Quantifying the Influence of Geosynthetics on Pavement Performance (2017) / Chapter Skim
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From page 13... ...
13 CHAPTER 4. EXPERIMENTS, MODELING, AND FINDINGS Introduction This chapter on the experiments, modeling, and findings of the research project reviews the tests that were used commercially and identifies the geosynthetic properties that were most relevant to pavement performance prediction.
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In by geotex separatio modulus Availabl Perform T physical layer and performa geosynth propertie Test stan F addition to tiles was an n reduced th of the base e Test Meth ance he geosynth properties, m aggregates/ nce-related etic properti s included: Tex-621-J than the m ASTM D ASTM D5 ASTM D6 ASTM D7 ASTM D7 ASTM D6 dards for ev ASTM D5 ASTM D4 the 15 per ASTM D4 igure 4.1. M the above m other impor e base cour course and t ods for Ev etic properti echanical p soils.
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15 ASTM D5493, to measure the permittivity, which influences the filtration function. ASTM D4595, to measure the tensile stiffness.
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Pullout T T using the embedde pullout fo variable d The pullo nonlinear geosynth the interf Equation G where G G is the maximum model co the interf δ est to Deter he interactio pullout test d in the base rce was rec ifferential t ut force ver stage, and etic interact acial shear m 4.3.
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17 Figure 4.3. Pullout Force versus Geosynthetic Displacement in a Pullout Test Laboratory Methodology for Quantifying Influence of Geosynthetics The application of geosynthetics had the potential ability to reduce the thickness of the base courses, improve performance, and extend the service life of the pavement structure.
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F T geosynth Universa radial def anisotrop (c) igure 4.4.
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T equations rewritten where strain in t A the loadin static stre employed the dynam resilient he loading p of the cros in the incre 1 1 rz r r rz r z E E E E 1 1 rz r r rz r z E E E E r is the stres he radial dir ccording to g protocol, ss states ass in the triax ic stress co strain was ac Figure 4.5 rotocol used s-anisotropic mental form rr r r z rz r r E E rr r r z rz r r E E s in the radi ection; and the cross-an including th ociated with ial test, as s nsisted of 1 hieved after . Configur in the triax aggregate in Equation r z r z al direction; z is the str isotropic co e compress correspond hown in Tab .5 seconds o 25 repetitio 19 ation of Ra ial test was specimens, a 4.5 for the z is the st ain in the ax nstitutive re ion, shear, a ing dynami le 4.1.
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20 Table 4.1. Triaxial Test Protocol for Determining Cross-Anisotropic Properties Stress State Static Stress (psi)
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21 Table 4.2. Influence of Geosynthetic on Material Properties -- Geosynthetic Location: Mid-Height Stress State Geosynthetic Type r geosynthetic r control E E (%)
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22 Table 4.3. Influence of Geosynthetic on Material Properties -- Geosynthetic Location: One-Quarter below the Middle Stress State Geosynthetic Type r geosynthetic r control E E (%)
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23 Table 4.4. Influence of Geosynthetic on Material Properties -- Geosynthetic Location: Bottom Stress State Geosynthetic Type r geosynthetic r control E E (%)
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24 In contrast to the geogrid, the application of a geotextile slightly reduced the vertical modulus of the UGM specimen but significantly raised the horizontal modulus. As a result, the geotextile increased the anisotropic ratio of the specimen by 20~60 percent, which indicated that the geotextile made the specimen more isotropic (54)
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25 In order to characterize the stress-dependent permanent deformation behavior of UGMs with and without geosynthetics, a new permanent deformation model was proposed, as shown in Equations 4.7 to 4.9. The proposed model was able to determine the accumulated permanent deformation at any specific stress state and number of load repetitions.
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26 Figure 4.7. Illustration of the Stress-Related Terms in the Proposed Model Table 4.5 shows the seven stress levels designed to determine the coefficients of the proposed rutting model.
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Table Stress S 8 9 T deformat comparis different Figure 4. errors (R general, a stress lev the meas Figur T predictio model co States 8 a the mode model wa 0 0 0 0 0 1 A cc um ul at ed P la st ic S tr ai n (% )
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28 of the UGM with and without geosynthetics. Table 4.7 lists the model coefficients determined for the tested UGM with and without geosynthetics.
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29 the permanent deformation behavior of the UGM. It was found that the geogrid placed in the middle of the UGM had a greater effect on the RPS than the geogrid placed at one-quarter below the middle of the UGM.
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30 Analytical Model for Quantifying the Influence of Geosynthetics The repeated load triaxial tests indicated that the placement of geosynthetics influenced the cross-anisotropic properties (i.e., the vertical and horizontal modulus) and the permanent deformation properties of the UGM.
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31 (a) Displacement Pattern of UGM Restraint by Geosynthetic (b)
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32 Equation 4.13 was used to calculate the maximum equivalent additional stress 3max .
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33 0 2 0 2 0 2 2 2 2 2 2 V UGM V Modified l V UGM V Modified V Effective h l V UGM V Modified E h E z dz l h h E h l E z dz E l h E h l E z dz l h h (4.15) where V UGME is the vertical modulus of the unreinforced base course; h is the thickness of the base course; and l is the distance between the geosynthetic layer and the bottom of the base course.
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34 (a) Predicted Horizontal Moduli vs.
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35 Experimental Plan and Setup Test Matrix Experimental Setup One of the largest tank containers in the United States was used to execute the experimental program. This modular cylindrical container, which measured 8 ft in diameter, was divided into three segments; each segment was 3 ft high.
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Figure F 4.14. FWD igure 4.15.
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Figu Sensors a V pavemen 3500, we the pavem 600 kPa. over a sm to captur one LVD drilled th of the ba concrete loading a and defor YMFLA Micro-el 5 g in thr geosynth geosynth recorded the surfac of the ge In each e LST for f re 4.16.
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38 in the various experiments. Figure 4.17 and Figure 4.18 illustrate a typically instrumented experiment for a reinforced flexible and rigid pavement, respectively.
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Figure 4.17. Instrumentation P X = lan for Fle 0 inch; (b)
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Figure 4.18. Instrumentation Y = Plan for Ri 0 inch; (b)
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T cells plac done afte excavate carefully material Figure 4. T attention were plac compacte mechanic desired d strings w point of t pressure the vario with appr T accelerom base (for the geosy displacem accelerat In geogrid, measure the geosy to specifi an ultra-f cyanoacr once the applying he instrume ed at a dept r compactin around the c on a leveled was placed c 19 shows th Figure 4.1 he instrume .
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glue drie while pla One gaug direction T temperatu heated as After coo This was the strain on top, an mixture w strain gau installatio concrete concrete Care was attached was pour Figure Data Acq A with 20 N channel s a system displacem involving double-in for furthe acquired, imported d, the gauge cing the CA e was place s. he instrume re sensors u phalt binder ling, a smal used to ensu gauge and t d a static pr as then pla ge on top o n of the PC and complet was placed taken to sec lead wire ca ed carefully 4.20.
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43 Data Analysis Methodologies The laboratory testing program for flexible and rigid pavements included a series of instrumentation that included accelerometers, LVDTs, earth pressure cells, and strain gauges. The instrumentation program was designed to assess several aspects of the influence of the base reinforcement on pavement responses under a variety of realistic pavement loading conditions.
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44 geotextile-reinforced base. The geosynthetic was located in both Experiments 4 and 6 at the middle of the CAB layer (i.e., 11 inches below the pavement surface)
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45 The superscripts "Above" and "Below" refer to the pressure cells above and below the geosynthetic. The horizontal stress in the CAB under the edge of the loading plate is referred to as B-Horiz.
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46 Figure 4.24 shows the horizontal stresses in the CAB layer measured along the edge of the loading plate. Noticeably lower horizontal stresses were observed with the reinforced base layer when compared to those measured in the control experiment (i.e., no reinforcement)
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Figure 4 .21. LST C and 5 onfiguratio )
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Figur Figure 4 e 4.24.
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53 Figure 4.28. Horizontal Stresses at the Edge of the Loading Plate for Thick CAB Layer (Experiments 2, 4, and 6)
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T measurem geosynth load leve compared not result T of the lon the AC la respectiv consisten The load strain me bottom o (Experim Figure 4 While a 6 variations among the layer thick measured he above fac ents. Figur etics for bot ls, respectiv to those m in a propor he horizonta g-term perf yer are pres ely.
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Figure 4.30.
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58 Figure 4.34. Tensile Strains at the Centerline of the Load and at the Bottom of the AC Layer (Experiments 2, 4, and 6)
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59 and the adjacent unbound material. The potential horizontal slippage was calculated at all applied load levels.
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62 Figure 4.37. Horizontal Slippage of the Geosynthetic and Adjacent Unbound Material in Experiments 3 and 5 for Various Load Levels -- Flexible Pavements -8 -6 -4 -2 0 2 4 6 8 0 12 24 36 Ax is Tit le Radial Distance (inch)
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65 Figure 4.40. Horizontal Slippage of the Geosynthetic and Adjacent Unbound Material in Experiments 4 and 6 for Various Load Levels -- Flexible Pavements Rigid Pavement Similar to the flexible pavement results section, a recap of pertinent key factors that had significant influence on the measured data is first provided below.
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66 Unlike with the tests on flexible pavements, a number of instruments malfunctioned. This problem limited the scope of the data interpretation described below.
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67 base. In contrast, a reverse trend was observed for the vertical stresses below the reinforcement location.
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Figure Figure Only E 4.41.
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Figure Figure Only S F wet CAB 4.46.
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73 noted that the measurements on the geotextile were not available due to malfunction of the strain gauges. The normal strains on the geogrid in the X-direction were consistently higher under the edge of the loading plate (i.e., at a radial distance of 12 inches in Figure 4.48a and Figure 4.48b)
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74 Concrete strain gauges embedded at the bottom of the PCC slab were used to assess the normal strains developed due to surface loading. A positive strain reflected a tensile response associated with a beam action, while a negative strain reflected a compression response due to reversed beam action.
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75 supporting base layer. The data collected were not reliable to carry out the integration algorithm, which might be attributed to difficulties associated with the installation of the accelerometers.
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Figure O Figure Only t 4.50.
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Figure Fig 4.53. Typic ure 4.54.
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F the LST t 0.08-inch the base the rigid reinforce shows th Figur igure 4.55 p est. The stru geosynthet course.
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Figur Characte In was a lin nonlinear material. LST test modulus compress The nonl material material estimated e 4.56.
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81 Table 4.9. Selected Laboratory Tests for Material Characterization Material Type Constitutive Model Lab Test Model Input HMA Viscoelastic Dynamic modulus test Prony-series parameters (Gi, Ki, and τi)
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82 result, the unknown parameters in Equation 4.19 could be determined based on the least square error criterion. As observed in Equations 4.17 and 4.19, the form of the Prony-series model in ABAQUS was slightly different from the model used for fitting the dynamic modulus test result.
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83 Figure 4.57. Comparison between the Measured Dynamic Moduli and the Fitted Dynamic Moduli As stated in the previous section, the RaTT was employed to determine the crossanisotropic properties of the UGM used in the LST test.
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T products tensile fo for mach and geote direction the cross compared found tha manufact (a) T Fig he direct ten used in the rce and the ine direction xtile had sm .
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85 Figure 4.59. Relationships between Tensile Force and Tensile Strain for Geosynthetics Table 4.12.
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86 11i i iy y ycomputedE E E (4.31) where iyE is the vertical modulus output from the i th iteration; 1iyE is the vertical modulus output from the (i−1)
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87 Figure 4.60. Flowchart of the Developed UMAT Subroutine Development of Goodman Model Friction Subroutine When surfaces of the geosynthetic and aggregate/soil were in contact, they usually transmitted shear and normal stresses across their interface.
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88 the shear stiffness, sk , was determined using the pullout test data. This tangential contact behavior was defined by the user subroutine FRIC in the ABAQUS software.
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89 Figure 4.61. Simulation of Lateral Confinement in Geosynthetic-Reinforced Pavement Structure Effect of Geosynthetic Reinforcement on Pavement Responses The current Pavement ME Design software predicts pavement performance based on the following computed critical responses by the embedded finite element program (62)
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90 subjected to a 9-kip load on a circular area with the radius of 6 inches. The figure shows that placing the geogrid and geotextile at the bottom of the base course cannot reduce the surface deflections of the flexible pavement, while placing the geogrid in the middle of the base course only slightly decreases the surface deflections.
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91 structures diminished the vertical compressive stresses within the base layer by 2–3 psi. The decrease of vertical compressive stresses within the base layer was beneficial for reducing the permanent deformation of the base materials.
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92 the bottom of the base course did not affect the compressive strain in the base layer, but they both diminished the compressive strain at the top of the subgrade significantly. This finding demonstrated that a geosynthetic reinforced in the middle of the base course reduced the permanent deformation of the base layer, while a geosynthetic reinforced at the bottom of the base course helped decrease the permanent deformation of the subgrade.
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93 Figure 4.65 shows the load-induced tensile stress at the top of the PCC slab for the geosynthetic-reinforced and unreinforced rigid pavements. There was no significant difference observed among the geosynthetic-reinforced and unreinforced rigid pavements.
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94 _ _ 100% _ Strain Control Strain Geosynthetic Normalized reduction of strain Strain Control (4.35) where _Strain Control is the computed critical strain in the control model; and _Strain Geosynthetic is the computed critical strain in the geosynthetic-reinforced model.
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95 (a) Computed Vertical Strain at the Top of Subgrade (b)
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96 (c) Normalized Reduction of Vertical Strain at the Top of Subgrade (d)
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97 (a) Vertical Strain at the Top of Subgrade (b)
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98 (a) Computed Vertical Strain at the Top of Subgrade (b)
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Figure 4 (a) Flexibl (b)
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F element m subjected LST mea deflection deflection indicated highly ac strain at t develope and unrei geotextil predicted flexible p element m explanati behavior shown in have bee Figure igure 4.71 sh odels and t to a 9-kip l surements f by LVDT at this loca that the dev curate when he bottom o d finite elem nforced pav e-reinforced vertical pre avements. M odels, exce ons existed of the subgr Figure 4.73 n due to arch 4.70.
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101 (a) Pavement Structures with 6-inch Base Course (b)
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102 (a) Pavement Structures with 6-inch Base Course (b)
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103 (a) Pavement Structures with 6-inch Base Course (b)
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104 in the base and subgrade. Note that some of the pressure sensor data (e.g., P1, P3, P6, and P8)
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105 and the reinforcement influence zone was important to develop accurate numerical models of geosynthetic-reinforced pavement structures. ANN Approach for Predicting Pavement Performance The current Pavement ME Design software predicted pavement performance based on the computed critical pavement responses from a linear isotropic and layered elastic program.
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106 normally required a large database of input and output variables (67)
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107 Table 4.15. Selected Input Parameters for Unreinforced Pavement Structures Influential Factors Level Input Values Load Magnitude 1 9 kip HMA Thickness 3 2, 4, and 6 inches HMA Modulus 3 300, 450, and 600 ksi Base Thickness 3 6, 10, and 15 inches Base Vertical Modulus 3 20, 40, and 60 ksi Base Anisotropic Ratio 2 0.35 and 0.45 Subgrade Modulus 3 5, 15, and 25 ksi Note: The number of total cases was 486.
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108 Figure 4.76. Illustration of Three-Layered Neural Network Architecture The pavement response database was first randomly divided into a training dataset and a validating dataset as the ratio of 80 percent and 20 percent, respectively.
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109 Figure 4.77. Comparison of Tensile Strain at the Bottom of the Asphalt Layer
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110 Figure 4.78. Comparison of Average Vertical Strain in the Asphalt Layer
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111 Figure 4.79. Comparison of Average Vertical Strain in the Base Layer
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112 Figure 4.80. Comparison of Vertical Strain at the Top of the Subgrade
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113 Figure 4.81. Comparison of Vertical Strain at 6 inches below the Top of the Subgrade Determination of Modified Material Properties The performance of geosynthetic-reinforced flexible pavements included fatigue cracking, permanent deformation, and international roughness index (IRI)
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114 were selected to predict the responses of the geosynthetic-reinforced and the control pavement structures. Subsequently, the responses of the geosynthetic-reinforced pavement structure were compared to those of the control structure.
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115 Figure 4.82. Flowchart of the Process of Predicting Pavement Performance Input GeosyntheticReinforced Pavement Structure Data (Layer Information and Material Properties)
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116 Case studies were performed on flexible pavements with geosynthetics (i.e., geogrid or geotextile) placed in the middle or at the bottom of the base course.
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117 Table 4.16. Material Properties of Geosynthetic-Reinforced Pavements for Case Studies -- Material Properties of Control Pavement Material Type Thickness (inch)
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118 occurred with the geotextile placed in the center of the base course, and it was dramatically higher than that in the unreinforced pavement section. This finding indicated that placing the geotextile in the center of the base course significantly reduced the fatigue life of the pavement structures.
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119 Figure 4.85. Effect of Geosynthetic Location and Geosynthetic Type on Rutting Depth Figure 4.86.
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120 Validation of the Proposed ANN Approach Using the proposed ANN approach, a geosynthetic-reinforced pavement with any given material properties needed to be made equivalent to an unreinforced pavement with the modified material properties to obtain the identical pavement responses. The process of validating the proposed ANN approach is illustrated in Figure 4.87 and involved the following steps: Identify the in-service geosynthetic-reinforced pavement sections from the LTPP database and Texas Pavement Management Information System (PMIS)
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121 Figure 4.87. Flowchart of the Process of Validating the Proposed ANN Approach After a thorough review of the in-service pavement sections in the LTPP database and PMIS, researchers found 74 pavement sections containing geosynthetics in the LTPP database and 51 pavement sections containing geosynthetics in the PMIS.
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122 dense-graded asphalt concrete, a 23.2-inch crushed gravel unbound base, and a semi-infinite subgrade, which was classified as AASHTO 7-5 soil. A 0.1-inch woven geotextile was placed at the interface between the unbound base and subgrade.
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123 Figure 4.89. Comparison of Fatigue Cracking between ANN Approach Prediction and Field Measurement for Pavement Section 16-9032 Figure 4.90.
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