creep behavior of geosynthetics using confined...

Creep behavior of geosynthetics using confined-accelerated tests

F. A. N. Franca1 and B. S. Bueno2

1Graduate student, Laboratory of Geosynthetics, Geotechnical Engineering Department, School of

Engineering of University of Sao Paulo at Sao Carlos, University of Sao Paulo, Sao Carlos, Sao Paulo,

13.566-536, Brazil, Telephone: +55 16 3373 9501, Telefax: +55 16 3373–9509, E-mail:

[email protected], Laboratory of Geosynthetics, Geotechnical Engineering Department, School of Engineering

of University of Sao Paulo at Sao Carlos, University of Sao Paulo, Sao Carlos, Sao Paulo, 13.566-536,

Brazil, Telephone: +55 16 3373 9501, Telefax: +55 16 3373 9509, E-mail: [email protected]

Received 7 January 2010, revised 2 December 2010, accepted 22 June 2011

ABSTRACT: The creep behavior of geosynthetics has commonly been determined using

standardized creep tests, which are time consuming and very expensive. In addition, these tests

involve the use of in-isolation specimens. Thus they are not likely to consider the overall effect of

soil confinement. Confined creep tests conducted at elevated temperature can be used to address

these negative aspects of standardized creep tests. This paper presents a pioneering laboratory

apparatus developed in order to conduct confined, accelerated and confined-accelerated creep tests.

In addition, preliminary tests were performed to assess the new equipment’s capability of

conducting confined and accelerated creep tests. These tests were performed using a biaxial

geogrid, a woven geotextile, and a nonwoven geotextile. The new equipment allowed different

conditions to be reproduced. The creep behavior of the nonwoven geotextile and the geogrid was

found to be very sensitive to soil confinement. On the other hand, the woven geotextile presented

a creep behavior independent of soil confinement. The geogrid results did not agree with reports in

the technical literature. Accordingly, these results showed the importance of characterizing the

effect of soil confinement in geosynthetics creep behavior. Additionally, these preliminary results

showed the potential of the new device to overcome the main negative aspects of standardized

creep tests on geosynthetics.

KEYWORDS: Geosynthetics, Creep behavior, Soil confinement, Elevated temperature

REFERENCE: Franca, F. A. N. & Bueno, B. S. (2011). Creep behavior of geosynthetics using confined-

accelerated tests. Geosynthetics International, 18, No. 5, 242–254. [http://dx.doi.org/10.1680/

gein.2011.18.5.242]

1. INTRODUCTION

Creep behavior plays an important role in geosynthetic-

reinforced soil (GRS) structures, because it reduces the

design short-term tensile strength of the reinforcement. In

addition, creep strains obtained in laboratory tests may

also be used to evaluate the behavior of GRS structures

prior to their construction. These laboratory tests have

commonly been used to determine the creep behavior of

geosynthetics. They are conducted at standard values of

temperature and relative humidity using in-isolation speci-

mens. Furthermore, the recommended test duration may

reach up to 10 000 h (e.g. ASTM D 5262, ISO/TR 20432

(ISO 2007)). Consequently, the full determination of

geosynthetics creep behavior is both time consuming and

very expensive. Moreover, these tests do not consider the

possibly significant effect of soil confinement on the

stress–strain behavior of geosynthetics, as in-isolation

specimens are used. Thus in-isolation creep tests may lead

to conservative results, which increase construction costs.

In order to overcome these drawbacks, many studies

have suggested different approaches to evaluate geosyn-

thetics creep behavior. Creep tests conducted at elevated

temperature may be used to reduce the test duration. In

addition, they may be performed under soil confinement,

in order to consider its effect on specimens’ creep behav-

ior. Both improvements in geosynthetics creep tests—soil

confinement and heating—are quite simple to implement

when they are considered by themselves. As presented in

the next section, several researchers have published results

obtained from improved test arrangements, which were

able to consider each of these effects one at a time.

Geosynthetics International, 2011, 18, No. 5

2421072-6349 # 2011 Thomas Telford Ltd

This paper presents a pioneering piece of equipment

that allows confined-accelerated creep tests to be con-

ducted. In addition, preliminary results obtained using

three different geosynthetics and four different types of

creep tests (conventional, confined, accelerated, and

confined-accelerated) are shown. The new creep-testing

equipment was developed in the Laboratory of Geosyn-

thetics of the School of Engineering of the University of

Sao Paulo at Sao Carlos, in Brazil. Further analyses, such

as time–temperature superposition plots, are not discussed

in this paper. Additional tests are planned, and the

analyses of their results will contribute to understanding

of the creep behavior of the material used so far in this

research.

2. THEORETICAL REVIEW OFGEOSYNTHETICS CREEP BEHAVIOR

Creep behavior refers to time-dependent deformations that

any material is subjected to under a constant load.

Polymers may present significant creep deformations,

owing to their molecular structure. Also, the creep behav-

ior of geosynthetics is one of the most important proper-

ties in the design of GRS structures. Therefore their long-

term performance should be determined (Koerner 2005) in

order to provide reliable design parameters.

Creep deformations in geosynthetics are commonly

associated with both the macro- and microstructure of

geosynthetics. Bueno et al. (2005) illustrated this by

discussing the creep behavior of nonwoven geotextiles.

These authors defined the intrinsic creep behavior, which

is related to elongation of polymeric fibers, and the

structural creep behavior, which is associated with the

slippage between geosynthetic fibers.

Creep strains may be divided into three distinct phases,

with noticeable strain-rate distinctions. Figure 1 presents

the result of a conventional creep test (ASTM D 5262)

performed using a polypropylene woven geotextile. The

specimen was subjected to 70% of its short-term ultimate

tensile strength (UTS) in the cross-machine direction. This

test was conducted up to specimen failure in order to

illustrate the creep phases. First, an immediate elastic

strain (�i) was noticed in the geosynthetic specimen, due

to tensile load application. As the tensile load was

maintained, creep strains (�cr) occurred. These strains are

permanent, and are divided into three different phases:

primary (�1), secondary (�2) and tertiary (�3) creep. The

main differences between these creep stages concerns their

rate of strain. As shown in Figure 1, the creep strain rate

decreased during primary creep, and remained approxi-

mately constant throughout secondary creep. Finally,

tertiary creep was characterized by a significant increase

in creep strain rate, which led to specimen rupture.

Creep data are usually represented by means of the

logarithmic function

� ¼ aþ bln tð Þ (1)

where � is the specimen elongation (in %), a and b are

logarithmic regression parameters, and t is the elapsed test

time (in hours).

In this function, parameter a corresponds to the creep

strain at unitary time, and parameter b indicates the rate

of strain. Equation (1) can also be used to calculate the

initial strains of the specimen, commonly considered

1 min after the beginning of tensile load application.

Therefore the comparison between parameters a and b

obtained from creep tests in different conditions can be

used to evaluate the creep behavior of geosynthetics.

Concerning creep data visualization, both primary and

secondary creep strains are approximately linear in a

semi-logarithm plot.

The creep behavior of geosynthetics has been consid-

ered in the design of GRS structures either as a reduction

of the UTS of the material, or as a constraint of the

maximum strain of the geosynthetic. In the first case,

reduction factors are used to decrease the allowable long-

term strength of the geosynthetic. These factors are related

to creep, to some degradation processes (e.g. installation

damage, and both chemical and biological degradation),

and to uncertainties regarding their determination. Of

these, the reduction factor due to creep is commonly the

most significant. As a result, the design tensile strength

becomes considerably lower than the UTS obtained from

short-term tensile tests (Kongkitkul et al. 2010). In

addition to this approach, the creep data can also be used

to determine the geosynthetic’s maximum allowable strain.

The corresponding load is taken from short-term tensile

test results, and is taken as the design tensile strength of

the field GRS structure.

Standard creep tests (e.g. ASTM D 5262, ISO/TR

20432 (ISO 2007)) have been used to determine the creep

behavior of geosynthetics. Referred to in this paper as

conventional creep tests, they consist in the application of

a constant tensile load on an in-isolation geosynthetic

specimen, while specimen elongation measurements are

performed and strains are computed over time. Conse-

quently, it is possible to either plot a curve similar to that

in Figure 1 or express the data in a semi-logarithmic plot.

Despite their widespread use, conventional creep tests

present two main negative aspects. First, owing to their

very long duration, they may be both time consuming and

expensive. Second, they do not consider the effect of soil

confinement, since in-isolation specimens are used in

these tests. These drawbacks may be addressed by means

102

104

106

108

0

10

20

30

0 2 4 6 8 10

Str

ain

rate

,(%

/s)

ε�

Axi

al s

trai

n,(%

)ε

Time, (h)t

Primary creep Secondary creepTertiary creep Creep strain rate

ε1

ei

ε2

ε3

Figure 1. Creep phases of geosynthetics

Creep behavior of geosynthetics using confined-accelerated tests 243


of two main approaches: tests performed at elevated

temperature and under soil confinement, respectively.

Conventional creep tests can be performed at elevated

temperature in order to expedite the characterization of

creep behavior (Bueno et al. 2005). Thus several conven-

tional creep tests may be conducted at the same load

level, but at different temperatures. Subsequently, time-

temperature superposition (TTS) techniques are used to

interpret their results: that is, creep strains recorded at

elevated temperature can be interpreted as creep strains at

reference temperature on a different timescale. This

improves the prediction of geosynthetics’ long-term creep

behavior at ambient temperature. Several successful re-

searches have been conducted concerning the acceleration

of geosynthetics’ creep response by means of elevated

temperature tests (Jeon et al. 2002; Zornberg et al. 2004;

Bueno et al. 2005; Jones and Clarke 2007; Tong et al.

2008; Yeo and Hsuan 2008). This issue is already well

established in the technical literature, and its discussion is

not within the scope of this paper. In addition, ASTM D

6992 presents the stepped isothermal method (SIM), ini-

tially developed by Thornton et al. (1998). In this ap-

proach, the creep behavior of geosynthetics is evaluated

by tests conducted with temperature increments in one

single specimen. Thus the material variability does not

interfere in the analysis of the results.

In some cases, geosynthetic stress–strain behavior is

strongly dependent on soil confinement. FHWA (1998)

suggest three mechanisms of soil–geosynthetic interaction

that may cause this. First, the internal friction between

fibers and yarns is restrained under soil confinement.

Second, the basic components of a geosynthetic specimen

may present themselves as tortuous, and soil confinement

may constrain alignment of these curved elements. Finally,

soil penetration into openings or apertures in a geosyn-

thetic may reduce the reorientation of fibers and yarns.

Regarding these assumptions, the stress–strain behavior of

nonwoven geotextiles is considered to be the most affected

by soil confinement, followed by that of woven geotextiles

and geogrids. Even though Boyle et al. (1996) found

woven geotextiles’ stress–strain behavior to be completely

independent of soil confinement, FHWA (1998) showed

some results where woven geotextile stress–strain behav-

ior was affected by soil confinement. Based on such

discrepancies, FHWA (1998) suggest that each geosyn-

thetic should be characterized regarding its confined

stress–strain behavior.

Similarly to short-term stress–strain behavior, the creep

behavior of geosynthetics may also be affected by soil

confinement. Thus, in order to overcome the second

negative aspect of conventional creep tests, they may be

performed with in-soil specimens. Tests conducted with

this approach are more likely to consider the overall effect

of soil confinement in the creep response of geosynthetics.

Two main types of equipment are described in the

technical literature to perform confined creep tests. In the

first, as used in a pioneering study conducted by McGown

et al. (1982), the geosynthetic is directly subjected to the

tensile load by means of clamps connected to it. The

second type of equipment comprises a different load

system, where the confining soil is subjected to a vertical

stress and is allowed to deform laterally. As a result, the

soil itself transmits the tensile load to the specimen.

Figure 2 schematically presents both types of confined

creep test apparatus. The confining stress is applied

throughout the test in both configurations.

As mentioned above, the specimen is directly loaded in

the first type of confined creep equipment. Thus an

external loading system is used, which usually consists of

deadweights. One of the main disadvantages of this

confined creep test setup consists in the reduction of

tensile load due to soil–geosynthetic friction. This may be

overcome either by imposing a low-friction boundary

condition on the confining soil–geosynthetic contact, or

by allowing the confining box to move during the test

(FHWA 1998).

In the second type of confined creep equipment, the

confining stress causes an increase in the soil vertical

stresses, which leads to an increase in the horizontal

stresses. Thus movable faces on both sides of the test box

will be subjected to a horizontal force. Since the specimen

is attached to these faces, the geosynthetic will be

subjected to a tensile load. Despite its reliable reproduc-

tion of field load application, this type of confined creep

test usually leads to very small tensile load values. There-

fore it would require very high vertical stress values.

Additionally, the loading process generally results in very

small loading rates (e.g. 0.05%/min), compared with those

in in-isolation creep tests. ASTM D 5262 prescribes that

the load be applied smoothly at a strain rate of

10% � 3%/min. Thus results from the two tests cannot be

compared, owing to the difference in strain rate during

load application, and the effect of the soil is not clearly

defined (FHWA 1998). This issue was also discussed by

Soil

Geosynthetic

Confining stress

Hardened portionof the specimen

Tensile load Soil Geosynthetic

Confining stress

Movable faces

Figure 2. Types of confined creep test devices

244 Franca and Bueno


Walters et al. (2002) in a study of reinforcement stiffness

in different conditions (in-isolation and in-soil speci-

mens).

FHWA (1998) presented several types of equipment

used in different studies to measure the confined stress–

strain properties of geosynthetics. These authors also

described some studies where pullout tests were used to

characterize the in-soil stress–strain behavior of geosyn-

thetics. Despite the convenience of pullout tests, FHWA

(1998) suggested that alternative tests be considered to

conduct confined creep studies, such as those presented in

Figure 2. Additionally, Boyle and Holtz (1996) empha-

sized the importance of direct measurement of reinforce-

ment tension in in-soil tests in order to consider some

reduction in the load while the geosynthetic continues to

strain. This may occur because of a soil tendency to set

up.

Some recent studies on confined creep tests are pre-

sented by Costa (2004), Mendes et al. (2007), Ding et al.

(2008) and Kamiji et al. (2008). Despite the number of

successful attempts, there is not a standard procedure to

conduct in-soil creep tests.

The approaches to address the two main negative points

of conventional creep tests have been comprehensively

published in the technical literature. However, the combi-

nation of the two techniques in one creep test would both

consider the effects of the confining soil surrounding the

geosynthetic and expedite the tests. Thus reliable results

would be produced, with a reduction of test duration and

costs.

3. EXPERIMENTAL

3.1. Creep-testing equipment

Confined, accelerated and confined-accelerated creep tests

were performed with the new equipment developed during

this research, which is illustrated in Figure 3. The central

metal box (400 mm 3 400 mm) may be filled with soil in

order to conduct confined and confined-accelerated creep

tests. Each side of the equipment has both clamping and

loading systems. Consequently, the specimens (200 mm

wide) were symmetrically loaded from both sides with the

same load level by using deadweights. Additionally, a

pulley system was used in order to multiply the applied

deadweight load by a factor approximately equal to 5.75.

Since some friction may occur between different parts

of the new equipment, a pair of calibrated load cells were

used to determine precisely the tensile load on both sides

of the specimen. The entire loading system was calibrated.

In this procedure, several different deadweight levels were

used to establish their respective readings in each load

cell. Then their relationship was used to compute the

necessary deadweight to reach the desired load to be

applied to the specimen.

Readings from two telltales installed on the specimen

allowed the calculation of geosynthetic strains during each

test. The initial length (Li) between the two telltale

fixation points needed to be measured before the test.

Each telltale was linked to a calibrated linear variable

differential transducer (LVDT) by means of inextensible

steel wires. Readings from each LVDT were taken con-

Pressurized air

5

6

1

4

9

11

10

7

8

2

12

3

1 – Geosynthetic specimen

2 – Reinforced-lubricated portion ofthe specimen

3 – Upper part of testing chamber(confining medium and specimen)

4 – Lower portion of testing chamber(heating system)

5 – Pressurized air bag

6 – Roller grip

7 – Load cell

8 – Telltale weight

9 – LVDT

10 – Pulley system

11 – Dead weight

12 – Hydraulic jack

(a)

(b)

(c)

6

7

11

9

810

Temperaturecontroller

Cushion

Figure 3. Overall view of the new creep-testing equipment: (a) schematic side view; (b) general view; (c) close-up view of roller

grip and load cell position



tinuously during every test with a precision of 0.01 mm.

Therefore the displacement of each fixation point was

constantly measured and registered throughout the test.

Finally, specimen strains were calculated by means of the

equation

� ¼ dA þ dB

Li(2)

where � is the specimen elongation (in %); dA and dB are

the readings from each telltale (in mm); and Li is the

initial length between the two fixation points of the

telltales (in mm).

Since the new creep test device was designed to conduct

creep tests at elevated temperature, it is also equipped

with a heating system. This comprises two thermocouples,

an expanded polystyrene cover, and a heating chamber

filled with loose sand, in which a set of electric resistances

is installed. Thermocouple 1 (TC-1) is located near the

electric resistances, and is used to control the programmed

temperature set point inside the heating chamber. TC-1

can also be used to perform any heating path. The second

thermocouple (TC-2) is placed 20 mm above the speci-

men. Therefore the specimen temperature was considered

to be equal that read by TC-2. The whole heating system

was calibrated in order to determine the relationship

between the readings from each thermocouple. As a result,

the temperature near the specimen was accurately deter-

mined in each test.

Regarding confined creep tests, the confining stresses

were reproduced by means of a pressurized air bag placed

on top of the surrounding soil. The pressure inside the bag

was kept constant during the tests. Two pieces of high-

density polyethylene geomembrane were used to reduce

soil–geosynthetics friction on both sides. In addition, the

portions of each specimen in contact with the geo-

membrane were reinforced, covered with a polyester

Mylar1 sheet (0.075 mm thick, manufactured by

DuPont1), and lubricated. This procedure resulted in a

geosynthetic free length of 100 mm. Thus the specimen’s

width/length ratio was equal to 2:1.

3.2. Geosynthetics

Three different categories of geosynthetics were used in

this research. They were selected in order to provide a

wide variety of geosynthetic types. Geosynthetic A was a

biaxial polyester geogrid with 28 mm wide apertures.

Geosynthetic B was a polypropylene woven geotextile

with mass per unit area (ASTM D 5261) equal to

276 g/m2 (COV ¼ 3.03%) and thickness (ASTM D 5199)

equal to 0.94 mm (COV ¼ 4.21%). Both geosynthetics A

and B were tested in the machine direction. Geosynthetic

C was a polyester nonwoven geotextile manufactured with

needled-punched short fibers. The thickness (ASTM D

5199) and mass per unit area (ASTM D 5261) of

geosynthetic C were 2.59 mm (COV ¼ 6.80%) and

313 g/m2 (COV ¼ 4.64%), respectively. Table 1 sum-

marizes the geosynthetics’ information.

3.3. Confining media: soils

Confined creep tests were performed with two different

soils used as confining medium. Their grain size distribu-

tions curves (ASTM D 422; ASTM D 6637) and some

geotechnical characteristics (ASTM D 698; ASTM D 854;

ASTM D 4253; ASTM D 4254) are shown in Figure 4.

Soil A is pure sand (SP; ASTM D 2487), and was used at

45% of relative density in dry condition. Soil B is clayed

sand (SC; ASTM D 2487), and was compacted in the test

chamber to 90% of the maximum dry density, and at

standard Proctor optimum moisture content.

3.4. Tests

The first step in the creep testing was to perform short-

term, wide-width tensile tests (ASTM D 4595) using each

geosynthetic in order to determine their actual UTS and

strain at break. Thus the load level in every creep test is

referred to in this paper as a percentage of the respective

UTS.

The creep behavior of the geosynthetics was character-

ized in this research by means of four different types of

test: conventional, confined, accelerated, and confined-

accelerated creep tests. Conventional creep tests were

those standardized by ASTM D 5262, where an in-

isolation specimen was submitted to a constant tensile

load while axial strains were registered during the test. In

addition, conventional creep rupture tests were conducted

with geosynthetic A at different load levels. Three speci-

mens were used at each load level, and the average time

to break of each load level was computed in this set of

tests. Conventional creep tests were performed using

standard creep devices in order to allow a comparison of

their results with current practice.

Three types of creep test were performed with the new

equipment. Accelerated creep tests were conducted at

elevated temperature with in-isolation specimens. The

heating system was programmed to reach the desired

temperature set point, and the specimen was loaded after

Table 1. Geosynthetics subjected to creep tests in this research

Geosynthetic Description Mass per unit area

(ASTM D 5261) (g/m2)

Nominal thickness

(ASTM D 5199) (mm)

Tested direction

A Biaxial polyester geogrid, 28 mm wide apertures n/a n/a Machine direction

B Polypropylene woven geotextile 276 0.94 Machine direction

C Polyester nonwoven geotextile, needle-punched 313 2.59 Cross-machine direction

n/a: Not applicable.



temperature equilibrium was reached. Confined creep tests

refer to those where in-soil specimens were used in the

new creep test equipment. These tests were conducted at

room conditions (temperature and relative humidity),

which were kept within a strict range. Finally, the

confined-accelerated creep tests were conducted at ele-

vated temperature and under soil confinement, simulta-

neously.

A specific set of creep tests was performed using each

geosynthetic (Table 2). None of the tests reported in this

paper was conducted up to 10 000 h, as prescribed by

ASTM D 5262 and ISO/TR 20432 (ISO 2007). However,

these tests were clearly appropriate to compare the creep

response of the geosynthetics studied so far in this research.

Wide-width specimens (geotextiles 200 mm wide and

seven-rib geogrid specimen) were used in every creep test,

except for one confined creep rupture test performed with

geosynthetic A at 90% of UTS. In order to reach such a

high load with the new equipment, this test was conducted

with a three-rib specimen. For this reason, a specific set

of in-isolation creep tests was performed with both three-

rib and seven-rib specimens in order to compare their

creep behavior.

Creep data are represented by means of Equation 1 in

this paper. Therefore creep data were evaluated by com-

paring the specimens’ initial strain and parameters a and

b. Additionally, readings were taken at very short intervals

(0.1–1.0 s), but only a few points are shown in the plots

presented in this paper, to illustrate the geosynthetics’

overall creep behavior. Nonetheless, every regression line

is plotted based on the whole set of readings taken during

the test.

4. RESULTS AND DISCUSSION

4.1. Geosynthetic A

4.1.1. Wide-width tensile tests

Wide-width tensile tests (ASTM D 4595) conducted with

geosynthetic A in the machine direction resulted in a UTS

of 37.25 kN/m (COV ¼ 1.34%) and a strain at break equal

to 10.87% (COV ¼ 3.44%). The following sections pre-

0

20

40

60

80

100

0.001 0.01 0.1 1 10Particle diameter, (mm)d

Soil A

Soil B

Property Soil A Soil B

γs (kN/m³)

γd,max (kN/m³)

γd,min (kN/m³)

16.7 18.1

---

wopt (%)

15.0

16.0---P

artic

les

smal

ler

than

siz

e sh

own,

(

%)

n

26.826.7

Figure 4. Grain size distribution curves and geotechnical properties of the soils used as confining media

Table 2. Creep tests conducted with each geosynthetic, and their main characteristics

Geosynthetic Conventional

creep test

Accelerated

creep test

Confined

creep test

Confined-accelerated

creep test

A 10 h

20–97.5% of UTS

10 h

50% of UTS

43.08C

10 h

50 & 90% of UTS

Soils A and B

50 kPa

10 h

50% of UTS

Soil A: 50 kPa

44.18C

B 10 h

20–80% of UTS

10 h

30% of UTS

38.18C

10 h

30% of UTS

Soil A

50 kPa

None

C 100 h

20% of UTS

None 100 h

20% of UTS

30 kPa

100 h

20% of UTS

Soil A: 40 kPa

49.48C



sent the results of creep tests performed using this

material.

4.1.2. In-isolation creep tests

First, geosynthetic A was submitted to conventional creep

tests conducted up to 10 h. Table 3 presents the results

obtained from these tests at several different load levels

(20–80% of UTS). As expected, parameters a and b are

dependent on the load level. Figure 5 presents the results

obtained in conventional creep rupture tests with geosyn-

thetic A. It appears that the variability of the time to break

values is higher at higher load levels. This behavior is

expected due to material variability. Any variation in the

specimen UTS related to sample mean value causes a

large time to rupture variation at higher load levels. The

creep rupture tests allowed the plot of the initial portion of

geosynthetic A creep rupture curve (Figure 5). Both the

literature (Jewell and Greenwood 1988; Jeon et al. 2002;

Koerner 2005) and the standards (ASTM D 5262; ISO

2007) recommend that extrapolation of the creep rupture

curve should not exceed one order of magnitude; however,

this plot was used in this paper to compare the creep

rupture response of geosynthetic A under in-isolation and

in-soil conditions, as presented in the following section.

One single accelerated in-isolation creep test was

performed with geosynthetic A at 50% of UTS. This test

helped to evaluate the new creep-testing equipment con-

cerning this type of test. It was conducted at 43.08C �1.58C over 10 h, and resulted in values of a ¼ 8.824 and

b ¼ 0.141, respectively (Figure 6). Unsurprisingly, the

comparison between conventional and accelerated creep

tests shows that parameters a and b are both greater at

higher temperatures. Additional tests at elevated tempera-

ture between room temperature and 438C would allow the

plot of the creep master curve of geosynthetic A, which is

not in the scope of this paper. However, these preliminary

results show the capability of the new creep-testing

equipment to perform accelerated creep studies.

4.1.3. In-soil creep tests

The results from both confined and confined-accelerated

creep tests performed with geosynthetic A at 50% of UTS,

over 10 h, subjected to vertical stress equal to 50 kPa are

presented in Figure 7. Although the initial strain values are

quite different for soils A and B, soil confinement was

effective in reducing creep strains. Since parameter b

indicates the creep strain rate, it is easily noticed that the

strain rate in confined specimens is a small fraction of that

in conventional creep tests (about 3%). This emphasizes

that long-term strength taken from conventional tests,

Table 3. Logarithmic regression parameters of conventional creep tests using geosyn-

thetic A

Load level

(% of UTS)

Parameter a Parameter b Coefficient of

determination

Initial strain

(%)

20 2.186 0.068 0.975 1.91

30 3.769 0.124 0.984 3.26

40 5.212 0.076 0.981 4.90

50 6.784 0.099 0.988 6.38

60 8.033 0.115 0.994 7.56

70 8.926 0.168 0.992 8.24

80 10.232 0.220 0.988 9.33

Per

cent

age

ofU

TS

,/

(%)

T T

f

0.0001 0.001 0.10.01 1 10

TTf

� � �1.828ln 85.943t

0.055 h 1.383 h

80

82

84

86

88

90

92

94

96

98

100

Time, (h)t

Average time to break

Actual time to break values

In-soil creep rupture test

Regression line

R 2 0.944�

Figure 5. Creep rupture tests on geosynthetic A



where in-isolation specimens are used, may not reproduce

the field behavior of this geogrid (geosynthetic A).

Initial strains were approximately identical in in-soil

creep tests conducted using soil A as confining medium,

but the values of parameter b were noticeably different. As

can be seen Figure 7, the increase in temperature in the

confined-accelerated creep test (temperature equal to

44.1 � 0.58C) markedly increased the creep strain rate.

Table 4 presents parameters a and b obtained from these

tests, and also compares them with the results from

conventional creep testing.

One confined (50 kPa) creep rupture test at 90% of

UTS was performed in order to compare the geosynthetic

creep rupture time with that obtained in in-isolation creep

rupture tests. The mean time to break in in-isolation creep

rupture tests at 90% of UTS was equal to 0.055 h

(3.3 min), as presented in Figure 5. The time to rupture in

confined creep rupture tests under 50 kPa was 1.383 h

(about 83.0 min). Similarly to other confined creep tests,

this outcome may indicate that soil confinement markedly

reduced the creep strains of geosynthetic A.

0.01 0.1 1 10

e � 0.099 ln 6.7840.988t

R�

�2

ε � ��

0.141 ln 8.8240.996t

R 2

5.0

6.0

7.0

8.0

9.0

10.0

Axi

al s

trai

n,(%

)e

Time, (h)t

Conventional creep test

Accelerated creep test

Figure 6. In-isolation creep response of geosynthetic A loaded to 50% of UTS

0.01 0.1 1 10

Axi

al s

trai

n,(%

)ε

ε � ��

0.002 ln 6.7960.180t

R 2

ε 0.003 ln 8.8070.412

� ��

tR 2

ε t� ��

0.066 ln 6.9020.953R 2

5.0

6.0

7.0

8.0

9.0

10.0

Time, (h)t

Confined creep test in soil A

Confined creep test in soil B

Confined-accelerated creep test in soil A

Figure 7. In-soil creep response of geosynthetic A loaded to 50% of UTS (50 kPa of overburden stress)

Table 4. Logarithmic regression parameters obtained in

different creep tests performed using geosynthetic A at

50% of UTS

Description of test Parameter

a

Parameter

b

Initial

strain (%)

Conventional 6.784 0.099 6.38

Confined: soil A 6.796 0.002 6.79

Confined: soil B 8.807 0.003 8.79

Confined-accelerated: soil A,

temperature ¼ 44.18C

6.902 0.066 6.63



The confined creep rupture test required a specific

specimen configuration in order to subject it to the desired

load level. Thus a three-rib specimen was used instead of

a seven-rib one. Previous comparison between three-rib

and wide-width (seven-rib) specimens was performed by

means of in-isolation creep tests, at 50.2% of UTS. These

conventional creep tests were performed with a seven-rib

specimen different from that presented in Section 4.1.2.

This measure was taken in order to ensure that the three-

rib and seven-rib specimens were extracted as closely as

possible from each other in the geosynthetic sample.

Figure 8 presents the regression lines for both tests.

Although the initial strains were somewhat different, the

creep strain rates in the two tests were nearly the same.

The values of parameter b in these two tests were also

similar to that presented in Section 4.1.2. Therefore the

two specimen configurations were in agreement for this

purpose.

4.1.4. Comparison between in-isolation and in-soil creep

behaviors

It is suggested in the technical literature that soil confine-

ment has either a small or no effect on the creep behavior

of geogrids. However, the results obtained so far indicate

the converse of this. The creep strain rates of the in-

isolation specimens were expressively larger than the ones

obtained with in-soil specimen at the same temperature.

Thus it is clear that creep strains of geosynthetic A under

soil confinement are quite lower than those in in-isolation

tests. This emphasizes the suggestion of FHWA (1998)

that every geosynthetic should be characterized regarding

its in-soil creep behavior.

4.2. Geosynthetic B

4.2.1. Wide-width tensile tests

The UTS of geosynthetic B obtained by means of wide-

width tensile tests (ASTM D 4595) in the machine

direction was 50.93 kN/m (COV ¼ 4.19%), and its strain

at rupture was 14.84% (COV ¼ 10.93%). The results of

creep tests conducted using this material are presented in

the following sections.

4.2.2. In-isolation creep tests

Geosynthetic B was subjected to 10 h long conventional

creep tests at different load levels (20–80% of UTS).

Table 5 presents parameters a and b, and the coefficient

of determination obtained in these tests, and Figure 9

presents their semi-log plot. As expected, both initial and

creep strains were directly related to the loading level.

From Figure 9, it can also be seen that most of the tests

resulted in a non-linear plot on the semi-log scale (�against ln t ). This indicates the occurrence of tertiary

creep, as previously discussed. Nevertheless, only the

specimen loaded to 80% of UTS failed before 10 h of

conventional creep test. Since the linear portion of each

curve corresponds to primary and secondary creep, the

regression results presented in Table 5 correspond to this

portion of each curve.

One accelerated in-isolation creep test (30% of UTS)

was conducted with geosynthetic B at 38.18C (� 1.08C)

and was used to evaluate the new creep-testing machine

concerning accelerated creep tests (Figure 9). Parameters

a and b obtained in this test are also presented in Table 5.

It was performed for 10 h, and its results would allow one

to plot the in-isolation creep master curve for geosynthetic

B, together with additional creep tests. Similarly to con-

ventional creep data, accelerated creep results did not

present good agreement with a logarithmic regression.

4.2.3. In-soil creep tests

Since this paper presents preliminary tests with the new

equipment, only one single confined creep test has been

performed with geosynthetic B so far. Soil A was used as

the confining medium in this test. As described earlier,

the logarithmic regression did not lead to good agreement

when the full data set was considered, owing to the

Axi

al s

trai

n,(%

)ε

0.01 0.1 1 10

ε 0.095 ln 7.1660.963

� ��

tR 2

ε � ��

0.096 ln 6.6370.990t

R 2

5.0

6.0

7.0

8.0

9.0

10.0

Time, (h)t

Wide-width specimen

Three-rib specimen

Figure 8. Comparison of geosynthetic A creep response regarding the width of the specimen (wide-width and three-rib

specimens)



occurrence of tertiary creep. Thus a power function

represented by

� ¼ ctd (3)

was used in order to compare the in-isolation and in-soil

creep test results by means of parameters c and d. Here �is the specimen elongation (in %), c and d are regression

parameters, and t is the elapsed test time (in hours).

Similar to parameters b and a in Equation (2), parameter d

indicates how rapidly creep strains are occurring, and

parameter c shows the strain at unitary time. In addition,

Equation (3) may be used to calculate initial strain values.

The conventional creep test at 30% of UTS resulted in

c ¼ 5.361 and d ¼ 0.116, whereas the confined test (under

50 kPa of vertical stress) resulted in c ¼ 4.226 and

d ¼ 0.111. Both results are shown in Figure 10, where the

vertical axis is presented on a different scale than that in

Figure 9. Despite the differences in initial strain values,

the creep strain rates are quite similar in the two tests.

Therefore it may be concluded that soil confinement did

not affect creep strains in this woven geotextile. Further

in-soil creep tests are planned using geosynthetic B,

including confined-accelerated tests. These will allow a

confined-creep master curve to be plotted, and the long-

term creep behavior of geosynthetic B under soil confine-

ment to be predicted.

4.3. Geosynthetic C

Geosynthetic C was submitted to the shortest set of creep

tests. Therefore the results from these tests are presented

in one single subsection. The UTS of geosynthetic C was

equal to 17.24 kN/m (COV ¼ 4.02%) in the cross-machine

direction, and the strain at break was equal to 90.85%

(COV ¼ 1.71%), obtained in wide-width tensile tests

(ASTM D 4595).

The results of creep tests conducted with geosynthetic

C are presented in Figure 11. In the conventional creep

test, the initial strain was 32.2%. Subsequently, creep

strains occurred, which increased the total strain slightly

up to 35.3% after 100 h, at a constant rate (parameter

b ¼ 0.243). This small value of creep strain is justified by

the low tensile load level (20% of UTS). The confined

Table 5. Logarithmic regression parameters of in-isolation creep tests using geosynthetic

B

Load level

(% of UTS)

Parameter a Parameter b Coefficient of

determination

Initial strain (%)

20 4.024 0.389 0.999 2.43

30 5.367 0.550 0.998 3.12

40 7.683 0.707 0.999 4.79

50 9.194 0.834 0.998 5.78

60 9.925 0.898 0.998 6.25

70 11.742 1.072 0.998 7.35

80 13.554 1.373 0.999 7.93

30 (temperature ¼ 38.18C) 6.558 0.936 0.999 2.73

Note: These parameters refer to the linear portion of each curve (primary and secondary creep).

0.01 0.1 1 102

6

10

14

18

22

Axi

al s

trai

n,(%

)e

Time, (h)t

20% of UTS

30% of UTS

40% of UTS

50% of UTS

60% of UTS

70% of UTS

80% of UTS

30% of UTS, T 38.1°C�

Figure 9. In-isolation creep response of geosynthetic B



creep test with geosynthetic C was conducted at room

temperature, with vertical stress equal to 30 kPa. The

initial specimen strain in this test was 15.9%, which is

considerably less than that found in the conventional creep

test. In addition, the creep strain rate was also reduced

(parameter b ¼ 0.040). The reduction of both initial and

creep strains is due to soil confinement, as expected.

One single confined-accelerated creep test was con-

ducted at elevated temperature (49.4 � 1.58C). Concerning

both in-soil creep tests, the accelerated test presented the

highest value of initial strain (17.37%). It also showed the

highest value of creep strain (1.3%) and creep strain rate

(parameter b ¼ 0.065). This behavior was expected, since

temperature increases both strains and creep strain rate.

Table 6 shows parameters a and b obtained from each

test conducted with geosynthetic C. As expected, the

conventional creep test provided the highest value of

parameter b, which indicates its high creep strain rate. In

addition, evaluation of parameters a and b for each test

clearly indicated the influence of soil confinement on

creep strains. Unsurprisingly, both parameters a and b

obtained at elevated temperature were greater than those

at room temperature in in-soil creep tests.

Similarly to the other geosynthetics used in this re-

search, a creep master curve would be plotted with

additional creep tests in both in-isolation and in-soil

conditions, conducted at different temperature values.

5. CONCLUSION

This paper has presented a pioneering piece of equipment,

developed to conduct confined and accelerated creep tests,

either simultaneously or separately. Preliminary tests were

0.01 0.1 1 10

ε 5.3610.999

��

0.116

2t

R

ε ��

4.2260.995

0.111

2t

R

2

3

4

5

6

7

8

Axi

al s

trai

n,(%

)e

Time, (h)t

In-isolation specimen

In-soil specimen

Figure 10. Conventional and confined creep test of geosynthetic B loaded to 30% of UTS

0.01 0.1 1 10 100

ε 0.243 ln t 34.2450.990

� ��R 2

ε � ��

0.040 ln 16.3290.953t

R 2

ε ln 18.244t ��

0.0650.974R 2

12

15

18

21

24

27

30

33

36

Axi

al s

trai

n,(%

)e

Time, (h)t

Conventional creep test data

Confined-accelerated creep test data

Confined creep test data

Figure 11. Geosynthetic C creep test results



conducted with three different types of geosynthetics. The

following conclusions are drawn from the present study.

• The new creep-testing equipment was able to

perform both in-isolation and in-soil creep tests. In

addition, the two approaches were used simulta-

neously, introducing the confined-accelerated creep

test.

• Soil confinement considerably reduced the creep

strain of both the geogrid and the nonwoven

geotextile. Concerning the nonwoven geotextile, this

reduction is widely mentioned in literature; however,

creep strain reduction in geogrids due to soil

confinement is not usually mentioned in the

technical literature.

• Concerning the geogrid (geosynthetic A), both soils

A and B were effective in reducing creep strains

under soil confinement. Also, one confined creep

rupture test conducted with geosynthetic A showed

an increase in time to break, owing to soil

confinement.

• The creep behavior of geosynthetic B (woven

geotextile) was not affected by soil confinement. The

in-isolation and in-soil creep tests using the woven

geotextile showed similar values of creep strain rate,

under the same tensile load level.

Soil confinement is not considered to be effective in

reducing creep strains of geogrids, and is considered to be

hardly capable of that in woven geotextiles. However, it

has a pronounced influence on the creep behavior of

nonwoven geotextiles, since it mitigates the necking

process that occurs in in-isolation specimens. Necking is

decidedly reduced in confined specimens, because of the

restriction in filament movements. Consequently, creep

strains are strongly decreased, as presented in the results

for geosynthetic C.

FHWA (1998) suggested that every geosynthetic should

be submitted to such studies before their use. The results

for geosynthetic A support this statement. In contrast with

the literature, creep strains in tests performed with this

geogrid were also drastically reduced by soil confinement,

which may have occurred because of its structure. Aper-

tures in geogrids may allow a different type of soil–

geosynthetic interaction during confined creep tests. Thus,

under soil confinement, it may reduce long-term strains.

On the other hand, the woven geotextile presents neither

apertures nor necking behavior: therefore its creep behav-

ior is not noticeably affected by soil confinement.

Additional tests are necessary to confirm these results,

to evaluate the new equipment further, and to creep master

curves to be plotted. However, the tests performed so far

show that the new equipment is capable of successfully

addressing both the negative aspects of conventional creep

tests, either simultaneously or separately. It allowed more

reliable and cost-effective creep tests to be performed.

ACKNOWLEDGEMENTS

The authors would like to thank the State of Sao Paulo

Research Foundation (FAPESP) and the Coordination for

the Improvement of the Higher Education Personnel

(CAPES) for the financial support of this research. The

authors are also grateful to J. G. Zornberg for his help,

discussions and advice.

NOTATIONS

Basic SI units are given in parentheses.

a, b logarithmic regression parameters

(dimensionless)

c, d power function regression parameters

(dimensionless)

dA, dB displacements of telltale fixation points

(m)

Li initial length between telltale fixation

points (m)

R2 coefficient of determination

(dimensionless)

T tensile load applied during creep tests in

geosynthetics (N/m)

Tf tensile load at failure obtained in short-

term tensile tests according to ASTM D

4595 and ASTM D 6637 (N/m)

t elapsed time (s)

wopt optimum water content (%)

ªd,max maximum index unit weight (N/m3)

ªd,min minimum index unit weight (N/m3)

� axial strain (dimensionless)

�cr creep strain (dimensionless)

�i initial strain (dimensionless)

�1 primary creep strain (dimensionless)

�2 secondary creep strain (dimensionless)

�3 tertiary creep strain (dimensionless)

ABBREVIATIONS

ASTM American Society for Testing and

Materials

COV coefficient of variation

Table 6. Logarithmic regression parameters of creep tests using geosynthetic C

Creep test type Parameter a Parameter b Coefficient of

determination

Initial strain

(%)

Conventional 34.245 0.243 0.990 33.25

Confined 16.329 0.040 0.953 16.16

Confined-accelerated 18.244 0.065 0.974 17.98



GRS geosynthetic-reinforced soil

LVDT linear variable differential transducer

TC-1, TC-2 thermocouples 1 and 2

TTS time-temperature superposition

UTS ultimate tensile strength

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