) test results from (a) replotted with Log t on x-axis, (c) test results from(b) with time subtracted from second and third temperature steps so as to match slopes, and(d) Demonstration that a single specimen SIM test “visits” what would have been aconventional creep test performed with multiple test specimens each tested at a differenttemperature.

200 Geotextiles

�0.2�C. The first creep exposure of the SIM procedure is a conventional creep test inthe sense that the creep test specimen does not have a history of creep loading. How-ever, the second and subsequent creep exposures are complicated by having thermalhistories of the previous steps; this complexity defines the SIM procedure. The strainresults of a conceptual SIM test performed at 40% of ultimate tensile strength areplotted as a creep diagram in Fig. 9.13(a).

Because each temperature step represents a new creep test, a unique starting time, t0,is assigned and is referenced to the original or starting clock. The time on the newclock, or the initiation time for each new creep test at the new test temperature, isobtained by setting t�t0 ¼ 0. The data are then replotted in log time as shown inFig. 9.13(b).

With the premise that the creep rate at the very end of any temperature step is thesame as the creep rate at the very beginning of the next temperature step, the data fromeach temperature step are shifted horizontally until the slope at the end of one matchesthe slope at the beginning of the next. In Fig. 9.13(c), the first two temperature steps ofthe conceptual SIM test are shifted horizontally to slope-match.

The creep curve segments are then shifted forward with no overlapping to achievethe master curve as shown in Fig. 9.13(d).

As the x axis, log (tet0) shows, the master curve represents accelerated predictedstrain. The SIM creep master curve produces creep segments of what would havebeen four individual creep curves using conventional testing methodology. Byemploying a single test specimen and multiple temperature creep tests, SIM generatescreep test results in very short time frames. Typically, test durations are only in the20- to 24-h range.

After a challenging beginning, the SIM test procedure has proven to be a powerfultool in measuring the creep and creep rupture behavior of geotextile reinforcements.Industry researchers have employed SIM and have proposed related modifications

Stress = constant 40% UTS14

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Stra

in, %

(d)

0 2 4Log (t–tʹ)

6 8 10

50°C40°C30°C20°CSim 50°CSim 40°CSim 30°CSim 20°CMaster curve

Figure 9.13 Continued.


specific to certain products. SIM is not yet allowed to be used solely to document ofcreep and creep rupture behavior, but instead must be used in concert with conven-tional testing to support regulatory reviews.

9.3.2 Additional ways to present creep test results

9.3.2.1 Presentation of creep data: SherbyeDorn plots

A SherbyeDorn plot of creep results is a presentation of the creep rate, on a logscale, plotted against strain on a linear scale. The curves in this figure correspondto the creep curves at specific loads. Curves that are linear or concave downwardindicate that only primary creep is occurring and no rupture is anticipated. Curvesthat are parallel to the strain axis indicate secondary creep. A curve that is concaveupward indicates that tertiary creep is occurring and rupture is imminent.

9.3.2.2 Presentation of creep data: isochronous curves

Because many geotextile reinforcement design codes specify a limit to allowable creepstrain, it is convenient to present the loads at which these strain limits are reached. Inthis context, the creep data may be presented in an isochronous plot consisting of anarray of loadestrain curves, similar to the one from a tensile test, but with each curverepresenting a different duration. Each isochronous (ie, constant time) curve is createdby taking load and strain levels from each creep curve at a given constant time andplotting them to form an isochronous curve. Isochronous curves are not an extrapola-tion tool, but instead are an interpolation tool.

9.3.3 Stress relaxation

Creep tests represent an idealized situation in which the load carried by the geotextile isconstant with time. Stress relaxation, on the other hand, is the counterpart to creep andrequires no movement of the geotextile reinforcement or surrounding soil, and maytherefore proceed independently of the soil. For example if a geotextile is prestressedbetween two anchor keys, the geotextile maintains its original length and shape but theprestress relaxes.

Stress relaxation is measured by first straining or stretching the geotextile reinforce-ment to a prescribed strain such as 2% or 3%, and then locking the displacement andmonitoring the measured load. In Fig. 9.14, a geotextile reinforcement has beenstrained to an elongation associated with loading at approximately 40% of UTS.The load is then removed but the strain is held constant. The relaxation of load, pre-sented at load T over the ultimate strength Tult is presented.

Stress relaxation has been evaluated by many researchers, which highlights that cur-rent design models assuming constant loading of the geotextile reinforcement may beconservative in nature. These evaluations have also been combined with studies of

202 Geotextiles

geotextile reinforcements that undergo a loading history with periods of loading, relax-ation, loading at a higher level, or perhaps dynamic loading. This is especially relevantin seismic design with anticipated sudden changes in loading and displacement, withrates of loading much higher than the rate of loading in conventional standardizedtensile test procedures. This results in much higher observed stiffness for geotextilereinforcements undergoing seismic loading. After a period of sustained or dynamicloading, the stiffness of a geotextile reinforcement may increase. Significantly,researchers have concluded that the simple unconfined creep rupture approachpreviously described is conservative (Kongkitul and Tatsuoka, 2007).

9.3.4 Compressive creep strain

Time-dependent strain is not limited to geotextile reinforcement. Geotextile andgeotextile-related drains (geocomposites) experience time-dependent compressionunder load which directly affects the function of drainage by reducing availableflow capacity. For civil engineering structures such as road embankments and landfillapplications, the planned service lifetime of geosynthetic drains may be as much as100 years. With regard to the long-term flow capacity of the geosynthetic drain, itmay be compromised in the following ways:

• Instantaneous loading and associated compression of the core• Instantaneous intrusion of adjacent materials (such as filter geotextiles) into the core• Time-dependent loading and associated compression of the core• Time-dependent intrusion of adjacent materials (such as filter geotextiles) into the core• Chemical, sediment, and biological clogging (discussed in Section 5.2 of chapter Geotextile/

Geosynthetic testing standards development organizations)

Figure 9.14 Example of stress-relaxation test results.Leshchinsky, D., Dechasakulsom, M., Kaliakin, V.N., Ling, H.I., 1997. Creep and stressrelaxation of geogrids. Geosynthetics International, 4 (5), 463e479.


Short-term reductions in flow are generally measured via short-term hydraulictesting of the geosynthetic drain (examples are ASTM D4716 and ISO 12958).However, the time-dependent compression and intrusion will lead to a reduction inflow capacity and are critical to the long-term determination of flow. Indeed, collapseof the core will lead to significant flow reduction.

The time-dependent reduction in thickness at a given load may be measured in away similar to tension creep and extrapolated to longer times. Testing of the coreafter lamination with geotextiles or other adjacent materials facilitates the measure-ment of increasing time-dependent intrusion. In a typical compression creep mea-surement, a square of drainage product is placed between two flat plates and aload is applied. Compression is measured by sensitive extensometers attached tothe plates. The rate of creep reduces with time and is generally plotted as percentcompression or retained thickness versus log time, as shown in Fig. 9.15.

Temperature is used to accelerate compressive creep, including use of SIM,which affords the prediction of compressive creep at temperatures other thanroom temperature. Compressive loading exceeding a product’s time-dependentcompression resistance will cause collapse and stoppage of flow completely ornearly completely. Shear loads are common in geosynthetic drains used on sideslopes. Shear forces may distort or roll the core and can lead to acceleration ofthickness reduction and in some cases separation of drainage layers such as thecore and laminating geotextiles. A compressive SIM creep test apparatus withthe core placed between two heated platens affording shear loading is shown inFig. 9.16.

By performing multiple tests, a relation can be established between compressiveload and time to collapse or the time to a given thickness, which can then be usedto design the maximum loading allowed on the drain.

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0.0001 0.001 0.01 0.1 1 10 100 1000 10,000 100,000Log time (h)

Per

cent

reta

ined

thic

knes

s

480 kPa

240 kPa

720 kPa

960 kPa

1200 kPa

1440 kPa

Figure 9.15 Example of compressive creep test results of a geosynthetic drain at various loadlevels.

204 Geotextiles

9.4 Installation damage

It is acknowledged that when a geotextile is installed it undergoes damage that imme-diately reduces its strength. Installation damage can harm a product’s ability toperform functions such as drainage and reinforcement. Installation damage includesholes and punctures and areas of stressed wrinkles. The type of damage of concernis related to the application. Holes and cuts will reduce the effectiveness of filtersand separators. Reinforcements will lose strength.

In the context of geotextile reinforcements, it is common to measure the installationdamage that will be experienced during application. In general, test procedures tomeasure resistance to installation damage differ slightly in their approach, but somecommon features enable testing and research laboratories to accommodate the require-ments established by all.

9.4.1 Large-scale installation damage testing

The most commonly employed procedure was first developed by Watts and Brady(1994) of the Transport Research Laboratory in the United Kingdom. This procedure

Figure 9.16 Compressive creep test apparatus with angled test configuration and elevatedtemperature features.


has met the general criteria defined by ISO 13437 and ASTM D5818. In this approach,the entire process of geotextile exposure and exhumation is made controllable andrepeatable by employing a small outdoor laboratory approach. Samples of the geotex-tile are placed perpendicular to the running direction of the compaction equipment,simulating most retaining wall and reinforced slope installation guidelines. A substra-tum of four steel plates, equipped with lifting chains is incorporated into the exposureregime. The layers of compacted soil aggregate are constructed on top of the plateswith the geotextile reinforcement installed on top. An additional layer of soil aggregateis then installed on top of the geotextile. The geotextile is then exhumed by raising oneend of the steel plates with lifting chains to about 45�. Soil located at the bottom of theplate is removed, and if necessary the back of the lifting plate is struck with a sledge-hammer to loosen fill. As the upper lift falls away, the geotextile is removed by rollingit away from the underlying lift (Fig. 9.17). This procedure significantly minimizesexhumation damage unrelated to installation damage.

The exhumed samples are then cut and tested in their soiled condition. Other unex-posed samples are also cut and tested from the same roll of material to establish base-line tensile strength. The evaluation of installation damage is based on the retainedstrength observed after being damaged. Most installation damage test exposures arebased on the use of numerous aggregates and fills differing by features (average par-ticle size, particle angularity, overall gradation and hardness [durability], etc.). Severalobservations have been recorded for installation damage-related strength losses:

• The coating on coated fiber reinforcements is critical to product protection. Most importantare the quantity and thickness of the coating. The thoroughness of coverage and thickness ofthe final coating can govern the resulting retained strength. Thin or breached coatings canfacilitate damage where the soils and aggregates are allowed to strike through to the coreyarns and rip or tear the material. Alternatively, complete and thick coverage can oftenfacilitate minimal damage.

Figure 9.17 Example of the installation damage exposure procedure.

206 Geotextiles

• Another feature of woven, coated geotextile reinforcements is the geometric configuration ofthe product. Sometimes the difference between a relatively lightweight, weaker product and astronger heavier product is the structure of the additional yarn bundles added to individualgeogrid ribs. These additional yarn bundles add weight and strength to the product. However,when these are added horizontally at the cost of aperture size, the exposed surface area tomass ratio of the product changes, sometimes resulting in more damage absorbed. Alterna-tively, ribs reinforced by vertical addition rather than the adjacent addition of yarns haveshown immunity to this additional damage and have sometimes not shown increased suscep-tibility to installation damage. This is believed to result from the increase in strength withoutthe increased surface area to mass ratio.

• Weave patterns of woven geotextiles are significant to observed damage because more openweave patterns involve damage from sharp particles that infiltrate the geotextile structure.

9.4.2 Laboratory small-scale installation damage testing

Because it is not always convenient to perform large-scale installation damageassessments of geotextile reinforcements, a laboratory test, ISO 10722, has beendeveloped to simulate damage on a small scale. Fig. 9.18 shows a small-scale instal-lation damage test apparatus. An open square frame is filled with an aluminum oxideaggregate of a defined gradation and hardness which is then compacted. A wide testspecimen is laid over the compacted aggregate. A second open frame is laid over thetest specimen and filled with aggregate. A plate of set dimensions is then used tocompact the aggregate dynamically to a specified load and number of load cycles.The aggregate is then removed and damage is assessed by determining the retainedstrength.

Although the laboratory-scale installation damage test does not develop retainedstrength values useable for reinforcement design, it has proved useful to index

Figure 9.18 ISO 10722 test apparatus.


products undergoing large-scale testing so that future product changes or new productdevelopments may be evaluated using this smaller-scale test.

9.4.3 Long-term dynamic loading (fatigue) of geotextiles

Geotextiles used in traffic applications experience dynamic loading or fatigue. Exam-ples include asphalt reinforcement and separation under railway ballast. Loading willbe in compression although some load will be transferred to tensile elements in thegeotextile reinforcement. The effect of dynamic loading can be to compromise thefunction of a filter or separator, or to reduce the strength of a geotextile reinforcementto the point of defined failure. Fig. 9.19 shows a geotextile after dynamic stressing at aspecified load. A reduction in retained tensile strength is plotted versus the number ofloading cycles.

Testing of geotextile fatigue almost always consists of the large-scale applicationof high dynamic loads, recording the number of cycles to a predefined failure, and theextrapolation of these results to lower service loads to predict the number of cycles tofailure in application. Testing is often accelerated by increasing the loading fre-quency, employing a higher number of cycles per unit time provided that higherstrain rates do not misrepresent material behavior or change the mechanism of fail-ure. This is common practice in large-scale trafficking studies in which trafficking

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Figure 9.19 Example fatigue test results.Gopal et al., http://nptel.ac.in.

208 Geotextiles

http://nptel.ac.in

loads are applied to geotextile-reinforced roads and time to rut depth is measured, asshown in Fig. 9.20.

9.5 Abrasion of geotextiles

Abrasion of geotextile materials is most often a concern in coastal erosion preven-tion applications in which wave action energy is a continual stress on a geotextileor geotextile-reinforced structure, although for any application in which dynamicloading is anticipated, abrasion should be a concern. The approach most commonlyused to measure a geotextile product’s resistance to abrasion is to expose the ma-terial to cyclical abrasion from a standardized abradant, such as sandpaper, under aload. A relationship between the applied load and the time or cycles to failure, orretained strength, is established. The ASTM D4886 abrasion apparatus is shownin Fig. 9.21.

Specific to coastal erosion applications, a hydraulic abrasion test outlined by theGerman Federal Waterways Engineering and Research Institute has proven respon-sive. In the test, an octagonal exposure chamber is filled with water and a specifiedmass of graded angular basalt. The sides of the exposure tank hold eight wide tensiletest specimens. Abrasion is fostered by rotating the tank at 16 rpm for 80,000 cycles,changing the direction of rotation every 5000 revolutions. A picture of the BAWexposure tank is shown in Fig. 9.22.

Figure 9.20 Features of a trafficking test laboratory with field instrumentation, roadway testareas and axle loading capability.


Figure 9.21 ASTM D4886 test apparatus.

Figure 9.22 German hydraulic abrasion test apparatus.

210 Geotextiles

Currently the expression of geotextile resistance to abrasion is limited to compli-ance with these and other related abrasion tests rather than acceleration and relatedmodeling and the prediction of abrasion-related design life.

9.6 Determination of durability of reinforcement anddrainage applications using reduction factors

As shown, a common use of durability testing is to make sure the projected service lifeof a geotextile is longer than the required design lifetime. However, in many cases areduction in geotextile function is determined to ensure a design expectation that neverexceeds available long-term product performance. In these cases, the use of a reductionfactor (RF) is employed to reduce the relevant geotextile property to reflect the drop inperformance at a specific design time. The concept of RFs is to include into themeasured test property influences that are not included in the test protocol, such aslong-term properties. The RF is calculated by dividing the short-term property valueby the measured or predicted value at the end of a service life. RFs are greater than1.0 or, if no degradation is anticipated, equal to 1.0.

The use of RFs is not always appropriate. For example, we have seen that when anantioxidant is depleted for a polyolefin geotextile, the loss of mechanical propertiesmay occur rapidly. In this case it is more appropriate to predict service life than to calcu-late an RF, ensuring that the projected service life is more than the required lifetime.

9.6.1 Reduction factors for geotextiles (and geogrid)reinforcement

Because reinforcement geotextile products serve as reinforcement only when theyassume load, and knowing they experience strain when loaded, the measurement oftheir strain behavior and its incorporation into design is required. In addition, manyother in situ conditions affect the performance of the geotextile reinforcement,including effects caused by installation damage, weathering, and chemical degrada-tion. The approach to including these phenomena in design is to place a multicompo-nent RF on short-term measured tensile strengths. This allowable or long-term strengthof a geotextile reinforcement may be determined in accordance with the followingequation:

Tal ¼ Tult=RF

where RF ¼ RFID�RFCR�RFD; Tal ¼ allowable strength, the long-term tensilestrength that will not result in rupture of the reinforcement during the requireddesign life, calculated on a load per unit of reinforcement width basis; Tult ¼ theUTS of the reinforcement determined from width tensile tests (ASTM D4595,ASTM D6637, or ISO 10319); RF ¼ the combined reduction factor to account for


potential long-term degradation owing to installation damage, creep, and chemicalaging; RFID ¼ the strength reduction factor to account for installation damage to thereinforcement; RFCR ¼ the strength reduction factor to prevent long-term creeprupture of the reinforcement; and RFD ¼ the strength reduction factor to preventrupture of the reinforcement due to chemical degradation (RFCH) and weathering(RFW).

These reduction factors are generally common to both ISO and American Associ-ation of State Highway and Transportation Official standard guidance and are deter-mined in accordance with the procedures and standard referenced here.

If a limit is established for the maximum allowable strain of the geotextile reinforce-ment, in addition to calculation of Tal there must be a calculation of long-term strain inparallel. For these reasons it is always beneficial to capture both strain and time increep-rupture testing.

9.6.2 Reduction factors for geosynthetic drains and filters

For geosynthetic drains, the main property of interest is hydraulic flow in the plane ofthe drain, which is affected by time-dependent compression and intrusion, and chem-ical, particulate, and biological clogging. The resulting allowable flow may then bedetermined as follows:

Qal ¼ Qult=RF

where RF ¼ RFPC � RFCC � RFBC � RFINT � RFCR; Qal ¼ allowable flow rate, thelong-term planar flow rate that will not result in failure of the drain during the requireddesign life, calculated as the flow per unit of drain width; and Qult ¼ the ultimate flowrate of the geosynthetic drain from hydraulic flow tests (ASTM D4716 and ISO12958). Chapter Hydraulic properties, behavior and testing of geotextiles shows atypical hydraulic flow test apparatus. RF ¼ the combined reduction factor to accountfor potential long-term clogging owing to a reduction in thickness, intrusion of adja-cent materials (for example, a geotextile), and particulate, chemical, and biologicalclogging; RFPC ¼ the flow reduction factor to account for soil clogging and blindingof the drain; RFCC ¼ the flow reduction factor to account for chemical clogging of thedrain; RFBC ¼ the flow reduction factor to account for biological clogging of the drain;RFINT ¼ the flow reduction factor to account for the intrusion of adjacent materialcausing blinding of the drain; and RFCR ¼ the flow reduction factor to account for areduction in the thickness of the drain.

Geosynthetic drains often exhibit most of their thickness reduction and associatedintrusion within the first 100 h of loading. As such, many determinations of allowableflow incorporate a 100-h flow (Q100) test result replacing the Qult. This Q100 serves tocapture the plastic deformation of the drain and associated intrusion whereas the creepand intrusion reduction factors are determined to have a duration of 100 h to design lifeduration.

212 Geotextiles

RFPC is related to the quality of the filter geotextile and can be assumed to be min-imal as long as the adjacent fill or soil next to the geotextile filter meets filtration designcriteria. Still, upstream soil particles may embed themselves in a thick filter geotextileand block flow above the geotextile’s voids.

RFCC considers that the permeating liquid might carry or precipitate chemicalswhich may clog the geotextile filter or drain. High-alkalinity groundwater willreadily precipitate calcium and magnesium in this regard. Total suspended solidswith values greater than 5000 mg/L require high reduction factors. Liquids highin microbial content, such as landfill leachates, agricultural wastewaters, andsewage biosolids, are all troublesome and result in high measured RFBC values.Values of biochemical oxygen demand greater than 5000 mg/L are consideredhigh and warrant concern.

Although chemical and biological clogging test procedures exist they are relativelycomplex, typically involve handling challenging materials, and are not performedroutinely. Still, enough testing has been performed to enable many researchers torecommend default reduction factors. As an example, Koerner recommended rangesof RFPC, RFCC, and RFBC based on specific drainage applications. Giroud et al. pro-posed ranges of values for RFCC and RFBC together with similar ranges for RFCR andRFINT.

9.7 Summary and conclusions

Significantly, every project employing geotextiles (including geotextiles) represents apotential durability test laboratory. Indeed, as many geotextile installations age, a rele-vant question is how much longer the geotextile can last or when the geotextile shouldbe replaced. Although laboratory assessments of geotextile durability via acceleratedtesting technologies will always be predictive in nature, evaluation of geotextile sam-ples retrieved directly from a working installation does not have this liability. To takeadvantage of this possibility for a meaningful investigation of durability, it is alwaysworthwhile to plan and implement the exposure of a series of sacrificial sample repre-sentative materials so that evaluation of geotextile properties may be revisited atvarious times.

Often the challenge of deriving meaningful remaining life information from sam-ples exhumed or extracted from a site is the lack of information regarding the unex-posed or baseline condition of the geotextile. A suitable baseline condition may beestablished by selecting buried samples that have not been exposed to temperatureextremes, loads, chemicals, air, or excessive abrasion: for example, a geotextile sam-ple secured from an anchor trench. This sample can then serve a representative of anearly unexposed or baseline sample with which an exposed sample may then becompared.

Geotextiles have proven to be durable construction products with growing use.Most reported failures have emphasized the need for better selection of the appropriate


geotextile product and the requirement for vigilant construction quality control andquality assurance. Indeed, the most durable of geotextiles cannot overcome the chal-lenge of inappropriate or problematic installation. Still, the durability of a geotextileremains the subject of great interest and study as geotextile applications grow andthe creativity of geotextile formulations evolves.

References

Allen, T.M., 1991. Determination of Long-Term Strength of Geosynthetics: a State-of-the-ArtReview. In: Proceedings of Geosynthetics ’91, Atlanta, GA, USA, vol. 1, pp. 351e379.

Allen, T.M., Bathurst, R.J., 2002. Observed long term performance of geosynthetic walls andimplications for design. Geosynthetics International 9 (5e6), 567e606.

Greenwood, et al., 2011. CUR building and infrastructure report 243 on the durability of geo-synthetics. CUR Committee C 187.

Hsuan, Y., Koerner, R., 1998. Antioxidant Depletion Lifetime in High Density PolyethyleneGeomembranes. Journal of Geotechnical and Geoenvironmental Engineering 6 (532),532e541. http://dx.doi.org/10.1061/(ASCE)1090-0241(1998)124.

Hsuan, Y.G., Koerner, R.M., 2005. Stage “C” Lifetime Prediction of HDPE GeomembraneUsing Acceleration Tests with Elevated Temperature and High Pressure. In: Proceedings ofthe GRI-18 Conference on “Geosynthetics Research & Development In-Progress” atGeoFrontiers, January 26, 2005. GII Publication, Folsom, PA.

Koerner, G.R., Hsuan, Y.G., Koerner, R.M., 2002. Field measurements of alkalinity (pH) levelsbehind segmental retaining walls. In: Delmas, Gourc, Girard (Eds.), 7ICG. Nice, France,pp. 1443e1446.

Koerner, G.R., Koerner, R.M., 2005. Alkalinity between masonry blocks of a segmentalretaining wall (SRW). In: Proceedings of the GRI-18 Conference on “GeosyntheticResearch & Development In-progress” at GeoFrontiers. GII Publication, Folsom, PA.

Koerner, G.R., Koerner, R.M., Hsuan, Y.G., 2006. Long term monitoring of alkalinity at thegeogrid-block interface at a full scale masonry block retaining wall. In: Proc. 8 IGSConference, Yokohama, Japan, pp. 1205e1208.

Kongkitul, W., Tatsuoka, F., 2007. A theoretical framework to analyze the behavior of polymergeosynthetic reinforcement in temperature accelerated creep tests. Geosynthetics Interna-tional 14, 23e38.

Lucas, R., et al., 2006. Environmental burden of disease series, no. 13: solar ultraviolet radiation,global burden of disease from solar ultraviolet radiation. In: Pr€uss-€Ust€un, A., Zeeb, H.,Mathers, C., Repacholi, M. (Eds.). World Health Organization, Public Health and theEnvironment Geneva.

Leshchinsky, D., Dechasakulsom, M., Kaliakin, V.N., Ling, H.I., 1997. Creep and stressrelaxation of geogrids. Geosynthetics International 4 (5), 463e479.

Oregon Dept of Transportation, 2003. The pH of Water in Contact with Fresh Concrete.Research Notes, RSN 03e02.

Van Schoors, L.V., 2007. Hydrolytic aging of polyester (polyethylene terephthalate) geotextiles:state of the art assessment. Bulletin des Laboratoires des Ponts et Chaussées 270, 271.

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http://dx.doi.org/10.1061/(ASCE)1090-0241(1998)124

Wan, X., Whitman, F.H., et al., 2013. Chloride content and pH value in the pore solution ofconcrete under carbonation. Journal of Zhejiang University Science A. Applied Physics &Engineering 14 (1), 71e78.

Watts, G.R.A., Brady, K.C., 1994. Geosyntheticsdinstallation damage and the measurementof tensile strength. In: Proceedings of the 5th International Conference on Geotextiles,Geomembranes and Related Products.

Washington State Department of Transportation, Standard Practice T 925, 2009.


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9. Geotextile durability9.1 Introduction to geotextile durability assessment9.2 Geotextile degradation modes9.2.1 Chemical resistance9.2.1.1 Oxidation of geotextiles: polyolefins9.2.1.2 Stress cracking of polyolefins9.2.1.3 Hydrolysis of geotextiles: polyesters

9.2.2 Weathering of geotextiles

9.3 Tensile creep and creep rupture9.3.1 Accelerated creep and creep-rupture testing9.3.1.1 A special case of time temperature shifting: the stepped isothermal method

9.3.2 Additional ways to present creep test results9.3.2.1 Presentation of creep data: Sherby–Dorn plots9.3.2.2 Presentation of creep data: isochronous curves

9.3.3 Stress relaxation9.3.4 Compressive creep strain

9.4 Installation damage9.4.1 Large-scale installation damage testing9.4.2 Laboratory small-scale installation damage testing9.4.3 Long-term dynamic loading (fatigue) of geotextiles

9.5 Abrasion of geotextiles9.6 Determination of durability of reinforcement and drainage applications using reduction factors9.6.1 Reduction factors for geotextiles (and geogrid) reinforcement9.6.2 Reduction factors for geosynthetic drains and filters

9.7 Summary and conclusionsReferences

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9 - Geotextile durabilitydownload.xuebalib.com/xuebalib.com.33681.pdfThe impact of geotextile degradation may be realized immediately, gradually, or suddenly after an extended period