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Full-scalefieldtestsongeosyntheticreinforcedunpavedroadsonsoftsubgrade

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Geotextiles and Geomembranes 24 (2006) 21–37

www.elsevier.com/locate/geotexmem

Full-scale field tests on geosynthetic reinforced unpaved roadson soft subgrade

Rudolf Hufenusa,�, Rudolf Rueeggerb, Robert Banjacc, Pierre Mayorc,Sarah M. Springmanc, Rolf Bronnimannd

aEMPA, Materials Science and Technology, CH-9014 St. Gallen, SwitzerlandbRueegger Systems, Solutions in Geotechnical Engineering, Vonwilstrasse 9, CH-9000 St. Gallen, SwitzerlandcInstitute for Geotechnical Engineering, Swiss Federal Institute of Technology, CH-8093 Zurich, Switzerland

dEMPA, Materials Science and Technology, CH-8600 Duebendorf, Switzerland

Received 16 February 2005; accepted 14 June 2005

Available online 20 October 2005

Abstract

A full-scale field test on a geosynthetic reinforced unpaved road was carried out, including compaction and trafficking, to investigate

the bearing capacity and its performance on a soft subgrade. The test track was built with three layers of crushed, recycled fill material.

The 1st layer was compacted statically, whereas the 2nd and 3rd were dynamically compacted. The geogrids were instrumented with

strain gauges to measure the short- and long-term deformations and the ongoing formation of ruts was assessed from profile

measurements. The various geosynthetics used for this reinforced unpaved road were found to have a relevant reinforcing effect only

when used under a thin aggregate layer on a soft subgrade. Under such conditions, ruts can form in the subgrade, mobilizing strains and

thus tensile forces in the geosynthetic. The achievable degree of reinforcement depends on the stiffness of the geosynthetic and is limited

by finite lateral anchoring forces.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Bearing capacity; Full-scale field test; Reinforcement; Rut formation; Soft subgrade; Unpaved road

1. Introduction

Geosynthetics have been used successfully to reinforceunpaved roads on soft subgrade for many years. Con-struction of reinforced temporary roads (Mannsbart et al.,1999) and bases for heavy machinery (Garcin and Murray,2003) are examples of short-term usage of the geosynthetic,where the main goal is to save fill material. In paved roads(Anderson and Killeavy, 1989; Zia et al., 2001) and railwaytracks (Ashpiz et al., 2002; Izvolt et al., 2001) the adoptionof geosynthetic reinforcement aims at a permanentimprovement of the bearing capacity and the longevity ofthe road. Geosynthetics are installed between subgrade androad to separate or to reinforce. If migration of fines is veryprobable, separation is an essential function (Al-Qadi

e front matter r 2005 Elsevier Ltd. All rights reserved.

otexmem.2005.06.002

ing author. Tel.: +4171 274 7341; fax: +41 71 274 7862.

ess: rudolf.hufenus@empa.ch (R. Hufenus).

et al., 1994; Al-Qadi and Appea, 2003), but with decreasingbearing capacity of the subgrade, the importance ofreinforcement increases significantly (Saathoff and Horst-mann, 1999).Numerous field trials and full-scale laboratory investiga-

tions have illustrated that geosynthetics used to reinforceunpaved roads on soft subgrade facilitate compaction(Bloise and Ucciardo, 2000), improve the bearing capacity(Floss and Gold, 1994; Huntington and Ksaibati, 2000;Meyer and Elias, 1999), extend the service life (Cancelli andMontanelli, 1999; Collin et al., 1996; Jenner and Paul,2000; Watts et al., 2004), reduce the necessary fill thickness(Bloise and Ucciardo, 2000; Cancelli and Montanelli, 1999;Huntington and Ksaibati, 2000; Jenner and Paul, 2000;Martin, 1988; Miura et al., 1990), diminish deformations(Chan et al., 1989; Jenner and Paul, 2000), and delay rutformation (Cancelli and Montanelli, 1999; Knapton andAustin, 1996; Meyer and Elias, 1999).

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Nomenclature

CBR CBR coefficientEV1 Young’s modulus for the 1st plate loading

(MPa)EV2 Young’s modulus for the plate reloading (MPa)

Evib dynamic stiffness (MPa)h road layer thickness (m)N number of standard axle passesT2% tensile strength at 2% strain (kN/m)w water content (%)gd dry density (kN/m3)

R. Hufenus et al. / Geotextiles and Geomembranes 24 (2006) 21–3722

The combination of geosynthetic reinforcement and fillhelp to spread the concentrated vertical loads and to inhibitlarge deformations and local failures (Su et al., 2002). Twomodes of action can be distinguished (Bourdeau, 1991;Miura et al., 1990):

�

Confinement: A vertical load induces lateral forces,which spread the aggregate particles and thus lead tolocal deformations of the fill. Due to frictional interac-tion and interlocking between the fill material and thegeosynthetic, the aggregate particles are restrained at theinterface between the subgrade and the fill (Jenner andPaul, 2000). The reinforcement can absorb additionalshear stresses between subgrade and fill (Floss andGold, 1994; Meyer and Elias, 1999), which wouldotherwise be applied to the soft subgrade (Houlsbyand Jewell, 1990). This improves the load distributionon the subgrade (Moghaddas-Nejad and Small, 1996)and reduces the necessary fill thickness. The confiningmechanism does not imply the need for significant rutdepths to form (Collin et al., 1996; Perkins and Ismeik,1997), and therefore is also of interest for permanentpaved roads (Sellmeijer, 1990). The effectiveness of thereinforcement not only depends on the adequate loadtransmission to the fill material (via friction andinterlocking), but also is improved by the higher stiffnessof the geosynthetic (Cancelli et al., 1996; Kinney andXiaolin, 1995). � Membrane effect (Giroud and Noiray, 1981): If an

unpaved road is pre-rutted during construction, ageosynthetic reinforcement at the fill-subgrade interfaceis distorted and thus tensioned (Meyer and Elias, 1999).Due to its stiffness, the curved geosynthetic exerts anupward force supporting the wheel load and thusimproving the bearing capacity (Perkins et al., 1999).It acts like a tensioned membrane, with the pressure onthe soft subgrade being smaller than the pressureapplied to the fill on the upper, concave side. Thereinforcement, while in tension, spreads the load over alarger area, leading to a reduction in the settlementbeneath the footing (Ghosh and Madhav, 1994;Moghaddas-Nejad and Small, 1996). The membraneeffect is predominant for small fill thicknesses (Kenny,1998) and at low values of shear stiffness of thegranular fill (Ghosh and Madhav, 1994). Signifi-cant rut depths (Perkins and Ismeik, 1997; Watn et al.,1996) and high stiffnesses of the geosynthetic (Flossand Gold, 1994) must be provided to initiate the

membrane effect and thus enhance the bearing capacityof the footing.

Geosynthetics reinforcing unpaved roads on soft sub-grade have been shown to reduce the necessary fillthickness by approximately 30% (Cancelli et al., 1996;Cancelli and Montanelli, 1999; Huntington and Ksaibati,2000; Kenny, 1998; Miura et al., 1990; Perkins et al., 1998;Watts et al., 2004). Giroud and Noiray (1981) suggestedthe following criterion to select the thickness of anunreinforced unpaved road:

logN ¼h CBR0:63

0:19, (1)

where N is the number of standard axle passes, h the roadlayer thickness (m) and CBR is the CBR coefficient.The empirical approach (1) is valid for Np10000 and a

maximum rut depth of 75mm, or 40mm with reference tothe initial level of the pavement, respectively (Jenner et al.,2002). It is widely applied (Espinoza, 1994; Ingold, 1994;Koerner, 1997) and has proven satisfactory in practice(Jenner et al., 2002; Meyer and Elias, 1999; Som and Sahu,1999).The impact of reinforcement on an unpaved road on a

soft subgrade is significant with fill heights less than 0.4monly (Collin et al., 1996; Meyer and Elias, 1999; Posposiland Zednik, 2002). With higher fills, the depth effect of a(wheel) load generally is too small to mobilize a noticeabletensile force within the geosynthetic (Gobel et al., 1994).On the other hand, in unpaved roads the geosynthetic mustbe covered by a minimum fill layer of 0.2m to preventdamage during trafficking (Hirano et al., 1990; Meyer andElias, 1999).The geosynthetic reinforcement should be placed in the

lower part of the fill height (Jenner and Paul, 2000),whereas the optimal placement position is dependent onthe subgrade, the fill thickness and the magnitude of theapplied loads. With a soft subgrade and a fill thickness lessthan 0.4m the optimal position lies at the base of the fill(Cancelli and Montanelli, 1999; Haas et al., 1988; Miura etal., 1990; Walters and Raymond, 1999). With higherbearing capacity of the subgrade, increasing fill height orsmaller trafficking loads, the optimal placement position ofthe geosynthetic moves upwards to approximately0.25–0.35m below the surface of the fill (Haas et al.,1988; Moghaddas-Nejad and Small, 1996; Perkins et al.,1999).

ARTICLE IN PRESS

Fig. 1. Layout of test track with divisions between test fields.

R. Hufenus et al. / Geotextiles and Geomembranes 24 (2006) 21–37 23

Biaxial geosynthetics are typically used to reinforceunpaved roads, because well-balanced tensile forces can bemobilized. Their most important property is the tensilestiffness at tensile strains between 1% and 2% (Bloise andUcciardo, 2000), causing the maximum expected perma-nent elongation of the geosynthetic (Haas et al., 1988). Dueto settlements, the elongation of the reinforcementincreases with decreasing bearing capacity of the subgrade(Ghosh and Madhav, 1994).

The reinforcement affects the fill somewhat proportion-ally to the tensile stiffness of the geosynthetic (Collin et al.,1996; Gobel and Lieberenz, 1997; Hirano et al., 1990;Miura et al., 1990), with the soil-geotextile interactionbeing a limiting factor (Espinoza, 1994; Palmeira andCunha, 1993). In road and railroad applications, the mobil-ized forces can be limited due to insufficient anchoring.Thus a very stiff geosynthetic does not necessarily give riseto an increased reinforcing effect (Bourdeau, 1991).

With stiff geosynthetics, an applied load results inrelatively small elongations and thus minor deformationsof the reinforced fill layer (Chan et al., 1989; Jenner andPaul, 2000; Meyer and Elias, 1999). On a soft subgrade(CBRp3), stiff geosynthetics are comparatively moreefficient, but the influence of the reinforcement tensilestiffness decreases with increasing bearing capacity of thesubgrade (Cancelli et al., 1996; Cancelli and Montanelli,1999). The aggregate particles restrain the tensile elementsof the installed geosynthetic and thus stiffen the reinforce-ment. The degree of stiffening depends on the type ofgeosynthetic, with nonwovens being the most susceptible toconfinement by interlocking particles (Bauer, 1997).

Due to the fact that only an elongated geosynthetic candevelop forces, the reinforcing effect of a geosyntheticinstalled in an unpaved road on soft subgrade often doesnot develop until trafficking and some resulting deforma-tion occur (Chan et al., 1989; Huntington and Ksaibati,2000; Jenner et al., 2002). However, large deformations areonly accepted in unpaved roads. With increasing maximumrut depth and decreasing bearing capacity of the subgrade,the impact of the reinforcement on the service life of a roadimproves (Cancelli et al., 1996; Cancelli and Montanelli,1999; Haas et al., 1988). The maximum number of axlepasses required to achieve a given rut depth can be up to 10times higher on a reinforced unpaved road, compared tothe unreinforced situation (Cancelli and Montanelli, 1999;Collin et al., 1996; Perkins et al., 1998).

The absence of an accepted design technique explainswhy this topic is still being researched despite the initiationof investigations over 20 years ago (Perkins and Ismeik,1997). Since the geosynthetic reinforcement interacts withthe soil over the whole width of the unpaved road, theresults of reduced-scale laboratory simulations cannot betransferred to practice reliably. Currently there are noincontrovertible indications from laboratory tests of theinfluence that the geosynthetic will have on the perfor-mance of the pavement under trafficking (Watts et al.,2004).

2. Experimental

2.1. Concept of field trials

Full-scale field trials were undertaken in the autumn of2002 in order to ascertain the effect of geosynthetics on theload-bearing capacity of an unpaved road on softsubgrade, which was levelled and prepared in order tocreate a track of uniform strength (Hufenus et al., 2004).Gravelly, angular backfill was used for the test track andcompaction and in-service tests were carried out on the fill,which were reinforced with various geosynthetics. The goalof the study was to establish the extent to which reinforcinggeosynthetics improve compaction, bearing capacity andserviceability.Lessons were learnt from a similar research project

(Schad, 2001) during which weather conditions andtrafficking procedures during construction caused majorvariations in the results, which limited assessment of theresults. Surprisingly, in that study no indication ofimproved load-bearing capacity was observed due to thegeosynthetic reinforcement (Wilmers, 1999).An area within a brickworks clay pit in Diessenhofen,

near to Schaffhausen, was available for use as a test track.The ground consisted of relatively homogeneous clayey silt.The subgrade is characterized as having a somewhatirregular bearing capacity, which complicated the inter-pretation of the results, but also revealed the dependence ofcompaction and serviceability (rut formation) on thesubgrade parameters.The test track was divided into 12 fields (1–12) of length

8m, into which one layer of a variety of reinforcinggeosynthetics was placed, and two preliminary test fields(V1 and V2), where no geosynthetic, or only a separator,was laid (Fig. 1). The geogrids were partly placed, incombination with a nonwoven separator underneath.Three 0.2m layers (Fig. 2) of relatively poorly compactablerecycled rubble were placed. The 1st layer was compactedstatically (25 kN tandem flat roller Bomag BW 120) and the

ARTICLE IN PRESSR. Hufenus et al. / Geotextiles and Geomembranes 24 (2006) 21–3724

2nd and 3rd layers were compacted dynamically withconstant energy (80 kN flat roller Bomag Variocontrol BW177 with overall dynamic compaction control).

The test track was constructed adjacent to an existingroad, with a length of about 130m. This allowed forinstallation from the side, so that the test track was notsubjected to traffic, nor loaded by any equipment prior tocompaction. The subgrade was prepared with a cross slope(gradient approx. 4%) to allow rain and any seepage waterto run off. Measures were taken such as irrigation andcovering to ensure that the ground was not permitted todry out.

Installation, compaction and traffic characteristics weretested for a section (Zones V1 and V2) that demonstratedequal or worse conditions compared with fields 1–12 toprepare for construction in this area. No geosyntheticmaterial (Zone V2), or only a separation geosynthetic(Zone V1), was included at the end of the field for thesepreliminary tests. The truck used for trafficking testscomprises two single wheel steering axles in the front andtwo twin wheel axels in the back. The air pressure of the30 cm wide truck tires was 8.5 bar.

The progress of the field trials is listed in Table 1. Thecondition of the track and the geosynthetics was monitoredby instruments from installation to removal, using CBR

Fig. 2. Typical cross section through the test track.

Table 1

Progess of the field trials

Layer Day of field test Action

Subgrade 1st–28th Adjustment and lev

29th–30th Geosynthetics and c

Layer 1 31st Test track was laid

31st Test track was com

35th–36th Static plate load an

tests, 2 further pass

Layer 2 43rd–44th 2nd layer was laid w

44th Dynamic compactio

48th–49th Plate load and traffi

10 passes with 280 k

Layer 3 57th 3rd layer was laid w

57th Dynamic compactio

76th–78th Plate load and traffi

Subgrade 78th–80th Ballast cleared to ap

for all profiles, final

measurements (CBR penetrometer), shear vane measure-ments (Pilcon), specific gravity measurements, static anddynamic plate load tests, a dynamic falling weightdeflectometer (FWD), the overall dynamic compactioncontrol and the profile measurements (ruts) and straingauges on the geogrids.

2.2. Selection of geosynthetics

As a general principle, products that are conventionallyused to reinforce roads were selected (Table 2). These arebiaxial products that are able to withstand approximatelythe same force in both directions. Details of the geogridrolling width, mesh width and strength mobilized due toaxial tensile strain in the machine (MD) and the crossdirection (XD) are given, together with the respectivenumber of strain gauges installed (Section 2.5).Seven different reinforcing geosynthetics were used (nos.

02, 27, 28, 32, 42, 44 and 46), to represent the variouspossible raw materials, type and manufacturing process.Two weaker materials, a nonwoven separating geotextile(no. 41) and a woven slit tape geotextile (no. 45), were alsoincluded. Nos. 32, 42 and 46 were incorporated with andwithout an additional nonwoven separator (no. 40), whileno. 27 was only included in combination with thenonwoven geotextile. The nonwoven separator no. 40was installed in the preliminary test field V1.The distribution of the geosynthetic samples on the test

track (0–96m) is shown in Fig. 3 (grey: grid underlaid withnonwoven separator), together with the orientation of thegeosynthetic material (arrow ¼ direction of production), aswell as the position of the strain gauges and the profile usedto measure the ruts (broken line in Fig. 3 at two locationsper field). Fill heights and rut depths were measured.

elling of subgrade

abling for the strain gauges were laid

with 0.25m loose ballast 8/64 (compacted depth 0.2m)

pacted purely statically with 25 kN roller, 3–4 passes

d trafficking test over 1st layer with 130 kN truck: 2 passes for plate load

es on Zones V1 and V2 and 6 more on fields 1–12 (total 4–8 passes)

ith 0.25m loose ballast 8/64 (compacted depth 0.2m)

n with 80 kN roller, 3–4 passes

cking tests over 2nd layer with loaded truck (10 driving passes with 220 kN,

N)

ith 0.25m loose ballast 0/32 (compacted depth 0.2m)

n with 80 kN roller, 3–4 passes

cking tests with loaded truck (61 driving passes with 280 kN in total)

prox. 0.05m over the subgrade (geosynthetics) with a hydraulic excavator

excavation by hand shovel

ARTICLE IN PRESS

Table 2

Geosynthetics used in field experiment

No. Field Type of geosynthetic Width (m) Grid (mm) Strain gauges Tensile strength at (kN/m)

2% 5% max.

MD XD MD XD MD XD

02 10 PP slit tape woven 5.15 — — 12 12 30 30 65 65

27 9 Biaxial extruded PP grid in 5 layers 4.50 60� 60 — 6 10 14 20 22 35

28 2 PVC-coated knitted PET grid 5.10 20� 20 4 9 9 14 14 55 55

32 5/6 PET flat rib grid 4.75 32� 32 8 10 10 20 20 30 30

40 V1 PP nonwoven (separation) 5.00 — — 0.2 0.1 0.3 0.2 10 10

41 12 PP nonwoven (reinforcement) 5.00 — — 0.4 0.3 0.6 0.4 20 20

42 3/4 PVC-coated knitted PVA grid 5.20 40� 40 8 12 12 32 32 40 40

44 11 PET yarn reinforced PP nonwoven 5.20 8.5� 8.5 — 7.5 7.5 22 22 50 50

45 1 PP slit tape woven 5.15 — — 2 2 8 8 30 30

46 7/8 Biaxial extruded PP grid 3.80 65� 65 12 11 12 22 25 30 30

Fig. 3. Test track set-up.

R. Hufenus et al. / Geotextiles and Geomembranes 24 (2006) 21–37 25

The load–strain behaviour of the geosynthetics has beentested either longitudinally or transversely, depending onthe alignment of the samples in the field trial. Geosyn-thetics installed transverse to the track have been tested inmachine direction, the rest in cross direction (Fig. 3). Fig. 4

shows the load–strain curves of the virgin as well as theinstalled and excavated geosynthetics, determined accord-ing to EN ISO 10319 (1996).The shape of the load–strain curves proved to be almost

unaffected by installation damage and subsequently bytrafficking, i.e. even with a small decrease of the ultimatetensile strength and the elongation at break, the tensilestiffness remained approximately the same, in accordancewith other studies (Hufenus et al., 2002). Predicted strainsof the geosynthetic reinforcement were far below theequivalent strain at failure so that installation damagedid not influence the load–strain behaviour.

2.3. Soil parameters and environmental conditions

The subgrade was classifiable as CM (medium plasticitysilty clay). The water content near the surface wasmeasured as w ¼ 38:6� 4:7%. The distribution of particlesizes can be seen in Fig. 5.A penetrometer was used to determine the CBR

coefficients at depths of approximately 0.3, 0.45 and0.6m before installation and after removal of the fill.Measurements were carried out for every profile of fields1–12 and V1, V2 (Fig. 3) along the axis of track, as well asat a distance of 0.5 and 1.0m on the left- and right-handsides, respectively. It was assumed that the layers near thesurface have a greater influence on the bearing capacity andthe deformation behaviour of the subgrade than the deeperlayers. Taking into account that the normal compressivestress under a plate load decreases progressively withsubgrade depth, a weighted average CBR coefficient wasdefined, with weightings of three, two and one according todepths of 0.3, 0.45 and 0.6m, respectively. Fig. 6 showsthus weighted average CBR coefficients determined afterremoval of the fill. CBR coefficients higher than 12(measuring range of the penetrometer exceeded) werereported as 12.

ARTICLE IN PRESS

0

10

20

30

40

50

0 4

Strain [%]

Ten

sile

str

eng

th [

kN/m

]

virgin installed in 6-2 installed in 5-2

no. 32

2 6 80

10

20

30

40

50

0 2 4 6

Strain [%]

Ten

sile

str

eng

th [

kN/m

]

virgin installed in 4-2 installed in 3-2

no. 42

1 3 5

0

10

20

30

40

50

Strain [%]

Ten

sile

str

eng

th [

kN/m

]

virgin installed in 1-2

no. 45

0 42 6 80

10

20

30

40

Strain [%]

Ten

sile

str

eng

th [

kN/m

]

installed in 7-1installed in 7-2 installed in 8-2

no. 46

0 42 6 8

virgin

Fig. 4. Load–strain-curve of the exhumed geosynthetics nos. 32, 42, 45 and 46, installed in fields 6-2, 5-2, 4-2, 3-2, 1-2, 7-1, 7-2, 8-2, respectively.

0102030405060708090

100

0.001 0.01 0.1 1 10 100

Particle size [mm]

Per

cen

t fi

ner

by

wei

gh

t [%

]

subgradefill (layer 1&2)fill (layer 3)

Fig. 5. Particle size distribution in the subgrade and fill.

R. Hufenus et al. / Geotextiles and Geomembranes 24 (2006) 21–3726

The subgrade was not uniform: some zones of soft tovery soft consistency were located next to stiffer areas,where the bearing capacity and shear strength were alsohigher.

Mixing and regrading the subgrade along the entire testtrack was not considered to be feasible, so the ground wasscarified and regraded in fields 1–4 in order to reduce thebearing capacity to that equivalent elsewhere along the testtrack. CBR measurements made after the track had beenremoved indicated that the subsoil in fields 1–4 had

reconsolidated to almost similar CBR values as in thepre-test soil conditions.The undrained shear resistance of the subgrade was

measured directly in the field using a Pilcon shear vane,where the maximum detectable shear resistance was124 kPa, corresponding to a CBR value of approx. 3–4(Jaecklin and Floss, 1988; Saathoff and Horstmann, 1999).Post-test measurements of CBR values were used for theinterpretation of the influence of the fill layers andreinforcement on rut formation due to compaction andtrafficking.Loose recycled rubble, consisting primarily of concrete

scrap and secondarily of brickwork scrap (poorly gradedgravel, GP), was used to form the fill layers. The materialwas broken down to a maximum grain size of 64mm, andthe fine portion (with a diameter ofo8mm) was sieved out.The particle sizes for layers 1 and 2 then range betweenapprox. 8 and 64mm (Fig. 5). Because the proportion ofsmall particles was limited, the material demonstrated lowsensitivity to changes in the water content, and wassufficiently porous so that meteorological and percolatingwater was conducted quickly into the lateral drainageditches.In contrast, finer grained recycled material, with a

particle size of 0–32mm (poorly graded sandy gravel, also

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1-11-22-12-23-13-24-14-25-15-26-16-27-17-28-18-29-19-2

10-1

10-2

11-1

11-2

12-1

12-2

V1-

1V

1-2

V2-

1V

2-2

-1-0.50 0.5

1

0

2

4

6

8

10

12

CB

R

Profile

Position [m

]

Fig. 6. Weighted average CBR coefficients after removal of the foundation.

R. Hufenus et al. / Geotextiles and Geomembranes 24 (2006) 21–37 27

GP) was used for the 3rd layer, in order to achieve animprovement in density and hence interlocking, so that lessparticle movement in the voids within the material occurswhen the ruts are driven over during trafficking.

The subgrade and 1st fill layer were artificially wateredduring a very dry period in the middle of August to ensurethat the consistency and hence shear strength of the siltyclay remained relatively constant and the clay did not dryout. The 2nd and 3rd layers have been levelled off with anexcavator shovel to remove the ruts developed prior tocovering with the top layer. Evaluation of the effect ofcrushing by comparing particle size distribution curvesbefore and after installation, compaction and traffickingshowed that there was no significant influence.

2.4. Compaction controls

The dry density gd and the water content w of the 1st,2nd and 3rd layer of the test track were determined usingthe Troxler apparatus. The method has not been calibratedto other standard tests, such as the sand replacementmethod. Thus, the results are only valid for comparitivepurposes. Proctor tests were not carried out due to the highpercentage of coarse grains.

Static plate load tests were carried out to reveal theYoung’s moduli EV1 (1st plate loading) and EV2 (platereloading), using a 300mm diameter device to determinethe deformability and load-bearing capacity of the filllayers. Measurements were completed for each profile(Fig. 3) and directly above the strain gauges in fields 2, 5and 6.

Some dynamic plate load tests (FGSV, 1997) werecarried out to reveal the Young’s moduli of each of the filllayers. However, performing the test on the unboundsurface of the test track led to unsatisfactory results.

Dynamic compaction control is a technique used tomeasure the load-bearing capacity of compacted ground byanalysing the dynamically excited roller (Floss, 2001).Conclusions can be drawn about the dynamic stiffness Evib

and/or the degree of compaction by measuring andanalysing the acceleration. The depth of measurement forthis is greater than the depth of compaction and in thisparticular case reaches the soft subgrade. Consequently thedynamic compaction results reflect primarily the propertiesof the subgrade and not those of the fill layer.

2.5. Deformation measurements

The profile measurement used to assess the formation ofruts on the trafficked fill layers and on the subgrade afterremoval of the fill was carried out using a cross bardeveloped specially for this field test. This cross bar rests onthe left and right measuring posts driven in on either side ofthe track (Fig. 2), and the distance of the crossbar abovethe track is measured with an accuracy of 75mm.The geosynthetics have been instrumented with electrical

resistance strain gauges (ERSG) to determine their short-and long-term deformations. Static measurements havebeen performed to investigate deformations and stressescaused by the plate load test. Dynamic measurements wereperformed to study the influence of trafficking oncompaction. The foil strain gauges consisted of a con-stantan grid on a polyamide film. The maximum tensilestrain was specified as 5%.A single component, cold curing adhesive made of

cyanacrylate (Z70), was used to fix the strain gauges to thegeogrids. The uneven, coated knitwear was treated with atwo component polyurethane adhesive/filler to flatten thesurface and to glue on the strain gauges. The strain gaugesneeded to be protected (Bathurst et al., 2002; Springmanand Balachandran, 1994). Protection against ingress of

ARTICLE IN PRESSR. Hufenus et al. / Geotextiles and Geomembranes 24 (2006) 21–3728

water was achieved by covering with a tough, kneadableputty that strongly adheres on nearly every material(AK22). For protection against installation damage, arubber foil was loosely attached to the geosynthetic aroundthe strain gauge without hindering the elongation.

To validate the instrumentation process, i.e. protectingthe strain gauges without influencing the measurement,prior to the field tests, a strip of geosynthetic had beeninstrumented and tested in a laboratory simulator for theinstallation process according to prEN ISO 10722-1, 2004.The performance of the installed strain gauges on thegeosynthetic has been compared to an extensometer in atensile testing apparatus before and after the simulatedinstallation, following the standard EN ISO 10319 (1996).The traverse speed was set to a reduced value of 3mm/minto achieve a better temporal resolution.

It was not possible to glue the strain gauges to the slittape woven material no. 2. The adhesive led to a stiffeningof the material resulting in a force–strain relationship,which is far too steep. Likewise, geogrids could beinstrumented with strain gauges, but not geowovens(Bathurst et al., 2002). Samples no. 32, 42 and 46 showeda very good agreement of the force-strain curves. Theinstallation simulation had neither negative influence onthe strain gauge nor on the adhesive joint. All strain gauges

Fig. 7. Positioning of the loading plate (static plate load test) and the

transverse strain gauges.

0

10

20

30

40

50

600 0.1 0.2 0.3 0.4 0.5 0.6

Vertical stress [MPa]

Set

tlem

ent

[mm

]

1st loadingunloadingreloading

Profile V1-2

Fig. 8. Vertical stress-settlement diagrams of the 1st fill layer, for pro

survived the field test without failure due to the protectivemeasures taken.The selection of the samples to be instrumented was

made according to technical practicability (the potential forattaching the strain gauges without altering the force-deformation characteristics of the geosynthetic). Fourstrain gauges were fitted in a line at right angles to thetrack axis for each sample under test. Fig. 7 shows theirpositions. Since the truck was not externally guided, thecourse of the track varied during trafficking. Therefore theposition of the strain gauges could not be chosen to beprecisely below the truck tires.Fig. 3 shows the position of all strain gauges installed. In

addition to the strain gauges mounted at right angles infields 2–8, four additional strain gauges were affixed in thedirection of travel in field 7, in order to measure thelongitudinal elongation of the geosynthetic.

3. Results and discussion

3.1. Compaction improvement

The dry density shows no relevant dependency on theproperties of the subgrade and the reinforcement. Therelatively low density of the 1st layer (gd ¼ 14.970.2 kN/m3, w ¼ 6.370.4%) and the 2nd layer (gd ¼ 14.370.3 kN/m3, w ¼ 7.470.4%) is due to the difficulty of compactingsuch coarse-grained fill (particle size between 8 and64mm), with significant void space, and also to the lighterspecific weight of the material stemming from the brokenmasonry components. Material from the same source wasused for the well-graded 3rd layer, which has beencompacted to a significantly higher density(gd ¼ 16:7� 0:1 kN=m3, w ¼ 10:5� 0:3%).Young’s moduli EV1 and EV2 were calculated from the

static plate load test data for the 1st loading and reloadingcycles, respectively (cf. Fig. 8). Because the subgrade was sosoft, it was rarely possible to achieve a maximum initialload of 0.5MPa. The 1st loading has been increased in

0

10

20

30

40

50

600 0.1 0.2 0.3 0.4 0.5 0.6

Vertical stress [MPa]

Set

tlem

ent

[mm

]

1st loadingunloadingreloading

Profile 3-1

files V1-2 (without reinforcement) and 3-1 (with reinforcement).

ARTICLE IN PRESS

0

20

40

60

1-1

1-2

2-1

2-2

3-1

3-2

4-1

4-2

5-1

5-2

6-1

6-2

7-1

7-2

8-1

8-2

9-1

9-2

10-1

10-2

11-1

11-2

12-1

12-2

V1-

1V

1-2

V2-

1V

2-2

Profile

EV

2 [M

Pa]

0

4

8

CB

R [

%]

Young's modulus of the plate reloading on the 1st layerYoung's modulus of the plate reloading on the 2nd layerweighted average CBR coefficient of the subgrade

Fig. 9. Young’s moduli EV2 of the 1st and 2nd fill layer, compared to the weighted average CBR coefficients of the subgrade.

R2 = 0.6

R2 = 0.2

R2 = 0.60

10

20

30

40

50

60

70

0 2 6 9 10weighted average CBR coefficient of the subgrade [%]

Yo

un

g's

mo

du

lus

EV

2 [M

Pa] grid alone, strong slit tape woven

grid with nonwoven separatornonwoven, weak slit tape wovennot reinforced

1 3 4 5 7 8

Fig. 10. Correlation between the Young’s modulus EV2 of the 2nd fill

layer and the weighted average CBR coefficient of the subgrade.

R. Hufenus et al. / Geotextiles and Geomembranes 24 (2006) 21–37 29

0.05MPa steps until a settlement of approx. 50mm wasachieved for tests on the first 0.2m thick layer. Reloadingwas carried out with 0.07MPa steps until the penultimatestress from the 1st loading cycle was achieved. Consolida-tion during the test also made it nearly impossible to expecta settlement change ofo0.02mm/min in accordance withthe standards.

Fig. 8 shows settlements of 17 and 51mm, respectivelyfor a 1st loading cycle to 0.35MPa, which is typical for thefields with geogrid (field 3) and without geogrid (field V1)reinforcement. For tests on the top layer, the influence ofthe reinforcement is marginal, since the deformations aretoo small to mobilize forces with significant verticalcomponents for just one loading, unloading and reloadingcycle.

Fig. 9 shows the Young’s moduli EV2 of the reloadingcycle of the static plate load test in comparison with theweighted CBR values following the removal of the track.Significantly higher CBR values of 6–8 were measured (inthe area of fields 2 and 3). This influenced both theinteraction with the 1st fill layer and improved thecompactability of the 2nd and 3rd layers in these fields.

The highest stiffnesses measured for the geogrid re-inforcement were without the separation layer in fields 2, 3,6 and 7. The amount of improvement in the EV2 valueappears to have been reduced when the separation layer(fields 4, 5, 8 and 9) was present, due to a combination oflower frictional resistance between the two geosyntheticsand less opportunity for interlocking with the geogrid andthe fill. Values of EV2 for field 8 that included the extrudedgeogrid with the separation layer were particularly poor,but they probably reflect the effect of the lowest CBRvalues.

No significant difference between the remaining fieldsand the one representing the unreinforced field (V2) wasapparent. Typically, similar elasticity moduli were mea-sured after the dynamic compaction of the 2nd layer from

fields 4–11 (CBRE0.5–1.5) and fields (V1 and V2)(CBRE2–4). This implies that the improvement in bearingcapacity of the subsoil due to the reinforcement may beequivalent to an increase in the CBR of DCBR � 1–2.A correlation between the reloading modulus EV2 on the

second layer and the weighted CBR coefficients of thesubsoil (averaged over the width of the test track) has beenpresented in Fig. 10. Although the data is somewhat varied,general tendencies can be noted.The ratio of EV2/EV1 from plate loading tests on the 1st

fill layer in fields 4–11, with reinforcement, was signifi-cantly smaller than values from more robust subsoil (fields2 and 3) or without any reinforcement (fields 1, 2, V1, V2).This indicates that the reinforcement on soft ground wasactivated and slightly recovered during the unloading.The recycled fill material was difficult to compact. This

was manifested in the higher values of the ratio of EV2/EV1

(E4–5) for the large fill layer thickness and the betterquality ground. The values of EV1E22MPa andEV2E110MPa were rather low for the recycled fill material

ARTICLE IN PRESS

Fig. 11. Results of the overall dynamic compaction control on the 2nd and 3rd layer.

R. Hufenus et al. / Geotextiles and Geomembranes 24 (2006) 21–3730

and certainly would not have reached the requirements fora fill layer of an uncompacted road.

The determination of stiffness Evib deduced from thedynamic compaction control measurement values from the4th roller pass over the left and right lane, respectively, onthe 2nd layer (d ¼ 0:4m) and 3rd layer (d ¼ 0:6m) areplotted in Fig. 11. Only some of the vibration energy fromthe dynamic compaction has been effective in compactingthe 1st and 2nd fill layers due to the weak subgrade.Consequently, the dynamic compaction results reflect thein situ subgrade properties, and hence there are analogieswith the static EV2 values, with the exception of the lack ofincrease over fields 5–7. The higher values shown in Fig. 11between fields 12 and V1 (chainages 96–120m) and beyondchainage 136m were largely due to having placed the firsttwo fill layers to a total thickness of 0.6m rather than0.4m, with the inevitable effect on the measured elasticitymoduli.

There appears to be no direct relationship between thedynamic stiffness of the 3rd layer and the subsoil properties(Fig. 11) because less energy reaches the natural grounddue to the total layer thickness of 0.6m. The reductions invalues measured along the edges of field 11 and 12 are dueto inadequate support on one side of the track, causinglateral spread during dynamic compaction.

The 3rd layer was found, in general, not to contribute tothe bearing capacity of the test track founded on the stifferground. Marked geosynthetic-specific Evib values were alsonot observed except for reaching similar values in fields V1(with only a separating layer) and V2 (without reinforce-ment) to the reinforced fields 4–9 on weaker ground.

3.2. Rut formation

A selection of profile measurement results is shown inFig. 12 (profile V1-2, nonwoven separator), Fig. 13 (profile1-1, slit tape woven, tensile strength 30 kN/m) and Fig. 14(profile 5-1, flat rib grid), where the thickness of the layersis measured in relation to the initial level of the subgrade( ¼ 0mm). These examples show the rut formation on anunpaved road on soft subgrade without reinforcement(Fig. 12), with a relatively weak geosynthetic (Fig. 13),and with a comparatively stiff geosynthetic reinforcement(Fig. 14), respectively.The 1st layer has been trafficked with an unloaded

130 kN truck. Deep ruts developed in the unreinforcedpreliminary test fields V1 and V2 after 4 passes, in spite ofthe better bearing capacity of the subgrade, compared tothe fields 3–11. Field 1, reinforced by relatively weak slittape woven, neared the state of failure after 8 truck passesover the 1st layer.Despite the relatively high bearing capacity of the

subgrade (CBR ¼ 8–12) in fields 2 and 3 as well as at thebeginning of field 4, considerable rut formation occurred.The influence of the bend in the track resulted in slightlateral spreading of the wheel loads, explaining therelatively shallow ruts in the fields 8–12. Comparatively,small ruts were found in the fields 6 and 7, reinforced bybonded geogrids without nonwoven underlay.The 2nd layer was trafficked 10 times with a 220 kN

truck and 10 times with a 280 kN truck, and the smallestruts were found in the reinforced fields 2 and 3. This mustbe due to the relatively high bearing capacity of the

ARTICLE IN PRESS

-200

-100

0

100

200

300

400

500

600

700

800

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Position in profile, from left to right measuring post [m]

Fill

th

ickn

ess

[mm

]

3rd layer trafficked 2nd layer trafficked 1st layer traffickedbefore installation after removal

Fig. 13. Rut formation with weak reinforcement (profile 1-1, sample 02).

-200

-100

0

100

200

300

400

500

600

700

800

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Position in profile, from left to right measuring post [m]

Fill

th

ickn

ess

[mm

]

3rd layer trafficked 2nd layer trafficked 1st layer traffickedbefore installation after removal

Fig. 12. Rut formation without reinforcement (profile V1-2, sample 40).

R. Hufenus et al. / Geotextiles and Geomembranes 24 (2006) 21–37 31

subgrade in this section of the test track, which resulted in awell-compacted fill. On the other hand, the high bearingcapacity of the subgrade in the unreinforced fields V1 andV2 did not inhibit the formation of ruts. The smallest rutswere found again in the fields 6 and 7-1 for the tracksections with low bearing capacity (Fig. 15).

The 3rd layer has been trafficked with a 280 kN truck,with measurements after 11 and 61 passes. Again, thesmallest ruts were found in the reinforced fields 2 and 3with high bearing capacity of the subgrade, as well as in thefields 6 and 7 with the bonded geogrids without nonwovenunderlay. The rut formation was increased in the fields8–12, with the stronger subgrade being effective in field 12.

Due to the load distribution through the granular fill, theruts formed on the subgrade surface are shallower and

wider than the track surface above. Considerable deforma-tion of the subgrade was measured after the excavation ofthe fill in the poorly reinforced fields 1 (weak slit tapewoven), 12 (strong nonwoven), V1 (separating nonwovenonly) and V2 (no geosynthetic). The low bearing capacityof the subgrade resulted in relatively deep ruts in the fields8 and 9 (extruded geogrids with nonwoven underlay), butdid not cause large deformations in the geogrid-reinforcedfields 4–7. The small ruts formed on the subgrade of thefields 3, 4, 10 and 11 are partly explained by thecomparably high bearing capacity of the subgrade (fields3 and 4) and by the lateral spreading of the wheel loads dueto the slightly curved track (fields 10 and 11), respectively.Fig. 15 compares the mean depths of the left and right

ruts with the Young’s moduli EV2 of the 2nd and 3rd layers

ARTICLE IN PRESS

-24

-20

-16

-12

-8

-4

0

1-1

1-2

2-1

2-2

3-1

3-2

4-1

4-2

5-1

5-2

6-1

6-2

7-1

7-2

8-1

8-2

9-1

9-2

10-1

10-2

11-1

11-2

12-1

12-2

V1-

1V

1-2

V2-

1V

2-2

Profile

Ru

t d

epth

[cm

]

0

4

8

12

16

20

24

EV

2 [1

0 M

Pa]

, CB

R [

%]

resp

ecti

vely

2nd layer after 10 passes 3rd layer after 11 passes2nd layer after 20 passes 3rd layer after 61 passesYoung's modulus layer 2 Young's modulus layer 3average CBR coefficient

Fig. 15. Rut formation, compared to Young’s moduli EV2 and CBR coefficients.

-200

-100

0

100

200

300

400

500

600

700

800

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Position in profile, from left to right measuring post [m]

Fill

th

ickn

ess

[mm

]

3rd layer trafficked 2nd layer trafficked 1st layer trafficked

before installation after removal

Fig. 14. Rut formation with stiff reinforcement (profile 5-1, sample 32).

R2 = 0.92

R2 = 0.96

100

200

300

400

500

600

700

800

1 10 100 1000Number of axle passes

Lay

er t

hic

knes

s [m

m]

reinforcedunreinforced

CBR = 1

CBR = 2

CBR = 0.5

Fig. 16. Number of axle passes achievable without formation of ruts

deeper than 40mm.

R. Hufenus et al. / Geotextiles and Geomembranes 24 (2006) 21–3732

and the average CBR values of the subgrade. The figureillustrates that the rut formation is primarily a reflection ofthe values of EV2, the latter being affected by the bearingcapacity of the subgrade and the reinforcement of the filllayer. The relatively deep ruts in the fields 7-2 and 8 can beexplained by the very low CBR values of the subgrade inthis section. On the other hand, the results of the fields 2and 4 illustrate that a high bearing capacity with CBR46increases the longevity considerably. Taking the variationsof the CBR values into consideration, the rut formation isminimal in the sections reinforced with flat rib grids orextruded grids.Fig. 16 illustrates the maximum number of truck axle

passes achievable before the rut depth reaches approx.40mm, as a function of the fill layer thickness. Hence theeffect of the reinforcement decreases with increasing

ARTICLE IN PRESSR. Hufenus et al. / Geotextiles and Geomembranes 24 (2006) 21–37 33

thickness of the layer, becoming insignificant beneath a fillwith a thickness exceeding 0.6m. The transitional straightlines represent the minimum thickness of an unreinforcedunpaved road as a function of the bearing capacity of thesubgrade, as proposed by Eq (1). They show that theimprovement of this road (approx. 0.4m thickness)corresponded to an assumed enhancement of the CBRcoefficient by approx. 0.7–1.

3.3. Strain development

Fig. 17 shows the results of the static strain measure-ments under the right wheel track (position 3) duringcompacting and trafficking of layers 1–3. The permanentdeformation was below 1%. Trafficking layer 2 resulted ina significant strain increase for all strain gauges attached tothe geogrid in field no. 8. It is possible that the confiningload provided by a 0.4m thick cover anchored thegeosynthetic so that more strain was generated under thewheel. On the other hand, the fill layer was still thin enoughfor the build up of lasting ruts, which are still apparent on

-0.2-0.10.00.10.20.30.40.50.60.70.80.9

1st layercompacted

1st layertrafficked

2nd layercompacted

2nd layertrafficked

3rd layercompacted

3rd layertrafficked

Str

ain

[%

]

field 2field 3field 4field 5field 6field 7field 8

Fig. 17. Strain gauge measurements in position 3 with respect to the

consecutive loadings.

0

10

20

30

400 0.1 0.2 0.3 0.4 0.5

Vertical stress [MPa]

Set

tlem

ent

[mm

]

1st loading unloading reloading

Fig. 18. Vertical stress-settlement (left-hand side) and corresponding strain ga

static plate load testing on 1st layer.

the subgrade. Compacting and trafficking of layer 3increased the permanent strain only insignificantly despitethe large number of truck passes.The static measurements were primarily used to assess

the impact of the plate load test on the geosyntheticsunderneath. The load plate could not be placed exactlyabove the strain gauges due to the setup of the experiment.Fig. 18 shows the vertical stress-settlement of the plate loadtest on the 1st layer (Section 3.1), as well as thecorresponding tensile and compressive strain of the 4strain gauges in field 2 (static, pointwise strain measure-ment during plate load test) for the profile at position 12m(sample no. 28). The negative strains can be explained by asidewise raise of the subgrade during the plate load test,resulting in a compression of the stain gauges fixed on therear side of the bulging geogrid.The plate load test on the 1st layer generated similar

strains in the geosynthetic as trafficking (dynamic peakloads below the immediate load). The deformationreturned to its original value by releasing the plate loadand the permanent pretension remained relatively small(see Fig. 17). Beginning with the 2nd layer (40.4m), theplate load test hardly produced any additional strain,because the load plate with a diameter of 0.3m had onlysmall influence at the depth of the strain gauge.The water-saturated subgrade behaved under dynamic

load during trafficking as if it were undrained, whichresulted in settlement below the wheels and heave alongsidethem (Giroud and Noiray, 1981). The deformation of theterrain led to wave-like bending of the geosynthetics. Sincethe strain gauges were installed on the bottom side of thegeosynthetics, i.e. below the neutral line, this bendinginduced additional strain, which was superimposed. There-fore, the strain gauges at the outer positions had to sustainconsiderable compressive strains. In view of the fact thatthe location of the plate load tests with respect to the straingauges was somewhat arbitrary, the results are biased andof limited significance.

-0.10

-0.05

0.00

0.05

0.10

0.0 0.1 0.2 0.3 0.4 0.5

Vertical stress [MPa]

Str

ain

[%

]

Pos. 1 Pos. 2 Pos. 3 Pos. 4

uge measurements (right-hand side) in the profile at position 12m during

ARTICLE IN PRESSR. Hufenus et al. / Geotextiles and Geomembranes 24 (2006) 21–3734

The purpose of the dynamic measurements was toinvestigate the dependence of short-term deformationsunder the influence of compacting and trafficking. Fig. 19serves as an illustration. It shows the strain measured atposition 3 (centre of the right lane) of field 7 during theentire duration of the tests. Minimum (min) and maximum(max) strain of each event (compacting or trafficking) aregiven.

The additional mobilized permanent strength, whichresults from installation and loading of layers 2 and 3 aregiven in Table 3. Compacting with the roller generated onlyshort-term strain in the geosynthetic, which decayed afterthe roller passed and the same occurred with strains in thedriving direction caused by trafficking with trucks.Permanent strain in the cross direction with the corre-sponding prestress of the reinforcement built up only if thesubgrade has been deformed correspondingly. The tem-porary strain generated during roller or truck pass,exceeded the permanent strain by a factor of two.

The permanent strain of the geogrid was usually below0.5% and exceeded 1% only in extreme cases. The strainrates measured during trafficking the fill layer were in the

Table 3

Approximate tensile strength induced by fill installation and loading

Field Subgrade Geosynthetic Nonwoven

underlay

A

s

2

2, 3 Relatively firm Knitted grid With 3

4 Soft Knitted grid Without 6

5, 6 Soft Flat rib grid Without/with 1

7 Soft Extruded grid Without 3

8 Soft Extruded grid With 8

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

31th 44th 49th 49th 57th 77

Day o

Str

ain

[%

]

compacting 1st layer

compacting 2nd layer

compact

tr

trafficking 2nd layer with 22

trafficking 1st layer

trafficking 2nd laye

Fig. 19. Deformations beneath the centre

same order of magnitude as in EN ISO 10319 (1996).Therefore, it was reasonable to estimate the forces(Table 3) mobilized by deformation of the geosyntheticsaccording to the load–strain diagram in Fig. 4. Largedifferences in the strain rate would produce erroneousprediction, since the stiffness of the geosynthetics dependsstrongly on the load speed (Walters et al., 2002).The maximum strains and corresponding forces had

been partly reached during trafficking of the first layer, andpartly during compacting and trafficking of layer 2 as theinvestigations show. Due to the higher load-bearingcapacity of the subgrade in fields 2 and 3, the maximumstrain was only insignificantly higher than the permanentstrains or forces (Fig. 17). Higher strains and forces hadbeen measured in field 4 (knitted grid with nonwovenunderlay), which had a soft subgrade.The deformation and corresponding forces remained

relatively low in the fields 5 and 6 with flat rib grids. Thehigher measured strain in the extruded geogrids in fields 7and 8 could have been caused by local loading. Bathurstet al. (2002) noted that local pressure caused by largerstones on the thin extruded elements of the geogrids had

dditionally mobilized permanent

trength (kN/m)

Permanent

strength (kN/m)

Maximum

strength (kN/m)

nd layer 3rd layer

1 7 11

3 9 17

1 5 9

1 6 10

1 10 18

th 78th 78th 78th 78th 78th

f field test

min

max

ing 3rd layer

afficking 3rd layer

t truck

r with 28 t truck

of the right lane (position 3) in field 7.

ARTICLE IN PRESSR. Hufenus et al. / Geotextiles and Geomembranes 24 (2006) 21–37 35

led to greater local straining. Alternatively, the larger gridsize (Table 2), relative to the soil grading (Fig. 5), will havepermitted dilatation to occur within the opening. Thehighest strain and forces have been measured with extrudedgeogrids over nonwoven underlay (field 8).

The formation of ruts in the subgrade can be used toevaluate the deformation in geosynthetics. The rut wasapproximated through the sections A-B, B-C and C-D forthe calculation of the elongation as shown in Fig. 20. Therelative elongation was then estimated from the relativechange of the original length A-D and the stretched lengthA-B-C-D.

Average values and the 95% confidence interval ofthe relative elongation calculated from measurements ofthe ruts as well as the permanent strain measured with thestrain gauges (average of positions 1–4) are shown inFig. 21. The comparison shows good agreement. The straingauge measurements are slightly higher than those derivedfrom the rut measurements, with the exception of the flatrib grid (sample 32) in field 6. A plausible explanation forthe large deformations in field 8 is that the extruded gridslid on the nonwoven underlay.

4. Conclusions

4.1. Impact on bearing capacity

A significant improvement in the bearing capacity of afill layer reinforced by a geosynthetic was found to be trueonly for thin layers (hp0:5m) on very weak ground(CBRp2). The influence on the bearing capacity forthicker fill layers, or on stiffer and stronger ground, wasmarginal.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

2 4 7

Field

Str

ain

[%

]

derived from subgrade deformation

derived from strain gauges

3 5 6 8

Fig. 21. Permanent measured geosynthetic strain vs. strain derived from

deformation of the subgrade.

Fig. 20. Assessing the rut outline.

Typical requirements for the subbase (EV1X11MPaand/or CBRX6) were achieved on some test track sectionsfor which a reinforcing geosynthetic contributed little, evenfor very thin fill layers. In this case, problems due toreaching the bearing capacity were not expected.

4.2. Impact on compaction

Compaction of the 1st and 2nd layers was primarilyaffected by the ground properties. The compactability ofthin layers (hp0:5m) could be improved by inclusion of areinforcing geosynthetic if the ground had CBRp3. Thisinteraction represents a hypothetical improvement in theground properties of DCBR � 1–2. Stiff flat rib andextruded grids appeared to have the greatest effect.Transverse strains in the geosynthetics under the ruts ofdepth up to 10 cm were between 0.5% and 1% for layerthicknesses hp0:5m, and these values were approximatelydoubled during dynamic compaction.The compactability of the layers hX0:5m, measured in

terms of the increase in the elasticity modulus, was moredependent on the compaction properties of the recyclingmaterials used for the fill than the ground response.Virtually no further tension was mobilised in the reinforce-ment during compaction for the third fill layers (hX0:6m).

4.3. Impact of reinforcement on rut formation

For thin fill layers (h � 0:4m), the rut formation onweak ground with geotextile reinforcement was signifi-cantly less than without reinforcement, so that eitherhigher axle loads are possible for reinforced tracks, or forthe same axle loads, reinforced fill layers can be traffickedby more passes until the same rut depths are reached. Rutdepths should be limited to less than 10 cm for tracks to betrafficked by trucks up to 400 kN. This field test demon-strated that this trafficability limit was reached very quicklyfor un-reinforced layers with h � 0:4m. Thicker fill layerswill be necessary for heavier transport with greater axleloads.The reinforcement reduced the rut depth even for layers

hX0:5m, as well as the number of trafficking cyclespossible before reaching the maximum allowed rut depth.However, time and cost limits meant that the number oftrafficking cycles were limited, and so extrapolation isnecessary to represent in-service conditions. It is recom-mended that some trafficking be carried out before thecompletion of the tracks, in order to cause some ruttingand to mobilise tension in the reinforcement. For groundwith good bearing capacity and CBRX3, reinforcement isonly essential to bridge over weak zones.

4.4. Choice of geosynthetic

The use of stiffer geosynthetics in the strain ranges of1–3% increased the bearing capacity and compactability ofa fill layer on soft ground. The inclusion of a reinforcement

ARTICLE IN PRESSR. Hufenus et al. / Geotextiles and Geomembranes 24 (2006) 21–3736

is effective for CBRp3 when hp0:5m. Geosynthetictensile strength requirements at 2% strain (T2%), both inlongitudinal and transverse production directions, shouldbe:

T2%X8 kN=m (2)

The stiffness of the geosynthetics within a strain range of1–3% was not affected significantly by the constructionprocess, and no reduction in the tension capacity arose dueto installation. Tensions rose to 8–15 kN/m by fullembedment in the granular layer, due to interlocking. Itis unnecessary to specify the use of an extremely stiffgeosynthetic because tensions of only 6–10 kN/m weremobilised for heights of fill between 0.2 and 0.5m.

The effect of the geogrids was found to be reduced whenused in direct combination with a separating layer, becauseoptimal interlocking with the coarse-grained fill layer wasprevented and the grid was able to slide on the geotextile.Nonetheless, to prevent mixing between the subsoil and thefill material, a separating layer should be used and thegeogrid should be laid 5 cm above it within the granularlayer, to improve both shear interaction and the bearingcapacity.

4.5. Benefits

The following benefits have been identified as a result oflaying a geosynthetic as a reinforcing layer between the filland the subsoil.

�

Reduction of the thickness of the fill layer by E30% forspecified compaction values and bearing capacities,although a minimum fill thickness of h ¼ 0:3m shouldbe recommended. � Reduction in the rut formation as a function of the

trafficking, increasing the serviceable life of the track.

Economic advantages of geosynthetic reinforcement layprimarily in the possibility to reduce the thickness of the filllayer or to limit the amount of subgrade to be removed.This saves on the use of granular materials and the amountof unsuitable material for removal and deposition else-where, which has both economical and ecological aspects,and may be useful for construction tracks laid withoutasphalt surfaces.

Acknowledgements

The authors thank the Swiss Federal Roads Authorityfor financial support and for permission to publish theresults. BP Amoco Fabrics, Fritz Landolt AG, Tensar Int.GmbH, Huesker Synthetic GmbH, Sytec Bausysteme AG,Polyfelt GmbH. and Naue Fasertechnik GmbH & Co KGkindly provided the geosynthetics and additional financialsupport. The contribution of G. Feltrin, K. Weingart, R.Rohr, P. Nater, H. Kung, U. Schrade, P. Barbadoro andV. Keller in the field and with the laboratory tests is

gratefully acknowledged. Hastag AG, Probst Maveg AGand Viagroup SA are thanked for their in-kind contribu-tion to the field trials. G. Laios’ contribution towards thecorrection of this manuscript is much appreciated.

References

Al-Qadi, I.L., Appea, A.K., 2003. Eight-year of field performance of a

secondary road incorporating geosynthetics at the subgrade-base

interface. 82nd Annual Meeting, Transportation Research Board.

Washington, CD-Rom, 21pp.

Al-Qadi, I.L., Brandon, T.L., Valentine, R.J., Lacina, B.A., Smith, T.E.,

1994. Laboratory evaluation of geosynthetic-reinforced pavement

sections. Transportation Research Report 1439, pp. 25–31.

Anderson, P., Killeavy, M., 1989. Geotextiles and geogrids—cost effective

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