Geotextiles and Geomembrane
Available online 20 October 2005
the road. Geosynthetics are installed between subgrade androad to separate or to reinforce. If migration of nes is veryprobable, separation is an essential function (Al-Qadi
(Bloise and Ucciardo, 2000; Cancelli and Montanelli, 1999;Huntington and Ksaibati, 2000; Jenner and Paul, 2000;Martin, 1988; Miura et al., 1990), diminish deformations
ARTICLE IN PRESS(Chan et al., 1989; Jenner and Paul, 2000), and delay rutformation (Cancelli and Montanelli, 1999; Knapton andAustin, 1996; Meyer and Elias, 1999).
0266-1144/$ - see front matter r 2005 Elsevier Ltd. All rights reserved.
Corresponding author. Tel.: +4171 274 7341; fax: +41 71 274 7862.E-mail address: firstname.lastname@example.org (R. Hufenus).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 ll 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 of
et al., 1994; Al-Qadi and Appea, 2003), but with decreasingbearing capacity of the subgrade, the importance ofreinforcement increases signicantly (Saathoff and Horst-mann, 1999).Numerous eld 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 ll thicknessAbstract
A full-scale eld test on a geosynthetic reinforced unpaved road was carried out, including compaction and trafcking, to investigate
the bearing capacity and its performance on a soft subgrade. The test track was built with three layers of crushed, recycled ll 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 prole
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 nite lateral anchoring forces.
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Keywords: Bearing capacity; Full-scale eld test; Reinforcement; Rut formation; Soft subgrade; Unpaved roadFull-scale eld tests on geosynon soft
Rudolf Hufenusa,, Rudolf RueeggSarah M. Springman
aEMPA, Materials Science and TechbRueegger Systems, Solutions in Geotechnical EnginecInstitute for Geotechnical Engineering, Swiss Federa
dEMPA, Materials Science and Techn
Received 16 February 2s 24 (2006) 2137
etic reinforced unpaved roadsbgrade
b, Robert Banjacc, Pierre Mayorc,Rolf Bronnimannd
gy, CH-9014 St. Gallen, Switzerland
g, Vonwilstrasse 9, CH-9000 St. Gallen, Switzerland
stitute of Technology, CH-8093 Zurich, Switzerland
y, CH-8600 Duebendorf, Switzerland
accepted 14 June 2005
is distorted and thus tensioned (Meyer and Elias, 1999).
The geosynthetic reinforcement should be placed in thelowwh
ARTICLE IN PRESSnd G22Due 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 pressurelocal deformations of the ll. Due to frictional interac-tion and interlocking between the ll material and thegeosynthetic, the aggregate particles are restrained at theinterface between the subgrade and the ll (Jenner andPaul, 2000). The reinforcement can absorb additionalshear stresses between subgrade and ll (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 ll thickness. The conningmechanism does not imply the need for signicant 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 ll 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 anunpaved road is pre-rutted during construction, ageosynthetic reinforcement at the ll-subgrade interfaceThe combination of geosynthetic reinforcement and lllp to spread the concentrated vertical loads and to inhibitge deformations and local failures (Su et al., 2002). Twodes of action can be distinguished (Bourdeau, 1991;iura et al., 1990):
Confinement: A vertical load induces lateral forces,which spread the aggregate particles and thus lead to
CBR CBR coefcientEV1 Youngs modulus for the 1st plate loading
(MPa)EV2 Youngs modulus for the plate reloading (MPa)
R. Hufenus et al. / Geotextiles aapplied to the ll 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 ll thicknesses (Kenny,1998) and at low values of shear stiffness of thegranular ll (Ghosh and Madhav, 1994). Signi-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
theaptha(Cal.besmthe0.21919er part of the ll height (Jenner and Paul, 2000),ereas the optimal placement position is dependent onmembrane effect and thus enhance the bearing capacityof the footing.
Geosynthetics reinforcing unpaved roads on soft sub-grade have been shown to reduce the necessary llthickness 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 h CBR0:63
where N is the number of standard axle passes, h the roadlayer thickness (m) and CBR is the CBR coefcient.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 signicant with ll heights less than 0.4monly (Collin et al., 1996; Meyer and Elias, 1999; Posposiland Zednik, 2002). With higher lls, 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 ll layer of 0.2m to preventdamage during trafcking (Hirano et al., 1990; Meyer andElias, 1999).
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/m
eomembranes 24 (2006) 2137subgrade, the ll thickness and the magnitude of theplied loads. With a soft subgrade and a ll thickness lessn 0.4m the optimal position lies at the base of the llancelli and Montanelli, 1999; Haas et al., 1988; Miura et, 1990; Walters and Raymond, 1999). With higheraring capacity of the subgrade, increasing ll height oraller trafcking loads, the optimal placement position ofgeosynthetic moves upwards to approximately
50.35m below the surface of the ll (Haas et al.,88; Moghaddas-Nejad and Small, 1996; Perkins et al.,99).