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Geogrid mechanism in low-volume flexible pavements: accelerated testing of full-scale heavily
instrumented pavement sections
Imad L. Al-Qadia*, Samer Dessoukyb, Erol Tutumluera and Jayhyun Kwonc
aDepartment of Civil and Environmental Engineering, University of Illinois, Urbana-Champaign, Urbana, IL 61801, USA; bDepartment
of Civil and Environmental Engineering, University of Texas-San Antonio, San Antonio, TX 78249, USA; cTensar International Corp.,Atlanta, GA 30328, USA
(Received 22 December 2009; final version received 2 September 2010)
This study uses full-scale accelerated testing to provide new insight into the effectiveness of geogrids on the performance oflow-volume flexible pavements. Although several previous studies reported that geogrids improve pavement performanceby enhancing its structural capacity and reducing distress potential, this study goes further to quantify the effectiveness ofgeogrids, specify the mechanism of the reinforcement they provide and identify the optimum placement of geogrid in low-volume flexible pavements. Full-scale, low-volume flexible pavement sections were constructed on weak subgrade(California bearing ratio 4%) and heavily instrumented with 170 sensors. The pavement was divided into three cells witheach cell having three sections. The granular base and hot-mix asphalt layer thicknesses varied, and each cell had at least onecontrol and one geogrid-reinforced pavement section. The instruments were embedded to measure stress, strain, deflection,moisture, pore-water pressure and temperature and were used to monitor pavement response to a moving load using the
Accelerated Transportation Loading ASsembly (ATLAS). The testing programme was divided into two parts: responsetesting and performance testing. The response testing considered tyre configuration, loading, inflation pressure, speed andtravelling offset. The performance testing considered number of passes to failure. This paper presents the various pavementresponses to different loading configurations and pavement performances when a repetitive moving dual-tyre assembly at8 km/h and 44 kN was applied. Based on the performance testing and visual observation of the pavement cross sections afterexcavation, the reinforced sections showed reduced rutting and delayed surface cracking compared to the control sections.Specifically, the pavements measured response showed that geogrid-reinforced pavement sections exhibited less verticalpressure and less vertical deflection in the subgrade when tested at a low speed. Therefore, the studys most notableconclusion is that geogrid reinforcement reduces the horizontal movement of the granular material, especially in thelongitudinal direction. The study also concludes the following about geogrid placement: (1) for a relatively thick granularbase layer, placing the geogrid in the upper one-third of the base reduces the shear strains in the longitudinal and transversedirections. (2) For weaker pavements, the geogrid reinforcement at the base subgrade interface reduces the verticaldeflection. In the second case, the effectiveness of geogrid shall be compared to the increase in pavement structure or usingother geosynthetic materials such as geotextiles.
Keywords: geogrid; full scale; instrumentation; pavement reinforcement
Introduction
Unbound materials in pavement systems usually show
evidence of incremental degradation under repeated
loading. Giroud et al. reported that deterioration in granular
base layers occurs due to cycles of lateral displacement at
the bottom of the layer. The movement of aggregate
weakens the interface with subgrade, as fine particles
contaminating the granular layer also cause the penetration
of base course materials into subgrade (Al-Qadi et al. 1998,
Al-Qadi and Bhutta 1999, Al-Qadi 2002). In addition,
aggregate particles may break due to repeated loading.
Selig (1987) noted that the lateral displacement tends to
lower the base layer stiffness and induce local failure. Leng
and Gabr (2002) remarked that the degradation is
manifested by the reduction of the base layers ability to
spread a load, which increases thevertical stress transferred
to the subgrade.
Geogrid base reinforcement appears to have the
potential for successful and beneficial application in low-
to-moderate volume roads with thin hot-mix asphalt
(HMA) surfaces. When placed in a granular base course,
geogrids are thought to provide tensile reinforcement by
preventing lateral spreading of the base layer. In addition,
the interlock provided by geogrids may cause a relativelystiffer layer to develop around the geogrids (Kwon et al.
2008, Kwon and Tutumluer 2009). The other possible
benefits from using geogrid in pavements, which have
been reported in previously published literature, include
the following:
ISSN 1029-8436 print/ISSN 1477-268X online
q 2011 Taylor & Francis
DOI: 10.1080/10298436.2010.535534
http://www.informaworld.com
*Corresponding author. Email: [email protected]
International Journal of Pavement Engineering
Vol. 12, No. 2, April 2011, 121135
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(1) Serving as a construction platform over weak
subgrade as it facilitates compaction (Al-Qadi et al.
1994, Bloise and Ucciardo 2000).
(2) Extending pavements projected service life (Barks-
dale et al. 1989, Cancelli et al. 1996, Collin et al.
1996, Cancelli and Montanelli 1999, Jenner and Paul
2000, Al-Qadi and Appea 2003, Watts et al. 2004,
Yang and Al-Qadi 2005, 2007).(3) Reducing granular base course thickness for a given
design and service life (Miura et al. 1990, Valentine
et al. 1993).
(4) Increasing soil and base-bearing capacity (Floss and
Gold 1994, Appea et al. 1998, Appea and Al-Qadi
2000, Huntington and Ksaibati 2000).
(5) Reducing base soil contamination, depending on the
aperture size of geogrids and the increase in particles
interlocking at interface (Austin and Coleman 1993,
Loulizi et al. 1999, Ghosh and Dey 2009).
(6) Delaying and reducing rutting deformations (Chan
et al. 1989, Knapton and Austin 1996, Cancelli and
Montanelli 1999, Appea and Al-Qadi 2000, Jenner
and Paul 2000).
G eo gr id i s b eli ev ed to in cr ea se th e li fe of
pavements through several mechanisms including
interlocking unbound materials that results in restricting
lateral strain, improving its bearing capacity and
providing a membrane effect (Bender and Barenberg
1978, Giroud et al. 1985, Hass et al. 1988, Wong and
Small 1994, Moghaddas-Nejad and Small 1996, Perkins
and Ismeik 1997).
Interlocking controls the rotation and movement of
aggregate and, hence, could cause local stiffening and
greater friction at the interface. This may result in reduced
secondary deformation in the subgrade. Lateral restraint
restricts the horizontal flow beneath repetitive vertical
loading. Moreover, the possible increase in the stiffness of
the unbound material just above the geogrid could
improve pavement-bearing capacity. The membrane
effect of geogrid is believed to improve the vertical
stress distribution due to the presence of a deformed
membrane.
A recent national survey in the USA pointed out the
following reasons for the limited extent of using
geogrids for unbound aggregate base course reinforce-
ment in flexible pavement: a lack of detailed knowledgeon the mechanisms by which geogrids provide
reinforcement, a lack of established cost benefit
information and no available acceptable design solutions
(Christopher et al. 2001). These situations still exist as
indicated by a recent study conducted by the Federal
Highway Administration that will be available in 2010.
Although geogrid could provide benefits when used in
granular materials in flexible pavements, the complexity
of its mechanism in pavements is yet to be quantified. In
addition, the optimum location for installed geogrid in
the granular layer has been debated among researchers.
Broms (1977) demonstrated that geogrids provide
improvement if placed at the centre of the layer.
Barksdale et al. (1989) supported this finding for thin
pavement sections constructed with low-quality aggre-
gate bases; but for pavements constructed on soft
subgrade, they suggested the bottom of the base as thepreferred position. However, Chan e t al. (1989)
recommended that geogrids be placed as high as
possible in the granular base to reduce rutting. Al-Qadi
(2002) and Al-Qadi et al. (2006) suggested that the
optimum location of geogrid is at the upper one-third of
the granular layer thickness for low medium volume
pavements and at the bottom of the granular layer for
low-volume pavements constructed on a very weak
subgrade.
Monitoring in situ pavement response and perform-
ance is crucial to understanding geogrids mechanism in
pavements. Geogrids effect on the performance of
flexible pavement can be quantified by measuring the
responses of instrumented full-scale pavement sections
that are exposed to various vehicular and environmental
loading conditions. The first full-scale instrumented
and geosynthetically stabilised low-volume flexible
pavement section was constructed in Bedford, Virginia,
and monitored over several years (Brandon et al. 1996, Al-
Qadi et al. 1997, and Al-Qadi and Appea 2003). However,
that study did not measure the lateral movement of
aggregate or install geogrids within the granular
base layer. In addition, the effect of truck tyre
configuration becomes important for thin pavements due
to the effect of non-uniformity of vertical andtangential surface contact stresses (Al-Qadi et al.
2005, 2007, 2008, Yoo et al. 2006, Yoo and Al-Qadi
2007, 2008).
Objective and scope
The main objectives of this study were to quantify the
effectiveness of geogrid in low-volume flexible pave-
ments, to understand its reinforcing mechanisms and to
identify its optimum placement location in a granular base
layer.
To accomplish the objectives of this study, nine
heavily instrumented full-scale low-volume flexiblepavement sections were designed and constructed on a
weak subgrade to measure pavement responses, monitor
pavement performance and quantify the effectiveness of
geogrid-reinforced flexible pavements. The sections were
exposed to various accelerated loading levels using the
Accelerated Testing Loading ASsembly (ATLAS) at the
Advanced Transportation Research and Engineering
Laboratory (ATREL) at the University of Illinois in
Urbana-Champaign.
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Pavement site description
The constructed pavement sections consist of three cells,
A, B/C and D, with various layer thicknesses, lengths and
reinforcement types and locations. Each cell is divided
into three sections as shown in Figure 1. The HMA layer
thickness is maintained constant at 76 mm except for
section C1, which has a HMA thickness of 127 mm. The
granular base layer thicknesses are 203, 305 and 457 mm
for the three cells, A, B/C and D, respectively. As shown in
Figure 1, a transition zone (T) was built to taper out the
depth change in pavement thicknesses between the
different pavement cells. The transition zones along with
extensions at both ends of the pavement testing cells are
used as supporting pads for ATLAS. The constructed
pavement is 88 m long by 3.2 m wide.
Geogrids were installed at the granular basesubgrade
interfaces and at one-third of the base thickness from the
HMA layer interface. The experimental design considered
the effect of the following variables on low-volume
pavement performance: geogrid type (two geogrids withdifferent strengths were evaluated), location of geogrid in
the granular layer and HMA thickness.
The HMA layer was constructed with two lifts of a 9.5-
mm nominal maximum aggregate size (NMAS) wearing
surface mix. In section C1, an additional 25-mm NMAS
HMA layer was used to increase the layer thickness. The
granular base layer is A6 crushed limestone in accordance
with the Illinois Department of Transportation dense-
graded base specifications. The subgrade was prepared to
achieve and maintain 4% California bearing ratio (CBR).
To maintain the subgrade CBR constant during construc-
tion and testing, the water content was carefully controlled
through the construction of a drainage system at both sides
of the pavement and transversally between the pavement
cells. In addition, a prime coat was applied to the subgrade
surface after compaction to prevent water evaporationfrom the subgrade or water entrance from possible rain.
The moisture change was monitored throughout the testing
period using a time domain reflectometer (TDR) and pore-
water pressure sensors.
Instrument installation
During construction, more than 170 sensors were
embedded in the pavement sections to monitor pressure,
deflection, strain, temperature, moisture and pore-water
pressure (Figure 2). A total of 18 pressure cells, 49 linear
variable differential transformers (LVDTs), 12 strain
gauges, 82 thermocouples, 10 TDRs and 2 peizometerswere installed. The pressure cells, LVDTs and strain
gauges were installed at the centreline of the pavement
lane, while environmental sensors were embedded at 1 m
offset of the centreline. The details of instrumentation and
material testing can be found elsewhere (Al-Qadi et al.
2008). All instruments survived until the study was
completed; no instrument failed during construction or
testing.
Figure 1. The three cells pavement test sections.
Figure 2. Example of embedded LVDT, pressure cell and strain gauge.
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Testing programme
The accelerated loading tests were applied using ATLAS.
ATLAS is a movable loading facility supported by two
winches at each end to hold a steel frame structure that
houses a tyre carriage. The loading system is capable of
applying vehicular loading in a span of 25.90 m, which
includes acceleration and deceleration distances of 1.5 mat each end. Loading was applied by pulling the tyre
carriage in one direction (Figure 3). However, as a
common drawback of most accelerated pavement testing
(APT), the exerted torque in the tyre axles was not
simulated. The pavement section lengths and transition
areas were designed to allow each cell to be loaded
separately and to support ATLAS winches during loading,
respectively.
The pavement instrument responses were obtained
from three-tyre configurations: a dual-tyre assembly
11R22.5, a wide-base tyre 425/65R22.5 (old generation)
and a wide-base tyre 455/55R22.5 (new generation).
During testing, five loading levels at 26, 35, 44, 53 and
62 kN at two speeds (8 and 16 km/h) were applied in one
direction. Three-tyre inflation pressures were considered:
550, 690 and 750 kPa. During pavement loading, the
centre of each tyre configuration was aligned with the
centreline of the pavement. In addition, the loading was
applied at two offsets from the centreline, 150 and
230 mm. Each individual test was repeated for 5 10
cycles at each loading condition. All loading conditions
were operator controlled, except for the field temperature.
After completing the above pavement response
programme, the performance testing programme was
performed. The dual-tyre assembly was used to load the
pavement sections using 44 kN at a tyre pressure of
690 kPa and a speed of 8 km/h. Testing was terminated at
50 mm surface rutting. However, section C1 did not show
any sign of failure after relatively significant loading, and
the testing of sections D1D3 was terminated at
approximately 25-mm of rutting.
Data collection and processing
Instruments were connected to a data acquisition system to
collect and filter signal noise and store and plot the
collected data. An in-house programme for data
acquisition was developed using Labvieww. The load-
associated instruments were only activated during load
application, while the environmental instruments were
activated continuously to monitor temperature and
moisture changes. Normalisation of the instrument loading
response was achieved by subtracting the value before the
initiation of the response from the peak response.
Collected data were corrected to a HMA referencetemperature. During testing, the temperature ranges for
cells A, B/C and D were 1222, 835 and 19478C,
respectively. An exponential function was developed to
shift measured loading responses at various temperatures
to values at a reference temperature of 258C (Al-Qadi et al.
2007). Validation was conducted by comparing measured
and predicted responses at the reference temperature.
Loading offset was considered in this study (Figure 4).
Offsets were located at zero (centre of tyre configuration),
150 mm (centre of single dual-tyre assembly),
230 mm (edge of wide-base 455 tyre) and 305 mm (edge
of dual-tyre assembly, but off the edge of the wide-base
tyre).
Loading response analysis
Influence of tyre configuration and loading parameters
Compared to the conventional dual-tyre assembly,
wide-base tyres are reported to improve truck fuel
Figure 3. ATLAS with the dual-tyre assembly (left) and wide-base tyre hooked up to the carriage (right).
Figure 4. Tyre imprints for dual-tyre assembly and wide-basetyre.
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efficiency, reduce emission, increase payload, exhibit
superior braking and comfort and reduce repair and
maintenance cost (Al-Qadi and Elseifi 2007). However,
concern about the damage wide-base tyres could cause to
flexible pavements which has discouraged state agencies
from widely promoting the use of wide-base tyres.
Conventional wide-base tyres sustained high-operating
inflation pressure to minimise tyre deflection; however,high vertical contact stresses resulted at the pavement
surface. This shortcoming has been resolved in the new
generation of wide-base tyres. The new generation of
wide-base tyres is inflated at the same tyre pressure as dual
tyres. However, the reduced contact area of a wide-base
tyre compared to the dual-tyre assembly is still a concern
in spite of the fact that a wide-base tyre provides
significantly better contact stress uniformity (Al-Qadi et al.
2005).
The majority of the studies comparing conventional
wide-base tyres (425 or 385) to a dual-tyre assembly have
reported that wide-base tyres cause more damage to
pavement systems (Huhtala et al. 1989, TFHRC 2006).
This study investigates the impact of tyre configuration
(old generation 425/65R22.5 and new generation
455/55R22.5 wide-base tyres) on the constructed low-
volume pavement sections. In case the wide base tyre
becomes more widely implemented, it is important to
know how it would affect geogrids performance.
Imprints of the dual-tyre assembly and the two wide-
base tyre configurations showed that the WB425 has the
smallest contact area at the surface. This results in
relatively greater response on the pavement. This is
evident in Figure 5 where the response of WB425 is 32%
higher than that of the dual-tyre assembly. The figurepresents the responses from the sensors under the
centreline of the wide-base tyre and the centreline between
the two tyres for the dual-tyre assembly. WB455 has
shown an average of 10% less response than the WB425
and 14% greater response than the dual-tyre assembly. The
maximum response from the dual-tyre assembly at the
interface is between the tyres, while at shallow depths it is
under the centre of one tyre. At shallow depth, the HMA
response for the WB455 is approximately 33 and 8% less
than that from the WB425 and dual-tyre assembly,
respectively, under the same loading conditions (Figure 6).
However, the pavement sections, control and geogrid-
reinforced, showed the same trend in response to various
tyre types, although WB425 was concluded to be the most
damaging tyre to low-volume flexible pavement sections.
Vehicle speed has significant effect on the pavements
strain and deformation (Figure 7). In addition, Figure 7
shows no significant influence (#2%) on inflation
pressures at deep layers, while at shallow layers strain
responses vary within 7%.
Stress distribution under a tyre has been traditionally
considered uniform within the tyre imprints vicinity. This
approximation has been used in many multilayer
analysis techniques. However, in reality, stress distribution
varies along the tyre width and length. Using an
instrumented pad, the contact stresses in 3D were
measured for each tyre at various loading and tyre
pressures. This study also examined the effect of
wandering on pavement response.Results show that as a tyre deviates from the
instrument positions, pavement responses decrease. This
validates that stresses and strains under the tyre centre are
greater than those measured under the edge of the tyre. The
most critical tyre positions depend on the pavement depth
of interest. At shallow depth, under the centre of one tyre
of the dual-tyre assembly and the centre of WB455 are the
most critical positions; while deep in the pavement, under
the centre of the WB455 and the dual-tyre assembly (due
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Dual WB425WB455 Dual WB425WB455 Dual WB425 WB455
D1 D2 D3
Dual WB425 WB455 Dual WB425 WB455 Dual WB425 WB455
D1 D2 D3
Subgradevertical
deflection(mm)
26 kN 35 kN 44 kN 53 kN 62 kN 26 kN 35 kN 44 kN 53 kN 62 kN
05
1015202530354045
50
Pressure(kPa)
Figure 5. Deflection and pressure responses at base subgrade interface at 8 km/h and 750 kPa.
0
200
400
600
800
1000
1200
HMAtransversestrain(micro)
Dual WB425 WB455
D1
Dual WB425 WB455
D2
Dual WB425 WB455
D3
26 kN 35 kN 44 kN 53 kN 62 kN
Figure 6. Strain responses at the bottom of HMA at 8km/h and750kPa.
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to stress distribution overlap) is the most critical position.
In general, the effect of limited wander is more
pronounced at shallow depths (Figure 8).
Figure 9 illustrates the responses of HMA strain
gauges due to temperature variation. The data presented
in Figure 9 were obtained using the exponential correction
factor for three reference temperatures: 15, 25 and 358C.
As the base layer increases, the effect of tyre configuration
at the basesubgrade diminishes (Figure 10).
Influence of pavement layers
To evaluate the effectiveness of geogrid in pavements, the
granular base and HMA thicknesses are also considered.
The granular base thickness has shown an effect on both
the HMA strain and subgrade pressure and deflection.
When the base layer was increased by 50%, the subgrade
deflection was reduced by 60%. However, the impact of
increasing the HMA thickness was much more
pronounced (Figure 11).
0
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26 kN 35 kN 44 kN 53 kN 62 kN
Figure 7. Inflation pressure and speed influence in pressure responses.
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estrain
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Atransversestrain
(micro)
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26 kN 35 kN 44 kN 53 kN 62 kN 26 kN 35 kN 44 kN 53 kN 62 kN
26 kN 35 kN 44 kN 53 kN 62 kN26 kN 35 kN 44 kN 53 kN 62 kN
Figure 8. Tyre offset influence on subgrade pressure and HMA strain for (a) dual-tyre assembly and (b) WB455.
0100200300400500600700800
900
35C 25C 15C 35C 25C 15C 35C 25C 15C
D1 D2 D3
HMAtransversestrain(m
icro)
26 kN 35 kN 44 kN 53 kN 62 kN
Figure 9. Temperature influence on pavement responses.
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Influence of geogrid reinforcement
The insertion of geogrids in the granular base layer has
improved the performance of the tested pavement sections
in this study. The reinforced sections showed a reduction
in pavement responses in the range of 2331%, depending
on the pavement structure.
The improvement was obviously more pronounced
for weak pavements compared with thick pavements
(Figure 12). In addition, for a relatively thick granular
layer, single reinforcement at the top one-third of the base
layer improved the base resistance to transverse and
longitudinal deformation, while addition of a geogrid at
the base subgrade interface improved subgrade stability.
To investigate the effect of the geogrid type, two
distinctive geogrids, GG1 and GG2, were installed in cell
A (Table 1). The GG2 has a 50% higher modulus, tensile
strength and thicker rib than GG1. GG2 has shown slightly
better performance by minimising the base layer responses
(Figure 13). Similarly, Collin et al. (1996), Hirano et al.
(1990) and Miura et al. (1990) have suggested that
reinforcement affects the granular base stability and is
proportional to the tensile stiffness of the geogrids.
Performance testing analysis
Performance testing was conducted after the response
testing was completed. Loading conditions using dual-tyre
assembly were fixed throughout the testing programme.
The tyre offset was also kept unchanged at the centreline to
monitor the maximum instrument response. Due to the fact
that tests were conducted at various temperatures, data
normalisation was necessary. It was evident that the
pavement damage was in the form of rutting and cracking.
However, section C1, which has 127 mm of HMA,
0
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HMA
transversestrain(micro)
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A3 (203 mm) B1 (305 mm) D3 (457 mm)
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A3 (203 mm) B1 (305 mm) D3 (457 mm)
Dual WB455Dual WB455Dual WB455
A3 (203 mm) B1 (305 mm) D3 (457 mm)
Subgradeverticaldeflection(mm)26 kN 35 kN 44 kN 53 kN 62 kN 26 kN 35 kN 44 kN 53 kN 62 kN 26 kN 35 kN 44 kN 53 kN 62 kN
Figure 10. Effect of base thickness on pavement responses for control sections.
0
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Dual WB455 Dual WB455
B1 (76 mm) C1 (127 mm)
Dual WB455 Dual WB455
B1 (76 mm) C1 (127 mm)
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B1 (76 mm) C1 (127 mm)
HMAtransversestrain(micro)
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everticaldeflection(mm)
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Subgradeverticaldeflection(mm)26 kN 35 kN 44 kN 53 kN 62 kN 26 kN 35 kN 44 kN 53 kN 62 kN 26 kN 35 kN 44 kN 53 kN 62 kN
Figure 11. Effect of HMA thickness on pavement responses for control sections.
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(mm)
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Baselongitudinaldeformation
(mm)
26 kN 35 kN 44 kN 53 kN 62 kN
26 kN 35 kN 44 kN 53 kN 62 kN
26 kN 35 kN 44 kN 53 kN 62 kN
Figure 12. Effect of geogrid reinforcement and its location on granular base layer responses.
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experienced only tyre imprint marks on the surface.
Control sections, in general, showed excessive shear
deformation in the granular base layer compared with their
reinforced counterparts (Figure 14).
Reinforced sections performance
In general, granular base deformations were less in thegeogrid-reinforced sections. Results obtained from sec-
tions B1 and B2 suggest that reinforcement could
significantly reduce granular base lateral shear movement.
Similarly, for pavement with a thick granular base layer,
such as in section D, reinforcing the upper part of the base
could be very effective in reducing lateral shear. The
double reinforcement may improve long-term perform-
ance more than the single layer. However, the use of a
double layer depends on cost and pavement structure.
Surface crackingTyre pavement contact stresses are three dimensional
(Al-Qadi et al. 2005). In addition to pavement surface,
these stresses are affected by the tyre tread structure,
configuration, position, speed, material and inflation
pressure. The tangential stresses have a significant effect
at shallow depths and diminish with pavement depth. The
WB455 tyre resulted in greater longitudinal than
transverse strains as illustrated in Figure 15 for section
D2. The transverse strain reflects a tension-only response,
while the longitudinal strain reflects a compression
followed by a tension peak as a tyre moves over the sensor.
In general, the HMA longitudinal strain is usually
greater than the transverse strain, especially for thick
HMA layers. The difference between them decreases as
the structure becomes weaker. In addition, because of the
relatively significant longitudinal granular base move-
ment, a series of transverse cracks, 0.15 0.30 m apart,
along the wheel path as well as two longitudinal cracks at
the path edges, have appeared due to the tyre loading
repetitions. The severity of the cracks increases, as the
load repletion increases. In addition, the crack severity
progressed more rapidly in the case of thin granular base
layer and almost did not exist in the case of relatively thick
granular or HMA layers. The initiation and propagation of
transverse cracks are believed to be due to the high shear in
the pavement as well as the basesubgrade deformation.
This could be more manifested in the field, as the APT
does not provide a driving gear or braking, which can
significantly increase the shear stresses in pavements
(Figure 16).
Permanent deformation
The loading of a section was terminated at 50-mm rutting,
except when the rutting rate was relatively very low as in
section C1 and the D sections. Rutting was measured at
two zones in each section. The sections with the 150-mm
granular base layer showed significant rutting at a rapid
rate. To compare section performance, the rate of rutting
Table 1. Geogrid properties.
Properties
GG1 GG2
Load capacity Test method Units MD TD MD TD
Initial modulus ASTM D6637-01 kN/m 250 400 400 650
Tensile strength at 2% strain ASTM D6637-01 kN/m 4.1 6.6 6.0 9.0Tensile strength at 5% strain ASTM D6637-01 kN/m 8.5 13.4 11.8 19.6Ultimate tensile strength ASTM D6637-01 kN/m 850 1300 1315 1975
Note: MD, machine direction (along roll length); TD, cross-machine direction (across roll width).
A1 (GG1) A2 (GG2) A1 (GG1) A2 (GG2)
Basetransverse
deformation(mm)
Baselongitudinal
deformation(mm)
00.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0
0.2
0.4
0.6
0.8
1
1.226 kN 35 kN 44 kN
26 kN 35 kN 44 kN
Figure 13. Effect of geogrid strength on pavement responses.
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(RR) was determined for all sections. Table 2
presentsthe rutting and corresponding number cycles for
each section. The relation between RR and pavement
section design is shown in Figure 17. The calculated RR
showed clearly that the most rapid rutting development
was noted for the lowest unreinforced granular base layer
thickness, while the slowest rutting development was
noted for the section with the thickest HMA layer. In each
cell, the reinforced section showed better performance;
this improvement diminishes as the granular base layer
thickness increases. Although the reinforced section
showed a significant improvement, increasing the HMA
layer thickness is the most significant method to sustain
pavement integrity.
0
0.5
1
1.5
2
0
0.5
1
1.5
2
0 1000 2000 3000 4000 5000
No. of passes
Basetransversedeformation(mm)
0
0.05
0.1
0.15
0.2
Basetransversedeformation(mm)
Basetransversedeformation(mm)
A1 A2 A3
0 200000 40000 60000 80000 0 200000 40000 60000 80000
No. of passes
B1 B2 C1
0.00
0.05
0.10
0.15
0.20
100000
No. of passes
0 1000 2000 3000 4000 5000
No. of passes
0 200000 40000 60000 80000 0 200000 40000 60000 80000
No. of passes
100000
No. of passes
0 1000 2000 3000 4000 5000
No. of passes
0 200000 40000 60000 80000 0 200000 40000 60000 80000
No. of passes
100000
No. of passes
0 1000 2000 3000 4000 5000
No. of passes
0 200000 40000 60000 80000 0 200000 40000 60000 80000
No. of passes
100000
No. of passes
0 200000 40000 60000 80000 0 200000 40000 60000 80000
No. of passes
100000
No. of passes
D1 D2 D3
Baselongitudinaldeformation(mm)
Baselongitudinaldeformation(mm)
Baselongitudinaldeformation(mm)
A1 A2 A3
0
0.25
0.5
0.75
1
0
0.25
0.5
0.75
1
B1 B2 C1
0.00
0.05
0.10
0.15
0.20
0.00
0.05
0.10
0.15
0.20
D1 D2 D3
Baseverticaldeflection(m
m)
Baseverticaldeflection(m
m)
B1 B2 C1 D1 D2 D3
0
1000
2000
3000
4000A1
A2
A3
0
500
1000
1500
B1 B2
HMA
transversestrain(micro)
0
500
1000
1500
HMA
transversestrain(micro)
HMA
transversestrain(micro)
D1 D2 D3
0
20
40
60
80
Subgradepressure(kPa)
Subgradepressure(kPa)
Subgradepressure(kPa)A1 A2 A3
0
50
100
150
200
B1 B2 C1
0
20
40
60
80
D1 D2 D3
Figure 14. Traffic testing responses of pavement sections.
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Geogrid placement
The optimal geogrid placement position is dependent on
the subgrade, the base thickness and the tyre loading
magnitude. Hirano et al. (1990) reported that geogrids
must be covered by a minimum base layer of 200 mm to
control damage during trafficking. Cancelli and Mon-
tanelli (1999), Hass et al. (1988), Miura et al. (1990) and
Walters and Raymond (1999) suggested that the optimal
position of geogrid is at the bottom of the base for a soft
subgrade with a base thickness less than 400 mm.
However, for relatively higher subgrade-bearing capacity
and thicker base, the optimal position is at 250 350 mm
below the surface (Hass et al. 1988, Moghaddas-Nejad andSmall 1996, Perkins et al. 1999). However, in this study,
the placement of additional reinforcement layer at the
bottom of a thick base layer with geogrid in the top one-
third was found not to reduce the rutting potential. For
comparison, the response difference at the beginning and
end of performance testing for each sensor was divided by
the total number of passes. This allows normalisation of
data for each cell (Table 3).
250
0
250
500
750
1000
0 5 10 15 20 25
HMAtransverse/
longitudinalstrain(micro)
250
0
250
500
750
1000
HMAtransverse/
longitudinalstrain(micro)
250
0
250
500
750
1000
HMAtransverse/
longitudinalstrain(micro)
Tyre position (m)
0 5 10 15 20 25
Tyre position (m)
0 5 10 15 20 25
Tyre position (m)
D2-Long
D2-Trans.
Dual-tyre(a) (b) (c)WB425 WB455
D2-Long
D2-Trans.
D2-Long
D2-Trans.
Figure 15. Longitudinal and transverse HMA strain responses to various tyre configurations for section D2 at 44 kN loading.
Cell A Cell B Cell D
(c)(b)(a)
Figure 16. Pavement transverse cracking at the loading path due to the surface longitudinal strain.
Table 2. Rutting measurements and number of passes at two locations in each section.
Cell A A1-1 A1-2 A2-1 A2-2 A3-1 A3-2Number of passes 4456 4456 4456Final rut depth (mm) 19 20 19 30 33 31
Cell B/C B1-1 B1-2 B2-1 B2-2 C1-1 C1-2Number of passes 48,390 50,003 62,297Final rut depth (mm) 49 49 39 32 5 8
Cell D D1-1 D1-2 D2-1 D2-2 D3-1 D3-2Number of passes 89,155 89,155 89,155
Final rut depth (mm) 22 22 23 23 29 32
10.00
8.00
6.00
4.00
2.00
0.00Rateofrutting,
RR
(0.00
1mm/cycle)
A1
RR 4.36 6.35 9.07 1.34 0.50 0.11 0.27 0.28 0.40
A2 A3 B1 V2 C1 D1 D2 D3
Figure 17. Comparison of RR among pavement sections.
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Pavement trenches
After testing was completed, deep trenches were cut in thezones that showed severe distress (Figure 18) to quantify
layer thicknesses, interface locations and conditions and
geogrid depth. Considering variation in layer construction
processes, most layer thicknesses were within 10% of the
proposed design shown in Figure 1. The layer profile at
that base subgrade interface was found to be highly
affected by loading, especially for the control and thin-
base sections (Figure 19). This is in agreement with
previous suggestions that as the granular base thickness
increases, the impact of reinforcement lessens (Gobel et al.
1994, Collin et al. 1996, Posposil and Zednik 2000, Al-
Qadi et al. 2007).The layer thickness prior to loading was measured at
the edge of the trench. Deformed layers were measured at
the centre of the trench. Depression was noticed in the
HMA and base layers as well as in the subgrade. As the
number of passes increased, the thickness of each layer
varied. Hence, normalisation of thickness reduction and
subgrade rutting was used to compare section perform-
ances. The change in layer thickness was divided by the
layer thickness and the number of passes, as presented inTable 4. In general, the control areas revealed a higher rate
of depression in HMA and base layers in all sections
except D3. The subgrade rutting in these sections was
twice as severe as in the reinforced ones.
Summary
The conclusions from this study build on previous research
that has presented geogrid as a worthy reinforcement for
flexible pavement by going further to explain the
mechanisms involved. To conduct this necessary research,
a heavily instrumented low-volume flexible pavementstructure, consisting of three cells, was constructed on a
weak subgrade (CBR 2 4%). Each cell contained three
pavement sections including control and geogrid-
reinforced sections. The sections were exposed to
pavement response-measuring and performance test
programmes using the Accelerated Transportation Load-
ing ASsembly (ATLAS). The response programme
considered tyre configuration, loading, inflation pressure,
speed and travelling offset. The performance programme
considered the number of passes to failure.
The various layers were instrumented with 170 sensors
to monitor environmental and load-associated responses.
Three-tyre configurations, wide-base tyres 425 and 455and dual-tyre assembly, were used in the response testing
programme. Pavement sections, controlled and reinforced,
showed the same trend in response to various tyre types.
WB425 was concluded to be the most damaging tyre to
low-volume flexible pavement sections. Loading wander
has significant impact on pavement response. Maximum
pavement responses are associated with the centre of the
wide-base tyre and are depth dependent on the dual-tyre
assembly. Tyre inflation pressure is significant at the
Table 3. Ultimate difference in sensors responses normalised to the number of passes.
Sensors
HMASubgrade Base deflection (mm)
SectionsTransverse
strainPressure
(kPa)
Verticaldeflection
(mm) Vertical Transverse Longitudinal Comment
A1 197 9244 536 NA 101 263 Different GG productA2 530 9154 307 NA 79 94A3 260 8748 74 NA 370 606 ControlB1 11 2038 22 13 4 15 ControlB2 25 1753 20 9 3 9C1 NA 971 7 4 1 6 Thick HMA layerD1 14 574 5 2 1 2 Reinf. at 1/3 base layerD2 8 612 6 2 1 2 Double reinf.D3 7 855 4 3 6 4 Control
Note: All numbers are multiplied by 1026 except transverse strain that is multiplied by 1023.
Figure 18. Trench cut in section D2.
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shallow depth and its influence diminishes as the depth
increases within the pavement. Response testing also
indicated that increasing granular base layer and HMAthicknesses would reduce pavement response. Although
increasing HMA thickness was concluded to be the most
effective way to reduce pavement responses, geogrid also
decreased pavement response.
In general, geogrid-reinforced pavement sections
constructed on soft subgrade exhibited less vertical
pressure on subgrade and less vertical deflection in the
subgrade when tested at a low speed. In particular, this
observation was more manifested in weak pavement
structures. It needs to be noted that although geogrid tends
to improve overall low-volume flexible pavement
performance, it is not recommended to use a geogrid inunder-designed pavement with hope of meeting the design
requirements.
For a relatively thick granular base layer, placing the
geogrid at the upper one-third of the base reduces the shear
strains in the longitudinal and transverse directions. On the
other hand, for weak pavements, the reinforcement at the
base subgrade interface reduces the vertical deflection.
Thus, the study reveals a notable mechanism of geogrid
700
600
500
400
300
200
100
0
100
0.5 1.0 1.5 2.0 2.5 3.0
Profile(mm)
700
600
500
400
300
200
100
0
100
Profile(mm)
700
600
500
400
300
200
100
0
100
Pro
file(mm)
700
600
500
400
300
200
100
0
100
Pro
file(mm)
700
600
500
400
300
200
100
0
100
Pro
file(mm)
700
600
500
400
300
200
100
0
100
Profile(mm)
700
600
500
400
300
200
100
0
100
Pro
file(mm)
700
600
500
400
300
200
100
0
100
Profile(mm)
Transverse distance (m)
0.5 1.0 1.5 2.0 2.5 3.0
Transverse distance (m)
0.5 1.0 1.5 2.0 2.5 3.0
Transverse distance (m)
0.0 0.5 1.0 1.5 2.0 2.5 3.53.0
Transverse distance (m)
0.0 0.5 1.0 1.5 2.0 2.5 3.53.0
Transverse distance (m)
0.5 1.0 1.5 2.0 2.5 3.0
Transverse distance (m)
0.0 0.5 1.0 1.5 2.0 2.5 3.53.0
Transverse distance (m)
0.5 1.0 1.5 2.0 2.5 3.0
Transverse distance (m)
Surface Bottom of HMAGG2 Bottom of base
D1 D2 D3
B1 B2
A1 A2 A3
Sur face Bottom of HMAGG2 Bottom of base
Sur face Bottom of HMABottom of base
Surface Bottom of HMA
Bottom of base/GG1
Surface Bottom of HMA
Bottom of base/GG2
Surface Bottom of HMA
Bottom of base
Surface Bottom of HMA
Bottom of base/GG2
Surface Bottom of HMA
Bottom of base
Figure 19. Pavement layers profile across the trenches.
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reinforcement: It reduces the horizontal movement of thegranular material, especially in the longitudinal direction.
For the performance programme, APT was conducted
until pavement failure. Distress was observed in all
sections except section C1, which has 127 mm of HMA.
The control sections exhibited relatively unstable shear
deformation in the granular base layer resulting in early
distress compared to their reinforced counterparts. The
distress was mainly in the form of longitudinal and vertical
deformations. Pavement transverse cracking, perpendicu-
lar to loading direction, occurred in the pavement sections
having relatively thin granular layers. The RR parameter
was used as a performance indicator. In general, high RRwas obtained for unreinforced thin sections, while low RR
was obtained for reinforced sections with thick bases. This
study brought to light a specific benefit of geogrid: It
minimises the lateral deformations in the base layer. This
result emphasises the outcome of the response testing
programme.
Trenches dug in the sections revealed that both base
and subgrade were deformed. The deformation was
localised under the loading path resulting in soil
contamination of the base aggregate. The rutting in the
subgrade of control sections was twice that of the
reinforced sections for the weak structure and became less
as the pavement structure improved.
In conclusion, geogrid shows great potential to reduce
shear flow and lateral deformations in the granular base
layer which results in increasing the strain at the bottom of
HMA and minimising rutting and/or cracking in a thin
pavement structure. The effectiveness of geogrid starts in
early stages of loading and continues by controlling the
granular material movement due to shear.
AcknowledgementsThe assistance of J. Baek, P.-J. Yoo, E. Fini, J. Meister, M. Elseifi,B. Harkanwal, J. Anochie-Boateng, C. Montgomery, K. Jiangand Z. Leng during pavement construction and instrumentation isgreatly appreciated. The content of this paper reflects the views ofthe authors, who are responsible for the facts and the accuracy ofthe data presented herein. This paper does not constitute astandard, specification or regulation. The financial supportprovided by Tensar International Co. is greatly appreciated.
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