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GEOGRID REINFORCEMENT OF FLEXIBLE PAVEMENTS:A PRACTICAL PERSPECTIVE

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Geogrid Reinforcement of Flexible Pavements: A Practical Perspective

By Aigen Zhao and Paul T. Foxworthy

Recent efforts by the AASHTO Subcommittee on Materials, Technical Section 4E, to develop ageogrid/geotextile specification for pavement reinforcement have initiated very positivediscussions. The Geosynthetic Materials Association has participated in the discussion and maderecommendations to AASHTO with the presentation of a draft “White Paper” addressinginstallation survivability and specifications. The overwhelming comments back from thereviewers of the “White Paper” clearly show the need to 1) demonstrate the performance andcost benefits of geogrid reinforcement, and 2) develop a design procedure incorporating geogridwith value-added benefits, in addition to the installation survivability aspects already welldocumented.

Geogrid reinforcement has been used in the design and construction of pavements for over adecade, yet there exists no design method incorporating geogrid mechanical properties as directdesign parameters. Due to the complexity of layered pavement systems and loading conditions,there may never be a simple design method identifying the properties of a geogrid as directdesign parameters for reinforced pavement systems. Rather, a series of performance based testsshould be conducted to evaluate the structural contribution of geogrid reinforcement to pavementsystems, from which design parameters could be derived and incorporated into a designmethodology.

This paper presents a practical perspective to address: 1) a modified AASHTO design method forreinforced pavements, 2) performance tests to support and verify the design parameters, and 3)cost benefit and constructability analyses. Performance data and analyses presented here arelimited to multilayered polypropylene biaxial geogrids.

Modified AASHTO Design Method for Geogrid Reinforced Flexible Pavements

Existing design methods for flexible pavements include: empirical methods, limiting shearfailure methods, limiting deflection methods, regression methods, and mechanistic-empiricalmethods. The current AASHTO method is a regression method based on the results of road tests.The AASHTO method utilizes an index termed the “structural number” (SN) to indicate therequired combined structural capacity of all pavement layers overlying the subgrade. Therequired SN is a function of reliability, serviceability, subgrade resilient modulus, and expectedtraffic intensities. The actual SN must be greater than the required SN to ensure long termpavement performance.

The actual SN value for a unreinforced pavement section is calculated as follows:

22211 mdadaSN ∗∗+∗= Eq. (1)

where a1 a2 are the layer coefficients characterizing the structural quality of the asphalticconcrete (AC) layer and the aggregate base course (BC) in a pavement system. A subbase layer

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can be included in Eq. (1) if desired. d1, d2 are their thicknesses; and m2 is the drainagecoefficient for the granular base.

A modification to equation (1) is introduced to account for the structural contribution of ageogrid reinforcement to flexible pavements.

Eq. (2)

where LCR is the layer coefficient ratio. Equation (2) can be used to calculate the base coursethickness for geogrid reinforced pavements by rearranging its terms:

Eq. (3)

When the layer coefficient ratio, LCR, is greater than 1, the thickness of the geogrid reinforcedbase course is reduced compared to unreinforced sections; similarly, if the base course thickness isheld constant, the structural number of the reinforced section increases. An increased structuralnumber implies an extended service life of the pavement for the same traffic level.

The concept of layer coefficient ratio was introduced over a decade ago (Carroll, Walls and Haas1987, Montanelli, Zhao, and Rimoldi, 1997) to quantify the structural contribution of a geogridin a flexible pavement. This concept was established based on the reinforcing mechanism thatgeogrid provides lateral confinement to the base course material and improves the layer coefficientof the reinforced base. The next section addresses the controlled laboratory pavement testsperformed to develop this design parameter for multilayered polypropylene biaxial geogrids. Thefollowing sections provide field verification through nondestructive tests and full-scale in-groundtests.

Controlled Laboratory Pavement Testing

Laboratory tests were performed to study flexible pavement systems under cyclic loadingconditions, and to quantify the structural contribution of a geogrid reinforcement. The test setupis shown in Figure 1. Cyclic loading was applied through a rigid circular plate with a diameter of300 mm. The peak load was 40 kN with an equivalent maximum stress of 570 kPa. Asphalticconcrete, aggregate base course and subgrade soil layers were included in the pavement sections.The asphalt thickness was 75 mm, and the base thickness was 300 mm. A multilayeredpolypropylene geogrid manufactured by continuous extrusion and orientation processing wasused in the test, its properties are listed in Table 1. The details of the laboratory tests arepresented by Cancelli et al. (1996).

SN a d LCR a d m= ∗ + ∗1 1 2 2 2* *

22

112

*

maLCR

daSNd

∗∗−=

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Figure 1. Controlled laboratory pavement tests

Table 1. Properties of the Multilayered Geogrid Used in the Tests

Machine Direction Cross Machine DirectionUnit weight g/m2 240Open Area % 75Peak tensile strength kN/m 13.5 20.5Tensile modulus @2% strain kN/m 220 325Tensile modulus @5% strain kN/m 180 260Junction strength kN/m 12.2 19.2

Figure 2 shows pavement surface rutting for both control and geogrid reinforced sections. Thenumber of loading cycles versus subgrade CBR is presented in Figure 3 for rut depths of 12.5mmand 25 mm respectively.

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100 1000 10000 100000CYCLE, [-]

0

50

100

150VERTICAL SETTLEMENT, [mm]

300 mm GRAVEL Unreinforced CBR 1% Reinforced CBR 1%Unreinforced CBR 3%Reinforced CBR 3%Unreinforced CBR 8%Reinforced CBR 8%Unreinforced CBR 18%Reinforced CBR 18%

Figure 2. Pavement surface ruts for control and reinforced sections.

10

100

1000

10000

100000

1000000

0 3 6 9 12 15 18CBR, [%]

CYCLE, [-]

Unreinforced 25mm RUTReinforced, 25mm RUTUnreinforced 12.5mm RUTReinforced, 12.5mm RUT

Figure 3. Loading cycle number for control and reinforced at two rut depth.

Figure 4 depicts the relationship between the calculated layer coefficient ratio and subgrade CBRbased on pavement testing data from both control and reinforced sections. The layer coefficient

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ratio was calculated for each subgrade CBR based on procedures contained in the AASHTOGuide for the Design of Pavement Structures (1993). First, the structural number of the controlsection was calculated using Eq. 1. Second, the total number of 18-kip equivalent single axleloads (ESAL) that the control section could be expected to sustain before failure wasbackcalculated from the AASHTO flexible pavement design curve, assuming reliability = 95%,standard deviation = 0.35, design serviceability loss = 2, layer coefficient of asphalt = 0.4, layercoefficient of aggregate base course = 0.14, drainage coefficient = 1, and subgrade resilientmodulus = 1500 * CBR value. Third, the load correction ratio was calculated by dividing thetotal expected ESAL by the actual number of rigid circular plate load applications required toreach the predetermined rut depth failure criteria (25mm, or 12.5mm). The failure criterion of a12.5 mm rut depth was used since under a CBR of 18 the pavement never reached a 25mm rutdepth. Fourth, this load correction ratio was used to calculate the expected total number of ESALto failure for each reinforced section with the same subgrade CBR. Fifth, the structural numberof each reinforced section was determined from the AASHTO flexible pavement designnomograph. Finally, the layer coefficient ratio for each subgrade CBR was then calculated bysolving Eqs. (1) and (2). The layer coefficient ratio is presented as a function of subgrade CBRvalues in Figure 4, and as shown, the lower the subgrade CBR, the greater the layer coefficientratio.

S ubgrade C B R

0 2 4 6 8 1 0 1 2 1 4 1 6 1 8 2 0

La

ye

r C

oe

ffic

ien

t R

ati

on

, L

CR

1 .0

1 .2

1 .4

1 .6

1 .8

2 .0

Figure 4. Layer coefficient ratio vs. subgrade CBR

Nondestructive FWD Tests

Nondestructive tests were conducted in Wichita, Kansas, to evaluate the effectiveness of geogridmaterials in improving the structural capacity of pavement sections using AASHTOnondestructive testing and analysis procedures. To accomplish this objective, several residential,collector, and arterial street segments, previously constructed using geogrid materials, were

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identified for nondestructive testing. Ground penetrating radar (GPR) and falling weightdeflectometer (FWD) tests were conducted on these existing geogrid reinforced paved roads. TheFWD load plate used for this project was 285 mm in diameter, and two tests targeted to producenominal loads of approximately 9,000 pounds were performed at each test location. FWD testswere conducted at 100 foot spacing in the outside wheel path of each travel lane and staggered toprovide 50 foot coverage along the street centerline.

The GPR tests were first conducted to identify individual “uniform” pavement sections alongeach street segment. A core sample of the asphaltic concrete surface and hand auger sample ofthe aggregate base course were obtained on Dallas Street to provide ground truth for calibrationof the GPR data. The GPR data was then analyzed at each FWD test point to produce layerprofiles for each street segment and to further delineate the uniform sections shown in Table 2.

Table 2. Summary of Uniform Sections

Street From ToAverage

ACAverage

BaseSegment Section Station

(ft)Station

(ft)Thickness

(in)CoV*(%)

Thickness(in)

CoV*(%)

Geogrid

Sterling 1 0+00 5+70 4.8 11 9.7 11 YESDallas 1 0+00 9+78 6.3 17 6.6 8 YES

31st 1 0+00 15+00 8.6 7 7.3 10 YES2 15+00 24+98 8.3 4 7.8 13 YES3 24+98 52+66 8.1 6 6.9 10 YES

Pawnee 1 0+00 5+50 10.5 24 7.8 8 None2 5+50 28+92 11.9 7 8.0 10 YES3 28+92 40+58 12.1 20 8.1 17 None

* CoV = Coefficient of Variation = Standard Deviation Divided by the Mean

Field roadbed soil resilient modulus Mr values for each FWD test location were backcalculatedfrom deflection data. The structural capacity of each uniform pavement section was thenevaluated in terms of an effective structural number (SNeff) using the nondestructive deflectiontesting approach outlined in the 1993 AASHTO Guide. The method essentially evaluates thetotal, or overall, stiffness for the pavement structure (Ep) using deflection data. The effectivestructural number (SNeff) then is a function of the total pavement thickness and its overallstiffness, Ep.

In an effort to conduct a meaningful evaluation of geogrid effectiveness from the testing data, thedesign structural number of each street segment in the project was assessed. Layer thickness datafor each street segment, shown in Table 3, was obtained from original construction drawings andassigned AASHTO layer coefficients based on recognized typical values for each material type.For AC materials, an AASHTO layer coefficient of 0.40 was selected based on experience withfield compacted mixes. For granular base course materials, a layer coefficient of 0.14 wasselected as representative of the densely graded, crushed rock materials used in Wichita streetconstruction. This thickness and material quality information was then used to calculate the

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design structural number for each street segment shown in Table 3, recognizing that no benefitwas assigned for using a geogrid at the interface between the base and subgrade. These designstructural numbers could then be compared with backcalculated effective structural numbersfrom GPR and FWD deflection data to determine the impact of the geogrid. Table 3 presents acomparison of the design structural number and effective structural number for each streetsegment.

Table 3. Structural Number Comparison

StreetDesign

ACDesignBase Design As-built Effective

Segment Section Thick(in)

Thick(in)

SN SN SN CoV*(%)

Geogrid

Sterling 1 5.0 5.0 2.70 3.27 2.81 18 YESDallas 1 5.0 10.0 3.40 3.44 4.40 25 YES31st 1 7.0 8.0 3.92 4.46 6.33 19 YES

2 7.0 8.0 3.92 4.41 6.13 14 YES3 7.0 8.0 3.92 4.20 5.40 17 YES

Pawnee 1 11.0 8.0 5.52 5.29 6.47 13 None2 11.0 8.0 5.52 5.88 7.05 9 YES3 11.0 8.0 5.52 5.97 6.59 21 None

* CoV = Coefficient of Variation = Standard Deviation Divided by the Mean

A comparison of design layer thicknesses in Table 3 with as-built layer thicknesses in Table 2revealed the street segments chosen for the project were generally built somewhat thicker thanoriginally designed. Therefore, it was appropriate to adjust the design structural number toaccount for actual layer thicknesses in the assessment of geogrid effectiveness. This wasaccomplished by calculating an as-built structural number for each street segment, using thesame assumed layer coefficients for the AC and BC layer, and then comparing it with theeffective structural number. Table 3 also presents these as-built structural numbers.

For streets such as Dallas, 31st, and Pawnee, geogrids contributed significantly to theimprovement of the effective structural numbers. A 1.0 improvement in SNeff is evident onDallas, while increases averaged 1.6, and 1.2 for 31st, and Pawnee, respectively. The data didexhibit some variance due to other factors such as field compaction of the AC and base, and curetime of the AC. These factors may have contributed to the low effective structural number forSterling Street. Although not specifically designed and built as control sections, Sections 1 and 3on Pawnee were reportedly constructed as Pawnee Section 2 but without a geogrid. Thus, thesesections are as uniform as can practically be expected except for the use of a geogrid in Section2. The SNeff for Pawnee Section 2 is about 0.5 greater than for Sections 1 and 3, a significantimprovement in the overall SNeff for the section that should result in an additional 2 to 3 years ofpavement service life.

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Full-Scale In-Ground Testing of Pavement Systems

Full-scale in-ground tests were also conducted to evaluate the structural contribution ofgeosynthetic reinforcement to pavement systems. Up to 56 sections were constructed, includingreinforced and unreinforced control sections, different subgrade CBRs, base thicknesses, anddifferent reinforcing geosynthetics. The asphalt thickness was 75 mm. The geogrid was placedunderneath the base layer. The details of the test, and a more comprehensive analysis arepresented by Cancelli and Montanelli (1999).

The road section shown in Figure 5 is 30 m long and 4 m wide. The outer edges of the curveswere slightly raised giving a “parabolic” effect to facilitate the test vehicle turning withoutdeceleration. Underneath the cross sections of the road, a 4 m wide 1.2 m deep trench wasexcavated and lined with an impermeable plastic membrane to maintain the fill soil moisture.

Figure 5: Plan view of the full scale in ground test road (m)

To facilitate the full-scale test, the vehicle followed a well-defined path given by the centerlinespainted along the AC layer. Thus, the wheels always traveled along the same path, so that theaxle wheel loads were channelized along the testing section. The vehicle used in the tests was astandard truck having a dual wheel rear axle and a single wheel front axle. The rear and the frontaxle were loaded with 90 kN and 45 kN respectively, with a tire pressure of 800 kPa.

Table 4 summarizes the test data for sections reinforced with a multilayered geogrid along withthe control sections. The effect of geogrid reinforcement was immediately evident from thebeginning of the test when the control section originally designed with 500 mm of aggregate

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base thickness and 700 mm of clay with CBR equal to 1, had to be excavated prior the placementof the AC course. The strength of the unreinforced section was not sufficient to support theweight of the paving vehicle. The base thickness was then increased to 1000 mm. The controlsections, with subgrade CBRs of 3 and 300 mm base thicknesses, reached rut depths of over 25mm within 50 traffic passes. After 500 cycles, the maximum rut depth was 142 mm. Thus it wasdecided to excavate the control section, re-grade the existing base by importing additional graveland re-pave the entire section with 75 mm of AC.

Table 4: Test Data for Sections with and Without a Geogrid Reinforcement

Figure 6 shows the surface rutting for the reinforced section with a subgrade CBR of 3 and basethickness of 300 mm after 2,000 total traffic passes. As a comparison, the surface rut is shown inFigure 7 for the control section after only 300 total traffic passes. Significant rutting occurs forthe control section. Figures 8 and 9 present the rut depth profile for the reinforced section andcontrol section with different traffic passes. Since none of the reinforced sections reached 25 mmof rut depth, and since the rut depth for reinforced sections with subgrade CBRs above 3 did noteven reach 12.5 mm, no layer coefficient ratios have been calculated from these full-scale in-ground tests. The structural contribution provided by a geogrid reinforcement is quite significant.

Total Traffic PassesSection CBR Base

Thickness0 50 100 300 500 1000 2000 4000 8000

(%) (mm) Maximum rut depth (mm)Reinforced 1 500 0.0 --- 10.2 12.8 13.2 15.4 16.5 18.2 21.0

Control 1 1000 0.0 --- 5.1 5.4 5.8 7.2 8.1 9.7 12.4Reinforced 3 300 0.0 --- 6.7 8.1 8.8 10.2 11.3 12.6 14.3

Control 3 300 0.0 26.5 44.4 90.5 142 --- --- --- ---Reinforced 3 400 0.0 --- 2.3 3.1 3.3 4.2 4.8 5.5 6.8

Control 3 400 0.0 13.8 15.7 18.3 19.4 20.3 21.5 23.2 25.0Reinforced 8 300 0.0 --- 2.0 2.6 3.2 4.3 5.1 5.8 6.8

Control 8 300 0.0 2.1 2.9 3.7 3.4 4.5 4.7 6.3 7.6

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Figure 6: Reinforced section after 2000 total traffic passes.(CBR=3, Base course thickness = 300mm)

Figure 7. Control section after 300 total traffic passes.(CBR=3, Base course thickness = 300mm)

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-14

-12

-10

-8

-6

-4

-2

0

2

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Road Width, mm

Ru

t D

epth

, mm

0

50

100

300

500

1000

2000

4000

8000

Figure 8: Rut profile for the reinforced section.(CBR=3, Base course thickness = 300mm)

-100

-80

-60

-40

-20

0

20

40

60

80

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Road Width, mm

Ru

t D

epth

, mm

0

50

100

300

500

Figure 9: Rut profile for the control section.(CBR=3, Base course thickness = 300mm)

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Cost Benefits and Constructability

The increase in the layer coefficient of the base course material by a geogrid reinforcementallows a reduction in base thickness. The cost savings realized from using geogrid reinforcementin pavement systems would vary by projects. For illustration purposes, assuming an average in-place cost of $19.6/m3 ($15/yd3) for graded aggregate base (GAB), and $3.0/m2 ($2.5/yd2) forgeogrid, ESAL=1,000,000. The same input data as in Figure 4 for reliability, standard deviation,design serviceability loss, and material layer coefficients were assumed in the calculations. Theasphalt layer thickness in this example is assumed to be 75mm. Subgrade resilient modulus =4500psi (CBR=3), then LCR = 1.5 from Figure 4. The thickness reduction in the base layer byusing a multilayered geogrid is 172 mm, corresponding to a cost savings about $0.85/m2

($0.31/yd2). The cost benefits of reinforced pavements described here are limited to reducedmaterials and construction costs. The long-term benefits of geogrid reinforcement for extendedservice life and reduced maintenance costs are not addressed here.

In addition to the material cost savings, the benefits of using geogrid reinforcement in pavementsystems include an improved workability for the construction platform over low CBR subgrades.The constructability benefit is well recognized by the full-scale field tests presented in theprevious section, and it is also supported by field experience. Figure 10 shows the installation ofa geogrid over a fully saturated subgrade on a major state highway. The average subgrade CBRfor this project is less than 1, while ESALs are over 7.6 millions. The design calls for two layersof geogrids. The first geogrid layer (as shown in Figure 10) is placed directly over the weaksubgrade to build a 625-mm subbase layer. This layer of geogrid is defined as subbasereinforcement in the draft “White Paper”. Without this geogrid it is difficult to support theconstruction traffic and achieve the target compaction unless a significantly larger amount ofsubbase fill material is used. The second layer of geogrid is placed on top of the subbase toreinforce and confine the 300-mm base course material. This geogrid layer is defined as basereinforcement in the draft “White Paper.” Figure 11 shows the installation of base coursematerial over the second layer of geogrid. This geogrid layer’s objective is to improve the servicelife and/or obtain equivalent performance with a reduced structural section. A 63-mm AC layeris then placed on top of the base course layer.

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Figure 10. Placement of a geogrid layer over a saturated subgrade

Figure 11. Placement of base course material over the second geogrid layer

CONCLUDING REMARKS

Presently various design methodologies are being used in practice for geosynthetic reinforcedpavements. A critical review of geosynthetic reinforced base course layers in flexible pavements,including various design methodologies, was presented by Perkins and Ismeik (1997). Thedesign method for geogrid reinforced flexible pavements presented here is a modified AASHTOprocedure, and does not seek to disqualify other methods. The design of geogrid reinforcedpavements, in the authors’ opinion, is rather difficult compared to the design of reinforced

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slopes/walls; where a mechanistic-based design method can be rigorously employed. Sincesimple design methods incorporating geogrid properties as direct design parameters are notavailable, a series of performance based tests have to be accomplished, and the structuralcontribution of a geogrid material has to be quantified accordingly and incorporated in the designmethodology. The performance tests presented in this paper are limited to multilayered geogrids,may not consider all factors in the testing design and analyses, but are nonetheless systematicallyconducted. In addition to performance tests, construction survivability of a geogrid, as presentedin the draft “White Paper”, must first and foremost be evaluated.

Acknowledgments

The authors would like to thank Karla Parker for editing this paper; Ghada Ellithy forrecalculating Figure 4; and three anonymous reviewers for their helpful comments.

Aigen Zhao, Ph.D., P.E., is Technical Director of Tenax Corporation, Baltimore, MD.Paul T. Foxworthy, Ph.D., P.E., is Director of Pavement Services, Terracon, Inc., Lenexa, KS.

References

American Association of State Highway and Transportation Officials, (1993). “AASHTO Guidefor Design of Pavement Structures”.

Carroll, R.G., Walls, J.C. and Haas, R., (1987). “Granular base reinforcement of flexiblepavements using geogrids” Proceeding of Geosynthetics ’87, IFAI, New Orleans, pp. 46-57.

Cancelli A. and Montanelli, F. (1999). “In-ground test for geosynthetic reinforced flexible pavedroads” Proceeding of Geosynthetics ’99, IFAI, Boston.

Cancelli A., Montanelli, F. and Rimoldi, P., Zhao, A. (1996). “Full scale laboratory testing ongeosynthetics reinforced paved roads”, Proc. Int. Sym. on Earth Reinforcement, 573-578.

Geosynthetic Materials Association, IFAI, (1998). “Geosynthetics in pavement systemsapplications, section one: geogrids (draft)”.

Montanelli, F., Zhao, A., and Rimoldi, P., (1997). “Geosynthetics-reinforced pavement system:testing and design” Proceeding of Geosynthetics ’97, IFAI, Long Beach, pp. 619-632.

Perkins, S.W., and Ismeik, M., (1997). “A synthesis and evaluation of geosynthetic-reinforcedbase layers in flexible pavements: part I”, Geosynthetics International, Vol. 4, No. 6, pp. 549-604.