Extending Flexible Pavement Life Using Geogrids

By Jim Penman, CGeol, FGS, and Joe Cavanaugh, P.E.
Expiration Date: Septembef 2007

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Overview

Geogrids have been in common use for more than 25 years. While they have gained widespread acceptance as a solution to problems associated with roads constructed on soft or problematic subgrades, their use on competent subgrades has been less common. Clear, well-established design methodology is now available that allows the design engineer to quantify the benefits of using geogrids to extend pavement design life. This approach can be applied for the design of major highways or light-duty pavements associated with local housing or retail store developments.

Introduction

Geogrid technology has developed steadily since the products were first introduced in the early 1980's. The initial geogrids rapidly gained popularity within the civil engineering industry, principally due to their ability to provide simple, cost-effective solutions in various roadway and grade separation applications.

A geogrid is a regular grid structure of polymeric material used to reinforce soil or other geotechnical engineering related materials. Products generally are classified as either uniaxial geogrids or biaxial geogrids, depending upon whether their strength is predominantly in one or two directions. Uniaxial geogrids are principally used in grade separation applications such as retaining walls and steep slopes; biaxial geogrids are used mainly in roadway applications. Examples of both geogrid types are shown in Figure 1.

Figure 1: Uniaxial (UX) and biaxial (BX) geogrid

This article is principally concerned with the use of biaxial geogrids in base reinforcement applications. In these situations, the existing subgrade is of a firm nature or has been rendered such through the use of a subgrade improvement technique. One of the principal failure mechanisms of a pavement under these firm subsoil conditions is rutting resulting from progressive lateral movement of the aggregate base course during traffic loading (Figure 2).

Figure 2: The inclusion of BX Geogrids provides lateral confinement of the base, which results in enhanced pavement performance - either an increase in the pavement life, a decrease in the required thickness of the pavement, or a combination of the two.

The amount of lateral movement can be reduced greatly by including a biaxial geogrid within, or at the bottom of, the base course layer, Partial penetration of coarse aggregate particles through the geogrid apertures and subsequent compaction results in "mechanical interlock" or "confinement" of the aggregate particles.

Geogrid technology

The principal benefit of using a geogrid within the unbound aggregate component of a flexible pavement is less rutting at the surface because of reduced lateral spreading of the unbound aggregate. However, an additional feature of the reinforcement is that the geogrid-confined aggregate results in a much stiffer base course layer and a lower dynamic deflection of the pavement structure during traffic loading. Fatigue cracking of the asphalt is therefore reduced because of the presence of the geogrid reinforcement.

In order for geogrids to work successfully in base reinforcement applications, they must have the capacity to facilitate efficient load transfer between the aggregate and the geogrid. Webster (1992) reported on a large-scale research program undertaken by the U.S. Army Corps of Engineers (Corps) to investigate and determine the key physical properties of a geogrid required to create optimal interaction and load transfer. A summary of the key material properties determined in the study are presented in Table 1.

Current design practice for flexible pavements

The American Association of State Highway and Transportation Officials (AASHTO) provides guidelines for the design of flexible pavements in its current design guide (AASHTO, 1993), The design methods described in the guide are based on a purely empirical approach following a set of large-scale tests undertaken in Ottawa, 111., in the late 1950s. The designer is required to know the following input parameters for a proposed pavement section:

Figure 3: Calculation of the Structural Number for a pavement section.

Standard Normal Deviate (Z_R) - This parameter determines the probability that a road will maintain an acceptable level of serviceability during its design life. Typical values of reliability recommended by MSHTO are presented in Table 4, and the relationship between reliability and the required input parameter, Z_r' is shown in Table 5.

Standard Deviation(S₀) - This parameter describes the reliability of the input parameters selected for the local conditions. Default values of 0.40 to 0.50 are recommended for flexible pavements.

Change In Serviceability (?PSI) - This describes the loss in serviceability during the design life of the road and is dictated by acceptable levels of cracking, rutting, etc. An initial serviceability, P_i of 4.2 is normally assumed, and AASHTO recommends a terminal serviceability, P_t of 2.S or higher for a major highway and 2.0 for highways with less traffic. Once P_i and P_t are determined, ?PSI =P_i - P_t.

Subgrade Resilient Modulus (M_R) - This defines the strength of the subgrade or foundation 1ayer on which the main pavement sits. Once these input parameters have been determined, it is possible to calculate the allowable traffic capacity, W₁₈ for a particular pavement section using the following equation:

The allowable traffic capacity determined using this equation is quoted in Equivalent Standard Axle Loads (ESALs). To put this into perspective, a typical, fully laden 20-ton truck would impose a load equivalent to approximately 5 ESALs.

Geogrids in flexible pavement designs

Geogrids were invented in the late 1970's and sold commercially for the first time in the early 1980s. Clearly, they were not used in the original road test used to develop the current AASHTO design methodology for flexible pavement design. However, guidance for incorporating geogrids for base reinforcement in flexible pavements is given in the Interim Standard PP46-01 published by AASHTO in 2001.

This document recognizes that geogrids used in flexible pavements provide one or both of the following benefits:

• Extension of pavement design life
• Reduction of pavement layer thickness.

It is further stated in AASHTO's PP46-01 that to quantify these performance benefits for a particular geogrid, it is necessary to undertake large-scale performance testing under carefully controlled conditions. A good summary of the testing undertaken during the first 20 years since geogrids were introduced is provided by Perkins and Ismeik (1999).

Figure 4: Performance benefits for extended design life

Irrespective of the type of test undertaken, the objective is the same - quantify the improved performance of geogridreinforced pavement sections compared with unreinforced test sections.

Quantifying extended design life for geogridreinforced pavements

Consider the two pavement sections shown in Figure 4. The sections are identical apart from the fact that the reinforced pavement contains a geogrid at the subgrade-base course interface.

The parameter generally used to quantify the extension of pavement design life using geogrids is the Traffic Benefit Ratio (TBR). This is defined as follows:

TBR=the No. of cycles for given deformation in reinforced section divided by the No. of cycles for given deformation in unreinforced section

The results shown in Figure 4 are from the Corps testing undertaken by Webster (1992). In this simple example, the TBR would be calculated as follows:

TBR = 500/106 = 4.72

In other words, the use of a geogrid in this pavement section extended the pavement design life (for a 1 -inch surface rut) by a factor of 4.72. The AASHTO design guide methodology described above can be used to calculate the allowable traffic for an unreinforced pavement. To determine the extended pavement life when using a geogrid in the same pavement section, this value is simply multiplied by the appropriate TBR value for the geogrid concerned. Keep in mind, however, that in accordance with the directions given in PP46-01, any TBR values used for a particular geogrid must have been determined using testing methods correlated to observed field performance.

Geogrid use in local subdivision developments

The previous sections described the general methods by which geogrids can be used to extend pavement design life. This section focuses on how this technology can be applied to solve a specific problem associated with relatively light duty pavements.

As the population of our towns and cities continues to expand rapidly, new or recently constructed housing, in the form of subdivision developments, is becoming increasingly commonplace. One of the more frequent problems associated with the roads in these developments is a direct result of their method of construction.

Figure 6: a typical subdivision road during construction
on which the thin surface asphalt layer has not yet been installed.

Phased construction (Figure 6) has become an extremely common practice, particularly so in residential developments. To build a roadway to gain site access, contractors initially place the aggregate component of the pavement and, usually, a thin asphalt layer on top. This technique is particularly useful when local trenches are required for installation of utility pipes and cables. Once the overall site development is completed, the remaining asphalt is placed, ensuring that the road is in pristine condition on Day 1 of its formal use. Or is it?

Figure 7: Condition of a subdivision road only two to three years after final paving

Pavement distress in the form of asphalt cracking at the surface is common on roads within subdivisions (Figure 7). In many cases, these cracks start to appear shortly after construction - perhaps as soon as one or two years. Once the cracking starts to develop, the deterioration accelerates very quickly. Pavement distress depicted by alligator cracking is the most common in such developments and points to an overall integrity problem as the pavement approaches the end of its design life. Under these circumstances, the current owner of the road will need to replace the road's surface layers at considerable expense. If ruts have developed, it will also be necessary to replace the granular foundation layer(s).

Consider the three pavement sections shown in Figure 5. The trafficking capacity in each case has been calculated using the AASHTO guidelines

Strange as it may sound, in this typical example, leaving off the 1.5 inches of asphalt surfacing during a phased-construction procedure (Section B) reduces trafficking capacity of the pavement by more than 80 percent. For subdivision roads, however, the majority of the total trafficking is experienced during construction of the road itself and the surrounding housing. Therefore, it is not surprising that when the surface layer is installed at the end of construction, the rest of the pavement structure is approaching the end of its design life. Placement of an additional thin surface layer results in some additional trafficking capacity, but a year or two later the road starts to show the sort of surface distress indicative of problems associated with the structural integrity of the lower layers.

Figure 5: Premature pavement failure

The simple solution to this problem is a layer of geogrid installed at the bottom or within the base course during initial construction. The allowable trafficking determined for Section C in Figure 5 was calculated by applying a TBR value of 6 (typical for a high quality geogrid with the required supporting performance data) to the trafficking capacity for Section B. The outcome is that the trafficking capacity of the thinner road used during the construction phase exceeds that for which the completed road (Section A) was originally designed. From the road owner's perspective, for relatively little additional expense at the start of construction, the lifetime of their road is extended enormously, and expensive and disruptive rehabilitation or reconstruction activities are avoided.

Geogrid use in retail store developments

Another use of geogrid technology can be found in the development of pavements around retail stores. Typically, thicker, heavy-duty pavements are adopted in the loading areas around such stores, while thinner, lighter-duty pavements are used for the car parking areas.

One of the main problems associated with this approach is the potential for a "bathtub" effect - the subgrade is at a lower level in the areas of the heavy-duty pavements, These areas are prone to water ingress and build up, resulting in a reduction in the long-term strength of the pavement. In colder regions, these areas are also more susceptible to the effects of freeze-thaw activity. Both of these situations reduce the design life of the pavement, but there are additional practical problems for the contractor associated with this more complicated method of construction.

Figure 8: Biaxial geogrids create uniform subgrade elevations.

Consider the two sets of pavement sections shown in Figure 8, In each case, the trafficking capacity of the geogridreinforced sections is at least as great as the unreinforced pavement sections. Clearly, the reinforced sections offer cost benefits because they are thinner and require less material. However, the major advantage of this scenario is gained from the fact that the light and heavy-duty sections are of the same thickness, which creates a uniform subgrade elevation, In addition to offering protection against the bathtub problems described above, the reinforced sections offer significant material cost savings. Additional benefits result from increased speed of construction fewer stake-out procedures, less undercut/disposal of fill, and simpler construction.

A glimpse into the future

As previously stated, the current design approach prescribed by AASHTO is based purely on empirical results from the large scale field tests undertaken in Ottawa, III., in the late 1950's. New pavement design approaches, based on mechanistic-empirical (M-E) principles, are now being developed and refined by AASHTO and other entities.

Essentially, M-E pavement design involves the use of numerical modeling techniques to predict accurately the stresses and strains developed in a particular pavement section as a result of traffic loading. The mechanistic (or theoretically- based) predicted performance is then calibrated with field tests (empirical data) to validate the methodology. Official publication of the new AASHTO design guide may still be several years away, but the availability of M-E-based design methods incorporating the use of geogrids within the pavement structure is imminent. Researchers at the University of Illinois at Urbana-Champaign are about to publish the results of a four-year project investigating the use of geogrids in base reinforcement applications. Although this type of work has been undertaken previously by several authors, the scale of the testing undertaken at the University of Illinois to develop accurate transfer functions is unprecedented. Similarly, the discrete element modeling approach used to define the interaction between the geogrid and surrounding soil is revolutionary.

6PDH This eagerly anticipated advancement in pavement design will be published by the University of Illinois at the 86th Annual Meeting of the Transportation Research Board (TRB) in Washington, D.C., Jan. 22-26, 2007.

Summary

Geogrids can be used successfully to extend the design life of flexible pavements in a variety of base reinforcement applications. The techniques are equally applicable to major highways and small subdivision roads. Significant life cycle cost savings can be achieved with relatively little additional up-front expenditure.

Current AASHTO design methods exist to determine appropriate pavement sections incorporating geogrids into pavement structures. However, new and extremely innovative, state-of-the-art techniques using M-E principles are just around the comer .

Quiz Questions

1. In base reinforcement applicatio ns (firm subgrade conditions), what is the main function of a geogrid in improving pavement performance?
   1. Enhance loild distribution resu lting in less vertical deflection of the subgrade.
   2. Reduce lateral movemeot of aggregate particles within the base course layer.
   3. Prevent pumping of the subgrade.
   4. Reduce dynamic deflection of the asphalt.
2. Which of the following sets of index properties for a geogrid would likely give the best indication of how the product will perform in roadway applications?
   1. Ultimilte temile strength; aperture size.
   2. Tensile strength at 2% strain; percentage open area.
   3. Aperture stability modulus (torsional rigidity); junction strength.
   4. True initial modulus in use; creep limited strength.
3. Which of the following are True statements?
   1. Geogrids can never provide effective separation between two layers because of the passage of fine mil through their apertures.
   2. Geogrids and geotextiles essentially use the same mechanisms to provide reinforcement
   3. The strength of the geogrid used will be the predominant factor that determines the extension of design life for a reinforced pavement
   4. None, all these statements are false.
4. which of the following is a True statement regarding the current AASHTO (1993) pavement design approach?
   1. large-scale trafficking studies conducted in the 1950s form the basis for the design method.
   2. The design approach is based primarily on theoretical predictions of pavement performance.
   3. Benefits of geogrids for base reinforcement are included in the method.
   4. None, all of the above are false.
5. Based on the current AASHTO (1993) design approach, with all other conditions remaining unchanged, what would be the effect of increasing the subgrade strength?
   1. An increase in the overall Structural Number (SN) of the pavement.
   2. A likely increase in the drainage properties of the pavement
   3. An increase in the allowable traffic for the pavement.
   4. All of the above.
6. Which of the following soil properties would be of greatest value to an engineer when designing a reinforced pavement on a relatively competent formation?
   1. Shear strength of the subgrade.
   2. Shear strength of the base course.
   3. Consolidation properties of the subgrade.
   4. All equally valuable.
7. What difference(s) would you observe in the visual distress between subgrade rutting and base course rutting?
   1. The alligator cracks will be larger on the section with base course rutting.
   2. The rut profile will be wider on the section with subgrade rutting.
   3. There is no difference, the rut profiles will be the same.
   4. None of the above, as rutting always develops in both layers equally, and therefore you cannot determine the origin of the rutting.
8. A subdivision road is characterized by alligator cracking but no surface rutting. What is the most likely mode of failure for the pavement?
   1. Reflection of previous cracks caused by thermal expansion/ contraction.
   2. Fatigue failure of the road as it reaches the end of its design life.
   3. Presence of a weak binder within the asphalt.
   4. All of the above are equally likely.
9. If the silty gravel subbase (overlying the subgrade, and supporting a dense-graded gravel base layer) is the critical layer in a five-layer pavement section, where would you best position the geogrid?
   1. At the subbase-subgrade interface.
   2. In the middle of the subbase layer.
   3. At the subbase-base interface.
   4. Any of the above, because the geogrid will provide the same contribution at all three locations.
10. Which of the following advantages apply to MechanisticEmpirical design methods compared with the purely empirical approach currently adopted by AASHTO?
    1. Designs can account for variations in material properties with time.
    2. The approach can more easily be adapted to take account of local soil and climatic conditions.
    3. Design methods adopted now will more easily adapt to changes in vehicle loads in the future as trucks develop.
    4. All of the above

Jim Penman, CGeol, FGS, Director of Biaxial products & applications for Tensar International, is a geotechnical engineer with more then 13 years of experience in geosynthetics field.

Joe Cavanaugh, P.E. Vice president of Technology for Tensar International Corporation, is a registered professional engineer in several states and has 14 years of experience in geosynthetics and geotechnical design/construction.

REFERENCES

• AASHTO, 1993, AASHTO Guide For Design of Pavement Structures, American Association of State Highway and Transportation Officials.
• AASHTO, 2001, Provisional Standard PP46-01: Recommended Practice For Geosynthetic Reinforcement of the Aggregate Base Course of Flexible Pavements, April 2001 Interim Edition.
• Perkins, S.W. and Ismeik, M., 1999, A Synthesis and Evaluation of Geosynthetic Reinforced Base Course Layers in Flexible Pavements: Part I Experimental Work, Geosynthetics International, Vol. 4, No.6, pages 549-604.
• Webster, S.L., 1992, Geogrid Reinforced Base Courses for Light Aircraft: Test Section Construction, Behavior Under Traffic, Laboratory Tests and Design Criteria, Geotechnical Laboratory, Department of the Army, Waterways Experiment Station, Corps of Engineers, Mississippi.