effect of geogrid geometry on interface resistance …...size. the geogrid with original aperture...

Effect of Geogrid Geometry on Interface Resistance in a Pullout Test

Jongwan Eun1, Ph.D., P.E.; Ranjiv Gupta2, Ph.D., P.E.; Jorge G. Zornberg3, Ph.D., P.E.

1Assistant Professor, Univ. of Nebraska-Lincoln, Lincoln, NE. E-mail: [email protected] 2Project Engineer, Geosyntec Consultants, Phoenix, AZ. E-mail: [email protected] 3Professor and W. J. Murray, Jr. Fellow in Engineering, Civil, Architectural & Environmental Engineering, Univ. of Texas at Austin, Austin, TX. E-mail: [email protected]

Abstract

This paper presents the effect of aperture size on the low displacement stiffness response of geogrids subjected to pullout loading. The aperture size of geogrid was varied by cutting ribs of geogrids in the pullout tests. Two types of geogrids were tested at two normal pressures (21 kPa and 35 kPa). The Soil-Geosynthetic Composite (SGC) model was used to compute the low displacement interface stiffness (KSGC) of the geogrids. Based on the analysis of laboratory tests using SGC model, the results showed response of geogrids was highly dependent on the aperture size. The geogrid with original aperture size showed the highest KSGC value. As the size of the aperture increased, the KSGC decreased possibly due to reduction in the passive resistance of transverse members and the loss of confinement at the junctions of the geogrid.

INTRODUCTION

Geogrids have been widely used in stabilization of pavements for several decades. The performance of reinforced flexible pavements is governed by the interaction mechanisms between soil and geosynthetic. The interaction of the geogrid with surrounding soils consists of the passive resistance due to the thickness of rib, the friction of the surface of rib and the confinement of soil in the aperture due to the rib. Generally, the interaction developed between the soil and the reinforcement is a function of soil type, reinforcement type and how they are linked with each other (Teixeira et al. 2007). Actually, these factors are interrelated, and the combined effect of these factors results in complex interactions. Hence, appropriate laboratory test incorporating these variables should be to quantify the interaction mechanisms between the soil and the reinforcement.

Previous studies have focused on investigating the performance of geogrid reinforcement in flexible pavements using laboratory confined tests (Sugimoto et al. 2001, Palmeira 2004, Bergado et al. 2008), because they can provide: (a) the ability to capture the mechanism of lateral restraint; (b) parameters for mechanistic-empirical design; (c) repeatability of test results; (d) a parameter that distinguishes between the performance of various geosynthetics; (e) sensitivity to low displacement

Geotechnical Frontiers 2017 GSP 280 236

© ASCE Eun, J., Gupta, R., and Zornberg, J.G. (2017). “Effect of Geogrid Geometry on Interface Resistance in a Pullout Test.” Proceedings of the Geotechnical Frontiers 2017 Conference, Geotechnical Special Publication (GSP) 280, Geo-Institute of ASCE, 12-15 March, Orlando, FL, pp. 236-246.

magnitudes; and (f) convenience to conduct in the laboratory. Based on these advantages, a pullout test in a confined soil with monotonic loading has been used to reduce the variability in test results and to allow for the realistic measurement of the interface mechanisms. The geogrid geometry is a significant factor influencing pullout behavior of geogrid embedded in soils. Geogrid with comparatively large apertures, unlike other reinforcements (e.g. geotextiles) can sustain outer loading by providing both passive and frictional resistance components by transverse and longitudinal members (Teixeira et al. 2007, and Palmeira et al. 2004, 2008). Stress distribution between transverse and longitudinal members of the geogrid is affected by the geogrid geometry. However, there is much uncertainty about the complex influence of geogrid geometry. This research evaluated the effect of the geogrid geometry associated with various factors on the pullout behavior by using pullout stiffness at the interface between the geogrid and soil. The Soil-Geosynthetic Composite (SGC) model was used to illustrate analytically the interfacial mechanism governing reinforced soil with geogrid. The interface stiffness (KSGC) obtained from the model was evaluated to quantify the effect of the geogrid. A series of pullout tests was conducted to examine the pullout behavior of the geogrid in a confined soil and to determine the stiffness. Based on the results, the combined effects of the geogrid geometry associated with the type of the reinforcement, the confining pressure on the specimen, and the orientation of the specimen on the pullout behavior were investigated. SOIL-GEOSYNTHETIC COMPOSITE (SGC) MODEL Geosynthetic load-strain relationship. A load-transfer mechanism of geosynthetic in a confined soil demands to properly simulate the shear stress generated at the interface between the geosynthetic and the soil. To model the mechanism, an infinitesimal geosynthetic element subjected to force (F) in the pullout direction and to the shear stresses (τ) along both surfaces of the geosynthetic element of length (dx) surrounded soil mass can be assumed (Figure 1). Then, the force equilibrium can be given in differential form as follows: ( ) − ( ) − ( ) = 2 ( ) (1)

( ) = ( ) (2)

On the other hand, assuming that strain ε(x) develops in the dx due to the

change in confined force between two points in the element, the confined force and strain are related through confined stiffness (Jc) of the geosynthetic and is given as:

( ) = ∙ ( ) = ( ) (3)

Because the strain developed in the dx can then be related, the F can be

described as a differential form.


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used to determine the response of geosynthetic for given displacement increment. This can then be translated to quantify the soil-geosynthetic response to obtaining a measurement for a lateral restraint mechanism developed in the reinforced flexible pavements by using pullout test data. Thus, the equations were solved to obtain the relation between confined force and displacement in terms of model parameters as shown below. ( ) = 4 ∙ ∙ ( ) = ∙ ( ) (8)

The force and displacement at any given point x throughout the geosynthetic can be related by model parameters i.e., the yield shear stress (τy) and confined stiffness (Jc) of the soil-geosynthetic system. A coefficient of interface stiffness (KSGI) enables to evaluate soil-geosynthetic interaction (Gupta 2009, Zornberg et al. 2009). MATERIALS AND METHODS

Geogrid. Two different geogrid products, GG1 and GG2 were used as a reinforcement for the pullout test series (Figure 2). GG1 is comprised of knitted polypropylene (PP) yarns, crafted into a stable, interlocking pattern, and then coated for protection from installation damage. On the other hand, GG2 is an integrally formed, punched-and-drawn polypropylene (PP) grid featuring raised protrusions at each rib intersection to provide a structural abutment when placed between soil layers. The properties of GG1 and GG2 were listed in Table 1. The geogrids were prepared with dimensions of 0.6 m length and 0.45 m width for pullout test (ASTM D6706). Four different geometries―the original geogrid, the geogrid with only half of the transverse members, the geogrid with a doubled opening size and the geogrid with only longitudinal members―were used in this study. The designated specimen was prepared by cutting transverse members using pliers. The GG2 with half transverse members was not tested due to its large aperture size. LVDT. Five LVDTs were used to measure displacements at locations with a horizontal spacing of 100, 200, 300, 450, and 600 mm from the front end of the specimen, named LVDT 1 to 5. The displacement profile throughout the length of the geogrid could be monitored by installing LVDTs at various locations. The displacement rate of testing was set to 1mm/min, (ASTM D6706). The displacement of the specimen occurred as the specimen started to move due to pullout force. Soil. Monterey No. 30 sand was used as the backfill material for pullout testing. Monterey No. 30 sand is a clean and poorly graded, which was classified as SP according to the Unified Soil Classification System (USCS) (Zornberg et al. 1998). The test was conducted at the relative density of 50%.

Pullout test. The pullout test equipment consisted of a steel box (1.5 m × 0.6 m × 0.3 m), reaction frame, and applying pullout system (Figure 3a). The front end of the box had an opening of 50 mm and had two sleeves of 75 mm length to minimize the influence of the frontal box wall on pullout test results. In the front of the pullout box, the roller grips and its support trolley were designed to avoid stress concentration at


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the geosynthetic reinforcement. In the pullout box, the steel plates were used as the reaction frame system with wooden boards (Figure 3b). Six air cylinders were used for applying normal pressure on the surface of soils. The reaction frame system is a reliable way to apply a constant confining pressure on top of the geosynthetic specimen. Two hydraulic pistons were attached to the both side of the pullout box to apply pullout force on the specimen. The electric pump enabled better control over the rate of testing, since it could be independently controlled using the flow valve attached to it. The displacement transducers were attached to the system enabling faster data acquisition.

Table 1. Properties of geogrids used in the study.

Property Test method Units GG1 GG2

Rib shape Observation - Rectangular Rectangular

Rib Thickness Calipers mm 0.5 0.76

Norminal Apeture size

Calipers mm 15 × 15 25 × 33

Junction efficiency GRI-GG2-87 % - 93

Flexural Rigidity ASTM D1388-

96 mg-cm 100,000 250,000

Aperture Stability modulus

Kinney (2001) m-N/deg 0.44 0.32

Minimum true initial modulus ASTM D6637-

01

kN/m 250 250

MD kN/m 350 400

XD Tensile strength at

2% strain ASTM D6637-01

kN/m 5 4.1

MD kN/m 7 6.6

XD

Figure 2. Geogrid used for a pullout test: (a) GG1; and (b) GG2.

(b)(a)


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AT SMALL

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L DISPLAC

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of the soil in

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CEMENT R

rameter (KS

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(b)

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as compared.dentical, the s not show ding to the the pullout

oss machine n that of the longitudinalaperture of

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fromfonthe of gpretest Effobtloncasgeoonlthatpul EffcomKSG

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Fty

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m the regrental pullout f

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geogrid geomssure on theting data in a

fect of geogtained for Gngitudinal on

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fect of geomparison of GC in high coference of Ke of the orig

ogrid includithe ribs is apults, the effistance when

Figure 6. Coype betweenessure betw

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GG1 (orig.)

GG2 (orig

GG1 (longi.)

ession analyforces (FiguC shows a remetry on pule specimen, all cases wer

grid geomeGG1 and GGnly membersg different g

her than that nal membersG1 has stronistributes mo

ogrid geomKSGI under

onfining preKSGC betweenginal geogriding longitudipplied stronfect of trann comparativ

omparison on GG1 and Geen 21 and

MD and

ogrid geomKSGC for dif

ginal and the

g.)

GG2 (longi.)

)

(a)

sis with leaure 5c) decreelatively linellout behaviothe geogrid

re interpreted

etry and gG2 with the

. The slope ogeometries. obtained for, the value o

nger longitudore to the lon

metry and different co

essure was hn high and ld without alinal only megly when th

nsverse memvely high con

of KSGC of diGG2 in 21 k35 kPa of Gd XD in 21

metry and fferent orien

e double aper

35 kPa

21 kPa (

ast square meased with iear slope witor as well as

d orientationd.

eogrid typeoriginal witof the GG1 iThe KSGC o

r GG2 by apof KSGC of Gdinal membengitudinal m

confiningonfining prehigher than tlow confininltering the apembers. Thishe transversember contribnfining pres

ifferent geokPa of confiGG1; and (ckPa of conf

geogrid orntation of gerture size. Th

a (orig.)

21 kPa (orig

35 kPa (lo

longi.)

method. The increasing dthin 2.5 mms other factorn, and the ge

e. Figure 5thout alterinis stiffer thaobtained forpproximatelyGG1 is arouners than tran

members in G

pressure. essure such athat in low cng pressure waperture thans was becaue members abutes much ssure is appli

ograid geomining pressuc) the orientfining pressu

rientation. eogrid combihe GG2 was

g.)

ongi.)

(b)

increasing displacement

m. To quantifrs such as theogrid type,

5a comparedng the apertuan that of ther GG1 for ty 50%. Hownd six times nsverse memGG1 than GG

Figure 6b as 21 and 3confining prwas much la

n that in the use the confinare present.

more to toied to the ge

metry: (a) theure; (b) the tation of GGure.

Figure 6c ined with ths tested unde

XD (orig.)

MD (orig.)

XD (

M

rate of the t. However, fy the effect he confining

the pullout

d the KSGC

ure and the e GG2 in all the original

wever, in the higher than

mbers, so the G2.

shows the 35 kPa. The ressure. The arger in the case of the

nement due From these

otal pullout ogrid.

e geogrid confining

G2 between

shows the he geometry er 21 kPa of

XD (doub.)

long.)

MD (doub.)

MD (long.)

(c)


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confining pressure. The area of transverse ribs in the original case without altering the aperture was increased if the specimen was tested in the MD instead of the XD. However, as the number of transverse member decreases and the area of aperture increases in other cases to control the geometry, the values of KSGC are reduced rapidly in the MD, because the confinement due to transverse members for passive resistance is less than in the XD. As in the previous case, the confinement due to ribs was found to strongly influence the KSGC. CONCLUSIONS This paper presents the effect of geogrid geometry, geogrid type, confining pressure, and geogrid orientation on the pullout behavior of geogrid reinforced soil. Based on results of a series of pullout tests, the interface stiffness (KSGC) was evaluated at small displacement range (< 2.5 mm). The value of KSGC was found to decrease with increasing aperture size of the geogrid specimen. This was to the reduction in the passive resistance of transverse members of the geogrid and loss of confinement at the junctions. The difference of KSGC between high and low confining pressure was much larger in the case of the original geogrid than that in the case of the geogrid with only longitudinal member. The values of KSGC are reduced rapidly in the MD because the confinement due to transverse members for passive resistance is less than in the XD Based on the results obtained in this study, it was found that geogrid geometry strongly influences the pullout behavior of geogrids under small displacements. REFERENCES Bergado, D.T., Teerawattanasuk, C., (2008), “2D and 3D numerical simulations of

reinforced embankments on soft ground”, Geotextiles and Geomembranes Vol. 26, No.1, pp 39–55.

Gupta, Ranjiv, (2009), “A Study of Geosynthetic Reinforced Flexible Pavement System”, Ph.D. Dissertation, the University of Texas at Austin, Texas, USA.

Li, C., (2005), “Mechanical Response of Fiber-Reinforced Soil”, Ph.D. Dissertation, the University of Texas at Austin, Texas, USA.

Sugimoto, M., Alagiyawanna, A.N.M. and Kadoguchi, K. (2001), “Influence of rigid and flexible face on geogrid pullout tests”, Geotextiles and Geomembranes, Vol. 19, No.5, 257-277.

Palmeira, E.M. (2004), “Bearing force mobilization in pullout tests on geogrids”, Geotextiles and Geomembranes, Vol. 22, No.6, pp 481-509.

Palmeira, E.M. (2008), “Soil-geosynthetic interaction: Modeling and Analysis”, Mercer Lecture, presented at 4th European Conference on Geosynthetics-EuroGeo4, Edinburgh 2008.

Teixeira, S.H.C., Bueno, B.S. and Zornberg, J.G., (2007), “Pullout Resistance ofIndividual Longitudinal and Transverse Geogrid Ribs”, Journal of Geotechnical and Geoenvironmental Engineering, Vol. 133, No. 1, January 1, 2007, pp. 37-50.


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Zornberg, J.G., Ferreira, J.A.Z., Gupta, R.V., Joshi, R.V., and Roodi, G.H. (2009). Geosynthetic-Reinforced Unbound Base Courses: Quantification of the Reinforcement Benefits. Center for Transportation Research (CTR), Report No. FHWA/TX-10/5-4829-1, Austin, Texas, December 2009, Revised February 2012, 170 p.

Zornberg, J.G., Sitar, N., and Mitchell, J.K. (1998). “Limit Equilibrium as Basis for Design of Geosynthetic Reinforced Slopes.” Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 124, No. 8, pp. 684-698.


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effect of geogrid geometry on interface resistance …...size. the geogrid with original aperture...

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