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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2019. Proposed Enhancements to Pavement ME Design: Improved Consideration of the Influence of Subgrade and Unbound Layers on Pavement Performance. Washington, DC: The National Academies Press. doi: 10.17226/25583.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2019. Proposed Enhancements to Pavement ME Design: Improved Consideration of the Influence of Subgrade and Unbound Layers on Pavement Performance. Washington, DC: The National Academies Press. doi: 10.17226/25583.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2019. Proposed Enhancements to Pavement ME Design: Improved Consideration of the Influence of Subgrade and Unbound Layers on Pavement Performance. Washington, DC: The National Academies Press. doi: 10.17226/25583.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

NCHRP Web-Only Document 264: Proposed Enhancements to Pavement ME Design: Improved Consideration of the Influence of Subgrade and Unbound Layers on Pavement Performance Robert L. Lytton Xue Luo Sajib Saha Yu Chen Yong Deng Fan Gu Meng Ling Texas A&M Transportation Institute The Texas A&M University System College Station, TX Contractor’s Final Report for NCHRP Project 01-53 Submitted February 2019 ACKNOWLEDGMENT This work was sponsored by the American Association of State Highway and Transportation Officials (AASHTO), in cooperation with the Federal Highway Administration, and was conducted in the National Cooperative Highway Research Program (NCHRP), which is administered by the Transportation Research Board (TRB) of the National Academies of Sciences, Engineering, and Medicine. COPYRIGHT INFORMATION Authors herein are responsible for the authenticity of their materials and for obtaining written permissions from publishers or persons who own the copyright to any previously published or copyrighted material used herein. Cooperative Research Programs (CRP) grants permission to reproduce material in this publication for classroom and not-for-profit purposes. Permission is given with the understanding that none of the material will be used to imply TRB, AASHTO, FAA, FHWA, FMCSA, FRA, FTA, Office of the Assistant Secretary for Research and Technology, PHMSA, or TDC endorsement of a particular product, method, or practice. It is expected that those reproducing the material in this document for educational and not-for-profit uses will give appropriate acknowledgment of the source of any reprinted or reproduced material. For other uses of the material, request permission from CRP. DISCLAIMER The opinions and conclusions expressed or implied in this report are those of the researchers who performed the research. They are not necessarily those of the Transportation Research Board; the National Academies of Sciences, Engineering, and Medicine; or the program sponsors. The information contained in this document was taken directly from the submission of the author(s). This material has not been edited by TRB.

The National Academy of Sciences was established in 1863 by an Act of Congress, signed by President Lincoln, as a private, non- governmental institution to advise the nation on issues related to science and technology. Members are elected by their peers for outstanding contributions to research. Dr. Marcia McNutt is president. The National Academy of Engineering was established in 1964 under the charter of the National Academy of Sciences to bring the practices of engineering to advising the nation. Members are elected by their peers for extraordinary contributions to engineering. Dr. John L. Anderson is president. The National Academy of Medicine (formerly the Institute of Medicine) was established in 1970 under the charter of the National Academy of Sciences to advise the nation on medical and health issues. Members are elected by their peers for distinguished contributions to medicine and health. Dr. Victor J. Dzau is president. The three Academies work together as the National Academies of Sciences, Engineering, and Medicine to provide independent, objective analysis and advice to the nation and conduct other activities to solve complex problems and inform public policy decisions. The National Academies also encourage education and research, recognize outstanding contributions to knowledge, and increase public understanding in matters of science, engineering, and medicine. Learn more about the National Academies of Sciences, Engineering, and Medicine at www.national-academies.org. The Transportation Research Board is one of seven major programs of the National Academies of Sciences, Engineering, and Medicine. The mission of the Transportation Research Board is to increase the benefits that transportation contributes to society by providing leadership in transportation innovation and progress through research and information exchange, conducted within a setting that is objective, interdisciplinary, and multimodal. The Board’s varied committees, task forces, and panels annually engage about 7,000 engineers, scientists, and other transportation researchers and practitioners from the public and private sectors and academia, all of whom contribute their expertise in the public interest. The program is supported by state transportation departments, federal agencies including the component administrations of the U.S. Department of Transportation, and other organizations and individuals interested in the development of transportation. Learn more about the Transportation Research Board at www.TRB.org.

C O O P E R A T I V E  R E S E A R C H  P R O G R A M S  CRP STAFF FOR NCHRP Web-Only Document 264 Christopher J. Hedges, Director, Cooperative Research Programs Lori L. Sundstrom, Deputy Director, Cooperative Research Programs Amir N. Hanna, Senior Program Officer Eileen P. Delaney, Director of Publications Natalie Barnes, Associate Director of Publications Kathleen Mion, Senior Editorial Assistant NCHRP PROJECT 01-53 PANEL Field of Design—Area of Pavements Zhong Wu, Louisiana DOTD, Baton Rouge, LA (Chair) Mohammad A. Ahammed, Manitoba Infrastructure and Transportation, Winnipeg, MB Judith Corley-Lay, Michigan State University, East Lansing, MI (formerly with North Carolina DOT) Richard Y. Ji, Federal Aviation Administration, Atlantic City, NJ Jianhua Li, Washington State DOT, Issaquah, WA Mehdi Parvini, California DOT, El Dorado Hills, CA Iliya R. Yut, Connecticut Transportation Institute, Storres, CT Y. Jane Jiang, FHWA Liaison Nancy M. Whiting, TRB Liaison

v Contents CHAPTER 1. INTRODUCTION ................................................................................................... 1  Introduction ................................................................................................................................. 1  Objective ..................................................................................................................................... 1  Research Scope and Approach ................................................................................................... 1  Organization of the Report ......................................................................................................... 1  CHAPTER 2. SYNTHESIS OF CURRENT KNOWLEDGE ....................................................... 3  Characteristics of Unbound Layers and Subgrade Used in Pavement ME Design .................... 3  Influence of Unbound Layers and Subgrade on Performance of Flexible and Rigid Pavements ................................................................................................................................... 5  Unbound Layer and Subgrade Models for Performance Influence .......................................... 10  CHAPTER 3. RESEARCH PLAN ............................................................................................... 18  Evaluation and Screening of Unbound Layer and Subgrade Models ....................................... 18  Soil Water Characteristics Curve of Base and Subgrade for Flexible and Rigid Pavements ................................................................................................................................. 18  Equilibrium Suction of Base and Subgrade for Flexible and Rigid Pavements ....................... 19  Resilient Modulus of Base and Subgrade for Flexible and Rigid Pavements .......................... 19  Modulus of Subgrade Reaction for Rigid Pavements ............................................................... 19  Shear Strength of Base and Subgrade Layers for Flexible and Rigid Pavements .................... 20  Permanent Deformation of Base and Subgrade Layers for Flexible and Rigid Pavements ................................................................................................................................. 20  Faulting of Base for Rigid Pavements ...................................................................................... 20  Prediction of Pavement Performance and Sensitivity Analysis ................................................ 21  CHAPTER 4. FINDINGS ............................................................................................................. 23  Introduction ............................................................................................................................... 23  SWCC of Base and Subgrade for Flexible and Rigid Pavements ............................................ 23  Development of ANN Models .............................................................................................. 24  Prediction of SWCC Fitting Parameters Using ANN Model ............................................... 26  Comparison of ANN Model with Other Regression Models ................................................ 29  Validation of the Developed ANN Models........................................................................... 33  Equilibrium Suction of Base and Subgrade for Flexible and Rigid Pavements ....................... 37  GIS-Based Map of TMI ........................................................................................................ 38  Relationship between TMI and Equilibrium Suction ............................................................ 42  Resilient Modulus of Base and Subgrade for Flexible and Rigid Pavements .......................... 44  Prediction of Suction at Test Specimen Water Content ........................................................ 46  Development of ANN Models for MR Model Coefficients .................................................. 48  Prediction of MR Model Coefficients Using ANN Model .................................................... 49  Comparison of ANN Model with Other Regression Models ................................................ 53  Validation of the Developed ANN Models........................................................................... 56  Modulus of Subgrade Reaction for Rigid Pavements ............................................................... 57  Development of Modified k-value Model ............................................................................ 59  Estimation of Modified k-value for LTP Pavement Sections ............................................... 64 

vi Development of ANN Model ................................................................................................ 69  Shear Strength of Base and Subgrade for Flexible and Rigid Pavements ................................ 74  Prediction Model for Unbound Base .................................................................................... 75  Prediction Model for Subgrade ............................................................................................. 76  Permanent Deformation of Base and Subgrade for Flexible and Rigid Pavements ................. 78  Mechanistic-Empirical Permanent Deformation Model for Unbound Materials ................. 78  Regression Models for Permanent Deformation Model Coefficients for Unbound Base Layers ........................................................................................................................... 80  Faulting of Base Layer for Rigid Pavements ............................................................................ 82  Illustration of Development of Faulting ............................................................................... 82  Development of Faulting Model in Jointed Concrete Pavement .......................................... 83  Model Development for Entire Faulting Progression ........................................................... 84  Critical Faulting Depth Model .............................................................................................. 84  Use of LTPP Data for Entire Faulting Progression............................................................... 85  Comparison between Measured and Predicted Faulting ....................................................... 85  Calibration of Model for Entire Faulting Progression .......................................................... 86  Faulting Modeling Based on Permanent Deformation Characterization .............................. 91  Determination of Stress State in the Faulting Model ............................................................ 93  Categorization of Traffic Loads ............................................................................................ 93  Comparison between Measured and Predicted Faulting Based on Permanent Deformation Characterization ............................................................................................... 94  Calibration of Model for Faulting on Deformation Characterization ................................... 94  CHAPTER 5. INTERPRETATIONS, APPRAISAL, AND APPLICATIONS ........................... 97  Introduction ............................................................................................................................... 97  Moisture-Sensitive, Stress-Dependent, and Cross-Anisotropic Resilient Modulus ................. 97  FE Model of Pavement Structures ............................................................................................ 97  Pavement Performance Predicted by the Proposed Model and Pavement ME Design Model ...................................................................................................................................... 101  Fatigue Cracking in Asphalt Mixtures ................................................................................ 101  Permanent Deformation in the Base Layer ......................................................................... 102  Comparisons of Proposed Models and Pavement ME Design Models .................................. 104  Check the Nonlinearity of the Unbound Granular Base ..................................................... 104  Check the Anisotropy of the Unbound Granular Base ....................................................... 105  Check the Nonlinearity of the Subgrade ............................................................................. 106  Comparison of Resilient Modulus Models for Unbound Granular Base ............................ 107  Comparison of Rutting Models in Unbound Granular Base ............................................... 107  Sensitivity Analysis of Proposed Models and Pavement ME Design Models ....................... 108  Different Loading Levels .................................................................................................... 108  Different Thicknesses of the Asphalt Layer ....................................................................... 109  Different Thicknesses of the Base Layer ............................................................................ 109  Different Moisture Conditions of the Base Layer ............................................................... 110  Sensitivity Analysis of Proposed Faulting Models ................................................................. 111  Use of Dowel ...................................................................................................................... 112  Type of Base Layer ............................................................................................................. 113  Thickness of Base Layer ..................................................................................................... 114 

vii Freeze-Thaw Cycles ............................................................................................................ 114  Annual Average Number of Days with Temperature Greater than 32°C ........................... 115  Effect of Climatic Zone ...................................................................................................... 116  Sensitivity Analysis of Proposed Modulus of Subgrade Reaction Model .............................. 116  Sensitivity Analysis of Rigid Pavement Performance to Proposed Models ........................... 121  CHAPTER 6. SUMMARY ......................................................................................................... 126  Conclusions ............................................................................................................................. 126  Soil Water Characteristics Curve of Base and Subgrade for Flexible and Rigid Pavements ........................................................................................................................... 126  Resilient Modulus of Base and Subgrade for Flexible and Rigid Pavements .................... 127  Modulus of Subgrade Reaction for Rigid Pavements ......................................................... 127  Faulting of Base Layer for Rigid Pavements ...................................................................... 128  Prediction of Pavement Performance and Sensitivity Analysis .......................................... 129  Future Work and Recommendations ...................................................................................... 130  REFERENCES ........................................................................................................................... 131  Appendix A. Annotated Bibliography of Influence of Unbound Layers and Subgrade ............. A-1  Appendix B. Definitions of Model Parameters of Unbound Layer and Subgrade Models ........ B-1  Appendix C. Evaluation and Screening of Unbound Layer and Subgrade Models .................... C-1  Appendix D. Moisture-Sensitive, Stress-Dependent, and Cross-Anisotropic Resilient Modulus ............................................................................................................................. D-1  Appendix E. Slab-Base Interface Shear Bonding Model ............................................................. E-1  Appendix F. Sensitivity Analysis of Modulus of Subgrade Reaction Model .............................. F-1  Appendix G. Determination of Stress State in the Faulting Model ............................................ G-1  Appendix H. Categorization of Traffic Loads State in the Faulting Model ............................... H-1  Appendix I. Subgrade Subroutine for Flexible and Rigid Pavements .......................................... I-1  Appendix J. Unbound Base Course Subroutine for Flexible Pavements ..................................... J-1  Appendix K. Unbound Base Course Subroutine for Rigid Pavements ...................................... K-1  Appendix L. Rigid Pavement Structure Model Subroutine ......................................................... L-1  Appendix M. Conversion 2-Layer Model for Rigid Pavements Subroutine ............................. M-1

ix List of Figures and Tables Table 1. Inputs from Unbound Layers and Subgrade in Pavement ME Design. ............................ 4  Table 2. Influential Factors of Unbound Layers on Performance of Flexible Pavements. ............. 6  Table 3. Influential Factors of Unbound Layers on Performance of Rigid Pavements. ................. 7  Table 4. Influential Factors of Subgrade on Performance of Flexible Pavements. ........................ 8  Table 5. Influential Factors of Subgrade on Performance of Rigid Pavements. ............................. 9  Table 6. Modulus Models of Unbound Layers and Subgrade. ..................................................... 11  Table 7. Permanent Deformation Models of Unbound Layers and Subgrade. ............................. 13  Table 8. Shear Strength Models of Unbound Layers and Subgrade. ............................................ 15  Table 9. Erosion Models of Unbound Layers. .............................................................................. 16  Table 10. Foundation Models of Subgrade. .................................................................................. 16  Table 11. Thickness Sensitive Models of Unbound Layers. ........................................................ 17  Figure 1. Illustration of Three-Layered Neural Network Architecture. ........................................ 26  Figure 2. Comparison of Measured versus Predicted SWCC Fitting Parameters Using ANN Model for Plastic Soils. ........................................................................................... 28  Figure 3. Comparison of Measured versus Predicted SWCC Fitting Parameters Using ANN Model for Non-plastic Soils. ................................................................................... 29  Table 12. List of Existing SWCC Fitting Parameter Prediction Models. ..................................... 30  Table 13. Prediction Accuracy of SWCC Fitting Parameter Models. .......................................... 31  Figure 4. Comparison of Measured versus Predicted Suction at Various Saturation Levels for Plastic Soils. ................................................................................................................ 32  Figure 5. Comparison of Measured versus Predicted Suction at Various Saturation Levels for Non-plastic Soils. ........................................................................................................ 33  Figure 6. Validation of Measured versus Predicted SWCC Fitting Parameters Using ANN Model for Plastic Soils. ..................................................................................................... 34  Figure 7. Validation of Measured versus Predicted SWCC Fitting Parameters Using ANN Model for Non-plastic Soils. ............................................................................................. 35  Figure 8. Validation of Measured versus ANN Predicted Suction at Various Saturation Levels for Unbound Materials. ......................................................................................... 36  Table 14. Input Parameters Collected from Literature for Model Validation. ............................. 36  Figure 9. Comparison of Measured versus Predicted SWCC Curves for Unbound Materials. .......................................................................................................................... 37  Figure 10. TMI Distribution in United States (121). .................................................................... 38  Figure 11. Average Annual (a) Precipitation; (b) Mean Temperature GIS Map (1981 to 2010). ................................................................................................................................ 39  Figure 12. Average Annual Potential Evapotranspiration GIS Map (1981 to 2010). ................... 40  Figure 13. GIS Based Contour Map of TMI (1981 to 2010). ....................................................... 41  Figure 14. GIS Based Equilibrium Suction Map. ......................................................................... 42  Figure 15. TMI versus Equilibrium Suction (pF) (cm) for (a) A-1; (b) A-2; (c) A-3; (d) A-4; (e) A-6; and (f) A-7-6 Soil. ....................................................................................... 43  Figure 16. Calculated versus Predicted Equilibrium Suction (pF). .............................................. 44  Figure 17. Comparison of Measured versus Predicted Saturation (%) at 0.1, 0.33, and 15 Bars Suction Level for Unbound Granular Base Materials Using ANN Model. .............. 47  Figure 18. Illustration of Three-Layered Neural Network Architecture (a) Plastic; (b) Non-plastic Soil. ............................................................................................................... 49 

x Figure 19. Predicted MR Model Coefficients of Plastic Base Materials from Physical Properties Using the ANN Approach. .............................................................................. 50  Figure 20. Predicted MR Model Coefficients of Plastic Subgrade Materials from Physical Properties Using the ANN Approach. .............................................................................. 51  Figure 21. Predicted MR Model Coefficients for Non-plastic Base Materials from Physical Properties Using the ANN Approach. ................................................................ 52  Figure 22. Predicted MR Model Coefficients for Non-plastic Subgrade Materials from Physical Properties Using ANN Approach. ...................................................................... 53  Table 15. Prediction Accuracy of SWCC Fitting Parameter Models. .......................................... 54  Figure 23. Comparison of ANN Model Predicted Resilient Moduli against Measured Values for Base Materials. ................................................................................................ 55  Figure 24. Comparison of ANN Model Predicted Resilient Moduli against Measured Values for Subgrade Materials. ......................................................................................... 55  Figure 25. Comparison of Measured versus Predicted Resilient Moduli Using Regression Models............................................................................................................................... 56  Table 16. Input Parameters Collected from Literature for Model Validation. ............................. 57  Figure 26. Validation of Measured versus ANN Predicted MR at Various Stress Levels for Collected Unbound Materials. .......................................................................................... 57  Figure 27. Foundation Models for Rigid Pavement (a) Winkler Model; (b) Pasternak Model. ............................................................................................................................... 58  Figure 28. Flowchart of Corrected Base Modulus due to Cross Anisotropy. ............................... 61  Figure 29. Illustration of Transformed-Section Method for a Cooperated Concrete Slab and Base Course System. .................................................................................................. 62  Table 17. Steps of Moment of Inertia Calculation for a Cooperated Slab and Base System. .............................................................................................................................. 62  Figure 30. Illustration of In-situ Shear Stress in the Base Course on the PCC-Base Interface Using a Mohr-Coulomb Failure Envelope. ....................................................... 63  Table 18. Collected MR Coefficients and the Simulated Stress Values at the Mid Depth of Base Layer for LTPP Section 27-4054. ............................................................................ 65  Figure 31. Base Resilient Modulus Convergence with Iteration Number. ................................... 65  Figure 32. Formulation of Friction Angle from Mohr Coulomb Failure Envelope for (a) Treated Base; (b) Unbound Base. ..................................................................................... 66  Figure 33. Comparison of Calculated Slab-Base Interface Degree of Bonding Ratio with the BBF Approach for (a) Treated Base; and (b) Unbound Base Layer. .......................... 68  Figure 34. Sensitivity of Slab-Base Degree of Bonding on Wheelpath Fault (mm). ................... 68  Figure 35. Comparison of Modified versus LTPP k-values. ........................................................ 69  Figure 36. (a) Schematic Plot of a Typical Pavement Structure; (b) Axisymmetric Model of Pavement in ABAQUS. ................................................................................................ 71  Table 19. Selected Range of Input Parameters in ANN Training Data Set. ................................. 72  Figure 37. Illustration of Three-Layered Neural Network Architecture for k-values. ................. 72  Figure 38. Target and Output k-values for Training, Validation, and Overall Data Sets for 1296 Simulation Cases. ..................................................................................................... 73  Figure 39. Comparison of Calculated versus Predicted Modified k-values. ................................ 73  Figure 40. Modified k-values at 0, 0.3, 0.6, and 1 Degree of Bonding for Selected LTPP Pavement Sections. ........................................................................................................... 74 

xi Figure 41. Schematic Plot of Mohr’s Circle Showing Dependence of Shear Strength on Matric Suction. .................................................................................................................. 75  Figure 42. Comparison of Predicted and Measured Shear Strength Model Parameters (a) c’ and (b) φ’. ..................................................................................................................... 76  Figure 43. Illustration of Three-Layer Neural Network Architecture to Predict c’ Parameter. ......................................................................................................................... 77  Figure 44. Target and Output c’ Values for Training, Validation, and Overall Data Sets for 432 Subgrade Soils. ..................................................................................................... 78  Figure 45. Comparison of Predicted and Measured Permanent Deformation Model Parameters (a) 0 , (b)  , (c)  , (d) m, and (e) n. ........................................................... 81 Figure 46. Schematically Illustration of the Development of Faulting. ....................................... 83  Figure 47. Illustration of Field Faulting Data, including the Critical Faulting Depth. ................. 84  Figure 48. Comparison between Measured and Predicted Faulting Depth. ................................. 86  Table 20. Results of Multiple Regression Analysis for Coefficients in the First Faulting Model. ............................................................................................................................... 88  Figure 49. Comparison between Measured and Predicted Coefficients in Faulting Model. ........ 89  Figure 50. Comparison between Measured and Predicted Faulting at Inflection Point. .............. 90  Table 21. Results of Multiple Regression Analysis for Faulting at Inflection Point. ................... 90  Figure 51. Mean Critical Faulting Depth with or without Dowels. .............................................. 91  Figure 52. Plastic Deformation Curve before the Inflection Point. .............................................. 92  Figure 53. Comparison between Measured versus Predicted Faulting before Critical Depth. ................................................................................................................................ 94  Table 22. Results of Multiple Regression Analysis for Coefficients in the Load-Related Faulting Model. ................................................................................................................. 95  Figure 54. Comparison between Measured and Predicted Coefficients in the Permanent Deformation Faulting Model. ........................................................................................... 96  Figure 55. The 2D Axisymmetric Model Used in FE Analysis. ................................................... 98  Table 23. Material Properties of Pavement Layers. ...................................................................... 98  Table 24. Example of Base Material Information. ....................................................................... 99  Table 25. Material Parameters of Pavement Layers. .................................................................. 101  Figure 56. (a) Tensile Strain at the Bottom of the Surface and (b) Average Compressive Strain in the Centerline of the Base under Different Loading Levels. ............................ 104  Figure 57. Vertical Modulus Contours in the Base Layer under the Loading Level (a) 201 kPa, (b) 566 kPa, (c) 756 kPa, and (d) 1006 kPa (Unit: MPa)................................. 105  Figure 58. Vertical Modulus Contours in the Base Layer at (a) Dry Condition, (b) Medium Condition, and (c) Wet Condition (unit: MPa). ................................................ 105  Figure 59. (a) Tensile Strain at the Bottom of the Surface; (b) Compressive Strain at the top of the Subgrade the Pavement with Different Anisotropy (1/n) of Base Layers. ..... 106  Figure 60. Vertical Modulus Contours in the Subgrade at Different Loading Levels. ............... 106  Table 26. Base Material Information for Rut Depth Calculation. .............................................. 107  Figure 61. Rut Depth in the Base Layer Using Different Models. ............................................. 107  Figure 62. Pavement Performance Including (a) Load Repetitions to the Fatigue Cracking Failure; (b) Rut Depth in the Base at Different Loading Levels. .................................... 108  Figure 63. Pavement Performance Including (a) Load Repetitions to the Fatigue Cracking Failure; (b) Rut Depth in the Base at Different Thickness of the Asphalt Layer. .......... 109 

xii Figure 64. Pavement Performance Including (a) Load Repetitions to the Fatigue Cracking Failure and (b) Rut Depth in the Base at Different Thickness of the Base Layer. ......... 110  Figure 65. Pavement Performance Including (a) Load Repetitions to the Fatigue Cracking Failure and (b) Rut Depth in the Base at Different Moisture Conditions of the Base Layer. ..................................................................................................................... 111  Figure 66. Pavement Structure for LTPP Section 1-3028. ......................................................... 112  Table 27. Basic Information of LTPP Section 1-3028 for Sensitivity Analysis. ........................ 112  Figure 67. Effect of Use of Dowels on Faulting Based on (a) Full Faulting Model and (b) Load-Related Faulting Model. ........................................................................................ 113  Figure 68. Effect of Types of Base Layer on Faulting Based on (a) Full Faulting Model and (b) Load-Related Faulting Model. ............................................................................ 113  Figure 69. Effect of Thickness of Base Layer on Faulting Based on (a) Full Faulting Model and (b) Load-Related Faulting Model. ................................................................ 114  Figure 70. Effect of Freeze-Thaw Cycles on Faulting Based on (a) Full Faulting Model and (b) Load-Related Faulting Model. ............................................................................ 115  Figure 71. Effect of Number of Days with Temperature Greater than 32°C on Faulting Based on (a) Full Faulting Model and (b) Load-Related Faulting Model. ..................... 115  Figure 72. Effect of Climatic Zone on Faulting Based on (a) First Faulting Model and (b) Second Faulting Model. .................................................................................................. 116  Table 28. Selected LTPP Pavement Sections and FWD Backcalculated Modulus Values for Each Layer. ................................................................................................................ 117  Table 29. Calculated MR Values at the Mid-depth of Base Layer at Different Moisture Conditions. ...................................................................................................................... 118  Figure 73. Sensitivity of Degree of Bonding on Subgrade k-value Using (a) ANN Model and (b) Pavement ME Design Model. ............................................................................. 119  Figure 74. Sensitivity of Moisture on Subgrade k-value Using (a) ANN Model and (b) Pavement ME Design Model. ......................................................................................... 120  Figure 75. PCC Slab-Base Interface Bond Sensitivity on (a) Tensile Stress at Top of Slab; (b) Tensile Stress at Bottom of Slab; and (c) Deferential Deflection on Transverse Joints. .............................................................................................................................. 123  Figure 76. Base Layer Moisture Sensitivity on (a) Tensile Stress at Top of Slab; (b) Tensile Stress at Bottom of Slab; and (c) Deferential Deflection on Transverse Joints. .............................................................................................................................. 125 

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The performance of flexible and rigid pavements is known to be closely related to properties of the base, subbase, and/or subgrade. However, some recent research studies indicate that the performance predicted by this methodology shows a low sensitivity to the properties of underlying layers and does not always reflect the extent of the anticipated effect, so the procedures contained in the American Association of State Highway and Transportation Officials’ (AASHTO’s) design guidance need to be evaluated.

NCHRP Web-Only Document 264: Proposed Enhancements to Pavement ME Design: Improved Consideration of the Influence of Subgrade and Unbound Layers on Pavement Performance proposes and develops enhancements to AASHTO's Pavement ME Design procedures for both flexible and rigid pavements, which will better reflect the influence of subgrade and unbound layers (properties and thicknesses) on the pavement performance.

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