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Geotextiles and Geomembranes

ep

ie

, v

s Te

fo

ne 2

em

tes

the anchorage zone; hence these properties have direct

PR 2Ls0V f b tan f0, (1)

reinforcement length in the anchorage zone; s0 the

different researchers (Ghionna et al., 2001; Moraci andMontanelli, 2000; Palmeira and Milligan, 1989; Wilson-Fahmy and Koerner, 1993), or by back-calculation from

ARTICLE IN PRESSpullout test results. In this case previous experimentalstudies (Ghionna et al., 2001; Moraci and Montanelli,2000; Palmeira and Milligan, 1989) have shown that the

0266-1144/$ - see front matter r 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.geotexmem.2006.03.001

Corresponding author. Tel.: +390965875263; fax: +390965875201.E-mail addresses: [email protected] (N. Moraci),

[email protected] (P. Recalcati).implications to the design of reinforced soil structures. Thetest method is intended to be a performance test conductedas closely as possible to replicate design or as builtconditions (ASTM D 6706-01).In order to analyse the internal stability of reinforced

earth structures, it is necessary to evaluate the pulloutresistance of reinforcement, mobilized in the anchoragezone.The pullout resistance in a pullout test can be described

by the following equations:

V

effective vertical stress; f0 the soil shear strength angle; fbthe soilgeosynthetic pullout interaction coefcient; mS/GSYthe soilgeosynthetic interface apparent coefcient offriction; F* the pullout resistance factor; a the scale effectcorrection factor account for a non-linear stress reductionover the embedded length of highly extensible reinforce-ments (FHWA, 2001).The soilgeosynthetic pullout interaction coefcient fb

may be determined by means of theoretical expressions(Jewell et al., 1985), whose limits have been investigated byeffective pressures. The different geogrids used in the research have been tested using unconned tensile tests performed at different

speeds, and, in particular, at the same speed of the pullout tests; granular soil have been characterized through classication, Proctor and

shear tests.

The pullout test results showed the inuence of the different parameters studied on pullout behaviour. Moreover, on the basis of the

test results it was possible to evaluate the peak and the residual pullout resistance and the apparent coefcient of friction mobilized in the

same conditions.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Pullout; Structure; Dilatancy; Interface; Extensibility; Conning stress

1. Introduction

Pullout tests are necessary in order to study theinteraction behaviour between soil and geosynthetics in

PR 2Ls0V mS=GSY, (2)PR 2Ls0V aF, (3)where PR is the pullout resistance (per unit width); L thedifferent HDPE extruded geogrids embedded in a compacted granular soil by varying the specimen lengths and the applied verticalFactors affecting the pullout bembedded in a com

Nicola Moracia,, PaMediterranea University of Reggio Calabria, Dep. MECMAT

bTechnical Manager, TENAX S.p.A., Geosynthetic

Received 5 December 2004; received in revised

Available onli

Abstract

In order to study the factors affecting the behaviour of reinforc

pullout test apparatus has been designed. More than 40 pullout24 (2006) 220242

haviour of extruded geogridsacted granular soil

rgiorgio Recalcatib

ia Graziella Loc. Feo di Vito, I-89060, Reggio Calabria, Italy

chnical Office, V.le Monza 1, 20127 Milano, Italy

rm 21 February 2006; accepted 1 March 2006

1 April 2006

ent geogrids embedded in granular compacted soils, a large-scale

ts have been performed, at constant displacement rate, on three

www.elsevier.com/locate/geotexmem

ARTICLE IN PRESSanNomenclature

a scale effect correction factor account for a non-linear stress reduction over the embeddedlength of highly extensible reinforcements(dimensionless)

d soil-interface friction angle (1)din displacement of the rst node of geogrid

specimen (mm)DSi differential displacement between two nodes

(mm)f0 soil shear strength angle (1)f0cv shear strength angle at constant volume (1)f0p peak shear strength angle (1)gdmax maximum dry unit weight (kN/m

3)mS/GSYR soilgeosynthetic residual interface apparent

coefcient of friction (dimensionless)mS/GSY soilgeosynthetic peak interface apparent coef-

cient of friction (dimensionless)s0V effective vertical stress (kN/m

2)Ab competent area of each rib element where the

passive resistance can be mobilized (mm2)Ar node embossment area (mm

2)At bar portion between two nodes area (mm

2)

N. Moraci, P. Recalcati / Geotextilesvalues of fb are largely inuenced by the choice of the valueof the soil shear strength angle.According to FHWA (2001) the scale effect correction

factor can be obtained from pullout tests on reinforcementswith different lengths or derived using analytical ornumerical load transfer models which have been cali-brated through numerical test simulations. In the absenceof test data, a 0:8 for geogrids and 0.6 for geotextiles.In absence of a clear indication regarding the choice of

the soil shear strength angle to be used for the determina-tion of fb (or of the development of new theoreticalexpressions that include the evaluation of all the para-meters that inuence the mobilisation of the interactionmechanisms), and to avoid the use of sophisticatednumerical analyses, the problem of the determination ofthe pullout resistance may be overcome by the use of thesoilgeosynthetic interface apparent coefcient of frictiondetermined by means of large-scale pullout tests, using thefollowing expression:

mS=GSY PR

2Ls0V. (4)

It is important to note that the determination of mS/GSYusing Eq. (4) can be performed without any assumptionabout the values of the soil shear strength angle mobilizedat the interface, since all the parameters of the aboveequation can be easily determined from the pullout tests.

Br node thickness (mm)Bt thickness of the bar portion between two nodes

(mm)d50 average grain size (mm)F* pullout resistance factorfb soilgeosynthetic pullout interaction coefcient

(dimensionless)h soil thickness above the geosynthetic (m)HPB pullout box height (mm)J2% tensile stiffness at 2% of strain (kN/m)J5% tensile stiffness at 5% of strain (kN/m)lo distance between two nodes (mm)L reinforcement length in the anchorage zone (m)LPB pullout box length (mm)LR specimen length (m)P pullout force (kN/m)Pin pullout trigger force (kN/m)PR peak pullout resistance (kN/m)PRR residual pullout resistance (kN/m)Td long-term tensile strength (kN/m)TF tensile strength (kN/m)U uniformity coefcient (dimensionless)W specimen width (m)WPB pullout box width (mm)wopt optimum water content (%)Wr node width (mm)Wt width of the bar portion between two nodes

d Geomembranes 24 (2006) 220242 221Nevertheless, it is important to dene the role of all thedesign (and test) parameters on the mobilisation of theinteraction mechanisms (frictional and passive) in pulloutcondition, including geosynthetic length, tensile stiffness,geometry and shape, vertical effective stress (acting at thegeosynthetic interface) and soil shear strength.This paper deals with some results of an experimental

research carried out in order to study the factors affectingthe pullout behaviour of extruded geogrids.

2. Previous experimental studies

In order to study the pullout behaviour of geosyntheticsembedded in soils several tests devices were developed bydifferent researchers. A pullout test apparatus is composedby a rigid pullout box, a vertical load application system,a horizontal force application device, a clamping system,and associated instrumentation. Table 1 summarizes theprincipal characteristics of the existing devices.From Table 1 it is possible to notice important

differences both in the box dimensions and in the methodsused to minimise the effects of the boundary conditions onthe test results. Other differences can be seen in terms oftesting procedures.Moreover, it is apparent that much of the difference in

pullout response reported in literature for similar consti-tuent material properties (soil and reinforcement) may be

(mm)

ARTICLE IN PRESS

s ch

ont

at t

e th

igid

al fo

at t

e th

wit

ont

ss s

ron

x

anTable 1

Summary of pullout test devices and testing characteristics

Research centre/references Dimensions

LPBWPBHPB(mm)

Test apparatu

California Department of

Transportation (Chang et al.,

1977)

1300 910 510 Removable fr

Asian Institute of Technology

(Voottipruex et al., 2000)

1270 762 508 Metal sleevessystem outsid

Nagota University of

Technology (Sugimoto et al.,

2001)

680 300 625 Flexible and racrylic materi

University of Porto (Lopes

and Ladeira, 1996)

1530 1000 800 Metal sleevessystem outsid

internal walls

University of British

Columbia (Fannin and Raju,

1993; Raju, 1995)

1300 640 600 Aluminium frglued with gla

pressures on f

outside the bo

N. Moraci, P. Recalcati / Geotextiles222attributed to differences in test procedures, devices and inassociated boundary effects (Juran et al., 1988, Moraci andMontanelli, 2000).In order to analyse the inuence of boundary conditions

it is possible to compare the results obtained by differentresearchers using instrumented pullout test devices (Changet al., 2000; Farrag et al., 1993; Farrag and Morvant, 2000;Johnston and Romstad, 1989; Palmeira and Milligan,1989; Raju, 1995; Sugimoto et al., 2001); the comparison ofthese tests allows the following factors affecting test resultsto be identied: boundary conditions at the upper surfaceof the soil specimen; boundary conditions at the front wall;friction between side walls and soil; pullout box dimen-sions; clamping device.The soil specimen is usually conned in a large box with

rigid base and lateral sidewalls. The vertical conningstress is applied by means of a rigid plate in contact withthe soil or by means of a exible membrane (usuallyrubber) lled with liquid or gas (air bag). The secondsystem allows constant normal stresses and the possibilityof free vertical displacement at every point of the soilsurface contact area.

Saga University (Alfaro et al.,

1995)

1600 600 500 Metal sleeves at tsystem inside the

membranes on in

Lousiana State University

(Farrag et al., 1993)

1520 900 760 Metal sleeves at tsystem inside the

than the box wid

Kyushu University (Ochiai

et al., 1992)

600 400 400 Lubricated insiderubber membrane

Oxford University (Palmeira

and Milligan, 1989)

254 150 500;1000 1000 1000

Different roughn

side wall

University of Reggio Calabria

(Moraci et al., 2004)

1700 600 680 Sleeves at the froinside the box; lu

means of adhesivaracteristics Soil specimen

preparation

Type of test

plates Mechanical

compaction

Constant

displacement rate

he front wall; clamping

e box

Mechanical

compaction

Constant

displacement rate

front wall; lateral walls in

r X-ray analysis

Air pluviation Constant

displacement rate


e box; revetment of the

h thick neoprene sheet

Mechanical

compaction

Constant

displacement rate

and rear walls; side walls

heet; measurement of

t wall; clamping system


displacement rate;

cyclic tests at

different amplitude

and frequency

(0.10.01Hz)

d Geomembranes 24 (2006) 220242The inuence of the two different vertical load applica-tion devices has been studied by Palmeira and Milligan(1989) who compared the results of pullout tests performedwith both rigid and exible devices. These tests showclearly (Fig. 1(a)) that using a exible type load applicationdevice the maximum pullout force is lower than the valuesobtained using a rigid load application device.It is important to note that the use of a exible

membrane allows a better and more uniform loaddistribution on the whole contact area and consequentlya more uniform distribution of the effective stresses at thespecimen upper surface (Farrag et al., 1993).The inuence of the boundary conditions at the front

wall was studied by many authors by means of pressurecells placed at the front wall at different positions. Thesemeasurements have shown that the pressure applied on thefront wall grows continuously as the test progresses (Changet al., 2000; Johnston and Romstad, 1989; Raju, 1995;Sugimoto et al., 2001); the peak value is found in thevicinity of the geogrid and than the pressure on the frontwall decreases symmetrically towards top and bottomboundaries.


box; lubricated rubber

ternal walls

Tamping and

mechanical

compaction

Constant

displacement rate


box; specimen narrower

th

Mechanical

compaction

Constant

displacement rate;

steps load

walls by means of greased

s


displacement rate;

cyclic test

ess of front wall perspex Air pluviation Constant

displacement rate

nt wall; clamping system

bricated inside walls by

e Teon lm

Tamping Constant

displacement rate

ARTICLE IN PRESS

nda

anBy comparing pullout test results carried out withdifferent roughness at the front wall (Fig. 1(b)) Palmeiraand Milligan (1989) have demonstrated that the apparentfriction angle mobilized at the interface increases with theinterface friction angle d between the front wall and the llsoil. Moreover, the experimental results demonstrate thatthe inuence of the stiffness of the front wall is lower forlarge pullout boxes and for boxes having a large distancebetween the rst conned section of the specimen and thefront wall (sleeves).In order to study the inuence of the stiffness of the front

wall, Sugimoto et al. (2001) performed pullout tests with aspecial equipment capable of providing both rigid and

2.0

1.0

0

P p:k

N/0

.15

m

0 5.0 10.0p : mm

Flexible top boundary

Leighton Buzzard sand 14/25Grid 1, Lr = 75 mmy = 25 kPa

Rigid top boundary

(a)

Fig. 1. Boundary effects on pullout tests: (a) effects of top bou

N. Moraci, P. Recalcati / Geotextilesexible boundary conditions. In the case of the rigidboundary no displacement at the front wall was allowed;vice versa, for the exible boundary (obtained by means oftwo membranes lled with air in pressure and arrangedbetween the soil and the front wall of the box), freemovements at the front wall was possible.Using X-ray techniques it was possible to study

(Sugimoto et al., 2001), for both the boundary conditionsat front wall, the behaviour at the soil reinforcementinterface. It was found that the stiffness of the front wallinuences the distribution of the displacements along thereinforcement length and therefore the mobilisation of theinteraction mechanisms. With a exible front wall there isan uniform distribution of the interaction mechanismsalong the reinforcement, while with a rigid front wall theinteraction mechanisms (the skin friction, between soil andreinforcement solid surface, and the bearing resistance, thatdevelops against transversal elements) along the reinforce-ment are not uniform.This effect is explained by Sugimoto et al. (2001) as

the consequence of the local increment of the relativedensity of the soil close to the front wall caused by animposed discontinuity in the displacements eld (rigidfront wall), that in the case of exible front wall is notsignicant.Generally, in order to minimise the friction effects at the

front wall, low friction materials glued to the walls areused. In addition, in order to avoid front wall effects thefront conned section of the reinforcement specimen ismoved to a suitable distance from the front wall by meansof metal sleeves xed to the front wall.Different researchers (Bolt and Duszynska, 2000; Farrag

et al., 1993; Lopes and Ladeira, 1996; Raju et al., 1996)have studied the inuence of the sleeve length on pullouttest results.From the comparison of pullout tests carried out

4.0

2.0 b/

y

00

20 40

: deg

y = 25 kPa

Grid 1, Lr = 75 mmLeighton Buzzard sand 14/25

b

y

(b)

ry; (b) effect of wall roughness (Palmeira and Milligan, 1989).

d Geomembranes 24 (2006) 220242 223without sleeves and with different sleeve lengths Farraget al. (1993) have observed that increasing the sleeve lengthcauses a reduction of the pullout resistance and of thepressure exerted on the front wall. On the basis of testresults performed with different sleeve lengths, the authorsproposed to use sleeve length of 0.30m in order tominimise front wall effects.Raju et al. (1996) have performed FEM analysis in order

to simulate different boundary conditions related to thepresence of a sleeve. From these analyses the presence ofthe sleeves causes a reduction of the mobilised apparentfriction angle (dened as F*a) with respect to values fromtests without sleeves. This reduction is not affected by thetype of contact (frictional or smooth) between the soil andthe front wall. It was determined that the procedures usedto reduce the friction at the front wall (smooth material orlubrication) are not enough to reduce the effect of thepresence of a rigid front wall if sleeves of sufcient lengthare not used (at least 40 times the geogrid transversal barthickness).Another important aspect are the boundary conditions

on the side walls of the pullout box. In test equipmentthe effective vertical conning stress acting at the

soilreinforcement interface is due to the normal stressapplied on the top soil specimen and to the weight of thesoil above the interface. Considering the thickness of thesoil layer above the reinforcement the friction developedalong the lateral walls of the pullout box can lead toconning stresses lower than expected.Johnston and Romstad (1989) measured the normal

stress close to the reinforcement by means of pressure cells;due to the friction along the lateral walls, the reduction ofthe vertical effective conning stress at the soilreinforce-ment interface can be reduced up to 35% with respect tothe net value applied to the soil on the top specimen surface

ARTICLE IN PRESSN. Moraci, P. Recalcati / Geotextiles an224(using a test device with a ratio h/W 0.27; where h is thethickness of the soil layer above the geosynthetic and W isthe box width). The same results were found by otherresearchers (Chang et al., 2000; Farrag et al., 1993) usingtest devices with the ratio h/W ranging from 0.42 to 0.44.Such phenomena could explain the increase of pullout

resistance observed as the width of the reinforcementspecimen decreases (Bolt and Duszynska, 2000; Changet al., 2000; Farrag et al., 1993).Also in these cases, in order to minimize the friction

effects at the side walls, low friction materials glued to thewalls (Teon, smooth aluminium, glass, lubricated rubbermembranes) were used.Theoretical and experimental studies carried out by

Hayashi et al. (1996) and by Ghionna et al. (2001) haveshown that for reinforcement specimens having a widthsmaller than the pullout box (narrow reinforcements) thetendency of soil dilatancy develops a three-dimensionaleffect. In fact, the non-dilating zone in the soil surroundingnarrower geogrid specimens (zone a in Fig. 2) behaves as arestrain against soil dilatancy in the dilating zone (zone b inFig. 2). This in turn generates shear stresses at the borderbetween the two zones and produces an increase of theeffective normal stress on the soilgeogrid interfaceand, consequently, an increase of pullout resistance. Byincreasing the specimen width, the above-cited effect isreduced because the soil area that blocks the dilatancydecreases, and the shear stresses cannot be generatedanymore, due to the smoothness of the box walls lined withTeon lm (Fig. 2).Fig. 2. Scheme of the pullout interaction for narrow and wide specimens

(Ghionna et al., 2001).Measurements of the vertical stress obtained by means ofsmall pressure cells distributed along the entire width of thereinforcement specimen and at a vertical small distance(20mm) from the reinforcement (Hayashi et al., 1997)showed that, provided that proper techniques are used toreduce the friction between soil and the lateral walls, auniform distribution of the initial conning stress can beobtained also for specimens having a width equal to thepullout box width.Similar results have been obtained by pullout tests

performed by Chang et al. (2000) on geogrid specimenshaving different width and varying the friction at the innerlateral wall interface by applying on the walls materialswith different roughness.From the analysis of these data it can be concluded that

use of narrow reinforcement, in order to reduce theinuence of the friction between soil and lateral pulloutbox walls, can result in overestimation of the interactionparameters obtained by laboratory tests; this overestima-tion is non-conservative for design when the geosynthetic isused in reinforced soil, where the reinforcement element isloaded under plain strain conditions.Referring to the pullout box dimensions ASTM (D6706-

01) recommends that the width of the box should be atleast 460mm, for smooth lateral walls, and should be thegreater than 20 times the D85 of the soil or 6 timesthe maximum soil particle size; the minimum length shouldbe at least 610mm and should be greater than 5 times themaximum geosynthetic aperture size. Moreover, the thick-ness of the soil above or below the geosynthetic should be aminimum of 150mm and at least 6 times the D85 of the soilor 3 times the maximum soil particle size.Other indications on the choice of the proper height of

the pullout box for open mesh geosynthetics (geogrid ormetallic meshes) are given by Moraci and Montanelli(2000), who observed that the choice of the height of thebox should be connected with the dimension of the passivewedges that develop from any transversal rib of thereinforcement; according to the authors, the maximumsize of the passive wedge can be taken as 40 times thethickness of the transversal rib of the geogrid. Moreover,the box width is a function of the structure of reinforce-ment specimens.Another important topic is the conguration of the

clamping device that is necessary to apply the pullouttensile force. The clamping system can be either outside thepullout box or inside it. With the second system theclamping device is inserted a given depth into the test box,in order to allow total connement of the reinforcementduring the test.A comparative analysis of the inuence of the type of

clamping device on the correct interpretation of test resultshas been done by Farrag and Morvant (2000). Accordingto Farrag and Morvant (2000), in the tests performed withan external clamping device pullout curves should be

d Geomembranes 24 (2006) 220242plotted referring to the displacement of a point of thespecimen that is as close as possible to the clamping device

but that remains within the conning soil during the wholeduration of the test. In fact, the rst nodal displacementinside the soil is less than the displacement in correspon-dence of the pullout force application point.Moreover, the adoption of an external clamping system

leads to a reduction of the specimen anchorage length asthe test proceeds; this reduction must be considered in theinterpretation of the test.The internal clamping device has two main advantages:

the anchorage length is constant for the whole duration ofthe test; the displacement measured at the clamping deviceis exactly the displacement of the rst conned section ofthe geogrid (assuming that no relative movement can occurwithin the clamp), and can therefore be used directly in thetest interpretation.This clamping system requires a series of preliminary

calibration tests, that have to be done with the sameboundary conditions, on the clamping system without anyreinforcement, in order to evaluate the pullout resistancedeveloped by the clamping system alone.

3. Experimental research

3.1. Test apparatus

The pullout box consists of steel plates welded at theedges; the front wall, at mid height, has an opening 45mmwide. This opening is necessary to allow the insertion of theclamping device and of the sleeves, 0.25m long, xed to thefront wall itself. A smaller opening (3mm wide) is providedat the back wall of the box in order to allow the connectionbetween the systems used to measure the internal displace-ments of the specimen and the transducers xed on theexternal wall of the box.An air lled cushion, in which the air pressure was

carefully controlled, applies the vertical load. A steel plateis used to restrain the air cushion on the upper side.An electric jack applies the pullout force, which is

measured using a load cell placed between the electric jackand the clamping system (Fig. 4).The apparatus is capable to produce conned failure of

the geosynthetic specimen using a clamp placed insidethe soil, well beyond the sleeve in order to keep thegeosynthetic specimen always conned in the soil for thetest duration (Fig. 5).Friction between the soil and the side walls of the box is

minimised by use of smooth Teon lms.The equipment incorporates two sleeves near the slot at

the front of the pullout box in order to avoid front walleffects as recommended by a number of researchers

ARTICLE IN PRESSN. Moraci, P. Recalcati / Geotextiles and Geomembranes 24 (2006) 220242 225The test apparatus is composed by a pullout box(1700 600 680mm), a vertical load application system,a horizontal force actuator device, a special clamp, and allthe required instrumentation, Fig. 3.

(1)Fig. 3. Scheme of pullout test apparatus: (1) frame; (2) steel plate; (3) a(Ghionna et al., 2001; Moraci and Montanelli, 2000;Palmeira and Milligan, 1989).The specimen displacements have been measured and

recorded using inextensible steel wires connected to at least

(2)(3)

(4)

(5)

(7)(6)ir bag; (4) electric engine; (5) reducer; (6) load cell; (7) electric jack.

six different points along the geogrid specimen. The wireswere connected to displacements transducers (rotaryvariable displacement transducersRVDT) xed to theexternal back side of the box (Fig. 5).All the measurements were digitally recorded on a

personal computer at dened constant time intervals.

3.2. Test materials

Pullout tests have been performed on three differentHDPE extruded mono-oriented geogrids (respectivelydescribed as GG1, GG2 and GG3).The three geogrids show similar geometrical character-

istics in plan view. They all have the same number of tensileelements per unit width and longitudinal rib pitch, andsimilar elliptical aperture shape. However, the threegeogrids have different cross sectional shape with maindifferences in rib and bar thickness.A more detailed analysis of the transversal bar geo-

metry has also shown a non-uniform shape with greaterthickness at the rib intersection. The passive interactionmechanisms develop both at the node embossments andat the transversal bars. Therefore, the node embossmentand the transversal bar geometry have been carefullydetermined to calculate the effective passive resistancesurfaces.

The results of this analysis are reported in Table 2,where Wr and Br are, respectively the node width andthickness; Wt and Bt are, respectively the width andthickness of the bar portion between two nodes (Fig. 6),and Ab is the effective area of each rib element (composedof the node embossment and of the bar portion betweentwo nodes At+Ar) where the passive resistance can bemobilized.Due to the physicalchemical properties of the constitu-

ent polymer (HDPE), the geogrids have a viscous-elasto-plastic behaviour when subjected to a tensile force. Due tothe viscous behaviour the geogrid mechanical propertiesdepend on test temperature and test rate. So the mechan-ical properties of the different geogrids were evaluated bywide width tensile tests (EN ISO 10319) performed atdifferent displacement rates of 1 and 100mm/min. Thesespeeds are respectively the pullout test rate and thestandard wide width tensile test rate. The tensile testresults at 1mm/min displacement rate are reported inTable 3.A granular soil was used in these tests. The soil was

classied as a uniform medium sand with uniformity

ARTICLE IN PRESSN. Moraci, P. Recalcati / Geotextiles and Geomembranes 24 (2006) 220242226Fig. 4. Electric jack and load cell.Fig. 5. Clamping system, sleeves acoefcient U d60/d10 1.5 and average grain sized50 0.22mm. Standard Proctor compaction tests gave amaximum dry unit weight gdmax 16.24 kN/m3 at anoptimum water content wopt 13.5%.Direct shear tests, performed at an initial unit weight

equal to 95% of gdmax (obtained at a water content of 9%),yield very high single values of the peak shear strengthangle f0p, in the range 481 (for s0V 10 kPa) to 421 (fors0V 100 kPa), Fig. 7. The shear strength angle at constantvolume f0cv was 341.

Table 2

Structural characteristics of the different geogrids

Geogrid Wr (mm) Wt (mm) Br (mm) Bt (mm) Ab (mm2)

GG1 11.26 6.6 3.80 3.57 66.35

GG2 11.86 6.0 4.65 4.48 85.35

GG3 12.36 5.5 5.16 4.85 90.45nd displacement transducers.

ARTICLE IN PRESS

tion

anFig. 6. Schematic cross sec

Table 33.3

T

(1)

(2)

Ten

disp

Geo

GG

GG

GG

tan

'

N. Moraci, P. Recalcati / Geotextiles. Test procedure

he pullout test procedure was the following:

preparation of the surfaces of the pullout box: in orderto minimize the friction between the soil and the box,all the box walls have been covered with adhesiveTeon lm;lling and compaction of the soil in the lower half ofthe box: the soil was initially dried in a oven at 105 1Cfor 24 h, and then prepared at a water content of 9.3%;after it was laid in the box in 100mm thick layers andmanually tamped to a nal thickness of 0.30m;

(5)

(6)

(7)

Plenwidpre

sile stiffness and strength of the different geogrids at 1mm/min

lacement rate

grid J2% (kN/m) J5% (kN/m) TF (kN/m)

1 946.5 719.5 73.06

2 1338.5 1049.0 98.99

3 1903.0 1354.8 118.29

0 20 40 60 80 100'v [kPa]

0.8

0.9

1

1.1

1.2 Soil

Fig. 7. Variation of tanf0 versus s0V for the tested soil.disprese

stuwerF

andwitthesubwitA

or aForwitullout tests have been performed varying the specimength (LR 0.40, 0.90, 1.15m) while keeping the specimenth constant (W 0.58m). Applied vertical effectivessures were equal to 10, 25, 50 and 100 kN/m2. Thelacement rate was 1.0mm/min in all tests. So, in thearch 36 different pullout test combinations were3.4.rotation effects, measurements were taken on theexternal edges (close to the lateral walls);lling and compaction of the soil in the upper half ofthe box;closing the pullout box and connection of the top coverand of the clamp with the load application devices(normal and axial) and with the displacement monitor-ing system;set-up of the control and data acquisition instrumenta-tion; start of the test.

Experimental planwidth reduction or rotation phenomena of transversalbars are less likely. In order to investigate possibleleast one monitoring point for every transversal bar ofthe reinforcement specimen. The inextensible steel wireswere placed as close as possible to each other andattached to the central part of the specimen, wheretransducers. The choice of the position of the measure-ment points has been done in order to guarantee at(3) positioning of the clamp and connection to the geo-grid specimen; the parallelism of the specimen withthe box length and perfect horizontality have beencarefully checked; a small preload was applied tothe geogrid in order to avoid any weaving in thespecimen;

(4) insertion of the inextensible wires into PVC tubes andconnection to the geogrid bars and to the electrical

AA of the geogrid bar.

d Geomembranes 24 (2006) 220242 227died (in order to assess the test repeatability some testse performed three times)or each test condition, the friction between the clampthe test soil was evaluated by performing the test

hout the geogrid, Fig. 8. The pullout force values due toclamp measured at each displacement level weretracted from the pullout force measured in the testh the geosynthetic at the same displacement.ll the tests have been performed until geogrid rupturetotal horizontal displacement of 100mm was achieved.all the tests, the geogrid specimens remained conned

hin the soil for its whole length.

ARTICLE IN PRESS

lac

one

an4. Analysis of test results

In a preliminary analysis of the results of the performedtests (Moraci and Recalcati, 2005) it has been possible tohave very simple yet important qualitative and quantitativeinformation about the pull-out mechanisms; in particular,the inuence of the conning stress on the tensile failureforce and the relevance of the dilatancy effect on the pull-out results have been observed; in this second phase of theresearch programme the results have been studied in amore complete and careful way, in order to give aninterpretation to the preliminary observations alreadydescribed and in general to all the obtained results.

4.1. Pullout resistance

From a pullout test it is possible to evaluate the peakpullout resistance PR, corresponding to the maximumvalue of pullout force measured in the test, and the residual

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

0 20 40Disp

P [kN

]

Fig. 8. Pullout curves for clamp al

N. Moraci, P. Recalcati / Geotextiles228pullout resistance PRR, corresponding to the ultimate valueof pullout force at large displacements.Fig. 9 shows, for the three different geogrids, the pattern

of the pullout force (P) versus the displacement, measuredat the edge attached to the clamp, for the longerreinforcement specimens (LR 1.15m). Fig. 10 shows thepullout curves for the shorter reinforcements specimens(LR 0.40m). The different curves on the graphs arereferring to the different applied conning pressures.Analysing the pattern of the pullout curves it is evident

that the pullout behaviour is strongly inuenced by theapplied conning stress and by the embedded geogrid length.In fact, for all the geogrids used, the tests performed with

long specimens (LR 0.901.15m) and conning pres-sure higher than 25 kPa show a strain-hardening beha-viour, with a progressive increase of the pullout resistancewith the increase of the displacement. In these tests, due tothe clamp inserted into the soil, the mechanism of pullouttensile failure, with the specimen constantly and fullyconned by the soil, has been properly studied.In the tests carried out on GG1 and GG2 specimensand at conning pressure equal to 100 kPa, pullouttensile failure occurred. The tensile strengths in pulloutconditions were very close to the tensile strength obtainedby the tensile tests performed at the same rate as thepullout tests. This means that, under these test conditions,the inuence of connement on tensile strength wasnegligible.The results of tests performed on short specimens

(LR 0.40m) and on long specimens under low conningstress (10 and 25 kPa) show a strain-softening behaviour,with a progressive decrease of pullout resistance after apeak value.The different trends in the pullout curves are due to the

effects of reinforcement length and of conning stress.Long reinforcement specimens under high connement

stresses show extensibility; this fact induces a progressivemobilisation of the interaction mechanisms that determinethe pullout resistance (Fig. 11(a)). On the other hand,

60 80 100ement [mm]

100 kPa50 kPa25 kPa10 kPa

at the different conning stresses.

d Geomembranes 24 (2006) 220242short reinforcement specimens or long specimens underlow connement stresses show lower longitudinal strains(Fig. 11(b)) and then an almost immediate development ofthe interaction mechanisms.According to these results, the effect of extensibility is

more evident for high values of the conning stresses andfor long specimens.The inuence on the pullout resistance of the conning

stress and of reinforcement length can be analysed bymeans of Fig. 12, in which the black symbols indicate thetensile failure.These charts represent the peak pullout resistance PR for

different values of the conning stress; it is possible to notea clear curvature in the experimental curves. This curvatureis more evident for short specimens, because underpullout conditions the interaction mechanisms (the skinfriction, between soil and reinforcement solid surface, andthe bearing stress, that develops against transversalelements, Jewell, 1996) probably develop simultaneouslyalong the whole specimen length.

ARTICLE IN PRESS

4050 kPa25 kPa

GG1

an80LR= 1.15 m 100 kPa

50 kPa25 kPa10 kPa

GG1

N. Moraci, P. Recalcati / GeotextilesThe curvature decreases with an increase in specimenlength, probably because of a non-uniform distributionof the pullout resistant mechanisms along the specimen.Fig. 12 also shows two horizontal asymptotes representing,respectively, the peak tensile force of the geogrid (TF) andthe long term tensile resistance (Td) measured in constant

0 20 40 60 80 100displacement [mm]

0

20

40

60

P [kN

/m]

0 20 40 60 80 100dispalcement [mm]

0

20

40

60

80

100

120

P [kN

/m]

LR = 1.15 m 100 kPa50 kPa


0

20

40

60

80

100

120

140

160

P [kN

/m]

LR = 1.15 m100 kPa50 kPa25 kPa10 kPa

GG2

GG3

25 kPa10 kPa

Fig. 9. Pullout curves for LR 1.15m.50LR =0.40 m 100 kPa

d Geomembranes 24 (2006) 220242 229tensile load unconned creep tests (Moraci and Montanelli,1995). These curves, obtained for different soil reinforce-ment combinations, could be used in design practice tocalculate, using adequate factors of safety, the minimum


0

10

20

30

P [kN

/m]

10 kPa


0

10

20

30

40

50

60

70

P [kN

/m]


0 20 40 60 80 100displacment [mm]

0

10

20

30

40

50

60

70

P [kN

/m]

LR =0.40 m 100 kPa50 kPa25 kPa10 kPa

GG3

GG2LR= 0.40 m

Fig. 10. Pullout curves for LR 0.4m.

ARTICLE IN PRESSanPR=81.77 kN/mPR=78.62 kN/mPR=62.79 kN/m

20

40

60

80

100GG3GG2GG1

Dis

plac

emen

t (mm)

'v=50kPa

N. Moraci, P. Recalcati / Geotextiles230anchorage length that is necessary to mobilise the long-term design strength, once the long-term resistance underpullout conditions is known.From the same charts it is possible to say that pullout

of reinforcement is the limit state for short specimens(regardless of the normal stress) and longer specimens withvertical stress s0Vo100 kPa, while for long specimens withconning stress s0V4100 kPa, the limit state is tensilefailure of the reinforcement.Furthermore, it is possible to note, for the all the

geogrids tested, that the pullout resistance for long geogrid

0 200 400 1000 12000

Position along the specimen (mm)800600

(a) (

Fig. 11. Displacement along the specimen

Td

0

20

40

60

80

100

Td

0 25 50 75 100 125 150

P R [kN

/m]

P R [kN

/m]

GG1

1.15 m

0.90 m

0.40 m

Pullout failure

0

20

40

60

80

100

120

140

'v [kPa]

TdTd

0 25 50

GG3

Pullout failure

'v

Fig. 12. Peak pullout resistance envelopes PR=41.17 kN/mPR=39.68 kN/mPR=30.95 kN/m

GG3GG2GG1

10

20

30

40

50

Dis

plac

emen

t (mm)

'v=50kPa

d Geomembranes 24 (2006) 220242specimens, at low conning stresses (1025 kPa), and forshort geogrids is lower than the long-term design strength.In Table 4, the peak and the residual pullout forces

obtained in the tests are summarised.From these data it is possible to note that the reduction

in pullout resistance from peak to residual values dependson the conning stress and on the geogrid length. Inparticular, the largest reductions are observed for shortspecimens under low conning stresses (DP 3040%);long specimens have a reduction from peak to residualpullout resistance (strain-softening behaviour) only at low

00 50 100 150 200 250 300 350 400

Position along the specimen (mm)b)s: (a) LR 1.15m; (b) LR 0.40m.

75 100 125 150

1.15 m

0.90 m0.40 m

[kPa]

P R [kN

/m]

0

20

40

60

80

100

120

140

Td

0 25 50 75 100 125 150

GG1

1.15 m

0.90 m

0.40 m

Pullout failure

'v [kPa]

(black points indicate tensile failure).

ARTICLE IN PRESS

ests

kPa

26

55

13

76

99

43

72

80

75

anconning pressure, and this reduction is between 7% and27%.In order to analyse the inuence of the anchorage length,

and therefore the extensibility of the reinforcement, pulloutcurves have been normalised with respect to LR.Fig. 13 shows example curves referring to pullout tests

on the different reinforcement types for different conningstresses. From the analysis of these curves it can beobserved that short reinforcement specimens (LR 0.40m) develop a greater normalised peak pullout resis-tance with respect to longer reinforcement. At ultimateconditions the P/LR values appear to be independent of thereinforcement length.These results conrm the inuence of the specimen

length, and therefore the extensibility of the reinforcingelement, on pullout behaviour, particularly at peak load.To further appreciate the inuence of the vertical

conning stress on pullout behaviour, the pullout curveshave been normalised with respect to s0V (Fig. 14).

Table 4

Peak (PR) and residual (PRR) pullout resistance (kN/m) measured in the t

Geogrid Spec. length (m) Normal stress s0V

10 kPa 25

PR PRR PR

GG1 0.40 9.62 5.63 20.

GG1 0.90 16.62 12.14 34.

GG1 1.15 20.00 14.76 37.

GG2 0.40 13.42 8.44 24.

GG2 0.90 21.32 15.43 39.

GG2 1.15 26.96 19.53 51.

GG3 0.40 12.84 7.36 22.

GG3 0.90 19.85 15.48 41.

GG3 1.15 24.35 19.61 47.

aSpecimen failure.

N. Moraci, P. Recalcati / GeotextilesFrom these charts it is possible to notice an importantreduction in the normalised resistance passing from low tohigh connement stress, both at peak and residual loadconditions.The experimental results can be explained by means of

the soil dilatancy phenomenon that develops due to thethree-dimensional passive failure surfaces at the nodeembossments and at the geogrid transversal bars.Due to this phenomenon, the effect of which decreases

with an increase in conning vertical effective stress, twomain effects develop: the rst is due to the different worknecessary to expand the dilatancy surface at differentvertical effective conning stresses; the second effect is dueto the restriction of the dilatancy connected to the nearbysoil stiffness (constrained dilatancy), which yields a localincrement of the effective conning stress.Fig. 15 shows, for the same test condition in terms of s0V

(10 and 100 kPa) and LR (0.40 and 1.15m), the pulloutcurves obtained for the three different geogrids. Thesegraphs show the inuence of reinforcement stiffness andstructural characteristics on pullout resistance. The experi-mental results, interpreted as a function of the differentlongitudinal tensile stiffness, do not show a correlation. Infact, the three geogrids have a different tensile stiffness, butthe differences in tensile properties cannot be associatedwith a corresponding difference in pullout resistance.By comparing the pullout curves for the same reinforce-

ment length and applied vertical effective stress, it ispossible to note that the lower values of the pulloutresistance are associated with geogrid GG1, while thehigher values are associated with geogrid GG2 or GG3.Therefore, while there is always an increase of the

pullout resistance by passing from geogrid GG1 togeogrids GG2 and GG3, the comparison between GG2and GG3 is less signicant with differences in the orderof 10%.Since geogrid GG2 and GG3 have similar structural

characteristics including similar bearing area Ab (effectivearea of each unit element: composed of the node

50kPa 100 kPa

PRR PR PRR PR PRR

13.29 30.95 18.93 39.79 26.43

29.79 52.53 50.34 78.44a

34.32 62.79 62.79 72.48a

15.43 41.18 24.04 56.59 37.51

32.14 70.07 62.46 103.91 103.91

44.00 75.62 75.62 106.91a

13.64 37.68 25.18 58.68 49.04

34.69 72.95 61.27 97.59 97.59

43.79 81.77 81.77 115.19 115.19

d Geomembranes 24 (2006) 220242 231embossments and the half bar portion between two nodes)against which the passive resistance is mobilised (Fig. 6 andTable 2), it is possible to suppose that the values of thepullout resistance are mainly inuenced by the structuralcharacteristics (geometry and shape) of the geogrids.In fact, by comparing the experimental results of the

tests carried out on the three different geogrids, with thesame anchorage length and normal stress, independent ofthe reinforcement extensibility and soil dilatancy effects, itis possible to observe that the maximum percentagedifference of pullout resistance is in the order of 2049%with an average value of 34%. These values are very closeto the percentage difference of the effective bearing areas(Ab) between geogrid types upon which the passiveresistance is mobilised.

4.2. Interface apparent coefficient of friction

Fig. 16 shows the trend of the peak pullout interfaceapparent coefcient of friction mS=GSY as a function of the

ARTICLE IN PRESS

0

10

20

30

40

50

60

70P/

L R [(k

N/m)

.m

-1 ]

0

10

20

30

40

50

60

70

80

P/L R

[(k

N/m)

.m

-1 ]

020406080

100120140160180200

P/L R

[(k

N/m)

.m

-1 ]

GG1 0.40 m0.90 m1.15 m

GG3

'v=25 kPa

'v=25 kPa

GG2 0.40 m0.90 m1.15 m

0.40 m0.90 m1.15 m

'v=100 kPa

Displacement (mm)0 20 40 60 80 100



Fig. 13. Normalised pullout curves (P/LR).

0 20 40 60 80 1000

0.25

0.5

1

1.25

P/

' V [(k

N/m)

.kP

a-1 ]

P/

' V [(k

N/m)

.kP

a-1 ]

GG1LR= 0.40 m

GG3LR= 0.90 m

GG2LR= 1.15 m

10 kPa25 kPa50 kPa

100 kPa0.75

0

0.5

1

1.5

3

3.5

2

2.5




P/

' V [(k

N/m)

.kP

a-1 ]

0

0.5

1

1.5

0.2

3

2.5

0 20 40 60 80 100

Displacement (mm)

Displacement (mm)Fig. 14. Normalised pullout curves (P=s0V).

N. Moraci, P. Recalcati / Geotextiles and Geomembranes 24 (2006) 220242232

ARTICLE IN PRESSan0 20 40 60 80 100

displacement [mm]

0

5

10

15

20

25P

[kN/m

]LR= 0.40 m

'v=10 kPa

GG3

GG2

GG1

30

40

50

60

70

80

P [kN

/m]

LR = 0.40m

'v=100 kPa

GG3GG2GG1

N. Moraci, P. Recalcati / Geotextilesvertical effective applied stress. In each graph, the resultsrefer to the three different specimen lengths used. In thesame graphs also the curve of the variation of tanf0 versuss0V for the tested soil are plotted.In all the analysed cases it is possible to observe a

reduction in the mobilized peak pullout interface apparentcoefcient of friction with an increase in applied verticaleffective stress. Moreover, it is possible to notice that thelower values of mS=GSY apply to longer specimens.These results are due to two different phenomena: the

rst, of greater importance, is related to the soil dilatancyphenomena; the second effect, of less intensity, is due to theextensibility of the reinforcement that modies the inter-face tangential stress distribution and the correspondingpullout strength.Moreover it is possible to observe that peak mobilized

interface apparent coefcient of friction is greater than tanf0 only in the case of short reinforcement at lowerconning pressures.Analysing the test results, for equal reinforcement

lengths, one can observe that the peak mobilized interfaceapparent coefcient of friction for low vertical effectiveconning stress (10 kPa) is much higher than the corre-sponding one measured at higher vertical effective stresses(50100 kPa) due to the dilatancy behaviour. In particular,

0 20 40 60 80

displacement [mm]

0

10

20

100

Fig. 15. Pullout curves for the different geogrids for0 20 40 60 80 100

displacement [mm]

0

5

10

15

20

25

30

35

40

45

P [kN

/m]

LR= 1.15 m

'v=10 kPa

GG3

GG2

GG1

60

80

100

120

140

160

P [kN

/m]

LR= 1.15 m

'v=100 kPa

GG3GG2GG1

d Geomembranes 24 (2006) 220242 233the percentage variations due to dilatancy are up to 148%for GG1, up to 137% for GG2 and up to 135% for GG3.The greater percentage increments are for the geogridshaving shorter anchorage lengths (LR 0.40m) and thelower increments are for the longer anchorage lengths(LR 1.15m).The reinforcement extensibility effects can be explored

further by comparing, for equal vertical effective conne-ment stress, the peak apparent interface coefcient offriction values determined for the long reinforcementspecimens (LR 1.15m) with those evaluated for theshort ones (LR 0.40m). In these cases the percentagedifferences due to the reinforcement extensibility are up to45% for geogrid GG1, up to 46% for geogrid GG2 and upto 52% for geogrid GG3.Fig. 17 shows the inuence of the reinforcement stiffness

and structural characteristics on the mobilized peak inter-face apparent coefcient of friction.In these charts, the peak interface apparent coefcient of

friction is plotted as function of the vertical effectiveconning stress. Also in these cases the experimentalresults, interpreted as function of longitudinal tensilestiffness, do not show a correlation. Vice versa, the testresults conrm that the values of the soilgeosynthetic peakinterface apparent coefcient of friction mS/GSY are mainly

0 20 40 60 80

displacement [mm]

0

20

40

100

different s0V (10100 kPa) and LR (0.401.15m).

ARTICLE IN PRESSan0 20 40 60 80 100'v [kPa]

0.4

0.6

0.8

1

1.2

1.4ta

n'

0.4

0.6

0.8

1

1.2

1.4

S/G

SY

GG1Lr =0.40mLr=0.90mLr=1.15mSoil

N. Moraci, P. Recalcati / Geotextiles234inuenced by the structural characteristics (geometry andshape) of the geogrids.In fact, by comparing the experimental results of the

tests carried out on the three different geogrids, with thesame anchorage lengths and normal stress, independent ofreinforcement extensibility and dilatancy effects, it ispossible to observe that the maximum percentage differ-ences of mS/GSY are very close to the percentage differenceof the effective bearing areas (Ab) between geogrid typesagainst which passive resistance is mobilized.The residual interface apparent coefcient of friction can

be evaluated by means of the following expression:

mRS=GSY PRR

2LR s0V. (5)

Fig. 18 shows, for the different geogrids, the trend of theresidual pullout interface apparent coefcient of friction asa function of the vertical effective applied stress. Theseresults show that the residual pullout interface apparent

0 20 40

'v [kP

0.4

0.8

1.2

1.6

tan

'

G

Fig. 16. Peak interface apparent friction coefcie0 20 40 60 80 100'v [kPa]

0.4 0.40.8

1.2

1.6

tan

'

0.8

1.2

1.6

S/G

SY

GG2

SoilLr=1.15mLr=0.90mLr =0.40m

d Geomembranes 24 (2006) 220242coefcient of friction does not depend on the reinforcementlength but only on applied conning stress.Furthermore, comparing the results obtained for the

three different geogrids it is possible to note that mRS=GSYdepends on geogrid geometry.

4.3. Displacements

Fig. 19 represents a qualitative distribution of the nodaldisplacements of the reinforcement specimen for differentvalues of the applied pullout forces. The slope of everycurve (DSi/lo) represents the local strain ei (Fig. 19). Bycomparing the slope, in the same location, of two differentnodal displacement curves (i.e. for two different values ofthe applied pullout force) it is possible to obtain theincrement of strain. The pullout phase, in which theinteraction mechanisms develop along the whole reinforce-ment length, is characterized by a progressive reduction ofthe strain increment rate until a limit state condition, in

60 80 100

a]

0.4

0.8

1.2

1.6

S/G

SY

G3Lr =0.40mLr=0.90mLr=1.15mSoil

nt vs. s0V for different reinforcement lengths.

ARTICLE IN PRESSan0 20 40 60 80 100 120

Normal stress 'V [kPa]

0

0.4

0.8

1.2

1.6

2

S/G

SYLR= 0.40 m GG3

GG2GG1

0.4

0.8

1.2

1.6

S/G

SY

LR = 1.15m

N. Moraci, P. Recalcati / Geotextileswhich no increment of strain along the reinforcementoccurs.The pullout condition is reached when two adjacent

curves, as the two at the top in Fig. 19, are parallel eachother.The curves in Figs. 2025 represent the distributions of

the nodal displacements of the reinforcement specimen fordifferent values of the applied pullout forces. These curvesrefer only to the maximum and minimum values of theapplied stress (10 and 100 kPa) and to the maximum andminimum values of the reinforcement length (LR 0.40and 1.15m).In particular, Figs. 20(a)25(a) refer to tensile load

transfer phase and the onset of pullout (trigger force).During the load transfer phase, the portion of the specimendeveloping the interaction mechanisms that determines thepullout resistance (active length) increases with the pulloutforce until it reaches a limit value that causes the movementof the last bar; this force can be dened as the pullouttrigger force, Pin.Figs. 20(b)25(b) represent the displacement distribu-

tions corresponding to different applied pullout forces: thepullout trigger force (Pin); an intermediate value betweenthe trigger force and the maximum pullout force measuredduring the test; the maximum pullout force measured

0 20 40Normal s

0

Fig. 17. Peak interface apparent coefcient0 20 40 60 80 100 120

Normal stress 'V [kPa]

0

0.4

0.8

1.2

1.6

S/G

SY

LR = 0.90m GG3GG2GG1

GG3

GG2

GG1

d Geomembranes 24 (2006) 220242 235during the test; the residual pullout force PRR measuredduring the test.It is important to note that for tests showing a strain

hardening behaviour in terms of pullout resistance (i.e. forlong reinforcements and high conning stressesFigs. 23(b) and 25(b)) the displacements distributions showa continuous increase in pullout force (from bottom to top).The pullout condition is reached when the top curves in

Figs. 20(b)25(b) are parallel each other as in the case ofshort reinforcements (LR 0.40m) and for low valuesof the connement stress (s0V 10 kPa). On the other hand,for long reinforcements (LR 1.15m) and for highconnement stresses (s0V 100 kPa), increments of theaverage strains along the whole specimen length are alsopresent at large displacements of the rst conned node. Inthis study, pullout conditions were also considered to bereached for high values of the displacement (100mm).The nodal displacement distributions conrm the differ-

ent pullout behaviour of reinforcement due to a variationof the length and the connement stress. In fact shortreinforcements at all connement stresses and longreinforcements for low values of s0V show an almostuniform distribution of nodal displacements; vice versalong reinforcement, at high connement stress, shownon-linearity of the nodal displacements distribution.

60 80 100 120tress 'V [kPa]

of friction vs. s0V for different geogrids.

ARTICLE IN PRESSan1.6

1.2

GSY

Geogrid GG1 0.40 m

0.90 m1.15 m

N. Moraci, P. Recalcati / Geotextiles236These differences are related to the effect of reinforce-ment extensibility on the mobilization of interface mechan-isms. In particular, for long reinforcements and for highvalues of s0V, a progressive mobilization of the interfaceinteraction mechanism develops; on the other hand, forshort reinforcement and for long reinforcement at lowvalues of s0V these mechanisms are activated almost at the

0.8

0.4

00 20 40 60 80 100 120

'v [kPa]

R

S/

R

S/G

SY

1.6

1.2

0.8

0.4

00 20 40

'v [k

Geogrid GG3

Fig. 18. Residual interface apparent coefcien

disp

lace

men

t

|0position along the specimen

s1

s2

s3 3=2

1

2P3

P2

P1

=2-1Pi=pullout load

Fig. 19. Nodal displacement curves.1.6

1.2

0.8

0.4

0

R

S/G

SY

0 20 40 60 80 100 120'v [kPa]

60 80 100 120

Pa]

Geogrid GG2

0.40 m

0.90 m

1.15 m

0.40 m

0.90 m1.15 m

d Geomembranes 24 (2006) 220242same time along the whole reinforcement. As a conse-quence, for long reinforcements with high values of s0V, themobilized interface shear strength angle seems to bebetween the peak and the constant volume values. Viceversa, for short reinforcement and long reinforcement andlow values of s0V, because of an almost uniform mobiliza-tion of the resistance along the reinforcing element, themobilized interface shear strength angle is close to the peakvalue.Furthermore, the nodal displacement curves can be used

in displacement stability analysis methods because fromthese curves it is possible to obtain, for different anchoragelength and connement stress, the pullout resistance valuesthat are mobilized for given displacement values of thesection that represents the boundary limit between theactive zone and the anchorage length, Fig. 26.From the displacements curves it is also possible to

obtain the pullout force that causes the movement of thelast bar, Pin. This is the load value that occurs at completemobilization of the anchorage length, and the correspon-dent displacement of the last node din.From the analysis of the experimental results it is

possible to note that, for geogrid GG1, Pin values fallbetween 4.58 and 19.86 kN/m for short reinforcements(LR 0.40m) and between 14.59 and 45.09 kN/m (valuemeasured at 50 kPa) for long reinforcements (LR 1.15m), Fig. 27. For geogrid GG2, Pin values are largerthan the GG1 ones and fall between 6.52 and 24.40 kN/m

t of friction vs. s0V for different geogrids.


N. Moraci, P. Recalcati / Geotextilesfor short reinforcements and between 18.61 and 90.53 kN/m for long reinforcements, Fig. 28. Finally, analysing theresults obtained for geogrid GG3, it is possible to note that

0 100 200 300 400Position along the specimen [mm]

0

0.4

0.8

1.2

displa

cem

ent [m

m]4.58 kN/m

4.03 kN/m

GG1LR = 0.40 m

'v = 10 kPa

(a)

0 100 200 300 400

Position along the specimen [mm]

0

20

40

60

80

100

Disp

lacem

ent [m

m]

5.63 kN/m9.62 kN/m5.63 kN/m4.58 kN/m

GG1

LR = 0.40 m

'v = 10 kPa

(b)

Fig. 20. Displacements measured along the specimen (for GG1 a

0 100 200 300 400


0

1

2

3

4

5

6

Dis

plac

emen

t [mm]

19.86 kN/m

12.63 kN/m

GG1LR = 0.40 m

'v = 100 kPa

(a)

0 2Position along t

0

20

40

60

80

100

Dis

plac

emen

t [mm]

26.36 39.78

(b)

Fig. 21. Displacements measured along the specimen (for GG1 an8

d Geomembranes 24 (2006) 220242 237Pin values are similar to the ones obtained for GG2; inparticular they fall between 5.07 and 27.51 kN/m forLR 0.40m and between 16.22 and 92.80 kN/m for

0 200 400 600 800 1000Position along the specimen [mm]

0

2

4

6

Dis

plac

emen

t [mm] 11.69 kN/m

11.05 kN/m

9.00 kN/m

4.84 kN/m

14.59 kN/mGG1

LR = 1.15 m

'v = 10 kPa

0 200 400 800 1000


0

20

40

60

80

100

Disp

lacem

ent [m

m]

14.76 kN/m20.00 kN/m14.75 kN/m

14.59 kN/m

GG1LR = 1.15 m

'v = 10 kPa

600

nd s0V 10 kPa): (a) tensile force transfer; (b) pullout phase.

0 200 400 600 800 1000


0

8

16

24

32

40

48

Dis

plac

emen

t [mm] 72.03 kN/m

42.39 kN/m

29.57 kN/m

14.32 kN/m

72.48 kN/mGG1LR= 1.15 m

'v = 100 kPa

failure

00 400he specimen [mm]

kN/mkN/m

26.36 kN/m19.86 kN/m

GG1LR = 0.40 m'

v = 100 kPa

d s0V 100kPa): (a) tensile force transfer; (b) pullout phase.

ARTICLE IN PRESSan2

2.4 [m

m]6.52 kN/m4.71 kN/m

GG2

' = 10 kPaLR = 0.40 m

N. Moraci, P. Recalcati / Geotextiles238LR 1.15m, Fig. 29. In every analysed case the lowervalues are obtained at low connement stress, and thehigher values at high connement stress.Moreover, it is possible to note that the trigger force

represents a percentage of the peak pullout force that is


0

0.4

0.8

1.2

1.6

Dis

plac

emen

t v

(a)

0 100 200 300 400


0

20

40

60

80

100

Dis

plac

emen

t [mm]

8.44kN/m14.42 kN/m8.45kN/m6.52kN/m

GG2LR =0.40 m'v = 10 kPa

(b)



0

1

2

3

4

5

6

Dis

plac

emen

t [mm]

24.40 kN/m

16.26 kN/m

GG2LR = 0.40 m

'v = 100 kPa

(a)

0 200 400Position along the specimen [mm]

0

20

40

60

80

100

Dis

plac

emen

t [mm]

37.51 kN/m

37.54 kN/m24.40 kN/m

GG2LR = 0.40 m

'v = 100 kPa

56.59 kN/m

(b)

Fig. 23. Displacements measured along the specimen (for GG2 an6

8

[mm] 13.52 kN/m

13.14 kN/m

18.61 kN/mGG2LR= 1.15 m

d Geomembranes 24 (2006) 220242between 35% and 73% for geogrid GG1, between 30% and72% for geogrid GG2 and between 32% and 72% forgeogrid GG3. In any case, the lower values of the ratioPin/PR refer to short reinforcements, while the highervalues refer to long reinforcement specimen lengths.

0 200 400 600 800 1000 1200Position along the specimen [mm]

0

2

4

Dis

plac

emen

t 12.14 kN/m7.51 kN/m

'v = 10 kPa

0 200 400 600 800 1000 1200


0

20

40

60

80

100

Dis

plac

emen

t [mm]

19.53 kN/m26.96 kN/m19.50 kN/m18.61 kN/m

GG2LR = 1.15m'

v = 10 kPa


0 200 400 600 800 1000Position along the specimen [mm]

0

10

20

30

40

50

60

Dis

plac

emen

t [mm] 65.49 kN/m

46.18 kN/m39.99 kN/m16.70 kN/m

90.53 kN/mGG2LR= 1.15 m

'v = 100 kPa

0 200 400 600 800 1000 1200


0

20

40

60

80

100

Dis

plac

emen

t [mm]

106.91kN/m

90.53 kN/m

GG2LR = 1.15m'

v =100 kPa

failure



N. Moraci, P. Recalcati / GeotextilesFrom the analysis of the displacement values that arenecessary to trigger the pullout mechanism, it is possible tonote that, these values increase with specimen length and


0

0.6

1.2

1.8D

ispl

acem

ent [m

m]5.07 kN/m

3.92 kN/m

GG3LR = 0.40 m

'v= 10 kPa

(a)


0

20

40

60

80

100

Dis

plac

emen

t [mm]

5.07 kN/m

GG3LR =0.40 m'v = 10 kPa

(b)

7.35 kN/m10.84 kN/m7.36k N/m


0 100 200 300 400


0

1

2

3

4

5

6

Dis

plac

emen

t [mm] 25.26 kN/m

GG3LR = 0.40 m'

v = 100 kPa

(a)

0 200 400Position along the specimen [mm]

0

20

40

60

80

100

Dis

plac

emen

t [mm]

GG3LR = 0.40 m'

v = 100 kPa

(b)

30.81 kN/m

49.08 kN/m58.68 kN/m49.08 kN/m30.81 kN/m

Fig. 25. Displacements measured along the specimen (for GG3 an6 GG3

d Geomembranes 24 (2006) 220242 239with effective connement stress. For the test performed ongeogrid GG1 the trigger displacements fall between 1.17and 4.03mm for LR 0.40m and between 5.43 and


0

1

2

3

4

5

Dis

plac

emen

t [mm]

LR= 1.15 m

'v = 10 kPa


0

20

40

60

80

100

Dis

plac

emen

t [mm]

GG3LR = 1.15m'

v = 10 kPa

5.76 kN/m7.05 kN/m9.63 kN/m12.94 kN/m16.22 kN/m

19.62 kN/m24.34 kN/m19.61 kN/m16.22 kN/m


0 600 10000

4

8

12

16

20

24

Dis

plac

emen

t [mm] 74.10 kN/m

29.25 kN/m

GG3LR= 1.15 m'

v = 100 kPa

0 200 800 1000Position along the specimen [mm]

0

20

40

60

80

100

120

Dis

plac

emen

t [mm]

GG3LR= 1.15 m'

v = 100 kPa

200

Position along the specimen [mm]800400 1200

75.11 kN/m

51.01 kN/m

10.43 kN/m

400 600

75.11 kN/m88.44 kN/m101.79 kN/m115.15 kN/m


ARTICLE IN PRESSan q

L

DISPLACEMENTSMETHOD

z

N. Moraci, P. Recalcati / Geotextiles24043.60mm for LR 1.15m. For geogrid GG2 the samevalues fall between 1.63 and 3.56mm for short reinforce-ments and between 5.66 and 53.09mm for long reinforce-ments. Finally, for geogrid GG3 the values fall between1.60 and 4.16mm for LR 0.40m and between 4.13 and55.08mm for LR 1.15m. In each of these cases the lowervalues refer to low values of the connement stress whilethe higher values refer to high values of the connementstress.

j

HM

I

II

Critical slip line

REINFORCED

SOIL MASS

Z

sheetj

Laj

Lajp

a

zj

j

j

j

Fig. 26. Denition of the displacements method: (I) active zone; (II)

anchorage zone; Lpaj anchorage length (Gourc et al., 1986).

0 20 40 60 80 100'v [kPa]

0

20

40

60

80

P in

[kN/m

]

GG1Lr=0.40mLr=1.15m

Fig. 27. Curves Pin vs. s0V for GG1.80

100GG2

Lr=0.40mLr=1.15m

d Geomembranes 24 (2006) 220242It is noteworthy to highlight that for every test in which aconned rupture of the specimen occurred, it happenedafter reaching the pullout trigger condition and then in thephase previously indicated as the pullout phase, apartform the test carried on geogrid GG1 with LR 1.15mand s0V 100 kPa: in this test the tensile failure of thereinforcing element has occurred before pullout trigger.Therefore, the greatest displacements almost always refer

to pullout forces close to the conned peak tensile load of

0 20 40 60 80 1000

20

40

60

P in

[kN/m

]'v [kPa]

Fig. 28. Curves Pin vs. s0V for GG2.

0 20 40 60 80 100

'v [kPa]

0

20

40

60

80

100

Pin

[kN

/m]

GG3

Lr=0.40m

Lr=1.15m

Fig. 29. Curves Pin vs. s0V for GG3.

the

5.

pa

pugrico

Far

Far

Gh

Go

ARTICLE IN PRESSanpercentage differences of interface apparent coefcientof friction (up to 150%), due to the dilatancy effects,were observed for the short reinforcement layers(LR 0.40m).

The experimental results have also shown that theextensibility of reinforcement has an inuence on peakpullout strength. In particular, extensibility effects aremore evident for long reinforcements and at highconning stresses (up to 50%). Under residual pulloutload conditions, the extensibility effects are negligible.

The empirical results also show an increase of peak andresidual pullout strength, and therefore of the mobilizedcient of friction mobilized at low vertical effectiveconning pressure (10 kPa) is higher than at highconning pressure (50 or 100 kPa). The dilatancyphenomenon is related to development of passive failuresurfaces, which are generated against the node emboss-ments and the geogrid transversal bars. The maximumthat the pullout interaction mechanism develops pro-gressively along the reinforcement specimens, with aprogressive increase of the pullout resistance with anincrease in displacement. The tests performed onshort specimens (LR 0.40m) and on long specimensunder low conning stress show a strain softeningbehaviour, with a progressive decrease of pulloutresistance after peak load. In this case, the interactionmechanisms develop almost at the same time along thewhole length of the specimen.The phenomenon that has the largest inuence onpullout strength and on the interface apparent coef-cient of friction (mS/GSY), both at peak and residual loadconditions, is the dilatancy of the soil at the interface.Due to dilatancy effects, the interface apparent coef-bedded length and vertical effective stress) on thellout behaviour of three mono-oriented extruded geo-ds embedded in a granular soil. In particular, the mainnclusions are:

The tensile strength in pullout conditions is very close tothe tensile strength obtained by in air tests performed atthe same rate of displacement as pullout tests. Thismeans that the inuence of soil connement onreinforcement tensile strength is negligible.The pullout behaviour depends on reinforcement lengthand on the applied vertical stress. In particular, the testsperformed with long specimens (LR 1.15m) andconning stresses larger than 25 kPa show a strain-hardening behaviour. In this case it is possible to sayemnnement stress values and the highest specimen length.

Conclusions

The test results clearly show the inuence of the differentrameters studied (reinforcement stiffness and structure,co

reinforcement, and they are obtained at the highest

N. Moraci, P. Recalcati / Geotextilesinterface apparent coefcient of friction, while increas-ing the competent bearing area of the each node (Ab),

HaWA-NHI-00-043, 2001. Mechanically stabilized earth walls and

reinforced soil slopes. Design and constructions guidelines. U.S.

Department of Transportation Federal Highway Administration.

ionna, V. N., Moraci, N., Rimoldi, P., 2001. Experimental evaluation

of the factors affecting pull-out test results on geogrids. Proceedings of

International Symposium: Earth Reinforcement. Fukuoka, Kyushu,

Japan.

urc, J.P., Ratel, A., Delmas, P., 1986. Design of fabric retaining walls:

the displacement method. Proceedings of Third International Con-

ference on Geotextiles, vol. 2, Wien, Austria, pp. 289294 and

10671072.FHMaterials, 8996.

rag, K., Acar, Y.B., Juran, I., 1993. Pull-out resistance of geogrid

reinforcements. Geotextiles and Geomembranes 12, 133159.Vancouver, Canada. pp. 633643.

rag, K., Morvant, M., 2000. Effect of clamping mechanism on pullout

and conned extension tests. In: Stevenson, P.E. (Ed.), Grips, Clamps,

Clamping Techniques, and Strain Measurement for Testing of

Geosynthetics, ASTM STP 1379, American Society for Testing andagainst which passive mechanisms are mobilized. Thedifference in values of interface apparent coefcient offriction related to the geogrid structure (shape andgeometrical characteristics) are up to 49% with anaverage value of 34%. These values are very close to thepercentage differences of the effective bearing areas (Ab)between the geogrids against which the passive resis-tance is mobilized.

The pullout resistance decrement after peak load isrelated to both reinforcement length and conningstress.

The residual interface apparent coefcient of frictiondepends only on applied vertical stress and geogridgeometry. In these conditions mS/GSY

R does not dependon reinforcement length.

The node displacements curves obtained by pullout testsmay be useful in stability analysis performed by meansof displacement methods.

References

Alfaro, M.C., Miura, N., Bergado, D.T., 1995. Soilgeogrid reinforcement

interaction by pullout and direct shear tests. Geotechnical Testing

Journal 18, 157167.

ASTM D 6706-01, 2001. Standard test method for measuring geosynthetic

pullout resistance in soil. ASTM, Philadelphia, PA, USA.

Bolt, A.F., Duszynska, A., 2000. Pull-out testing of geogrid reinforcement.

Proceedings of the Second European Geosynthetics Conference

EUROGEO 2000, vol. 2, Bologna, Italy, pp. 939943.

Chang, J.C., Hannon, J.B., Forsyth, R.A., 1977. Pullout resistance and

interaction of earthwork reinforcement and soil. Transportation

Research Record 640. National Research Council, Washigton, DC,

pp. 17.

Chang, D.T.T., Chang, F.C., Yang, G.S., Yan, C.Y., 2000. The inuence

factors study for geogrid pullout test. In: Stevenson, P.E. (Ed.), Grips,

Clamps, Clamping Techniques, and Strain Measurement for Testing of

Geosynthetics, ASTM STP 1379. American Society for Testing and

Materials, pp. 129142.

EN ISO 10319, 1992. Geotextile wide-width tensile test. International

Organization for Standardization, ISO, Geneve.

Fannin, R.J., Raju, D.M., 1993. Large-scale pull-out test results on

geosynthetics. Proceedings of Geosynthetics 93 Conference, vol. 2.

d Geomembranes 24 (2006) 220242 241yashi, S., Alfaro, M.C., Watanbe, K., 1996. Dilatancy effects of

granular soil on the pullout resistance of strip reinforcement.

Proceedings of the International Symposium: Earth Reinforcement,

Fukuoka, Kyushu, Japan, pp. 3944.

Hayashi, S., Shaliu, J.T., Watanbe, K., 1997. Effect of restrained dilatancy

on pullout resistance of strip reinforcement. Proceedings of Geosyn-

thetics Asia97, vol. 1, Bangalore, India, pp. 3944.

Jewell, R.A., 1996. Soil reinforcement with geotextiles. CIRIA Special

Publication 123, Thomas Telford ISBN:0-7277-2502-5.

Jewell, R.A., Milligan, G.W.E., Sarsby, R.W., Dubois, D., 1985.

Interaction between soil and geogrids. Proceedings from the Sympo-

sium on Polymer Grid Reinforcement in Civil Engineering, Ed.

Thomas Telford, London, pp. 1830.

Johnston, R.S., Romstad, K.M., 1989. Dilation and boundary effects in

large scale pull-out tests. Proceedings of 12th International Conference

on Soil Mechanics and Foundation Engineering, vol. 2, Rio De

Janeiro, Brasil, pp. 1263l266.

Juran, I., Knochenmus, G., Acar, Y.B., Arman, A., 1988. Pullout response

of geotextiles and geogrids (synthesis of available experimental data).

Symposium on Geosynthetics for Soil Improvement, vol. 18, ASCE

Geotechnical Publication, pp. 92111

Lopes, M.L., Ladeira, M., 1996. Role of specimen geometry, soil height

and sleeve length on the pull-out behaviour of geogrids. Geosynthetics

International 3 (6), 701719.

Moraci, N., Montanelli, F., 1995. Comportamento a breve ed a lungo

termine di due geosintetici con funzione di rinforzo. Associazione

Geotecnica Italiana. IXX Convegno Nazionale di Geotecnica. vol. 1,

Pavia, pp. 369380.

Moraci, N., Montanelli, F., 2000. Analisi di prove di sf`lamento di

geogriglie estruse installate in terreno granulare compattato. Rivista

Italiana di Geotecnica N. 4/2000, pp. 521.

Moraci, N., Recalcati, P., 2005. Pullout resistance of extruded geogrids

embedded in a compacted granular soil. Sixteenth International

Conference on Soil Mechanics and Geotechnical Engineering, Vol. 3,

Osaka, Japan, pp. 13891392.

Moraci, N., Romano, G., Montanelli, F., 2004. Factors affecting the

interface apparent coefcient of friction mobilised in pullout condi-

tion. Third European Geosynthetics Conference, vol. 1, Monaco,

pp. 313318.

Ochiai, H., Hayashi, S., Otani, J., Hirai, T., 1992. Evaluation of pull-out

resistance of geogrid reinforced soils. Proceedings of the International

Symposium on Earth Reinforcement Practice, Fukuoka, Kyushu,

Japan, 141146.

Palmeira, E.M., Milligan, G.W.E., 1989. Scale and other factors affecting

the results of pull-out tests of grid buried in sand. Geotechnique 11 (3),

511524.

Raju, D.M., 1995. Monotonic and cyclic pullout resistance of geosyn-

thetic. Ph.D. Thesis. The University of British Columbia, Vancouver,

Canada.

Raju, D.M., Lo, S.C.R., Fannin, R.J., Gao, J., 1996. Design and

interpretation of large laboratory pullout tests. Proceedings of the

Seventh AustraliaNew Zealand Conference on Geomechanics,

Adelaide, Australia, pp. 151156.

Sugimoto, M., Alagiyawanna, A.M.N., Kadoguchi, K., 2001. Inuence of

rigid and exible face on geogrid pullout tests. Geotextiles and

Geomembranes 19, 257277.

Voottipruex, P., Bergado, D.T., Ounjaichon, P., 2000. Pullout and

direct shear resistance of hexagonal wire mesh reinforcement in

weathered Bangkok clay. Geotechnical Engineering Journal 31 (1),

4362.

Wilson-Fahmy, R.F., Koerner, R.M., 1993. Finite element modelling of

soilgeogrid interaction with application to the behavior of geogrids in

a pullout loading condition. Geotextiles and Geornembranes 12,

479501.

ARTICLE IN PRESSN. Moraci, P. Recalcati / Geotextiles and Geomembranes 24 (2006) 220242242

Factors affecting the pullout behaviour of extruded geogrids embedded in a compacted granular soilIntroductionPrevious experimental studiesExperimental researchTest apparatusTest materialsTest procedureExperimental plan

Analysis of test resultsPullout resistanceInterface apparent coefficient of frictionDisplacements

ConclusionsReferences

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