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PERKINS AND ISMEIK D Geosynthetic-Reinforced Bases in Flexible Pavements: Part II 

1 INTRODUCTION 

Geosynthetics have been proposed and used as reinforcement in the base course layers 

of flexible pavements for the past 15 years. Studies have been performed to examine 

the feasibility of this application and concentrated on the use of experimental test sections 

to demonstrate the performance of geosynthetic-reinforced roadways. The test results 

of these experimental studies are analyzed and discussed in a companion paper 

written by the authors of the current paper (Perkins and Ismeik 1997). In the majority 

of the studies reviewed, it was concluded that an appreciable increase in performance 

could be realized by using geosynthetic reinforcement, and that optimal performance 

is dependent on many factors. 

Improvement has been attributed mainly to the prevention of lateral spreading of the 

aggregate base. An improvement is defined as an increase in pavement service life and/ 

or a reduction in base course thickness. For properly designed sections, improvement 

is typically seen for all rut depths and not only for rut depths that render the pavement 

inoperable. Suitable frictional and interlocking interaction between the geosynthetic 

and the surrounding base course soil are required to prevent spreading of base course 

soil. For this reason, geogrids have typically offered greater improvement in performance 

as compared to geotextiles. The torsional rigidity of geogrids has an impact on 

performance, with torsionally stiff products generally offering better performance. 

The importance of separation and filtration was illustrated in several of the studies 

reviewed by Perkins and Ismeik (1997); if the section was not properly designed for separation 

and filtration, the reinforcement benefit of the geogrid could be minimized. The 

test results from these studies also revealed that sections, which were designed to fail 

under the operation of traffic repetitions characteristic of permanent roads, experienced 

greater improvements in performance as compared to sections which failed in a rapid 

manner. For properly designed sections, improvement was seen for a variety of subgrade 

conditions, pavement section layer thicknesses, and geosynthetic location and 

layering configuration. These studies taken as a whole suggest that the optimal design 

of reinforced pavements depends on the following factors: geosynthetic type, manufacturing 

process, mechanical properties, placement location, and layering; base course 

thickness and quality; asphalt concrete (AC) thickness; subgrade type, strength, and 

stiffness characteristics; and load magnitude and frequency of application. 

Solutions have been proposed for the design of geosynthetic-reinforced pavements 

and have been based either on empirical or analytical considerations. No design solutions 

which propose a general analytical solution that has been validated by experimental 

data have been identified. The importance of the many variables defined above 

which impact performance implies that a successful empirical solution will need to be 

developed from an extensive series of experiments involving these variables. 

Analytical solutions developed within the framework of a finite element method 

(FEM) can potentially account for many of the loading- and materials-related features 

thought to impact geosynthetic-reinforced pavement performance. Many of these features 

are the same as those which must be incorporated into general, pavement response 

FEM models. The FEM model description of loading should account for the time-dependent 

nature of the load. Constitutive models employed for the pavement system materials 

should account for the time-dependent behavior of the AC layer and the cyclic 

behavior of the base and subgrade soils under repetitive loads. Similar considerations 

606 GEOSYNTHETICS INTERNATIONAL S 1997, VOL. 4, NO. 6


are necessary for the geosynthetic materials, while geosynthetic strength and stiffness 

anisotropy should also be considered. Interaction at the soil-geosynthetic interface is 

also important and should be considered. Studies involving the development of FEM 

models for reinforced pavement systems are summarized in Section 3. 

2 EXISTING DESIGN TECHNIQUES 

Five studies that describe a design approach for reinforced pavements have been 

identified. Haas et al. (1988), Montanelli et al. (1997), and Webster (1993) present empirical 

design approaches based on experimental findings from their respective studies. 

Davies and Bridle (1990) and Sellmeijer (1990) present design approaches based on 

analytical considerations and were not necessarily verified by experimental results. 

2.1 Penner et al. (1985) 

Penner et al. (1985) presented an empirical design approach based on the experimental 

work by Haas et al. (1988). The design approach was based on the pavement design 

guidelines published by the American Association of State Highway and Transportation 

Officials (AASHTO 1981) and was used for comparing results and developing base 

course equivalency charts. This design approach is similar in concept to that used by 

Al-Qadi et al. (1997) for comparing experimental results, which is described in the 

companion paper by Perkins and Ismeik (1997). 

The structural number (SN) of each control section was calculated assuming layer 

coefficients of 0.4 for the asphalt layer and 0.14 for the granular base layer. The subgrade 

soil support value (S) was determined from the subgrade California Bearing Ratio 

(CBR) strength and ranged from 4.3 to 5.7. The values for SN and S were then used in 

the AASHTO design method to determine the total equivalent 80 kN single-axle load 

applications, which ranged from 60,000 to 10,000,000 applications. 

A load correction factor was calculated for each section by dividing the number of 

80 kN single-axle load applications by the actual number of load applications necessary 

to cause failure (failure was defined as a 20 mm rut depth). This load correction factor 

was intended to account for differences in loading conditions between the laboratory 

experiments and moving wheel loads in the field. For the control sections, the load 

correction factors ranged from 3.5 to 10, which were comparable to the values obtained 

by Al-Qadi et al. (1997) (5 to 12.5) using the AASHTO method. The use of these correction 

factors can be explained as follows: if the laboratory section was subjected to actual 

field loads, a 20 mm rut depth would have developed after a number of load applications 

that is equal to the number of load applications required in the laboratory experiments 

multiplied by the load correction factor. 

This load correction factor was then taken to apply to the reinforced sections within 

a particular test series. The load correction factor for each reinforced section was then 

used to calculate the 80 kN single-wheel load applications by multiplying the actual 

number of load applications experienced in the laboratory tests by the corresponding 

correction factor. From the AASHTO method, a SN value for that section was determined. 

A SN value for the reinforced granular base was then calculated by subtracting the 

asphalt layer SN component from the total SN value. A reinforced layer coefficient was 

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then calculated by dividing the SN value for the reinforced base by its corresponding 

thickness. The ratio of the reinforced to unreinforced layer coefficients was calculated 

by using the layer coefficient for the unreinforced granular base, i.e. the control section. 

For equivalent base layer SN values, the ratio of the reinforced to unreinforced layer 

coefficients is equal to the ratio of unreinforced to reinforced base layer thickness. The 

ratio of the reinforced to unreinforced layer coefficients was plotted against the reinforced 

base thickness. The graph decreased as the reinforced base thickness approached 

250 mm when the geogrid was placed at the bottom of the base. Additional improvement 

in the layer coefficient ratio was noted for a base thickness of 250 mm when the 

geogrid was placed in the middle of the base. Based on these results, Figure 1 was developed 

and shows the relationship between the thickness of a reinforced base to an unreinforced 

base when Geogrid A is used (the properties of Geogrid A are listed in the 

companion paper). 

2.2 Montanelli et al. (1997) and Webster (1993) 

Montanelli et al. (1997) also used the AASHTO design method to determine a layer 

coefficient ratio for the granular base, which is equal to the ratio of the reinforced to unreinforced 

layer coefficients. Values of this ratio ranged from 2 to 1.5 and were determined 

from experiments using one geogrid and subgrades with different CBR strengths. 

The values greater than 1.5 were calculated for subgrade CBR strengths less than 3. The 

layer coefficient ratio value was used as a multiplication factor for the depth of the reinforced 

base in the equation used to calculate the structural number. This implies that for 

an equivalent structural number, the unreinforced base could be reduced by 33 to 50%. 

Webster (1993) produced a design chart similar to that of Haas et al. (1988) by directly 

comparing and extrapolating test results for sections of equivalent base course thickness. 

The original design chart included the 50 mm thick AC layer used in the experiments. 

The authors of the current study have modified the original chart by excluding 

the 50 mm thick AC layer which resulted in the chart shown in Figure 2. 

Reinforced base thickness (mm) 

400 

Geogrid at midpoint of base layer 

300 

Geogrid A 

200 

Geogrid at bottom of base layer 

100 

0 

0 100 200 300 400 500 600 

Unreinforced base thickness (mm) 

Figure 1. The flexible pavement design chart proposed by Haas et al. (1988) (from 

Penner et al. 1985). 



Reinforced base thickness (mm) 

400 

300 

200 

100 

0 

0 100 200 300 400 500 600 

Unreinforced base thickness (mm) 

Figure 2. The flexible pavement design chart proposed by Webster (1993). 

2.3 Davies and Bridle (1990) 

Davies and Bridle (1990) developed an analytical technique to calculate the amount 

of permanent deformation (rut depth) of reinforced pavements with load cycle. The displacement 

response of the pavement under a single monotonic load application was predicted 

using an energy method. An expression for the potential energy of the pavement 

system was developed as a function of the central displacement of the applied load. The 

general shape ofthesurface displacement profile wasassumed tobe thesame asdisplacement 

profiles observed in previously published studies. The geosynthetic layer provided 

an additional energy component to the system as it deformed and was shown to increase 

the component of strain energy provided by the base layer of the pavement. Both the 

base layer and the geosynthetic were assumed to provide a component of strain energy 

as structural members in bending, even though both materials have little flexural rigidity. 

The development of permanent deformation with increasing load cycle was predicted 

by varying the stiffness parameters of the subgrade. Permanent deformation was assumed 

to be negligible in the base layer. The stiffness parameters of the subgrade during 

loading were assumed to be less than those during unloading. The stiffness parameters 

were assumed to vary with increasing load cycle, with the difference between those for 

loading versus unloading becoming less at an decreasing rate. 

The net effect of this material model was a prediction of rut depth which increased 

with load cycles at a decreasing rate. The values and variation of the material stiffness 

parameters were determined primarily from the results of repeated load experiments 

on reinforced pavement test sections. Calibration of material parameters from model 

pavement experiments limits the utility of the design approach due to the time and expense 

associated with model construction. Full use of this technique would be facilitated 

by relating the material stiffness parameters to material properties, such as 

resilient modulus, that can be more readily determined from element tests. 

2.4 Sellmeijer (1990) 

Sellmeijer (1990) formulated a model for the behavior of a soil-geotextile-aggregate 

system which accounted for both the membrane action and the lateral restraint function 


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of the geotextile. While an AC layer was not specifically included in the model, the 

model was said to be suitable for paved roads because it can analyze cases in which only 

small rut depths were permissible. An elastic-plastic model was used for the aggregate. 

The subgrade was assumed to be a rigid, perfectly plastic material. Interaction between 

the soil and geotextile was accounted for using a simple law of friction. The function 

of lateral restraint was shown to increase the mean stress and stiffness in the aggregate 

layer. The model was not compared to experimental results. 

3 REVIEW OF FINITE ELEMENT STUDIES 

A number of studies have been conducted to examine the usefulness of finite element 

programs to predict the response of roadways reinforced with geosynthetics. Several 

of these studies were performed in conjunction with the experimental studies described 

in Section 2 such that model predictions could be compared to experimental results. The 

development of a finite element model to accurately predict performance is attractive 

because the model can be used to examine various experimental test configurations that 

have not been modeled experimentally. A well-formulated finite element model can potentially 

be used to perform an extensive parametric study, with the results of this study 

being used to develop a design methodology. Table 1 is a summary of the major features 

of each study reviewed in this section. 

3.1 Barksdale et al. (1989) 

Barksdale et al. (1989) adapted an existing finite element model to predict the response 

obtained in the experimental portion of their study. The prediction of tensile 

strain in the base material was essential in determining the amount of tensile strain developed 

in the geosynthetic, which in turn determined, in part, the benefit provided by 

the reinforcement. The cross-anisotropic model used for the base was the only model 

capable of simultaneously predicting the lateral tensile strains in the bottom of the base 

and the small vertical strains in the bottom and upper sections of the base, as observed 

in the laboratory experiments. 

The finite element model was calibrated and verified using the test data generated 

from test series 3 of the Barksdale et al. (1989) study and unreinforced pavement section 

data from Barksdale (1984). The unreinforced pavement section used for calibration 

was strong in comparison to the sections in test series 3. The finite element model was 

capable of predicting measured variables to within ±20% for the strong unreinforced 

section. For the weaker sections used in the study, the finite element predictions were 

not as good. The strain in the geosynthetic was over predicted by approximately 33% 

when the geosynthetic was located in the bottom of the base layer and was under predicted 

by approximately 14% when located in the middle of the layer. The vertical stress 

and vertical strain on the top of the subgrade were under predicted by approximately 

50%. The lateral strains were also under predicted by approximately 50%. 



Table 1. Summary of the finite element model studies analyzed in the current paper. 

Barksdale et al. 

(1989) 

Burd and Houlsby 

(1986) 

Burd and Brocklehurst 

(1990) 

Author 

Burd and Brocklehurst 

(1992) 

Dondi 

(1994) 

Miura et al. 

(1990) 

Wathugala et al. 

(1996) 

Analysis 

type 

Axi-symmetric Plane strain Plane strain Plane strain Threedimensional 

Axi-symmetric Axi-symmetric 

AC 

constitutive 

model 

Isotropic, 

nonlinear elastic 

None None None Isotropic, 

linear elastic 

Isotropic, 


Isotropic 

elastoplastic, 

Drucker-Prager 

AC 

thickness 

(mm) 

Variable None None None 120 50 89 

Base 

constitutive 

model 

Anisotropic, 


Isotropic, 


Matusoka 

Isotropic, 


Matusoka 

Isotropic, 

lastoplastic, 

Matusoka 

Isotropic, 



Isotropic, 


Isotropic, 



Base 

thickness 

(mm) 

Variable 75 300 300 300 150 140 

Geosynthetic 

constitutive 

model 

Linear elastic Isotropic, 


Isotropic, 


Isotropic, 


Isotropic, 


Isotropic, 


Isotropic, 


von Mises 

Geosynthetic 

element 

type 

Membrane Membrane Membrane Membrane Membrane Truss Solid 

continuum 

Geosynthetic 

thickness 

(mm) 

None None None None None None 2 

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Table 1. Continued 

Interface 

elements and 

model 

Linear elasticperfectly 

plastic 

None None Elastoplastic, 

Mohr-Coulomb 

Elastoplastic, 

Mohr-Coulomb 

Linear elastic 

joint element 

None 

Subbase 

constitutive 

model 

None None None None None Isotropic, 


Isotropic, 


HiSS δo 

Subbase 

thickness 

(mm) 

None None None None None 200 165 

Subgrade 

constitutive 

model 

Isotropic, 

non-linear 

elastic 

Isotropic, 


von Mises 

Isotropic, 


von Mises 

Isotropic, 


von Mises 

Isotropic, 


Cam-Clay 

Isotropic, 


Isotropic, 


HiSS δo 

Load 

application 

Monotonic Monotonic, 

footing width 

=75mm 

Monotonic, 

footing width 

= 500 mm 

Monotonic, 

footing width 

= 500 mm 

Monotonic, two 

rectangular 

areas, 240 mm 

× 180 mm 

Monotonic, 

200 mm 

diameter plate 

Single cycle, peak 

pressure = 725 kPa 

on a 180 mm 

diameter plate 

Remarks on 

observed 

improvement 

Base layer 

could be 

reduced in 

thickness by 4 - 

18%, greater 

improvement 

seen for 

sections with 

weak subgrade 

Improvement 

seen after 

penetration of 4 

mm, model 

overpredicted 

improvement 

beyond 4 mm 

displacement 

Improvement seen after 

penetration of 12 mm, 

improvement increased 

with increasing 

geosynthetic stiffness 

Improvement seen 

after penetration of 25 

mm 

15 - 20% 

reduction in 

vertical 

displacement, 

fatigue life of 

section 

increased by a 

factor of 2 - 2.5 

5% reduction 

in vertical 

displacement, 

improvement 

level did not 

match 

experimental 

results 

20% reduction in 

permanent 

displacement 

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The accuracy of the model predictions was contrasted against the conclusion that the 

calculated relative changes in the observed response between the three sections in the 

test series examined showed comparable trends to the model predictions. This indicates 

that relative comparisons should be reasonably good, thereby allowing the model to be 

used to perform a sensitivity (parametric) study to examine relative benefits of various 

pavement section configurations. It should be noted that Barksdale et al. (1989) state 

that the analytical predictions of tensile strain in the bottom of the AC layer were not 

validated by experimental results because these results were inconsistent. Thus, reducing 

the thickness of base course layers based on this measure may be erroneous. 

In the sensitivity study, the finite element model was used to calculate the lateral tensile 

strain at the bottom of the AC layer and the vertical strain at the top of the subgrade 

for a single load application. Lateral tensile strain was used to evaluate fatigue resistance 

of the AC layer, while the vertical strain was used to predict the degree of rutting, 

which in turn was used to evaluate improvement in pavement performance for unreinforced 

and reinforced sections. An improvement in performance due to reinforcement 

was quantified as the reduction in aggregate base thickness for a reinforced roadway 

giving the same tensile strain (fatigue) and vertical strain (reflecting permanent deformation) 

as that for the unreinforced section. Improved performance increased with 

increasing geosynthetic stiffness and decreased with increasing subgrade stiffness and 

asphalt thickness. Optimal improvement occurred when the geosynthetic was placed 

between the bottom of the base and 1/3 up into the base layer. 

Barksdale et al. (1989) used the 1972 AASHTO design method (AASHTO 1972) to 

determine design thicknesses for sections with subgrade CBR strengths ranging from 

3 to 10 and for two different traffic loading conditions. Using the more stiff geosynthetic, 

reductions in base course thickness ranged between 4 to 16% when improvement 

was based on equal lateral strain in the bottom of the AC layer, and 6 to 18% when improvement 

was based on equal vertical strain at the top of the subgrade. In general, more 

improvement was observed for sections with a weak subgrade and a thinner AC layer. 

These results are not necessarily confirmed when viewed in light of other experimental 

studies discussed in the companion paper. Barksdale et al. (1989) stated that the mechanisms 

modeled were more suited for geotextiles, and that additional research was required 

to define the mechanisms of improvement associated with geogrids and to develop 

suitable models. 

3.2 Burd and Houlsby (1986) 

Burd and Houlsby (1986) developed a large strain finite element model that was used 

to analyze experimental results from reinforced unpaved road test sections but could 

be extended to include material elements representing an asphalt layer. The large strain 

formulation was included to account for the large rut depths that can develop in unpaved 

roads. Interface elements were not included in the model, which implies perfect fixity 

between the soil layers and the geosynthetic. 

The model was used to predict the response of a footing resting on a base layer with 

a geosynthetic placed between the base and the underlying subgrade. The model predictions 

were compared to experimental results and a reasonable correspondence was 

achieved. The experimental results showed a slight improvement in the load-displacement 

curve for the reinforced footing for footing penetrations less than 4 mm, while the 


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model did not show improvements of this kind until the footing penetration exceeded 

4 mm. For a footing penetration greater than 4 mm, the improvement exhibited by the 

reinforced footing became significant for both the model and the experimental results; 

the model over predicted the experimental results at larger displacements, and the over 

prediction became more significant as the footing displacement increased. 

3.3 Burd and Brocklehurst (1990) 

Burd and Brocklehurst (1990) applied this same model to a larger footing. Similar to 

the results obtained by Burd and Houlsby, the model did not show improvements in the 

load-displacement curve until 12 mm of settlement had occurred. The model was used 

in a parametric study to demonstrate the influence of geosynthetic stiffness on improving 

performance. 

Burd and Brocklehurst (1990) extended this model to include interface elements. The 

model was used to predict the response of a footing placed ona base layer with reinforcement 

between the base and subgrade. The finite element analyses predicted negligible 

improvement in the load versus displacement response until a displacement in excess 

of 25 mm was achieved. In general, the model with interface elements showed less improvement 

than the model without interface elements. In light of the comparison of 

model and experimental results performed by Burd and Houlsby (1986), it appears that 

interface elements were required only when large footing displacements were present. 

3.4 Dondi (1994) 

Dondi (1994) used the commercial program ABAQUS (Hibbit, Karlsson and Sorenson, 

Inc. 1994) to model a geosynthetic-reinforced pavement. Load was applied to the 

pavement surface over two rectangular areas measuring 240 mm by 180 mm and representing 

a single pair of dual wheels. The wheels were 120 mm apart. Each rectangular 

area experienced a peak loading pressure of 1500 kPa. Due to the loading geometry, a 

three-dimensional finite element analysis was performed. A cohesion of 60 kPa was assigned 

to the base course soil to avoid numerical instabilities. Different friction coefficients 

were used between the geosynthetic and the base and subgrade soils. Sections 

were analyzed with and without the geosynthetic layer and for two geosynthetics of differing 

elastic modulus. 

The measured stress and strain on elements in the base and in the subgrade revealed 

that the base layer experienced moderate increases in load carrying capacity for the reinforced 

cases, while the strain in the subgrade decreased substantially for the reinforced 

cases. The model indicated that the geosynthetic layer reduced the shear stresses 

and strains experienced by the subgrade. The vertical displacement of the loaded area 

was reduced by 15 to 20% by the inclusion of the geosynthetic. The displacement of 

the unreinforced section was not stated. An empirical power expression involving tensile 

strain in the AC layer was used to evaluate the fatigue life of the sections, showing 

that the service life of the reinforced sections could be increased by a factor of 2 to 2.5 

as compared to the unreinforced section. 



3.5 Miura et al. (1990) 

Miura et al. (1990) performed a finite element analysis of a reinforced paved road to 

compliment their laboratory and field experimental program which is described in the 

same paper. The section layer thicknesses were chosen so as to correspond to the laboratory 

test sections. The results from the analysis of reinforced and unreinforced sections 

showed general agreement with results from the laboratory test sections where surface 

displacement and strain in the geosynthetic were plotted against distance from the centerline 

of the load. The improvement in the surface displacement for the reinforced section 

as compared to the unreinforced section was greatly underestimated by the finite 

element model when compared to the experimental results. The finite element model 

showed a reduction in displacement of 5% while the experimental results showed a 35% 

reduction. The monotonic loading results from the finite element analysis were 

compared to the experimental results at 10,000 cycles of applied load. With regard to 

this comparison, the finite element model was not intended to be an exact representation 

of the experiments, but was used to develop a better understanding of the mechanisms 

involved in reinforcement. 

3.6 Wathugala et al. (1996) 

Wathugala et al. (1996) also used the commercial program ABAQUS (Hibbit, Karlsson 

and Sorenson, Inc. 1996) to develop a finite element model of a geogrid-reinforced 

pavement. The model was formulated in anticipation of being used to predict the response 

of the experiments being performed by the Louisiana Transportation Research 

Center (LTRC 1996). The select base course and embankment soils were modeled using 

the constitutive model developed byDesai et al. (1986)and Wathugala and Desai (1993). 

This model can account for the nonlinear behavior of soil materials during nonvirgin 

loading, which is particularly appropriate for cyclic loading applications. This feature 

was not used, however, with nonvirgin loading modeled by a linear elastic response. A 

geogrid-soil interface model was not incorporated into the overall model. The geogrid 

was given a thickness of 2.5 mm and the pavement section was analyzed with and without 

the geogrid layer. The addition of the geogrid reduced the permanent rut depth by 

approximately 20% for a single cycle of load. This level of improvement was most likely 

due to the flexural rigidity of the geosynthetic, which is an artificial feature arising 

from the material and element model used for the geosynthetic. 

4 STUDIES IN PROGRESS 

In this section, various on-going studies of reinforced flexible pavements reported in 

the literature or known to the authors of the current paper are highlighted. This presentation 

of information is intended to inform readers of the possible future publications of 

results arising from these studies. 

Brandon et al. (1996) have described the construction and monitoring plans for nine 

instrumented flexible pavement test sections along a rural secondary road in southwest 

Virginia. The 15 m long sections were built to examine the effects of geogrid and geotextile 

reinforcement. Three geotextile-reinforced, three geogrid-reinforced, and three 


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unreinforced control sections were constructed. A nominal asphalt thickness of 90 mm 

was used with three base course thicknesses (100, 150, and 200 mm) which allowed a 

control, geogrid, and geotextile section to be built for each base layer thickness. The 

geosynthetics were placed between the subgrade and the base course layer. The A-6 

subgrade (low plasticity silt, ML) had an in-place CBR of approximately 7%. The sections 

have been instrumented with stress cells, soil and asphalt strain sensors, strain 

gages attached to the geosynthetics, thermocouples, and soil moisture cells. The 

construction of the test sections was completed in August 1994. Monitoring of the test 

sections was scheduled for a 3 year period. 

Preliminary results have been reported by Al-Qadi et al. (1997). Results from the 

three sections with the thinnest structural layers (89 mm thick AC layer, 100 mm thick 

base layer) show that after 26 months of traffic, the control section has reached a 25 mm 

rut depth, while the geogrid and geotextile sections have reached 19 and 16 mm rut 

depths, respectively. Ground penetrating radar data suggests that fines contamination 

of the base course layer is most significant for the control section and least significant 

for the geotextile-reinforced section. 

Hayden and Dunn (1996) have proposed the construction of 12 test sections to examine 

the use of various geosynthetics for functions of reinforcement, separation, and 

drainage. The test sections are being constructed along a 3 km portion of Route 1A in 

the towns of Frankfort and Winterport, Maine, USA. The test sections will be visually 

inspected on a periodic basis. Performance measurements will be made using the falling 

weight deflectometer (FWD) test on a yearly basis. The test sections will be instrumented 

to measure stress and strain; readings will be taken every three months. The 

project is expected to be monitored for a 10 year period. A preliminary report will be 

published at the end of the second year. 

Kennepohl (1993, p. 2) described a 0.65 km long field trial section constructed along 

TransCanada Highway 17 approximately 52 km north of the Town of Wawa, Ontario, 

Canada. The area contains a silty subgrade with a high water table. Severe rutting of the 

roadway has occurred in the past due to the heavy logging traffic in the area. The existing 

AC layer was pulverized and mixed into the existing base course layer. A geogrid, Geogrid 

A, was placed over this surface, with an additional 150 mm of select base material 

placed over the geogrid. (A description of Geogrid A is given in Table 3 in the companion 

paper by Perkins and Ismeik (1997).) A control section without geogrid-reinforcement 

was also constructed. As of November 1996, no distress (i.e. rut development or 

fatigue cracking in the asphalt layer) was measured in either section (Kennepohl 1996). 

A program study titled “Construction and Comparison of Louisiana’s Conventional 

and Alternative Base Courses under Accelerated Loading” is being conducted at the 

Pavement Research and Accelerated Loading Facility at the Louisiana Transportation 

Research Center (LTRC) (LTRC 1996). In this program, several test sections were devoted 

to the study of geosynthetic reinforcement of the base layer. Lanes 2 and 3 of the 

Phase I study incorporated a geotextile and a geogrid, respectively. Each lane contained 

90 mm of AC and an A-4 subgrade. Lane 2 was comprised of a 215 mm thick crushed 

base layer over a nonwoven geotextile. Lane 3 was comprised of a 140 mm thick base 

layer on top of a geogrid which is directly on top of the geotextile. Lane 3 failed prematurely 

due to shear between the wearing course and binder layer in the AC layer (Rasoulian 

1996). The failure of the AC layer was not related to the section design. LTRC is 

planning to retest Lane 3 at a later date. 



Laboratory experiments on pavement systems are being conducted by the authors of 

the current paper (Perkins 1996). The pavement sections are constructed in a large concrete 

box and subjected to a cyclic load that is applied using a 300 mm diameter plate. 

Geogrid A and Geotextile A are being used as reinforcement (Perkins and Ismeik 1997). 

Variables included in the testing program include subgrade type and strength, geosynthetic 

type, geosynthetic placement position, and base thickness. Asilty sand and a highly 

plastic clay are the two types of subgrade being used. An extensive array of stress, 

strain, temperature, and moisture content sensors are being used to monitor the response 

of the sections. To date, three preliminary sections have been constructed, loaded, and 

monitored. The load frame allows for the load to be moved to various locations across 

the pavement surface such that a single cycle of load can be applied at various distances 

from a particular sensor to assess the reasonableness of the response of each instrument. 

This is done before the repeated load test. For the repeated load test, the load is placed 

at the center of the tank. These tests have also provided interesting data on the dynamic 

response of the as-constructed pavement section (Perkins et al. 1998). 

Figure 3 shows a three-dimensional view of the peak strain response of a geogrid specimen 

placed approximately at the bottom of a 200 mm thick base layer in a test section 

with a 100 mm thick AC layer. This graph is an example of the data obtained from the 

tests performed by the authors to-date. The section contained a silty sand subgrade with 

a CBR value of approximately 15. The peak strain has been determined as a function 

of distance from the applied load by moving the load around the tank, applying a single 

cycle of load, and measuring strain response at the same location on the geogrid. The 

Strain (%) 

Figure 3. The development of strain in Geogrid A in a pavement system subjected to a 

single, 40 kN load pulse (Perkins et al. 1998). 


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radial and tangential distance axes denote directions where a radial and tangential strain 

is being measured. The plot shows that a maximum strain of 0.25% is obtained directly 

beneath the load center. In the radial direction, the strain goes to zero at a 200 mm radius, 

then, at greater radial distances the strain becomes compressive, and gradually returns 

to zero at a 1 m radius. In the tangential direction, the strain remains tensile at all points 

and reduces to zero at a 750 mm radius. 

Finally, Cancelli et al. (1996) and Montanelli et al. (1997) are continuing their work 

by constructing full-scale pavement sections which will be subject to moving wheel 

loads. No additional details of this work were made available. Work is also being conducted 

at the University of Alaska, Alaska, USA (Kinney et al. 1998a, 1998b), as a continuation 

of the work reported by Collin et al. (1996). 

5 CONCLUSIONS 

Of the design approaches proposed, the empirical methods of Penner et al. (1985), 

Montanelli et al. (1997), and Webster (1993) are limited to the conditions associated 

with the experiments of the study. The design methods do not appear capable of accounting 

for the influence of wide variations in variables such as geosynthetic type, load 

magnitude, asphalt concrete and base layer thickness, and subgrade type. The analytical 

method proposed by Davies and Bridle (1990) does not account for movement in the 

base layer soil, which appears to largely control the development of rut depth for properly 

designed sections and must be calibrated from actual pavement loading experiments. 

The analytical approach of Sellmeijer (1990) appears promising due to its ability to account 

for the effect of increased confinement on the base course layer as a result of interaction 

with the geosynthetic. 

Varying degrees of success have been achieved in developing a finite element model 

to predict the response of reinforced flexible pavements. Barksdale et al. (1989) and 

Miura et al. (1990) have been able to incorporate certain laboratory/field test section 

behavior into their design models. Perhaps the most significant limitation of all of the 

models developed to date is the inability to predict pavement section response over a 

number of repeated load cycles. All of the models were developed to predict pavement 

response under a single load application. 

Typical mechanistic-empirical design procedures for conventional pavements allow 

the response measured during a single load application to be empirically extended to 

predict the development of rut depth or fatigue cracking in the asphalt layer with continued 

load cycles. This approach may not be appropriate for reinforced flexible pavements 

for several reasons. First, the difference in response between a reinforced and 

unreinforced flexible pavement may not be appreciably different for a single cycle of 

load when compared to the difference seen over many cycles of load. This creates a potential 

problem with regard to the sensitivity of the empirical expressions used. Second, 

the amount of experience necessary to develop these empirical relationships for reinforced 

pavements is likely insufficient at this point in time. Thus, a mechanistic model 

which can predict performance for any number of applied load cycles is required. 

The modeling implications for the various pavement layer materials in a reinforced 

system for such an approach are significant. For the base and subgrade materials, a material 

model capable of calculating continued plastic deformations with increasing load 



cycle number is necessary. Conventional elastic-plastic models are not capable of predicting 

such plastic deformations when a load of constant magnitude is repeatedly cycled. 

These models predict the plastic strains during the first applied load and predict 

recoverable elastic strains for each following load cycle. Similar features are necessary 

for the geosynthetic materials, along with features which model the torsional rigidity, 

creep, stress relaxation, and dynamic soil-geosynthetic interaction characteristics of the 

materials. Additional work appears to be necessary to develop a finite element method 

model capable of describing these effects. Such a model would be useful for developing 

design tools suitable for use by pavement designers. 

ACKNOWLEDGEMENTS 

The authors gratefully acknowledge the financial support of the Montana Department 

of Transportation through Grant Number 8126. The authors are indebted to many of the 

authors of the papers cited and other members of the research community who reviewed 

this paper for accuracy and/or provided additional details on their work. Included in this 

list are I. Al-Qadi, A. Anderson, R. Bathurst, P. Dunn, K. Henry, G. Kennepohl, M. Rasoulian, 

G. Richardson, P. Rimoldi, and A. Zhao. 

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