Your Federal Quarterly Tax Payments are due April 15th Get Help Now >>

AASHTO Method for Flexible Pavement Design by vrb11676

VIEWS: 1,571 PAGES: 15

        Technical Reference GRID-DE-6

                        A PRACTICAL PERSPECTIVE

1999                                            TENAX Corporation
                                                4800 East Monument Street
                                                Baltimore, Maryland 21205
                                                tel: (410) 522 - 7000
                                                fax: (410) 522 - 7015
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 a
geogrid/geotextile specification for pavement reinforcement have initiated very positive
discussions. The Geosynthetic Materials Association has participated in the discussion and made
recommendations to AASHTO with the presentation of a draft “White Paper” addressing
installation survivability and specifications. The overwhelming comments back from the
reviewers of the “White Paper” clearly show the need to 1) demonstrate the performance and
cost benefits of geogrid reinforcement, and 2) develop a design procedure incorporating geogrid
with value-added benefits, in addition to the installation survivability aspects already well

Geogrid reinforcement has been used in the design and construction of pavements for over a
decade, yet there exists no design method incorporating geogrid mechanical properties as direct
design 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 direct
design parameters for reinforced pavement systems. Rather, a series of performance based tests
should be conducted to evaluate the structural contribution of geogrid reinforcement to pavement
systems, from which design parameters could be derived and incorporated into a design

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

Modified AASHTO Design Method for Geogrid Reinforced Flexible Pavements
Existing design methods for flexible pavements include: empirical methods, limiting shear
failure methods, limiting deflection methods, regression methods, and mechanistic-empirical
methods. 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 the
required combined structural capacity of all pavement layers overlying the subgrade. The
required SN is a function of reliability, serviceability, subgrade resilient modulus, and expected
traffic intensities. The actual SN must be greater than the required SN to ensure long term
pavement performance.
The actual SN value for a unreinforced pavement section is calculated as follows:
                                      SN = a1 ∗ d1 + a2 ∗ d 2 ∗ m2                       Eq. (1)
where a1 a2 are the layer coefficients characterizing the structural quality of the asphaltic
concrete (AC) layer and the aggregate base course (BC) in a pavement system. A subbase layer

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

A modification to equation (1) is introduced to account for the structural contribution of a
geogrid reinforcement to flexible pavements.
                                                                                            Eq. (2)
                                   SN = a1 ∗ d1 + LCR∗ a2 * d 2 * m2

where LCR is the layer coefficient ratio. Equation (2) can be used to calculate the base course
thickness for geogrid reinforced pavements by rearranging its terms:
                                                                                            Eq. (3)
                                              SN − a1 * d1
                                        d2 =
                                             LCR ∗ a2 ∗ m2

When the layer coefficient ratio, LCR, is greater than 1, the thickness of the geogrid reinforced
base course is reduced compared to unreinforced sections; similarly, if the base course thickness is
held constant, the structural number of the reinforced section increases. An increased structural
number 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 Haas
1987, Montanelli, Zhao, and Rimoldi, 1997) to quantify the structural contribution of a geogrid
in a flexible pavement. This concept was established based on the reinforcing mechanism that
geogrid provides lateral confinement to the base course material and improves the layer coefficient
of the reinforced base. The next section addresses the controlled laboratory pavement tests
performed to develop this design parameter for multilayered polypropylene biaxial geogrids. The
following sections provide field verification through nondestructive tests and full-scale in-ground

Controlled Laboratory Pavement Testing

Laboratory tests were performed to study flexible pavement systems under cyclic loading
conditions, and to quantify the structural contribution of a geogrid reinforcement. The test setup
is shown in Figure 1. Cyclic loading was applied through a rigid circular plate with a diameter of
300 mm. The peak load was 40 kN with an equivalent maximum stress of 570 kPa. Asphaltic
concrete, 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 multilayered
polypropylene geogrid manufactured by continuous extrusion and orientation processing was
used in the test, its properties are listed in Table 1. The details of the laboratory tests are
presented by Cancelli et al. (1996).

                      Figure 1. Controlled laboratory pavement tests

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

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

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

                   VERTICAL SETTLEMENT, [mm]
                         300 mm GRAVEL
                       Unreinforced CBR 1%
                       Reinforced CBR 1%
                       Unreinforced CBR 3%
             100       Reinforced CBR 3%
                       Unreinforced CBR 8%
                       Reinforced CBR 8%
                       Unreinforced CBR 18%
              50       Reinforced CBR 18%

               100                1000           10000                 100000
                                             CYCLE, [-]

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

                   CYCLE, [-]



            1000                                         Unreinforced    25mm RUT
                                                         Reinforced,     25mm RUT
                                                         Unreinforced    12.5mm RUT
             100                                         Reinforced,     12.5mm RUT

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

        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 CBR
based on pavement testing data from both control and reinforced sections. The layer coefficient

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

                                                       2 .0
       L a y e r C o e ffic ie n t R a tio n , L C R

                                                       1 .8

                                                       1 .6

                                                       1 .4

                                                       1 .2

                                                       1 .0
                                                              0    2      4     6      8        10       12   14   16   18   20

                                                                                       S u b g ra d e C B R

                                                                  Figure 4. Layer coefficient ratio vs. subgrade CBR

Nondestructive FWD Tests

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

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

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

                            Table 2. Summary of Uniform Sections

                                             Average             Average
    Street             From         To         AC                 Base
  Segment Section Station Station Thickness CoV* Thickness CoV* Geogrid
                         (ft)       (ft)       (in)      (%)       (in)      (%)
   Sterling     1       0+00        5+70        4.8       11       9.7        11     YES
    Dallas      1       0+00        9+78        6.3       17       6.6         8     YES
     31st       1       0+00       15+00        8.6        7       7.3        10     YES
                2      15+00       24+98        8.3        4       7.8        13     YES
                3      24+98       52+66        8.1        6       6.9        10     YES
   Pawnee       1       0+00        5+50       10.5       24       7.8         8     None
                2       5+50       28+92       11.9        7       8.0        10     YES
                3      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 backcalculated
from deflection data. The structural capacity of each uniform pavement section was then
evaluated in terms of an effective structural number (SNeff) using the nondestructive deflection
testing approach outlined in the 1993 AASHTO Guide. The method essentially evaluates the
total, or overall, stiffness for the pavement structure (Ep) using deflection data. The effective
structural number (SNeff) then is a function of the total pavement thickness and its overall
stiffness, Ep.

In an effort to conduct a meaningful evaluation of geogrid effectiveness from the testing data, the
design structural number of each street segment in the project was assessed. Layer thickness data
for each street segment, shown in Table 3, was obtained from original construction drawings and
assigned 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 with
field compacted mixes. For granular base course materials, a layer coefficient of 0.14 was
selected as representative of the densely graded, crushed rock materials used in Wichita street
construction. This thickness and material quality information was then used to calculate the

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

                           Table 3. Structural Number Comparison

                       Design Design
  Street                 AC        Base Design As-built Effective
 Segment      Section Thick Thick             SN       SN         SN       CoV*     Geogrid
                         (in)      (in)                                     (%)
  Sterling       1        5.0       5.0      2.70      3.27      2.81        18      YES
   Dallas        1        5.0      10.0      3.40      3.44      4.40        25      YES
    31st         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      YES
                 3        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      None
                 2       11.0       8.0      5.52      5.88      7.05         9      YES
                 3       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 2
revealed the street segments chosen for the project were generally built somewhat thicker than
originally designed. Therefore, it was appropriate to adjust the design structural number to
account for actual layer thicknesses in the assessment of geogrid effectiveness. This was
accomplished by calculating an as-built structural number for each street segment, using the
same assumed layer coefficients for the AC and BC layer, and then comparing it with the
effective structural number. Table 3 also presents these as-built structural numbers.

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

Full-Scale In-Ground Testing of Pavement Systems

Full-scale in-ground tests were also conducted to evaluate the structural contribution of
geosynthetic reinforcement to pavement systems. Up to 56 sections were constructed, including
reinforced and unreinforced control sections, different subgrade CBRs, base thicknesses, and
different reinforcing geosynthetics. The asphalt thickness was 75 mm. The geogrid was placed
underneath the base layer. The details of the test, and a more comprehensive analysis are
presented 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 curves
were slightly raised giving a “parabolic” effect to facilitate the test vehicle turning without
deceleration. Underneath the cross sections of the road, a 4 m wide 1.2 m deep trench was
excavated 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 centerlines
painted along the AC layer. Thus, the wheels always traveled along the same path, so that the
axle wheel loads were channelized along the testing section. The vehicle used in the tests was a
standard truck having a dual wheel rear axle and a single wheel front axle. The rear and the front
axle 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 with
the control sections. The effect of geogrid reinforcement was immediately evident from the
beginning of the test when the control section originally designed with 500 mm of aggregate

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

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

                                                    Total Traffic Passes
  Section    CBR      Base    0      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     12.4
Reinforced     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     25.0
Reinforced     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

Figure 6 shows the surface rutting for the reinforced section with a subgrade CBR of 3 and base
thickness of 300 mm after 2,000 total traffic passes. As a comparison, the surface rut is shown in
Figure 7 for the control section after only 300 total traffic passes. Significant rutting occurs for
the control section. Figures 8 and 9 present the rut depth profile for the reinforced section and
control section with different traffic passes. Since none of the reinforced sections reached 25 mm
of rut depth, and since the rut depth for reinforced sections with subgrade CBRs above 3 did not
even 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.

            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)



  Rut Depth, mm

                     -4                                                                                50
                     -6                                                                                300



                          0       200    400      600     800       1000    1200    1400     1600   1800         2000

                                                            Road Width, mm

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




    Rut Depth, mm


                      -20                                                                                  0


                              0    200     400     600     800       1000    1200   1400     1600   1800         2000

                                                                Road Width, mm

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

Cost Benefits and Constructability

The increase in the layer coefficient of the base course material by a geogrid reinforcement
allows a reduction in base thickness. The cost savings realized from using geogrid reinforcement
in 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) for
geogrid, 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. The
asphalt 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 by
using 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 reduced
materials and construction costs. The long-term benefits of geogrid reinforcement for extended
service life and reduced maintenance costs are not addressed here.

In addition to the material cost savings, the benefits of using geogrid reinforcement in pavement
systems 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 the
previous section, and it is also supported by field experience. Figure 10 shows the installation of
a geogrid over a fully saturated subgrade on a major state highway. The average subgrade CBR
for this project is less than 1, while ESALs are over 7.6 millions. The design calls for two layers
of geogrids. The first geogrid layer (as shown in Figure 10) is placed directly over the weak
subgrade to build a 625-mm subbase layer. This layer of geogrid is defined as subbase
reinforcement in the draft “White Paper”. Without this geogrid it is difficult to support the
construction traffic and achieve the target compaction unless a significantly larger amount of
subbase fill material is used. The second layer of geogrid is placed on top of the subbase to
reinforce and confine the 300-mm base course material. This geogrid layer is defined as base
reinforcement in the draft “White Paper.” Figure 11 shows the installation of base course
material over the second layer of geogrid. This geogrid layer’s objective is to improve the service
life and/or obtain equivalent performance with a reduced structural section. A 63-mm AC layer
is then placed on top of the base course layer.

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

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


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

slopes/walls; where a mechanistic-based design method can be rigorously employed. Since
simple design methods incorporating geogrid properties as direct design parameters are not
available, a series of performance based tests have to be accomplished, and the structural
contribution of a geogrid material has to be quantified accordingly and incorporated in the design
methodology. 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 systematically
conducted. In addition to performance tests, construction survivability of a geogrid, as presented
in the draft “White Paper”, must first and foremost be evaluated.


The authors would like to thank Karla Parker for editing this paper; Ghada Ellithy for
recalculating 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.


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

Carroll, R.G., Walls, J.C. and Haas, R., (1987). “Granular base reinforcement of flexible
pavements 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 paved
roads” Proceeding of Geosynthetics ’99, IFAI, Boston.

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

Geosynthetic Materials Association, IFAI, (1998). “Geosynthetics in pavement systems
applications, 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-reinforced
base layers in flexible pavements: part I”, Geosynthetics International, Vol. 4, No. 6, pp. 549-


To top