GEOGRID REINFORCEMENT OF FLEXIBLE PAVEMENTS: A PRACTICAL PERSPECTIVE

Transcription

1 TENAX Technical Reference GEOGRID REINFORCEMENT OF FLEXIBLE PAVEMENTS: A PRACTICAL PERSPECTIVE 1999 TENAX Corporation 4800 East Monument Street Baltimore, Maryland tel: (410) fax: (410)

2 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 documented. 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 methodology. 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 = a Eq. (1) 1 d1 + a2 d2 m2 where a 1 a 2 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

3 can be included in Eq. (1) if desired. d 1, d 2 are their thicknesses; and m 2 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. SN = a d + LCR a * d * m Eq. (2) 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: d 2 SN a1 * d1 = LCR a m 2 2 Eq. (3) 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 tests. 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).

4 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 Open Area % 75 Peak tensile strength kn/m Tensile strain kn/m Tensile strain kn/m Junction strength kn/m 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.

5 VERTICAL SETTLEMENT, [mm] 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% CYCLE, [-] Figure 2. Pavement surface ruts for control and reinforced sections CYCLE, [-] Unreinforced 25mm RUT Reinforced, 25mm RUT Unreinforced 12.5mm RUT Reinforced, 12.5mm RUT 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

6 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 ratio. 2.0 Layer Coefficient Ration, LCR Subgrade CBR 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

7 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 Street From To Average AC Average Base Segment Section Station Station Thickness CoV* Thickness CoV* Geogrid (ft) (ft) (in) (%) (in) (%) Sterling YES Dallas YES 31 st YES YES YES Pawnee None YES None * CoV = Coefficient of Variation = Standard Deviation Divided by the Mean Field roadbed soil resilient modulus M r 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 (SN eff ) 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 (E p ) using deflection data. The effective structural number (SN eff ) then is a function of the total pavement thickness and its overall stiffness, E p. 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

8 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 segment. Table 3. Structural Number Comparison Street Design AC Design Base Design As-built Effective Segment Section Thick Thick SN SN SN CoV* Geogrid (in) (in) (%) Sterling YES Dallas YES 31st YES YES YES Pawnee None YES 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 SN eff 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 SN eff for Pawnee Section 2 is about 0.5 greater than for Sections 1 and 3, a significant improvement in the overall SN eff for the section that should result in an additional 2 to 3 years of pavement service life.

9 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

10 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 Section CBR Base Thickness Total Traffic Passes (%) (mm) Maximum rut depth (mm) Reinforced Control Reinforced Control Reinforced Control Reinforced Control 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 inground tests. The structural contribution provided by a geogrid reinforcement is quite significant.

11 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)

12 2 0 Rut Depth, mm Road Width, mm Figure 8: Rut profile for the reinforced section. (CBR=3, Base course thickness = 300mm) Rut Depth, mm Road Width, mm Figure 9: Rut profile for the control section. (CBR=3, Base course thickness = 300mm)

13 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 inplace cost of $19.6/m 3 ($15/yd 3 ) for graded aggregate base (GAB), and $3.0/m 2 ($2.5/yd 2 ) 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/m 2 ($0.31/yd 2 ). 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.

14 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 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

15 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. Acknowledgments 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. References 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 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, 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 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