Geogrid Mechanism in Low-Volume Flexible Pavements Accelerated Testing of Full-scale Heavily...

7/31/2019 Geogrid Mechanism in Low-Volume Flexible Pavements Accelerated Testing of Full-scale Heavily Instrumented Pav

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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.

I.L. Al-Qadi et al.122


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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.

International Journal of Pavement Engineering 123


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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

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Dual WB425WB455 Dual WB425WB455 Dual WB425 WB455

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deflection(mm)

26 kN 35 kN 44 kN 53 kN 62 kN 26 kN 35 kN 44 kN 53 kN 62 kN

05

1015202530354045

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Pressure(kPa)

Figure 5. Deflection and pressure responses at base subgrade interface at 8 km/h and 750 kPa.

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HMAtransversestrain(micro)

Dual WB425 WB455

D1

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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).

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Figure 7. Inflation pressure and speed influence in pressure responses.

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estrain

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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

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35C 25C 15C 35C 25C 15C 35C 25C 15C

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icro)

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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,

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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 10. Effect of base thickness on pavement responses for control sections.

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Figure 11. Effect of HMA thickness on pavement responses for control sections.

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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.



9/16

(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



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)



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


0

500

1000

1500

HMA


HMA


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.



10/16

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/


250

0

250

500

750

1000

HMAtransverse/


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.



11/16

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.



12/16

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


0.5 1.0 1.5 2.0 2.5 3.0


0.0 0.5 1.0 1.5 2.0 2.5 3.53.0


0.0 0.5 1.0 1.5 2.0 2.5 3.53.0


0.5 1.0 1.5 2.0 2.5 3.0


0.0 0.5 1.0 1.5 2.0 2.5 3.53.0


0.5 1.0 1.5 2.0 2.5 3.0


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


Bottom of base/GG2


Bottom of base


Bottom of base/GG2


Bottom of base

Figure 19. Pavement layers profile across the trenches.



13/16

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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Normalised values multiplied by 106.



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