Shinoda & Bathurst v22 G&G 2004

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Geotextiles and Geomembranes 22 (2004) 205222

Lateral and axial deformation of PP, HDPE and

PET geogrids under tensile load

Masahiro Shinodaa, Richard J. Bathurstb,*a

Railway Technical Research Institute, Foundation and Geotechnical Engineering Structures, TechnologyDivision, 2-8-38, Hikari-cho, Kokubunji-shi, Tokyo 185-8540, JapanbGeoEngineering Centre at Queens-RMC, Civil Engineering Department, Sawyer Building, Room 2085,

Royal Military College of Canada, 13 General Crerar, Kingston, Ont., Canada K7K 7B4

Received 20 March 2003; received in revised form 10 March 2004; accepted 15 March 2004

Abstract

The paper describes a series of short-term in-isolation tensile tests that were carried out to

investigate the loadstraintime performance of typical knitted polyester (PET), biaxial

polypropylene (PP) and uniaxial high-density polyethylene (HDPE) geogrid products. The

specimens were subjected to constant rate of strain (CRS) loading and variable rate of strain

(VRS) loading. Variables between test specimens included specimen length and specimen

aspect ratio (e.g. number of ribs). A novel feature of these tests was that internal strains in the

specimens were recorded using non-contact measurements taken with a video-extensometer

apparatus. The data shows that at large strains the PP and HDPE geogrid specimens exhibited

large lateral strains while lateral strains were negligible for the PET product. Results of CRS

tests carried out with different specimen aspect ratios showed that there was no change in

tensile strength, axial strain and lateral strain at rupture for HDPE and PET materials. The PP

material in this investigation did show an increase in axial and lateral strain at rupture for a

specimen tested at an aspect ratio approaching one. The polyolefin geogrids (PP and HDPE)were shown to exhibit tensile stiffness and ultimate tensile load capacities that increased with

rate of strain. Implications of the contractive behaviour of the polyolefin geogrid products to

the selection of in-soil reinforcement stiffness values and rheological modelling are identified.

r 2004 Elsevier Ltd. All rights reserved.

Keywords: Geosynthetics; Geogrids; Tensile strength; Axial strains; Lateral strains; Stiffness; Video-

extensometer

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*Corresponding author. Tel.: +1-613-541-6000 ext 6479/6347/6391; fax: +1-613-545-8336.

E-mail address: [email protected] (R.J. Bathurst).

0266-1144/$ - see front matterr 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.geotexmem.2004.03.003


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1. Introduction

In-isolation tensile loading of geosynthetic reinforcement products according to

ASTM test methods D 4595, D 6637 and D 5262is routine practice to characterisethe loadstrain properties of these materials.

The importance of measurement of tensile load-strain properties of geotextiles and

geogrids under soil confinement has been noted in the literature ( McGown et al.,

1982, 1994, El-Fermaoui and Nowatzki, 1982, Siel et al., 1987, Christopher et al.,

1986, Leshchinsky and Field, 1987). One important finding from in-soil tests was

that there can be a significant increase in the stiffness and strength of some

geotextiles confined in soil compared to the unconfined condition (Ling et al., 1992,

Boyle et al., 1996).Yuan et al. (1998)performed in-soil constant rate of strain tests

on woven and nonwoven polypropylene (PP) geotextiles, an extruded high-density

polyethylene (HDPE) geogrid and a woven polyester (PET) geogrid. Walters et al.

(2002)reinterpreted their test results and concluded that soil confinement has a large

effect on the secant stiffness values (measured at 5% axial strain) for nonwoven

geotextiles (an increase of up to 500% depending on the confining pressure and soil

type), a small but measurable influence on the stiffness values of woven geotextiles

and geogrids (an increase of 530% over the in-isolation value), and no influence on

the extruded HDPE geogrid stiffness response.

In-isolation tensile tests on nonwoven geotextiles show that necking or lateral

contraction occurs during axial extension (Rowe and Ho, 1986). Surprisingly, the

onset and magnitude of lateral contractions for both geotextiles and geogridreinforcement products during testing has not been the subject of quantitative

investigation. Geogrid reinforcement products are routinely used in reinforced soil

retaining wall construction. The selection of an appropriate reinforcement stiffness

value has been demonstrated to be a precursor to the correct estimation of

reinforcement loads from in situ strain readings with consideration of the duration of

loading (Allen and Bathurst, 2002). Lateral contraction of geogrids under tensile

load is, therefore, of interest if the argument is accepted that constrained lateral

deformation of these materials in-soil can lead to a stiffer axial response of these

materials compared to the same response in air. Evidence of lateral contraction of

typical geogrid reinforcement products tested in-isolation and the axial strain levelsat which lateral contraction is initiated could lead to the need to re-examine current

assumptions regarding the applicability of in-isolation tensile test results to the in-

soil environment.

2. Objective

The principle objective of the investigation described here was to examine the in-

isolation lateral and axial deformation of three typical geogrid reinforcementproducts up to rupture using different specimen dimensions and different axial

loading paths and loading rates. A novel feature of the testing program was the

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M. Shinoda, R.J. Bathurst / Geotextiles and Geomembranes 22 (2004) 205222206


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measurement of local axial and lateral displacements using a non-contact video-

extensometer technique (Shinoda and Bathurst, 2004).

3. Test program

3.1. Geogrid materials

Specimens of biaxial PP geogrid, uniaxial HDPE geogrid and a knitted PET

geogrid were used in the current investigation (Table 1). These materials are typical

geogrid reinforcement products used in soil reinforcement applications. All three

materials have been tested previously at RMC as part of an on-going program of

full-scale testing of retaining walls and slopes (Bathurst et al., 2002). The visco-elasticbehaviour of the PP geogrid material used in this study has also been investigated by

Kaliakin et al. (2000)andThornton (2001).

3.2. Loading apparatus

A MTS servo-controlled hydraulic actuator with a stroke of 160 mm and rated to

100 kN was used to carry out the tensile load tests. The controller for this actuator

provided strain rate and load control functions and the ability to switch between

modes without interruption. Roller clamps described by Bathurst and Cai (1994)

were used to grip the geogrid specimens (Fig. 1a). A load cell mounted between the

actuator piston and top loading clamp was used to record the tensile load during

each test. Axial displacements at the top clamp were recorded by an LVDT mounted

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Table 1

Geogrid properties

Property Materials

Polymer type PP PET HDPEStructure Punched sheet and drawn Knitted Punched sheet and drawn

Coating Uncoated PVC Uncoated

Mass/unit area (g/m2) 215 114 NA

Aperture size (mm)

Machine direction 25 27 140

Cross-machine direction 33 22 15 (maximum)

Thickness (mm)

At longitudinal member 1.0 1.1 1.0

At junction 2.9 1.2 2.7

Wide-width tensile strength (kN/m)

At 5% strain 8.3 4.4 35.7

Ultimate 12.5 17.5 68.9

NA = Not available; PP = polypropylene; PET = polyester; HDPE = high-density polyethylene; PVC

= polyvinyl chloride. Sourcemanufacturers literature.

M. Shinoda, R.J. Bathurst / Geotextiles and Geomembranes 22 (2004) 205222 207


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in the MTS actuator piston. A 0.2-kN-preload was applied to each specimen prior to

testing to remove any initial slack in the specimen, particularly at the roller clamps.

All tests were carried out at 2072C consistent with related ASTM specifications.

3.3. Strain monitoring

A high-resolution digital Charged Coupled Device (CCD) camera (video-

extensometer) and ancillary data acquisition system were used to record specimen

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Fig. 1. General arrangement of wide-width strip tensile loading apparatus, test specimens and strain

monitoring points on specimens. Note: Targets on HDPE specimens placed vertically from midpoint oftransverse member to midpoint of longitudinal members. (a) Cross-section view of test apparatus, (b) PP

specimen, (c) PET specimen, (d) HDPE specimen.



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axial load and displacements of targets mounted directly on longitudinal members of

the test specimens. An advantage of this approach is that local displacements of

geogrid points can be tracked in XandYdirections at locations well removed from

the specimen clamps. Another advantage of this equipment is that specimen responseis not influenced by the measurement technique as may be the case for clip-on

extensometers and strain gauges glued directly to the surface of geosynthetic test

specimens. In this investigation the targets comprised 2-mm-diameter painted white

circles. The video-extensometer camera and software locks on to points defined by

the contrast between the white targets and the black geogrid surface. The Xand Y

displacements of each monitoring point are recorded and specimen strains calculated

from selected pairs of targets. The system used in this investigation is capable of

tracking multiple points at a frequency of up to 25 Hz and plotting the displacements

and computed strains against load in real time. The resolution of the measurements

is a function of the size of the field of view. In this investigation the measurements

were taken at a resolution of 0.64mm. The ease of use, flexibility and high resolution

of strain measurements using a video-extensometer system has been demonstrated by

Jones (2000) who used similar equipment to monitor load-extension tests on

geotextiles. Additional details of the equipment and monitoring technique used in

the current study have been reported byShinoda and Bathurst (2004).

Preliminary tests showed that axial and lateral strains calculated from video-

extensometer targets placed on adjacent junctions or at points midway between

junctions of PET and PP specimens gave the same values indicating that strain

distributions were uniform for these materials. Local axial tensile strains reportedhereafter were calculated using displacement measurements for targets a and b

and lateral strains using targets a and d located close to the centre of PET and

PP specimens (Fig. 1b and c). Non-uniform strain distributions were observed for

the HDPE specimens due to local variation in the cross-sectional area and modulus

of the drawn polymer material. Non-uniform axial strain distributions in thick

uniaxial drawn HDPE geogrids have also been noted by McGown et al. (1994). In

the current investigation, six targets were placed on the surface of the HDPE

specimens in the pattern illustrated in Fig. 1d. Targets a and d were located at

approximately the midpoint of the longitudinal members between transverse

members. Average axial tensile strains were calculated using deformations recordedbetween points a and c, and d and f. Lateral strains were calculated by

taking the average of horizontal deformations for pairs of points at the same

elevation on the specimen. This array of six targets allowed average strains

corresponding to one aperture spacing in longitudinal and transverse directions to be

calculated as well as to investigate the distribution of lateral strains between

longitudinal members.

3.4. Loadextensiontime paths for tensile tests

3.4.1. Constant rate of strain (CRS) testsAs part of the development of the experimental methodology, reference

conventional CRS tests at 10% strain/minute (based on cross head speed) were

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carried out on each material type (Fig. 2a). These tests were repeated three times to

confirm that test results were repeatable (Shinoda and Bathurst, 2004). In the current

investigation, two series of CRS tests were performed on specimens trimmed to

different lengths and widths. The first series comprised 200-mm-wide PP and PET

geogrid specimens trimmed to lengths of 224, 314, 432 and 540 mm and 226, 286, 395and 496 mm, respectively. A second series of tests was carried out at 10% strain/

minute on specimens trimmed to a constant length of approximately 200 mm but

with different widths (representing different number of longitudinal members). The

PP specimens were prepared with 2, 4, 5 and 6 longitudinal members. Four PET

geogrid specimens were prepared with 2, 4, 6 and 8 longitudinal members and six

HDPE geogrid specimens with 2, 4, 6, 8, 9 and 10 longitudinal members. The

influence of strain rate on PP, PET and HDPE geogrid specimen response was

investigated by applying strain rates of 0.1, 1.0, 10 and 100% strain/minute up to

rupture. In some cases the actuator stroke was exceeded before rupture could be

achieved.

3.4.2. Variable rate of strain (VRS) tests

The results of variable rate of strain load testing have been used byHirakawa et al.

(2003) to verify an advanced three-component rheological model for different

polymer geogrid reinforcement materials. The influence of variable strain rate during

load-extension of PP, PET and HDPE geogrid specimens was investigated in the

current study by changing the axial strain rate (based on cross head speed) during the

test (Fig. 2b). The strain rate was varied in the sequence of 10, 1.0, 0.1, and 1.0%

strain/min to rupture with each strain rate increment held for about 20, 450 and4500 s, respectively for the PP and PET specimens, and 20, 250 and 2600 s for the

stiffer HDPE specimens.

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Time

Axial

strain

Time

Axialload

Time

Axialstrain

Time

Axialload

(a) (b)

Fig. 2. Loadextensiontime paths for test program. (a) Constant rate of strain (CRS) test, (b) variable

rate of strain (VRS) test.



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4. Test results

4.1. Influence of specimen dimensions on loadstrain response

Fig. 3 shows the influence of specimen length on load-extension results for

specimens with constant width (200 mm). The tensile strength at rupture (or large

strain) was reasonably constant between specimens of each type and did not show a

dependence on aspect ratio (Fig. 3a). With the exception of the shortest test

specimen, the PP specimens were not taken to rupture due to the stroke capacity of

the actuator. The average tensile strength of the PP and PET specimens was 12.9 and

17.3 kN/m, respectively, which are close to the manufacturers reported values of

12.5 and 17.5 kN/m based on wide-width strip tensile tests (ASTM D 4595)(Table

1). Small deviations from loadstrain response using mechanical extensometers and

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0 10 20 30 405

0

-5

-10

-15

PET geogrid

PP geogrid

X = Specimen rupture0.40 (6)

0.51 (6)

0.70 (6)

0.88 (6)

0.37 (8)

0.64 (8)0.46 (8)

0.90 (8)

Lateralstrain(%)

Axial strain (%)

0 10 20 30 400

5

10

15

20

Width to length ratio

Number of longitudinal members

X = Specimen rupture

0.37 (8)

0.46 (8)

0.64 (8)

0.90 (8)

PP geogrid

PET geogrid

0.88 (6)0.70 (6)0.51 (6)

0.40 (6)

Ax

ialload(kN/m)

Axial strain (%)(a)

(b)

Fig. 3. Influence of specimen length on CRS load-extension results for specimens with constant width

(200 mm). (a) Axial load versus axial strain, (b) lateral strain versus axial strain.



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the non-contact measurement technique employed here may be one source of the

discrepancy as demonstrated byShinoda and Bathurst (2004). Another source of the

difference may be variations in material properties between product rolls. The load-

extension response of the specimens of each material over the monitored axial strainrange were sensibly the same and appear to be independent of the aspect ratio of the

specimens for these CRS tests.Fig. 3bshows plots of lateral strain versus axial strain

for the same tests. The PET specimens exhibited very little lateral contraction

(maximum of 1.5%) while lateral strains for the integral PP specimens were

pronounced after about 10% axial strain. At axial strains >10%, the lateral strains

varied linearly with increasing axial tensile strain. The axiallateral strain responses

of the PP specimens over the range of strains investigated (i.e. local slope of the

curves) were independent of specimen dimensions.

Fig. 4 shows the influence of specimen width on load-extension results for

specimens with constant length (200 mm). The rupture strengths of the specimens of

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0 10 20 30 400

5

10

15

20


Number of longitudinal members


0.88 (8)

0.53 (6)

0.36 (4)

0.13 (2)

0.90 (6)

0.79 (5)0.60 (4)

0.21 (2) PP geogrid

PET geogrid}PP and PET geogrid

Axial

load(kN/m)

Axial strain (%)

0 10 20 30 405

0

-5

-10

-15

0.88 (8)

0.53 (6)0.36 (4)0.13 (2)

0.90 (6)

0.79 (5)0.60 (4)0.21 (2)

PP and PET geogrid

PP geogrid

PET geogrid


Lateralstrain(%)

Axial strain (%)

(a)

(b)

Fig. 4. Influence of specimen width on CRS load-extension results for specimens with constant length

(200 mm). (a) Axial load versus axial strain, (b) lateral strain versus axial strain.



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both types were the same (i.e. independent of specimen width) (Fig. 4a). However,

the tensile strain at rupture for the widest PP specimen (180 mm0.9 aspect ratio)

was notably larger (25% axial strain) than the narrower specimens (E 12% axial

strain).Fig. 4bshows that the lateral contraction of the PP specimens was greater forthe specimens with a width less than 180 mm (0.9 aspect ratio) at the same axial

strain. The greater lateral contraction (i.e. greater aperture distortion) may be the

reason that the narrower specimens failed at a lower axial strain. The load-extension

curves for the PET specimens were independent of specimen dimension (Fig. 4a) and

lateral contractions were small (p0.05%) (Fig. 4b). The largest contraction at

rupture was recorded by the widest specimen with 8 longitudinal members.

Nevertheless, this value is about one-half of the values reported for the longer

specimens in the previous data set. The small amount of contraction recorded for the

PET specimens in both data sets is thought to be due to straightening of the multiple

longitudinal filaments during axial extension and appears to increase with specimen

length and to a lesser degree with specimen width.

The results of CRS tests on HDPE specimens with constant length but variable

width are plotted inFig. 5. No significant systematic variation in the load-extension

response and load or strain values at rupture can be seen in Fig. 5a. The average

lateral strains recorded for the same tests are plotted in Fig. 5b. Lateral strains at

rupture were reasonably constant for all specimens and varied from 40% to 80% of

the axial strains at rupture. Otherwise there was no consistent trend in the average

lateral strain at rupture with specimen width. The lateralaxial strain curve for the

narrowest specimen shows apparent dilation at small axial strains. However, this isthought to be an erroneous result related to the difficulty that was encountering

securing the very narrow specimen in the large roller clamps used in this

investigation.

Fig. 6ashows a summary of the rupture strengths for all specimens versus aspect

ratio. Taken together, the data shows that tensile strength at rupture was

independent of specimen dimensions for each of the three material types tested.

Fig. 6bsummarises axial strain at rupture versus aspect ratio for all tests. For PET

and HDPE specimens the strain at rupture was reasonably constant. The widest PP

specimen showed a very much higher strain at rupture compared to narrower

specimens. Fig. 6c shows that the trend in data noted above for axial strains atrupture is preserved for the lateral strains at rupture with only the PP material

showing a large increase in lateral strain at rupture for the widest specimen.

4.2. Influence of strain rate on loadstrain response

The load-extension results discussed in this section correspond to CRS and VRS

tests carried out on specimens of all three material types prepared to the same

dimensions (i.e. 220 mm long and 200 mm wideaspect ratio=0.9). The continuous

curves in the figures correspond to CRS tests carried out at a constant strain rate

(Fig. 2a). The stepped curves correspond to VRS tests carried out at variable targetstrain rates (measured at the cross-head) of 10, 1.0, 0.1 and 1.0% strain/min to

rupture (Fig. 2b). The small differences in magnitude between target strain rate

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values and those noted on the following figures is that the latter were recomputed

after the specimens had been subjected to the initial preload while the former werebased on the initial distance between the cross-heads prior to preloading. For the

purpose of this investigation, these differences are considered inconsequential. The

constant VRS loadstrain responses (Fig. 7a and b) show that the polyolefin

materials were very sensitive to rate of loading and axial strains at rupture decreased

with increasing rate of strain. The variable strain rate VRS tests showed that with

decreasing strain rate increments the loadstrain segments rapidly approached and

fell on top of the corresponding constant rate of strain curves. The corresponding

data for the PET specimens shows that the loadstrain response of the PET material

was sensibly independent of the strain rate-time loading path. The observations for

axial load axial-strain plots for CRS and VRS data described here also hold true forthe corresponding axial load versus lateral strain curves but are not reproduced here

for brevity.

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0 5 10 152

0

-2

-4

-6

-8

-10

0.66 (10)

0.58 (9)

0.52 (8)

0.40 (6)

0.25 (4)

0.12 (2)

X = Specimen ruptureHDPE geogrid

Av

eragelateralstrain(%)

Axial strain (%)

0 5 10 150

20

40

60

80

Number of

longitudinal mem bers



0.66 (10)

0.58 (9)

0.52 (8)

0.40 (6)

0.25 (4)

0.12 (2)HDPE geogrid

Axialload(kN/m)

Axial strain (%)(a)

(b)

Fig. 5. Results of CRS tests on HDPE specimens with constant length but variable width. (a) Axial load

versus axial strain, (b) average lateral strain versus axial strain.



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0.0 0.2 0.4 0.6 0.8 1.00

20

40

60

80

* Average value

PP geogrid*12.8 kN/m

Aspect ratio

*17.2 kN/m

*60.9 kN/m

PET geogrid

HDPE geogrid

Tensilestrength(kN/m

)

0.0 0.2 0.4 0.6 0.8 1.00

10

20

30

PET geogrid

*9.1 %

*15.9 %

PP geogrid

* Average value

Aspect ratio

HDPE geogridAxialstrainatrupture(%)

(a)

(b)

(c)

0.0 0.2 0.4 0.6 0.8 1.05

0

-5

-10

-15

-20

* Average value

PP geogrid

Aspect ratio

*6.6 %

*0.5 %

HDPE geogrid

PET geogridLateralstrainatruptu

re(%)

Fig. 6. Influence of specimen aspect ratio on tensile strength and strains at rupture. (a) Tensile strengthversus aspect ratio, (b) axial strain at rupture versus aspect ratio, (c) lateral strain at rupture versus aspect

ratio.



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0 10 20 30 40

0

5

10

15

20

99.3% strain/minute

10.0% strain/minute

1.0% strain/minute

0.1% strain/minute

1.1% strain/minute

0.1% strain/minute

1.1% strain/minute

10.6% strain/minute

PP geogrid X = Specimen rupture

Axialload(kN/m)

Axial strain (%)

0 5 10 15 20 250

20

40

60

80

98.0% strain/minute

10.1% strain/minute

1.0% strain/minute

0.1% strain/minute

10.3% strain/minute

1.0% strain/minute

0.1% strain/minute

1.0% strain/minute

10.3% strain/minute

HDPE geogrid


Axia

lload(kN/m)

Axialload(kN/m)

Axial strain (%)

0 10 20 30 400

5

10

15

20

103.4% strain/minute

11.1% strain/minute

0.9% strain/minute

0.1% strain/minute

9.6% strain/minute

1.0% strain/minute

0.1% strain/minute

9.5% strain/minute

1.0% strain/minute

PET geogrid


Axial strain (%)

(a)

(b)

(c)

Fig. 7. Influence of strain rate and strainrate path on load-extension response (a) PP geogrid specimens,

(b) HDPE geogrid specimens, (c) PET geogrid specimens.



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The general trend in the lateral strainaxial strain CRS curves for the biaxial PP

specimens (Fig. 8a) was increasing magnitude of lateral strain with increasing axial

strain rate taken at the same axial strain value greater than 10% (i.e. the material

lateral stiffness decreased with increasing strain rate, or equivalently, necking of thespecimens increased). This trend was reversed for the HDPE material (Fig. 8b) which

may be due to the very different structure of the uniaxially drawn material and the

effect of averaging the lateral strains over three different rows of monitoring points.

The VRS curve segments for the same tests that are superimposed on the figures do

not consistently fall on the corresponding CRS test curves. For example, the VRS

curves continue to track over the 10% strain/min CRS curve regardless of axial

strain rate. Lateral strain response of the PET specimens was small (p0.08%) and

strain-rate path effects were not detectable at this level of deformation (Fig. 8c).

The variation in normalised tensile strength at rupture versus axial strain rate from

CRS tests is summarised in Fig. 9. The data shows a general trend of increasing

strength with increasing strain rate for polyolefin materials and less strain-rate

sensitivity for the PET material.

5. Conclusions

The influence of specimen dimensions and strain rate on the loadextension

response of geotextiles in wide-width tensile testing performed in-isolation has been

reported by others (Rowe and Ho, 1986, Shrestha and Bell, 1982). Similarinvestigations of the loadstrain response of geogrids with different specimen

dimensions and the influence of specimen geometry on strength and strains in both

lateral and axial directions have not been reported in the literature to the best of the

writers knowledge. It appears that lateral contractions of typical geogrid

reinforcement products have been assumed to be negligible in magnitude and,

perhaps for this reason, of little practical interest in the area of soil-reinforcement

interaction. This paper describes the results of a series of careful tests carried out to

examine the local load-extension response of three typical geogrid reinforcement

(integral PP and HDPE geogrids and one knitted PET geogrid) in both axial and

lateral directions. The specimen strain measurements were made using a non-

contacting video-extensometer technique. The following observations were made:

(1) For the range of strain rates investigated, the strength at rupture of the PP,

HDPE and PET specimens was not sensitive to specimen aspect ratio.

(2) The axial strain and lateral strains at rupture for the HDPE and PET specimens

were also independent of specimen dimensions. However, the axial and lateral

strains at rupture were greater for the PP geogrid specimen with the largest width

(180 mm or 6 longitudinal members) compared to narrower specimens.

(3) Lateral strains at rupture were about 6080% of the axial strains at rupture for

the HDPE material and about 4% of the axial strain values at rupture for thePET materials. The lateral strains at rupture for the PP specimens varied from

about 3050% of the axial strain at rupture.

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0 10 20 30 405

0

-5

-10

-15

99.3% strain/minute

10.0% s train/minute

1.0% strain/minute

0.1% strain/minute

Test result with

variable strain rate

PP geogrid


Lateralstrain(%)

Axial strain (%)

0 10 20 30 405

0

-5

-10

-15

98.0% strain/minute

10.4% strain/minute

1.0% strain/minute

0.1% strain/minute

Test result with variable strain rate

HDPE geogrid X = Specimen rupture

Lateralstrain(%)

Axial strain (%)

(a)

(b)

(c)

0 5 10 15 202

0

-2

-4

11.1% strain/minute

0.9% strain/minute

Test result with variable strain rate

X = Specimen rupturePET geogrid

Lateralstrain(%)

Axial strain (%)

Fig. 8. Lateral strainaxial strain response from CRS load-extension tests. (a) PP geogrid specimens, (b)

HDPE geogrid specimens, (c) PET geogrid specimens.



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(4) Loadstrain response in both orthogonal directions for the polyolefin materials

was observed to be rate-of-strain dependent while the PET material was not.

(5) Lateral contraction of the widest PP specimens (180 and 200 mm wide) was

observed to be negligible at strains less than about 810% axial strain but to

increase almost linearly with axial strains thereafter.

(6) Lateral contraction (or necking) of the HDPE specimens was initiated at theonset of the load-extension tests.

(7) Variable rate-of-strain loadstrain paths for the polyolefin materials were

observed to fall on the corresponding curves for constant rate-of-strain

tests provided each VRS load segment was maintained for a sufficient length

of time.

6. Implications

The results of this investigation have implications to the understanding of soil-

structure interaction for the types of reinforcement used in this study when used in-

situ and consequently for design. Fortunately, for ultimate limit state design of

reinforced structures in which the reinforcement selection is based on a reference in-

isolation rupture strength of the material (e.g.ASTM D 6637), the rate of strain and

the aspect ratio of the specimens do not appear to be a factor. However, for

serviceability design the conversion of strain to load requires selection of a suitable

stiffness value. The polyolefin materials in the current study have demonstrated rate-

of-strain dependence under both CRS and VRS load paths which makes the

selection of a suitable time-dependent stiffness value problematic. Walters et al.(2002) gave examples that showed the equivalent rate-of-strain loading of

reinforcement layers in actual walls was 4 or 5 orders of magnitude slower than

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0.1 1 10 1000.50

0.75

1.00

1.25

1.50

PP geogridsPET geogrids

HDPE geogrids

Tensilestrength/

Tensilestrengthat10%s

train/minute

Strain rate (%)

Fig. 9. Normalised tensile strength at rupture versus axial strain rate from CRS load-extension tests.



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that of the standard CRS test carried out at 10% strain/min (ASTM D 4595, D

6637). Walters et al. proposed that for design purposes, CRS tests could be carried

out at different strain rates and this data used to calculate a 2% secant stiffness

value (J2%) as a function of time. This value can then be used to calculate anend-of-construction reinforcement load (or post-construction loads at a prescribed

design life).

The rate of loading of a reinforcement layer is also an issue for seismic design

(Bathurst and Alfaro, 1996). The stiffness, rupture strength and strain at rupture are

functions of rate of strain for polyolefin materials as demonstrated in the tests

reported here. Of particular importance is the trend toward lower rupture strains

with increasing rates of axial strain.

The loadlateral strain response records reported here give indirect evidence of the

possible influence of soil confinement on reinforcement stiffness. Reinforcement

contractions observed in-isolation may be expected to be restrained in-soil due to soil

confinement, particularly in a stiff dense soil. Using this line of argument suggests

that for PP geogrids of the type investigated here, the confinement effect may not be

pronounced until axial strains have achieved strains in excess of (say) 10% (e.g.

Fig. 8a). Fortunately, this is well beyond any serviceability strain limit for design.

Allen and Bathurst (2002) have estimated that a maximum reinforcement strain of

about 3% is required to generate contiguous failure zones in reinforced soil retaining

walls constructed with high-quality and well-compacted granular soil backfills and

proposed that this strain level be used for design against a soil failure limit state. The

3% reinforcement strain associated with this limit state is much less than the strainlevels observed in this investigation to generate contractive behaviour in the PP

geogrid specimens. However, for HDPE geogrids, in situ stiffness increases due to

constrained contraction may develop at the onset of axial load (e.g. Fig. 5b). Hence,

in-soil stiffness values may be higher at all load levels for this material under

operational conditions. For the knitted PET geogrid tested in this study there were

no significant lateral contractions of the specimens and hence restrained contraction

is not a likely contributor to increased in-soil soil stiffness for this material.

Nevertheless, other mechanisms such as the interaction between geogrid transverse

members and the confining soil (i.e. passive bearing) and interface friction between

the surfaces of the geogrid members have been identified as contributors to pulloutcapacity of integral drawn polyolefin reinforcement products (Palmeira and

Milligan, 1990). The structure of these materials and their interaction with the soil

may also play a part to increase in-soil stiffness of these materials that is independent

of the restrained contraction effect described above.

Finally, the results of the tests reported here have implications to the development

of rheological models for planar polyolefin geogrid reinforcement materials such as

the products used in this investigation. Advanced loadstraintime models for

planar geosynthetic reinforcement products are exclusively one-dimensional. The

load-extension data for the PP and HDPE geogrid products reported in this study

have demonstrated a pronounced two-dimensional response under axial tensileloading that should be considered if rheological models are to be used to predict

reinforcement behaviour, particularly at large strains.

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Acknowledgements

The first author is grateful for the financial support from Integrated Geotechnical

Institute Limited of Tokyo, Japan to carry out the work described in this paper whilea visiting Research Fellow with the GeoEngineering Centre at Queens-RMC at the

Royal Military College of Canada, Kingston, Ontario, Canada. Financial support

was also provided by the Natural Sciences and Engineering Research Council of

Canada and the Department of National Defence (Canada) in the form of

equipment and operating grants awarded to the second author.

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