sp 4 pro paved design method

Performance Indicating Research

& Testing

TIF/Enhanced Confinement Effect

MSL Coefficient

SN = a1D1+a2D2m2

AASHTO ‘93 Log10(W18) =

….SN….

Full Scale Research & Field

Performance

Tensar International

SpectraPave4 PRO™ v3 Tensar TriAx® Geogrid Paved Applications Design Method

June 2010

© Copyright 2010, Tensar International -2 –

SpectraPave4 PRO™ Paved Application Design Method Introduction The objective of this document is to familiarize engineers with TriAx® Geogrids and the process used to arrive at a reinforced pavement solution within SpectraPave4 PRO™ Software as developed by Tensar International. The Paved Applications module facilitates analysis and design of flexible pavements in accordance with the AASHTO Guide for Design of Pavement Structures (1993). The AASHTO (1993) method is empirically based and models a flexible pavement as a series of layers which have a combined structural capacity to carry a certain number of traffic loads (ESAL’s) with pre-determined minimum levels of serviceability and statistical confidence. Traditionally, geosynthetic reinforcement of pavements has concentrated more on projects involving unpaved roads. However, the rising cost of asphalt and aggregates and increasing environmental pressure have caused government agencies and road builders worldwide to focus their attention on using similar techniques for permanent, surfaced pavements. To illustrate the level of acceptance within the pavement engineering community more than half of the State Departments of Transportation in the U.S. have published specifications for the use of geogrid reinforcement in roads. Tensar geogrids improve the performance of flexible pavement when placed within the aggregate base layer and/or at the aggregate base-subbase interface. For a given base thickness and allowable surface rut depth, the traffic carrying capacity can be increased through the use of geogrids, compared to a similar pavement with the same thickness of unreinforced aggregate base. Additionally, with a given base layer thickness and trafficking, rutting is significantly less for the reinforced pavement. Alternately, the reinforced pavement section can be designed and constructed with a reduced quantity of base material, to the extent that for the same trafficking, performance of a thicker unreinforced pavement and a thinner geosynthetic-reinforced pavement are the same. TriAx® Geogrid Geogrid usage has steadily evolved since the products were first introduced in early 1980’s. Tensar biaxial geogrids have gained widespread acceptance in the Americas over the last 25 years primarily as a solution to problems associated with pavements, haul roads and working surfaces constructed on soft or problematic soils. By examining all the design characteristics of biaxial geogrids, and through independent testing and research, Tensar International has identified the key geogrid parameters that affect performance with Tensar geogrids. These parameters are the profile of the rib section, rib thickness, junction efficiency, aperture size and in-plane stiffness. This research evolved into a revolutionary change from a rectangular to a triangular grid aperture. This fundamental change to the grid structure, coupled with an increase in rib thickness and junction efficiency, gives greatly improved aggregate confinement and interaction, leading to improved structural performance of the mechanically stabilized layer (MSL). The new TriAx® Geogrid outperforms Tensar biaxial geogrid for the following reasons: Load Distribution

Load distribution is 3-dimensional and is distributed radially throughout the aggregate.

For a stabilized layer to be effective it must have the ability to distribute load through 360 degrees. To ensure optimum performance, the geogrid reinforcement in a Mechanically Stabilized Layer (MSL) should have a high radial stiffness throughout the full 360 degrees.

Junction Integrity

TriAx® geogrid starts as an extruded sheet of polypropylene. The unique TriAx® structure is the result of punching an array of holes in the polymer sheet and stretching it to its final form. This patented process, coupled with the design of the junctions, results in a product with high junction strength and stiffness.

Junction Efficiency

Rigorous testing has been conducted in line with each of the three rib directions. In each direction tested, the junction strength is found to be essentially equal to the rib strength - giving a junction efficiency greater than 90%.


Multi -Directional Properties

As the name implies and because of their bi-directional structure, biaxial geogrids have tensile stiffness predominantly in two directions. TriAx® geogrids exhibit three principal directions of stiffness, which is further enhanced by their rigid triangular geometry. This produces a significantly different structure than any other geogrid available on the market today and provides high strength 360 degrees stiffness. TriAx® is truly a multi-axial product with near isotropic properties and proven multi-directional performance.

Proving the importance of rib profile

TriAx® geogrids have greater rib depth compared with conventional biaxial geogrids of similar weight.

To compare performance advantages between both forms of geogrid with various rib depths, Tensar International commissioned trafficking tests and analytical modeling. The results were conclusive in confirming that an improved structural performance of a mechanically stabilized layer (MSL) demonstrated the advantage of the TriAx® geogrids deeper rib depth. In addition, numerical modeling techniques confirmed the importance of geogrid rib thickness on aggregate confinement and load dissipation.

Background Information Readily available research suggests that the two main types of geosynthetic reinforcement, geogrids and geotextiles, perform differently due to a different set of mechanisms that become mobilized under the influence of vehicular traffic. Evaluation of the effects associated with the use of geosynthetics in paved applications is based on pavement trials undertaken in both small-scale laboratories and full-scale field-testing. Extensive research summaries from such work are provided by Perkins and Ismeik (1997) and within GMA White Paper II (2000). Further, past versions of Tensar’s commercial software package, SpectraPave, relied on AASHTO’s guidance as found in its publication entitled “Recommended practice for Geosynthetic Reinforcement of Aggregate Base Course of Flexible Pavement Structures” (AASHTO Designation PP 46-01 [2001] and renamed R 50-09 [2009]). Within R 50-09 and its predecessor, PP 46-01 a single Traffic Benefit Ratio (TBR) or Base Course Reduction (BCR) value approach is used to account for the benefit derived from inclusion of a geogrid within a flexible pavement structure. R 50-09 and PP46-01 provide no guidance on how to conduct a study to arrive at a TBR/BCR range. In addition, variations in pavement structure geometry and loading conditions are noticeably missing. Furthermore, these guidelines do not delineate the applications of subgrade stabilization and base course reinforcement. Research results presented in the Tensar Technical Note TTN: BR-96 (“Design Guideline for Flexible Pavements with Tensar Geogrid Base Layers”) report, as well as other research conducted since this time, led to the identification of additional sets of traffic improvement values. Within this document, the collection of numerous individual traffic improvement factor (TIF) values for each separate pavement geometry (thicknesses & material types) and subgrade condition form the basis of Tensar’s catalogue of pavement structures (Formally referred to as a TBR range). Although not expressed as a TIF data set in the past, values associated with traffic improvement for a given set of experimental conditions for Tensar’s BX1100 and BX1200 geogrids ranged from 1.5 to 6 respectively for use in SpectraPave2™ and SpectraPave3™ software. This early work may have led to the incorrect perception that TBR values are constant for all pavement sections independent of variables such as asphalt thickness, subgrade support value, aggregate thickness and aggregate quality. Tensar International has conducted numerous empirical or accelerated pavement studies and from recent and past data, it is evident that the TIF value for a single product is not constant. Instead, it varies depending upon several important factors. These factors include but are not limited to, thickness of aggregate in which the geogrid is placed, location of the geogrid relative to the pavement surface, initial strength of the subgrade soils and the quality of aggregate used in construction of the pavement section. Factors affecting each experimental TIF value include:

• Aggregate Thickness • Aggregate Quality • Location of Geogrid (Stabilization and/or Base Course Reinforcement) • Asphalt Thickness (Thick vs. Thin) • Partially confined zone and fully confined zone of aggregate above the geogrid (MSL Stiffness) • Subgrade strength or resilient modulus


Keeping these factors in mind, identification of a stiffness increase in the unbound materials surrounding Tensar geogrid was documented at the University of Illinois-Urbana Champaign, (Kwon, et. al., 2008 and 2009) and the enhanced confinement effect associated with use of TriAx® geogrid was field verified by Iowa State University (White, 2010) through subgrade stabilization of an unpaved road project. As depicted in Figures 1 and 2 below, a base course aggregate was placed over a 2% CBR subgrade in two (2) compacted 12-inch lift thicknesses. Figure 1 illustrates the layout of horizontal and vertical stress cells. Figure 2 depicts the readings of horizontal stress within the subgrade versus the passage of construction and truck traffic during test pad construction and trafficking.

Figure 1 – Cross Section of Instrumentation Installation for all Test Sections

Figure 2 – Horizontal Stress within the subgrade layer after roller compaction and test vehicle passes Evident within Figure 2, is the minimal amount of post traffic stress remaining within the subgrade in comparison to the level of horizontal stress exhibited in the control section. The lateral stress below the TriAx® geogrid is a little over 5 kPa versus 20 kPa for the control test section. This equates to a stress state value that is 25% of the control stress state thus indicating a high level of subgrade protection.

Subbase Lift 2

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Figure 3, below, depicts the horizontal stress state post trafficking exhibited within the first 12-inch lift of subbase material. In contrast to the control, TriAx® geogrid confines the unbound aggregate leading to an increased lateral stress within the aggregate and a higher resilient modulus for the reinforced subbase.

Figure 3 – Horizontal Stress within the subbase layer 1 after roller compaction and test vehicle passes

This work demonstrated an enhanced fully confined zone above the TriAx® resulting in uniform vertical stress across the subgrade resulting in less lateral stress. This mechanism is graphically depicted in Figure 4. This field study also demonstrated the difference between performance of TriAx® and biaxial geogrids. This information formed the logic behind development of SpectraPave4 PRO™ around only the TriAx® geogrid. As such, other geosynthetics are not included within SpectraPave4 PRO™ design software.

Figure 4 – Graphic Representation of the TriAx® Enhanced Aggregate Confinement Mechanism

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The horizontal stress values shown in Figures 2 and 3 were confirmed through field determination of the following relative density values for the second or upper subgrade lift after the completion of 21 truck passes: Control = 90.2% TX160 = 98.5% These numbers demonstrated that the aggregate placed over the TriAx® geogrid can be compacted to a much higher degree than for a control section or even one containing a Biaxial geogrid. This in turn results in a higher resilient modulus for the aggregate immediately above the TriAx® geogrid. A pavement designer can now confidently account for the presence of TriAx® geogrid through consideration of this enhanced confinement mechanism. The result of increased mechanical interlock in and lower subgrade and/or subbase stresses leads to an increase in the resilient modulus of aggregate adjacent to the TriAx® geogrid. As such, aggregate used specifically with a TriAx® geogrid is referred to as a mechanically stabilized layer or MSL within the SpectraPave4 PRO™ software.

Paved Road Boundary Conditions

Observing pavement performance is similar to looking at a fingerprint for a pavement type in that each pavement type has a unique set of performance curves. In addition, the change in riding quality, as shown in Figure 5, will be directly related to how well traffic loading is transferred to the road subgrade. It is important to acknowledge this because the design performance models serve to predict the service life of pavements based on expected performance. By applying a single traffic benefit ratio (TBR) value to pavement performance prediction for a variety of asphalt thicknesses the designer would be assuming that the geogrid is providing the same level of benefit in each case. Full-scale evidence collected from both real world installations and accelerated pavement tests (APT) confirms performance of geogrid-reinforced pavements is in fact quite variable. Because of this fact, Tensar International has divided pavements into three categories, namely thick, standard and thin flexible pavements. The dividing line between the thick and standard pavement categories is an asphalt thickness of 5”. The dividing line between standard and thin pavements is 3”. Tensar does not recommend that pavement designers using SpectraPave4 PRO™ make a comparison across these boundaries. For example, a thick pavement should not be compared to a standard pavement design section. Comparison within each category ensures that the critical distress mechanism for unreinforced and reinforced pavement sections remain the same.

Figure 5: Comparison between reinforced and unreinforced pavement sections.


07.8log32.2

)1(109440.0

5.12.4log

20.0)1(log36.9)(log 10

19.5

10

101810 −+

++

−∆

+−++= RoR M

SN

PSI

SNSZW

New Paved Road Design Approach It is evident from full-scale and small-scale performance indicating research that a new method of flexible pavement design was required to provide the pavement engineer with an appropriate TIF for each TriAx® geogrid-reinforced pavement section created within SpectraPave4 PRO™. Figure 6 summarizes the TIF concept as it relates to aggregate thickness and subgrade strength. Figure 7 summarizes the entire Tensar International flexible pavement design approach related to development of TIF adjusted MSL coefficients for its TriAx® geogrids. Ultimately, Performance indicating research and testing defines how Tensar differentiates between Tensar International geogrids. As such, SpectraPave4 PRO™ starts with the automatic generation of a TIF value that is appropriate for the pavement section considered in design.

Figure 6: TIF/TBR vs. Aggregate Thickness Relationship for increasing Subgrade CBR values In the AASHTO 1993 empirical design formula (Equation 1 below), the predicted pavement life is a function of the structural number (SN), serviceability limits, and reliability. As such, pavement life using a TriAx® geogrid is calculated based on an enhanced SN. The MSL Coefficient, or “a” value, of the TriAx® geogrid-reinforced pavement section is the key component of the enhanced SN value used within the AASHTO empirically based SN equation (Equation 2). The “a” value is representative of aggregate quality and degree of enhanced confinement achieved with a particular geogrid. Calibration of this “a” value has been done with an extensive catalogue of pavement structures (thicknesses & material types), subgrade conditions, and TIF data. Complex algorithms that are based on the “a” value calibrations have been created and programmed into SpectraPave4 PRO™. The program automatically assigns the proper calibrated “a” value to the TriAx® MSL for the user defined input conditions.

(Equation 1)

SN = a1D1 + a2D2m2 + a3D3m3 (Equation 2) Where; ai = layer coefficients representative of surface, base and subbase courses; Di = actual thickness (in inches) of surface, base and subbase courses, and; mi = drainage coefficients for base and subbase courses.


Figure 7: Tensar International Paved Road Design Protocol Summary

The TIF is calibrated to an appropriate modified layer coefficient for the Mechanically Stabilized Layer (MSL) within which the TriAx® Geogrid is incorporated. This approach more accurately accounts for the variable performance benefit (effective TIF values) associated with the enhanced confinement effect. Layer coefficients presented in the AASHTO 1993 Design Manual for pavement materials are empirically derived correlations to material properties. As such, the layer coefficient is a measure of the relative ability of the material to function as a structural component within the pavement. It is important to note that the new increased layer coefficient is not a reflection of the aggregate material alone, but is adjusted to account for the improved long-term performance due to inclusion of the TriAx® geogrid, yielding a stiffened composite of aggregate and geogrid. In addition, current AASHTO correlations for the resilient modulus of a granular base layer and its layer coefficient are not valid for a composite material that consists of granular aggregate material and Tensar TriAx® geogrid reinforcement. Because of increased contact forces and stresses around the geogrid, stiffness of the unbound aggregate increases significantly and improves overall pavement performance. The increase in aggregate stiffness is evident from review of Figures 2 and 3 discussed earlier. This increase in, and retention of, stiffness results in a reduction in the amount of rutting and increased fatigue life of the pavement. In addition to a stiffness enhancement, the MSL provides the drainage benefit associated with an unbound aggregate layer as well as the ductility that is not present in treated or untreated unbound aggregate materials. Validation of the TIF values for TriAx® Geogrid Empirical research has been used to validate the performance of TriAx® geogrid, which was performed with a MSL incorporating TriAx® geogrid. The type of research used to support the TIF values comes from accelerated testing which includes:

• Model accelerated pavement testing (APT) • Full-scale accelerated asphalt surfaced pavement testing

Performance Indicating

Research & Testing

TIF/Enhanced Confinement

Effect

MSL Coefficient

SN = a1D1+a2D2m2

AASHTO ‘93 Log10(W18) =

….SN….

Full Scale Research & Field

Performance


Model Accelerated Pavement Testing

The small-scale trafficking facility at Tensar International’s Technical Centre in Shadsworth, UK is commonly referred to as a fixed model loading device that applies a load to a 6” thick aggregate base course (ABC) layer on a soft subgrade. The pressure applied to the ABC surface is the same tire pressure as a full scale wheel, ≈ 600 kPa. The geogrids are located at the interface of the subgrade and base course materials. The following curves in Figure 8 and Figure 9 show the improvement in performance of the TriAx® geogrids over the control section.

Figure 8: Shadsworth, UK Accelerated Pavement Testing

Figure 9: Shadsworth, UK Accelerated Pavement Testing

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Trafficking Test SummaryAverage Deformation At Sensor 2 Position (Centre) With 0-25mm Aggregate

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Full-Scale Accelerated Pavement Testing at ERDC in Vicksburg, MS

Documented within the USACE Interim Report (Jersey, S.R. and Tingle, J.S., 2010) are the testing protocol and results for the TX140 with 2-inches of asphalt and two (2) control sections with 2-inches and 3-inches of asphalt. Further, the asphalt was placed on a foundation that consisted of an 8-inch aggregate base layer and a design subgrade CBR of 3%. The profile for test sections is found as Figure 10. The strength profiles obtained after construction of the subgrade indicate a variation of 0.5% in the Subgrade support values. Shown in Figures 11 and 12 are photographs of the excavations performed full depth through the trafficked pavements after 100,000 ESAL’s for the control and TX140, respectively. TBR values for these pavement conditions provided by TX140 geogrid reinforcement are derived through examination of rutting performance from Figure 13 and are as computed within Table 1 in relation to the 2” AC control section.

Figure 10: Profile of Accelerated Pavement Testing Program at the USCOE Facility in Vicksburg, MS.

Figure 11:TX140 Section After 100,000 ESAL’s

Item 3


Figure 12: Control Section with 2” AC After 24,000 ESAL’s

From this research and the use of SpectraPave4 PRO™, it is possible to demonstrate the correlation between full-scale research and the program output using pavement design parameters found within the USACE Interim Report. The screen shot from an adjusted cross section is shown below in Figure 14. Based on a predicted design of 41,000 ESAL’s for the unreinforced pavement section, the expected design life of the reinforced section is 247,000 ESAL’s.

Figure 13: Accumulated Rutting for TX140, Control with 2” of AC, and the 3” AC Section

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Table 1: Traffic Improvement Factors Versus the 2” AC Control Section Test Item Treatment 0.25 inch 0.50 inch 0.75 inch 1.0 inch

Item 1 TX140 20 19+ 8+ 5+

Item 4 Control (2 inch AC) - - - - Item 5 3 inch AC 3 2 2 1

Figure 14: Screen Shot from SP4 PRO™ Showing the Approximate Correlation to USACE Research

Design of Pavements on Soft Soil Subgrades

The design of a paved road over a soft subgrade is a two-step process. Based on Tensar International’s experience, stabilization of the subgrade is required for soils exhibiting a resilient modulus of less than or equal to 5000 psi (CBR of approximately 3). For these field conditions, the stabilization layer should be designed using the Giroud-Han method as incorporated within the unpaved module of the SpectraPave4 PRO™ software. For stabilization of soft soil, the designer needs to consider axle load, tire pressure and the required maximum rut depth associated with placement of the aggregate stabilization layer. Site-specific soil strength conditions for the proposed stabilized road section can be evaluated through use of the soil strength chart presented in Table 2. Using the rut depth criteria for granular soil (rutting on the proposed stabilization layer) is conservative based on the lack of cohesion associated with aggregate that is commonly used for construction of a stabilization layer. Following placement of the aggregate subgrade stabilization layer, referred to as the mechanically stabilized layer (MSL), a resilient modulus of 9000 psi at the top of the MSL. This value is then used to undertake a conventional paved road design.

After completion of stabilization design, the paved road base course reinforcement design can be performed using the AASHTO ’93 design procedure with incorporation of an “improved” subgrade modulus that is deemed acceptable to the pavement design engineer. Again, based on TIC’s experience this value would be set to 9000 psi for the conditions described above and the default values found within the unpaved application module of the SpectraPave4 PRO™ software. Keep in mind that base course reinforcement will require a second layer of geogrid. As such, within SpectraPave4 PRO™ software, the unreinforced paved module case represents use of one layer at the subgrade interface and the reinforced paved module design case represents one geogrid layer at the subgrade interface and one geogrid layer beneath the base course layer.


Table 2: Guide for Estimating Subgrade Soil Strength

References

AASHTO. (2009).”Standard Practice for Geosynthetic Reinforcement of the Aggregate Base Course of Flexible Pavement Structures.” AASHTO Publication R 50-09. American Association of State Highway and Transportation Officials, Washington, D.C.

AASHTO. (2001).”Recommended Practice for Geosynthetic Reinforcement of the Aggregate Base Course of Flexible Pavement Structures.” AASHTO Publication PP46-01. American Association of State Highway and Transportation Officials, Washington, D.C.

AASHTO. (1993). AASHTO Guide for Design of Pavement Structures. American Association of State Highway and Transportation Officials, Washington, D.C.

AASHTO, (1993), AASHTO Guide for Design of Pavement Structures; Part I, Chapter 3: Economic Evaluation of Alternative Pavement Design Strategies,

Al-Qadi, I. L., Dessouky, S. H., Kwon, J. and Tutumluer, E. (2008). “Geogrid in Flexible Pavements: Validated Mechanism,” Transportation Research Record 2045, TRB, National Research Council, Washington, DC, USA, pp. 102-109.

Barksdale, R.D., Brown, S.F. & Chan, F. (1989). “Potential Benefits of Geosynthetics in Flexible Pavement Systems.” National Cooperative Highway Research Program Report 315. Transportation Research Board, Washington, D.C.


Brown, S.F., Jones, C.P.D. & Brodrick, B.V. (1983). “Use of Non-Woven Fabrics in Permanent Road Pavements.” Proceedings of the Institution of Civil Engineers, Part 2, Volume 73, pp. 541-563. London, England, United Kingdom.

Collin, J.G., Kinney, T.C. & Fu, X. (1996). “Full Scale Highway Load Test of Flexible Pavement Systems with Geogrid Reinforced Base Courses.” Geosynthetics International , Vol. 3, No. 4, pp. 537-549.

Fannin, R.J. & Sigurdsson, O. (1996). “Field Observations on Stabilization of Unpaved Roads with Geosynthetics.” Journal of Geotechnical Engineering, Vol. 122, No. 7, pp. 544-553. American Society of Civil Engineers.

FHWA, (1998), Life Cycle Cost Analysis in Pavement Design, Publication No. FHWA-SA-98-079,

Gabr, M. (2001). “Cyclic Plate Loading Tests on Geogrid Reinforced Roads.” Research Report to Tensar International Corporation., NC State University.

Giroud, J.P., Ah-Line, C. & Bonaparte, R. (1985). “Design of Unpaved Roads and Trafficking Areas with Geogrids.” Proceedings of Polymer Grid Reinforcement Conference, pp. 116-127. Thomas Telford Ltd., London, England, United Kingdom,.

Giroud, J.P. & Han, J. (2004a). “Design Method for Geosynthetic-Reinforced Unpaved Roads: Part I – Development of Design Method.” Journal of Geotechnical and Geoenvironmental Engineering, in press. American Society of Civil Engineers.

Giroud, J.P. & J. Han. (2004b). “Design Method for Geosynthetic-Reinforced Unpaved Roads: Part II – Calibration and Applications.” Journal of Geotechnical and Geoenvironmental Engineering, in press. American Society of Civil Engineers.

Giroud, J.P. & L. Noiray. (1981). “Geotextiles-Reinforced Unpaved Road Design,” Journal of Geotechnical Engineering, Vol. 107, No. 9, pp. 1233-1253. American Society of Civil Engineers.

GMA (2000). GMA White Paper II: Geosynthetic Reinforcement of the Aggregate Base/Subbase Courses of Pavement Structures. Geosynthetic Materials Association, Industrial Fabrics Association International.

Jersey, S.R. and Tingle, J.S. (2010) " Full-Scale Accelerated Pavement Tests Geogrid Reinforcement of Thin Asphalt Pavements Phase 1 Interim Report", USACE Engineer Research and Development Center, Vicksburg, MS.

Knapton, J. & Austin, R.A. (1996). “Laboratory Testing of Reinforced Unpaved Roads,” Earth Reinforcement. Ochiai, Yasufuku, and Omine (eds). Balkema, Rotterdam.

Kwon, J., Tutumluer, E. and Al-Qadi, I.,(2009) “A Validated Mechanistic Model for Geogrid Base Reinforced Flexible Pavements,” ASCE Journal of Transportation Engineering, Volume 135, Issue 12, pp. 915-926

Kwon, J., Tutumluer, E., and Konietzky, H., (2008). “Aggregate Base Residual Stresses Affecting Geogrid Reinforced

Flexible Pavement Response,” International Journal of Pavement Engineering, Volume 9, Issue 4, pages 275-285.

Kwon, J. and Tutumluer, E. (2009). “Geogrid Base Reinforcement with Aggregate Interlock and Modelling of the Associated Stiffness Enhancement in Mechanistic Pavement Analysis,” Transportation Research Record 2116, Transportation Research Board, National Research Council, Washington, D. C., pp. 85-95.

Perkins, S.W. & Ismeik, M. (1997). “A Synthesis and Evaluation of Geosynthetic-Reinforced Base Layers in Flexible Pavements: Part I.” Geosynthetics International, Vol. 4, No. 6, pp. 549-604.

Perkins, S.W. (1999). “Geosynthetic Reinforcement of Flexible Pavements: Laboratory Based Pavement Test Sections.” Final Report, FHWA/MT-99-001/8138. United States Department of Transportation, Federal Highway Administration, Washington, D.C.

Qian, Y. (2009). “Experimental Study on Triangular Aperture Geogrid-Reinforced Bases Over Weak Subgrade Under Cyclic Loading”, Master’s Thesis, University of Kansas, Lawrence, KS Steward, J., Williamson, R. & Mohney, J. (1977). “Guidelines for Use of Fabrics in Construction and Maintenance of

Low-volume Roads.” Report FHWA-TS-78-205. United States Department of Transportation, Federal Highway Administration, Washington, D.C.


Tensar (1996). “Design Guideline for Flexible Pavements with Tensar Geogrid Base Layers.” Tensar Technical Note, TTN: BR96, p. 77. The Tensar Corporation, Atlanta, Georgia.

Tensar (1998). “Design Guideline for Unpaved Applications under Dynamic Loading with Tensar Geogrids.” Tensar Technical Note, TTN:BR5, p. 30. The Tensar Corporation, Atlanta, Georgia.

Watts G.R.A., Blackman, D.I. & Jenner, C.G. (2004). “The Performance of Reinforced Unpaved Sub-bases Subjected to Trafficking.” Proceedings of the Third European Geosynthetics Conference. Munich, Germany.

Webster, S.L. (2000). Personal Communication with Mr. Robert B. Anderson. Tensar International Corporation.

Webster, S.L. (1992). “Geogrid Reinforced Base Course for Flexible Pavements for Light Aircraft: Test Section Construction, Laboratory Tests and Design Criteria.” US Army Corps of Engineers Report No. DOT/FAA/RD-92-25. United States Army Corps of Engineers, Washington, D.C.

White, D.W. (1990). “Literature Review of Geotextiles to Improve Pavements for General Aviation Airports.” US Army Corps of Engineers Report No. DOT/FAA/RD-90/26. United States Army Corps of Engineers, Washington, D.C.

White, D.W. (2010). “Instrumentation Results from an Evaluation of the Response of Aggregate Base Course Material Used for Subgrade Stabilization .” Interim report from Earthworks Engineering Research Center Iowa State University, Ames, IA

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