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Geotechnik im BauwesenGeotechnical Engineering

Univ.-Prof. Dr.-Ing. Martin Ziegler

Investigation of the Confining Effect of Geogrid Reinforced Soilwith Plane Strain Model Wall Tests

ABSTRACT Large scale laboratory testing of unreinforced and geogrid reinforced soil has been carried out atRWTH Aachen University to investigate the confining effect of geogrids in reinforced soil inprinciple. With a series of triaxial tests the development of the additional confining effect due tothe reinforcement has been confirmed. This confining effect is necessary for the increase of theload bearing capacity of the soil.

Furthermore, to determine the development of shear zones within the geogrid reinforced soil atest apparatus with a transparent sidewall has been constructed to monitor soil particledisplacements and rotations during a controlled facing movement. The evaluation of the soil

particle movements confirmed the development of the confining effect and explains thedevelopment of an earth pressure from the composite material which is far less than the activeearth pressure from the unreinforced soil.

1. IntroductionThe immense contribution of geogrids to the strength of reinforced soil is well known in bothscience and practice. The vast amount of research that has been carried out in this area isreflected in the German design standard for geosynthetic reinforced soil (EBGEO, 2009) thathas been updated recently. The earth pressure acting on the facing of geogrid reinforcedretaining walls is assumed herein to be far less as it used to be in the former version and assuggested by other standards, such as (BS 8006, 1995). The beneficial effect of geogrids to thestability of soil structures as well as economic factors also lead to an increasing acceptance ofgeogrid reinforced soil as a construction technique. (Ziegler, 2009) reported growth rates of upto 15 % for the use of geogrids in Germany.

The mechanism of the geogrids leading to the increased strength of the soil has been identifiedas confining effect (Ling & Tatsuoka, 1994). To investigate this confining effect in principle andespecially in order to visualize it, large scale triaxial and plane strain laboratory testing has beencarried out.

2. Triaxial testingLarge scale triaxial tests have been carried out to investigate the influence of the stress level,the type of filling soil and the vertical reinforcement spacing to the reinforcing effect of geogrids.The dimensions of the triaxial cell have been chosen to allow testing at 1:1 scale.

Test Set-upThe soil specimens were 500 mm in diameter and approximately 1100 mm high (Fig. 1). Theconfining pressure has been applied by vacuum. Two types of soil have been tested, i.e. acrushed base course material (d 50 = 12 mm) and a medium sand (d 50 = 0.5 mm) that have beencompacted to both 95% and 100% proctor density.

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Tests have been conducted with a constant strain rate of 0.1 %/min, while plunger load, geogridstrains and average radial strains have been recorded. The analysis made in this paper is basedon results of tests with a biaxial polypropylene geogrid with 30 kN/m nominal strength made ofwelded pre-stretched flat bars. The aperture size of the grid was 32 mm x 32 mm and the tensileforce at 2 % strain was 12 kN/m as stated by the manufacturer.

Test ResultsThe test results that have been reported earlier by (Ruiken & Ziegler, 2008) indicate a significantincrease of the bearing capacity and reduction of deformations due to the geogrid reinforcement.The distribution of average radial strains over the height of specimen with a varying number ofreinforcement layers has shown that the development of radial strains is reduced significantlydue to the geogrids, even without attaching the geogrids to the elastic cover.

Fig. 1. Principal sketch of Fig. 2. Deformed unreinforced andtriaxial cell. reinforced triaxial specimens at the same load

level and corresponding mechanical models.

This confining effect of the geogrids, that has been observed also by (Moghaddas-Nejad &

Small, 2003) and (Eiksund et al., 2004), can be explained with the mechanical model given inFigure 2. The effect is similar to an additional confining pressure 3 acting homogeneously overthe whole height of the specimen if the vertical spacing between the reinforcement layers issmall enough. The magnitude depends on the degree of activation of the geogrids.Corresponding stress paths that take the additional confinement into account have beendetermined and drawn for the reinforced triaxial tests in (Ziegler & Ruiken, 2009).

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3. Plane Strain Testing Additional large scale plane strain testing of geogrid reinforced soil has been carried out todetermine the development of shear zones within the geogrid reinforced soil and to investigatethe advantageous reinforcement effect of reducing the lateral earth pressure on the facing,which has been suggested various times, e.g. (Clayton et al., 1993) and (Soong & Koerner,1997), and which has recently been measured by (Yang et al., 2009).

The basic mechanism suggested in (Soong & Koerner, 1997) for the stress condition of thebackfill soil directly behind the facing corresponds with the effect that has been observed duringtriaxial testing, i.e. a zone of unconfined soil between the geogrid layers, which is therefore notrestricted in deformation by the composite material itself, but has to be supported externallyeither by the confining pressure at triaxial testing or by the front facing of geogrid reinforcedretaining structures, respectively.

Fig 3. Apparatus for plane strain testing of reinforced soil.

Test Set-upThe apparatus shown in Fig. 3 has been constructed to carry out reinforced soil wall modeltesting under plane strain conditions as well as plane strain element testing under a constantconfining pressure. The maximum dimensions of the soil specimen are (H WD) 1m x 1m x0.45m. Front and back facing can be retracted independently, whereas the fixed sidewallsprovide the plane strain conditions. The 106 mm thick glass sidewall is designed for a maximumdeflection of 0.1 mm under surcharge loads up to 50 kPa that are being applied uniformly with a

compressed air cushion. The geogrids can be installed either without or with connection to thefacing, where the connection loads can be recorded.

Test have been conducted with the above described sand, which has been deposited with arainfall technique to more than 100 % proctor density. The friction parameters have beendetermined with direct shear tests to 7 between sand and glass at the sidewalls and to lessthan 2 between the facing elements and a thin latex membrane lubricated with silicone grease.

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Test Results

Horizontal earth pressure developmentThe horizontal earth pressure distribution over the height of the facing has been obtained from

20 elements (each 5 cm high). Redundancy was achieved by comparing the sum with the resultof one large load cell supporting the whole facing. In Figure 4 the earth pressure development isgiven for unreinforced and reinforced specimens, where the geogrids were not attached to thefacing. The curves start with a difference in the initial values. This is due to slight movement (upto 0.7 mm) of the facing elements during loading, which results in the activation of the geogridsalready during the loading sequence. For the comparison always the sum of the lateral stressesacting on the whole facing has been considered.

Fig. 4. Development of the horizontal earth pressure (grids not attached to the facing).

The results indicate a significant reduction of the active earth pressure down to 40% of those ofthe unreinforced soil, even without connecting the geogrids to the facing. The effect increaseswith an increasing number of reinforcement layers. (Tsukamoto et al., 1999) also carried outplane strain model wall tests with geogrid reinforced sand and observed similar significantreductions for the coefficient of the lateral earth pressure K a .

The distribution of the earth pressure over the height of the facing indicated a significantreduction of the lateral stresses below the upmost geogrid layer. The coefficient of the lateralearth pressure Ka seems to decrease therefore with depths. This observation supports theassumption made in the recently updated (EBGEO, 2009) for the earth pressure acting on theback of the facing of geogrid reinforced retaining structures with deformable facing elements,i.e. K a /2 at the lower 60 % of the structure.

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Particle displacements and rotationsIn order to visualize the confining effect of the geogrids and the development of shear zoneswithin the reinforced soil, digital pictures of the specimens have been taken during testing.These pictures have been processed afterwards evaluating particle displacements and rotations(PIV-method). The digital camera used for this purpose was remote controlled to avoidmisalignment from touching for the release and had a fixed focal length lens to achieve aminimum distortion. The resolution of the photographs has been identified as being theparameter with the biggest influence for the quality of the PIV-evaluation. Therefore, the camerahad a resolution of 15 Mpixel.

Fig. 5. Shear zone development from particle rotations (PIV)in unreinforced and reinforced specimens (grids not attached to the facing).

The evaluation of soil particle rotations after a facing retraction of 10 mm is shown for anunreinforced specimen as well as for specimens reinforced with two and five geogrid layers inFig. 5. The soil particles in the slip surface rotate counter-clockwise indicating a downwardmovement of the failure wedge behind the facing. It can clearly be seen that an increasingdegree of reinforcement results in a decreasing distance between slip surface and front facing.The small failure wedges of the reinforced specimens cause less pressure on the facing asthose of the unreinforced soil, which corresponds to the results of the lateral pressure presentedin Fig. 4.

The confining effect of the reinforcements is of major concern in this study. Therefore, by

focusing on a detail section of the specimen (0.3 m x 0.2 m) at the height of the upper geogrid atthe facing high resolution photographs of the sand were achieved. The total particle displace-ments within the same detail section are shown in Fig. 6 for the unreinforced and for thereinforced soil. The results clearly confirm the closer position of the slip surface to the front in thereinforced soil.

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Fig. 6. Total particle displacements of unreinforced and reinforced soilwithin the detailed section at the facing.

In Fig. 7 the horizontal displacements (left plot) and the soil particle rotations (right plot) aregiven for the detail section of the reinforced specimen after 10 mm facing retraction. With regardto the total displacements and the rotations of the sand particles, it is clear that in contrast to thedevelopment of a single failure plane in the unreinforced soil, various failure planes have beendeveloped due to the geogrid. However, it has to be noted that the various slip surfaces do notappear at once but develop one after the other, always at the position with the lowest resistanceof the soil. Once a sliding plane reaches a geogrid it is subsequently activated resulting in a localincrease of the strength of the composite material. The failure plane then jumps to anotherposition, wherever the resistance of the composite material of soil and geogrid is lower.Remarkable in this content is the fact that the failure plane seems to be orthogonal to thegeogrid, trying to avoid its activation. In other words, in the vicinity of the geogrids the soil isconfined horizontally, which becomes very apparent with regard to the evaluation of thehorizontal displacements in the left plot of Fig. 7.

Fig. 7. Horizontal particle displacements and particle rotationswithin the detailed section at the facing.

Considering the evaluation of the rotations of the sand particles in the right plot of Fig. 7, thedifferent colours of the slip surfaces indicate a different sense of rotation of the soil particles.Primarily, this indicates relative downward movement of the soil between the pink major shearzone (A) and the facing. It is the main failure wedge. Furthermore, the orange/red (B) colourindicates a secondary sliding plane within the main failure wedge, which is leading from the free

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end of the geogrid down- and inwards. The clockwise sense of rotation indicates a relativedownward movement of the soil between (A) and (B), which means that the soil underneath (B)is not confined by the geogrid and therefore pushed towards the facing.

For the whole model wall the mechanism observed in the detail section corresponds with anarching effect of the soil between the geogrid layers. This effect that has been assumed alreadyfrom the presented results of the triaxial testing, from FE-calculations for reinforced plane straincompression tests (Peng et al., 2000) and reinforced retaining structures (Pachomov et al.,2007) can be seen in the sliding planes in Fig. 5 that have been developed in the specimenreinforced with five geogrid layers.

The described mechanisms and the results of the horizontal displacements led to the idealizedmechanical model presented in Fig. 8 for the reinforced soil with geogrids that are not attachedto the facing. It is analogue to those derived from the triaxial tests (Fig. 2). Together with the factthat the primary sliding plane is shifted towards the facing due to the confining effect of the

geogrids, it delivers the explanation why the lateral pressure of geogrid reinforced soil has to befar less than those of the unreinforced soil.

Fig. 8. Mechanical model for the reinforced soil (geogrids not attached).

Summary and DiscussionThe confining effect of the geogrids has been shown with large scale triaxial tests and has beenvisualized in plane strain model wall testing. Both types of testing have shown a considerablereduction of deformations in the vicinity of the geogrids.

The confining effect led to a significant increase of the bearing capacity of the triaxial specimen.During plane strain model wall testing, the horizontal earth pressure on the facing has beenreduced by the geogrids. An arching effect and the corresponding zone of the soil between thegeogrids that cannot be supported has been identified.

From a soil mechanical point of view the assumption of a very low to moderate horizontal earthpressure on the back of the facing of reinforced retaining structures with deformable facings istherefore justified.

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AcknowledgementThe Authors would like to thank the Naue GmbH & Co. KG and Colbond B.V. for providing thegeogrids and for their financial support. Special thanks go to the Geosynthetic Institute that alsosupported part of this work under its GSI Fellowship Program.

ReferencesBS 8006 (1995): Code of Practice for Strengthened/Rei nforced Soils and other Fills. BSi ,

Milton Keynes, UK.EBGEO (2009): Berechnung und Dimensionierung von Er dkrpern mit Bewehrungseinlagen

aus Geokunststoffen. DGGT , Essen, Germany, Draft 02/2009 (in German).Clayton, C.R.I.; Milititsky, J. & Woods, R.I. (1993): Earth Pressure and Earth -Retaining

Structures. Blackie Academic & Professional , Glasgow, UK, 2 nd Edition.Eiksund, G.; Hoff, I. & Perkins, S. (2004): Cyclic Triaxial Tests on Reinforced Base Course

Material. Proc. EuroGeo3 , DGGT & igs, Munich, Germany, Vol. 2, 619-624.Ling, H.I. & Tatsuoka, F. (1994): Perfor mance of Anisotropic Geosynthetic-Reinforced Cohesive

Soil Mass. J. of Geotechnical Engineering , Vol. 120, No. 7, pp. 1166-1184.Moghaddas-Nejad, F. & Small, J.C. (2003): Resilient and Pe rmanent Characteristics ofReinforced Granular Materials by Repeate d Load Triaxial Tests. Geotechnical TestingJournal , ASTM, Vol. 26, Issue 2.

Pachomov, D.; Vollmert, L. & Herold, A. (2007): Der Ansatz des horizontalen Erddrucks auf dieFront von KBE- Konstruktionen. J. geotechnik , special issue 2007, pp. 129-136 (inGerman).

Peng, F.-L.; Kotake, N.; Tatsuoka, F.; Hirakawa, D. & Tanaka, T. (2000): Plane StrainCompression Behaviour of Geogrid- Reinforced Sand and its Numerical Analysis. J. Soilsand Foundations , Japanese Geotechnical Society, Vol. 40, No. 3, pp.55-74.

Ruiken, A. & Ziegler, M. (2008): Effect of Reinforcement on the Load Bearing Capacity ofGeosynthetic Reinforced Soil. Proc., 4th European Geosynthetics Conf., igs, Edinburgh,UK, Proc. on CD.

Soong, T.-Y. & Koerner, R.M. (1997): On the Required Co nnection Strength of GeosyntheticallyReinforced Walls. J. Geotextiles and Geomembranes , Vol. 15, pp. 377-393.

Tsukamoto, Y.; Ishihara, K.; Higuchi, T. & Aoki, H. (1999): Influence of Geogrid Reinforcementon Lateral Earth Pressures against Model Retaining Walls. J. Geosynthetics Int. , Vol. 6,No. 3, pp. 195-218.

Yang, G.; Zhang, B.; Lv, P. & Zhou, Q. (2009): Behaviour of Geogrid Reinforced Soil RetainingWall with Concrete- Rigid Facing. J Geotextiles and Geomembranes 27 (2009) , pp. 350-356.

Ziegler, M. (2009): Bodenbewehrung der Markt mit den gr ten Steigerungsraten. J. tis Tiefbau Ingenieurbau Straenbau, 9/2009, p. 32-34.

Ziegler, M. & Ruiken, A. (2009): Mater ialverhalten des Verbundbaustoffs "geogitterbewehrterBoden" aus groen triaxialen Druckversuchen. J. Geotechnik 32 (2009) , No. 3, p. 148-155.

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