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Geotextiles and Geomembranes 28 (2010) 403–408

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Geotextiles and Geomembranes

journal homepage: www.elsevier .com/locate/geotexmem

Technical Note

Modeling of soil–woven geotextile interface behavior from direct sheartest results

Anubhav*, P.K. BasudharDepartment of Civil Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India

a r t i c l e i n f o

Article history:Received 23 May 2009Received in revised form12 November 2009Accepted 3 December 2009Available online 24 December 2009

Keywords:GeotextileInterfaceDirect shear testNon-linear modelSoftening behavior

* Corresponding author. Tel.: þ91 512 259 7029; faE-mail addresses: anubhavs@iitk.ac.in (A

(P.K. Basudhar).

0266-1144/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.geotexmem.2009.12.005

a b s t r a c t

Apart from other factors, the performance of geosynthetic reinforced soil structures depends also on thecharacteristics and behavior of the interface between soil and geosynthetic. Experiments were conductedin a direct shear test apparatus to study the shear force–displacement behavior at the soil–geotextileinterface using two differently textured woven geotextiles. Analyzing the data so obtained a non-linearconstitutive model has been presented for predicting both the pre-peak and the post-peak interfacebehavior. The predictions made by the developed model are found to be in good agreement withexperimental data obtained from direct shear tests.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

In reinforced soil-slopes, the soil–geotextile interfaces areusually the weakest portions of the system. If at the interface thepeak shear strength exceeds the residual shear strength, there isa possibility of progressive failure arising out of large postconstruction deformations. Based on laboratory model test, Kriegerand Thamm (1991) demonstrated that knowledge of the values ofthe coefficients of friction of the geotextile–soil and geotextile–geotextile interfaces are essential for a realistic failure assessmentof geotextile reinforced soil walls. Soil–reinforcement interfaceproperties have significant influence on the lateral displacement inupper layers of a reinforced soil wall. Thus, study of the interactionat the soil–geotextile interface is very important from stabilityconsiderations and drawing special attention of the geotechnicalengineering community engaged in designing such structure.Several investigators (Desai and El-Hoseiny, 2005; Karpurapu andBathurst, 1995; Rowe and Ho, 1996; Sawwaf, 2007) incorporatedsuch interface properties in numerical simulation of reinforced soilwalls and slopes.

The frictional behavior at the soil–geosynthetic interface isusually obtained by direct shear tests. Though the shear strength of

x: þ91 512 259 7395.nubhav), pkbd@iitk.ac.in

All rights reserved.

soil–geosynthetic interface has been investigated by conductingother tests, such as tilt table tests (Wu et al., 2008), direct shear testis still the most common testing method for geotextiles and hasbeen used by several investigators (Bergado et al., 2006; Lee andManjunath, 2000; Palmeira, 2009). The direct shear tests have alsobeen used to study the interface shear strength of soil with othertypes of geosynthetic reinforcements such as geogrids, tire shreds,rubber chips, geofoam (Bernal et al., 1997; Liu et al., 2009; Xenakiand Athanasopoulos, 2001).

For simulating the interface behavior a linear elastic model withMohr–Coulomb criterion is commonly used. Experimental obser-vations made from such direct shear tests usually show that theforce–displacement relationship is non-linear till a peak is attained,beyond which softening behavior is observed. Few studies on themodeling of geosynthetic interface are due to Esterhuizen et al.(2001), Gilbert and Byrne (1996), Reddy et al. (1996), Seo et al.(2004). Reddy et al. (1996) used hyperbolic model for non-linearbehavior, however, the strength softening at large displacementwas not considered. Strength softening was considered by Ester-huizen et al. (2001) and they suggested that plasticity models aremore suitable than elasticity models. Based on disturbed stateconcept Seo et al. (2004) presented models for both pre-peak andpost-peak behavior of geomembrane–geotextile interface. Thesemodels were verified for clay–geomembrane or geotextile–geo-membrane interfaces.

Experimental studies on sand–geotextile interface behaviorhave been conducted using a modified direct shear box. Two

Table 1Properties of Kalpi sand.

D10 (mm) D30 (mm) D50 (mm) emax emin Gs Cu Cc

0.26 0.43 0.58 0.78 0.54 2.66 2.58 1.06

0

50

100

150

200

250

0 50 100 150 200 250Normal Stress (kPa)

Shea

r St

ress

(kP

a)

p 46=φ

r 36=φ

Fig. 1. Direct shear test results for Kalpi sand at Dr¼ 70%.

Anubhav, P.K. Basudhar / Geotextiles and Geomembranes 28 (2010) 403–408404

differently textured multifilament woven geotextiles were used inthis study. Based on the experimental observations a constitutivemodel appropriate for geosynthetic interfaces has been presentedhere. The model parameters can be obtained for any specificproblem by conducting such tests. Using these model parametersthe back fitted stress–displacement response has been obtainedand compared with the direct shear test data.

2. Experimental studies of soil–geotextile interface

2.1. Test materials

Kalpi sand has been used in this study. It is a natural river sandfrom Kalpi region of Yamuna river basin situated in the northernpart of India. Kalpi sand is abundantly available in this region. Thebasic properties of Kalpi sand are given in Table 1. As per UnifiedClassification System, Kalpi sand is characterized as poorly gradedsand (group symbol – SP). Scanning Electron Microscopy showedthat the shapes of the particles are highly angular. However, forreasons of space and brevity these are not presented here.

In reinforced soil structures or foundation beds, the geo-synthetic reinforcements are generally enmeshed in between twocompacted sand layers. Therefore, all experiments were conducted

Fig. 2. Enlarged scanned view of coar

by placing the sand at a relative density of Dr¼ 70%. Shear strengthparameters for Kalpi sand were obtained from direct shear test. AtDr¼ 70%, the peak friction angle and residual friction angle wereestimated as 46� and 36� respectively (Fig. 1).

Two types of multifilament (polyester) woven geotextiles onewith fine texture (FTG) and other with coarse texture (CTG) wereused in the present study. Fig. 2 shows the magnified scannedviews of the two geotextile specimens. This was obtained byplacing the specimens directly on a photo scanner (HP ScanjetG4010). The difference in texture can be identified from Fig. 2. FTGhas lower thickness and mass per unit area as compared to CTG.The apparent opening size of FTG is 0.1 mm whereas CTG has noopenings. The properties of the geotextiles as determined are givenin Table 2.

2.2. Test procedure for interface shear displacement properties

As per ASTM D 5321 (2008), both square and rectangular shearboxes could be used for finding the interface shear displacementcharacteristics. These boxes should have a minimum dimensionthat is greater than 300 mm; 15 times the D85 of the coarse soilused in the test, or a minimum of five times the maximum openingsize (in plan) of the geosynthetic tested. These dimensions areguidelines based upon requirements for testing most combinationsof geosynthetics and soils. However, smaller shear boxes could beused if it can be shown that the data generated using smallerdevices contains no scale or edge effects when compared to theabove-mentioned minimum size devices. It was observed byvarious researchers (Jewell and Wroth, 1987; Palmeira, 1988;O’Rourke et al., 1990; Takasumi et al., 1991) that apparatus size doesnot affect significantly the friction angles for cohesionless sandshaving ratio of mean particle size to length of the box in the rangeof 50–300.

The sand used in present study has D50¼ 0.58 mm &D85¼ 0.95 mm and maximum geotextile opening size is 0.1 mm.Considering the grain size of Kalpi sand and geotextile opening size,smaller size shear box (60 mm� 60 mm) also meets the minimumrequirement for interface testing and the same was used in thepresent study after suitably modifying the box for clamping thegeotextile specimens. The schematic diagram of the modified shearbox is shown in Fig. 3.

The specimens from the roll were cut using sharp cutting edgeand clamped in the lower box. All precautions were taken toproperly fix the geotextile specimen such that there were nowrinkles in the specimen. To prevent the geotextile sample fromstretching along smooth dummy base, the base was made rough bygluing sand paper on it. The upper half of the shear box was then

se and fine textured geotextiles.

Table 2Properties of geotextiles.

Geotextile Thickness (mm) Mass per unitarea (gm/m2)

Apparent openingsize (mm)

Tensile strength (kN/m) Puncher resistance (kN)

M/c direction Cross M/c direction

ASTM D 5199 ASTM D 5261 ASTM D 4751 ASTM D 4595 ASTM D 4833

FTG 0.35 145 0.1 mm 25.3 28.2 0.5CTG 0.57 226 Nil 30.2 42.8 0.85

Fig. 3. Modified shear box assembly.

Anubhav, P.K. Basudhar / Geotextiles and Geomembranes 28 (2010) 403–408 405

placed on the lower one and assembled properly by inserting pins.The soil (Kalpi sand) was then placed over the geosynthetic in theupper half of shear box and compacted by tamping to a desiredrelative density of 70%. The specimen was sheared at a displace-ment rate of 0.125 mm/min. Tests were conducted till large defor-mation occurs. In this assembly, the entire lower box is coveredwith geotextile and hence no area correction is required.

3. Test results

For the two woven geotextile–sand interfaces, the shear stressversus horizontal displacement curves for different normal stressare shown in Fig. 4(a) and Fig. 5(a). It is observed that for both thetypes of geotextiles, as expected, the shear displacement required tomobilize peak shear strength increases with increase in normalstress. Figs. 4(b) and 5(b) show the variation of the peak and residualshear stress with normal stress for FTG and CTG respectively.

Similarly, the peak friction angle value of sand–CTG interface(42.2�) has been observed to be higher than that of sand–FTGinterface (36.7�) and the corresponding adhesion intercepts wereobtained as 6 kPa and 4 kPa respectively. In contrast the value offriction angle at large displacement for CTG (32.7�) and FTG (31.7�)showed only marginal difference; no adhesion intercepts wereobserved here.

4. Modeling of interface behavior

The complete shear stress–displacement response of the soil–geotextile interface is considered and modeled by breaking it up intwo segments namely a pre-peak behavior and a post-peakbehavior. Peak and large displacement (residual) shear stresses arecomputed using Mohr–Coulomb failure envelope. The generalapproach followed by Esterhuizen et al. (2001) has been used inthis study but the modeling of post-peak softening response ofgeotextile–soil interface is modified so that the predictions areconsistent with the observations made.

4.1. Pre-peak behavior

It can be seen from Figs. 4 and 5 that the initial shear stiffnessdepends on the normal stress and the tangent shear modulus varieswith the displacement. The procedure proposed by Duncan andChan (1970) was modified for modeling the non-linear pre-peakbehavior at the geosynthetic interface. The method was originallydeveloped for simulating stress–strain behavior obtained fromtriaxial test, which was subsequently modified and used for shearstress displacement behavior of interfaces (Reddy et al., 1996; Seoet al., 2004). The peak and residual shear stress for the interface arecomputed from linear failure envelope which can be representedby Mohr–Coulomb criteria:

sp or sr ¼ cþ sntan f (1)

where sp¼ peak shear stress, sr¼ large deformation shear stress,c¼ apparent cohesion intercept, sn¼ normal stress and f¼ frictionangle.

The non-linear normal stress dependent interface shearbehavior is represented by a hyperbolic equation of the formproposed by Kondner (1963):

s ¼ d

1Eiþ d

sult

(2)

where, s¼ shear stress, d¼ shear displacement, Ei¼ initial tangentshear modulus, sult¼ ultimate shear strength. For soil–geotextileinterfaces, an increase in normal stress will result in steeper shearstress–relative displacement curves and a higher strength and thevalue of Ei and sult therefore increase with increasing normal stress.This stress dependency is taken into account by using empiricalequations to represent the variation of Ei and sult with normalstress. The variation of Ei with sn is represented by an equation ofthe form suggested by Janbu (1963):

Ei ¼ KPa

�sn

Pa

�n

(3)

where, K¼modulus number, n¼modulus exponent, both aredimensionless numbers and Pa¼ atmospheric pressure. Variationof sult with normal stress is accounted for by correlating failureshear stress obtained by Mohr–Coulomb criteria to it as:

sp ¼ Rf $sult (4)

where, Rf¼ failure ratio and as sp is always smaller than sult, thevalue of Rf will always be less than unity. As proposed by Duncanand Chan (1970), the instantaneous slope of the shear stress–displacement curve (Et) for any normal stress can be expressed by:

Et ¼ KPa

�sn

Pa

�n�1� Rf

scþ sn tan f

�2

(5)

Pre-peak interface behavior can be well represented by theabove hyperbolic model. For obtaining the peak shear strengthMohr–Coulomb failure envelope (eq. (1)) has been used.

4.2. Post-peak behavior

The generalized stress–displacement relationship for clay–geo-membrane interface is shown in Fig. 6(a). Immediately after

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10Horizontal Displacement (mm)

)aPk(ssertS

raehS

0

50

100

150

200

0 50 100 150 200

Normal Stress (kPa)

Shea

r St

ress

(kP

a)

Shear stress vs. Horizontal Displacement Shear Stress vs. Normal Stress

07.364

==

p

kPacφ

07.31=rφ

165 kPa

115 kPa

65 kPa

25 kPa

a b

Fig. 4. Kalpi sand and FTG interface behavior.

Anubhav, P.K. Basudhar / Geotextiles and Geomembranes 28 (2010) 403–408406

reaching the peak, clay–geomembrane interfaces show sharpreduction in shear strength and after some displacement, the rateof shear strength degradation reduces and finally reaches toresidual strength value. Considering above post-peak behavior,Esterhuizen et al. (2001) developed a model for post-peak responseof clay–geomembrane interfaces following the general procedureoutlined by Turnbull and Hvorslev (1967) to represent the softeningbehavior of soils with displacement along the failure surfaces. Seoet al. (2004) also used a similar approach in one of the models forrepresenting the post-peak behavior of interfaces. In both the casesthe normalized shear strength degradation and normalizeddisplacement softening curves were represented by hyperbolicrelationship.

The experimental observations from the present study forsand–geotextile interfaces show a different post-peak stress–displacement relationship (Figs. 4 and 5) as compared to clay–geomembrane interface. The generalized stress–displacementrelationship as observed in this study for sand–geotextile inter-faces is presented in Fig. 6(b). In this case, after the peak, thestrength decreases slowly in the initial stages. Therefore, the post-peak response of sand–geotextile interface cannot be representedby the hyperbolic relationship suggested for clay–geotextileinterfaces. As such, in this study, taking into account the experi-mental observations, the approach suggested by Esterhuizen et al.(2001) has been modified and adopted for an improved simulationof the shear stress–shearing displacement behavior at the inter-face over a large displacement.

The post-peak shear strength reduction (sp� s), post-peakplastic shear displacement dp are defined in Fig. 6. Post-peak shearstrength is bounded by peak shear strength and residual shear

0

2040

6080

100120

140160

180

0 2 4 6 8 10Horizontal Displacement (mm)

Shea

r St

ress

(kP

a)

Shear stress vs. Horizontal Displacement

165 kPa

115 kPa

65 kPa

25 kPa

a

Fig. 5. Kalpi sand and CT

strength. Post-peak shear strength reduction (sp� s) is normalizedby the shear strength reduction from peak to residual value(sp� sr), represented by residual factor (Skempton, 1964):

R ¼ sp � ssp � sr

(6)

The experimental observation from present study of geotextile–sand interface shows that the relationship between plastic sheardisplacement and residual factor (normalized shear strengthdegradation) follows a S-Shaped curve (with R value ranging from0 to 1) which can be effectively represented by following equation:

R ¼ 1� exp��Adz

p

�(7)

The above equation is commonly used for growth and decayprocess. This equation is similar to the expressions suggested forcomputing the disturbance function in case of Disturbed stateconcept models (Park and Desai, 2000; Seo et al., 2004).

4.3. Determination of the model parameters and back fittedbehavior

The shear stress vs. displacement curves for FTG–sand and CTG–sand interfaces are respectively shown in Fig. 4(a) and Fig. 5(a).Using the procedure as described in Section 4.1, the pre-peakshear–displacement behavior was obtained. The model parametersused to describe the pre-peak behavior of FTG–Sand and CTG–Sandinterfaces are given in Table 3. To model the post-peak behavior,residual factors (normalized shear strength degradation) werecalculated from the experimental data and plotted against plastic

0

50

100

150

200

0 50 100 150 200Normal Stress (kPa)

Shea

r St

ress

(kP

a)

Shear Stress vs. Normal Stress

07.32=rφ

02.426

==

p

kPacφ

b

G interface behavior.

Clay-Geomembrane Interface

Esterhuizen et al. (2001)

Sand-Geotextile Interface

pτ

rτ

δ

τ

Residual

Peak

rp τ−ττ−τp

pδ

pτ

rτ

δ

τ

Residual

Peak

rp τ−ττ−τp

a b

Fig. 6. Generalized stress–displacement relationship for geosynthetic interface.

Table 3Model parameters for pre-peak and post-peak behavior.

Interface Pre-peak model parameters Post-peak model parameters

K Rf n A z

FTG–Kalpi sand 1374 0.51 0.5 0.879 1.510CTG–Kalpi sand 1815.7 0.57 0.51 0.754 1.851

0.0

0.2

0.4

0.6

0.8

1.0

0 1 2 3 4 5 6 7

δ p (mm)

R

FTG-Kalpi Sand Interface

).exp(1 zpAR δ−−=

Fig. 7. Normalized strength degrada

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10

Horizontal Displacement (mm)

)aPk( ssertS raehS

ExperimentalPredicted

FTG-Kalpi Sand

165 kPa

115 kPa

65 kPa

25 kPa

a b

Fig. 8. Experimental and pred

Anubhav, P.K. Basudhar / Geotextiles and Geomembranes 28 (2010) 403–408 407

shear displacement beyond peak (dp). For Sand–FTG and Sand–CTGinterfaces, the experimental values and fitted curves (using Eq. (7))are shown respectively in Fig. 7(a) and (b). The coefficients A and zappearing in eq. (7) were obtained by non-linear regression anal-ysis using MATLAB. Values of these coefficients for both the inter-faces are also given in Table 3. Very good fits were obtained for bothFTG and CTG, as indicated by their respective coefficient of deter-mination values (R2) 0.93 and 0.95. Using the model parametersgiven in Table 3, complete shear stress–displacement relationshipwas predicted for different normal stresses. The comparison ofexperimental results and fitted curves for Sand–FTG interface areshown in Fig. 8(a) exhibiting excellent agreement.

0.0

0.2

0.4

0.6

0.8

1.0

0 1 2 3 4 5 6 7 8

δ p (mm)

R

CTG-Kalpi Sand Interface

).exp(1 zpAR δ−−=

tion with plastic displacement.

0

20

40

60

80

100

120

140

160

180

0 2 4 6 8 10

Horizontal Displacement (mm)

Shea

r St

ress

(kP

a)

ExperimentalPredicted

CTG-Kalpi Sand

165 kPa

115 kPa

65 kPa

25 kPa

icted interface behavior.

Anubhav, P.K. Basudhar / Geotextiles and Geomembranes 28 (2010) 403–408408

It is seen that for CTG–Sand interface the post-peak behaviorexhibits appreciable strain softening response. The experimentalshear stress–displacement curves for CTG–Sand interface (withdifferent normal stresses) and back fitted curves as obtained fromthe model described above are shown in Fig. 8(b). Here also, theback fitted curves showed excellent agreement with experimentaldata over the complete stress–displacement range. However, it isto be noted that the accuracy of the predictions is very muchdependent on the computed values of peak shear strength. Forboth CTG–Sand and FTG–Sand interfaces, linear Mohr–Coulombstrength envelope was found to be suitable for predicting thepeak and residual stresses and the same was used in the model.Many researchers reported that geosynthetic–soil interfacebehavior measured by direct shear apparatus shows non-linearshear stress–normal stress (s–s) relationship (Giroud et al., 1993;Esterhuizen et al., 2001). In such cases, non-linear failure envelopsmay be used for prediction of peak shear strengths.

5. Conclusions

Based on the above study, following conclusions can be drawnfor the interface behavior of two differently textured multifilamentgeotextiles as selected and the Kalpi sand (poorly graded mediumsand).

Peak interface shear strength is significantly higher for CTG(coarse textured geotextile with no openings) as compared to FTG(fine textured geotextile with apparent opening size of 0.1 mm). But,CTG–Kalpi sand interface shows greater degree of strain softeningthan the FTG–Kalpi sand interface. Peak and large displacementshear strength envelope as well can be defined by Mohr–Coulombfailure criterion for both CTG– and FTG–sand interface.

The parameters of the developed simplified model to repre-sent the interface behavior can be estimated by back analyzingthe test data. With the use of these estimated parameters theback fitted curves showed excellent agreement with the experi-mental data over the complete stress–displacement ranges. As theshear stress–displacement relationship depends upon the specificproperties of the geotextile and soil, the developed model canonly be used for the interfaces which show similar characteristicsand response.

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