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Suggested Citation:"CHAPTER 1. INTRODUCTION." 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:"CHAPTER 1. INTRODUCTION." 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.

1 CHAPTER 1. INTRODUCTION INTRODUCTION The American Association of Transportation Officials’ (AASHTO’s) pavement mechanistic-empirical (ME) design software, AASHTOWare Pavement ME Design, and the AASHTO Mechanistic-Empirical Pavement Design Guide Manual of Practice (MEPDG) provide a methodology for the analysis and performance prediction of pavements and overlays. 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 indicated 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 Pavement ME Design need to be evaluated. In addition, proper enhancements should be developed to appropriately account for the influence of subgrade and unbound layers on the performance of flexible and rigid pavements. These enhancements will be incorporated into the Pavement ME Design procedures, which improve the analysis and design of flexible and rigid pavements. OBJECTIVE This research project proposes and develops enhancements to the 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. These enhancements include modifications of the models contained in Pavement ME Design and/or the development of new models. RESEARCH SCOPE AND APPROACH The project required the development and synthesis of several components: (a) evaluation and screening of unbound layer and subgrade models; (b) development of soil water characteristics curve models of base and subgrade; (c) development of resilient modulus (MR) models of base and subgrade; (d) development of modulus of subgrade reaction model; (e) development of faulting model of base layers; and (f) conduction of performance prediction and sensitivity analysis. ORGANIZATION OF THE REPORT This report is organized into six chapters:  The first chapter presents the introduction and research approach used in this research project.  The second chapter presents a synthesis of current knowledge of characterizing and modeling subgrade and base layers to quantify their influence on the performance of flexible and rigid pavements.  The third chapter presents the research plan of this project.  The fourth chapter presents the major findings with respect to the different models proposed in this project, including the model development, model calibration, and validation.

2  The fifth chapter presents interpretation, appraisal, and application of the findings discussed in the fourth chapter. More specifically, the sensitivity analysis of the proposed models will be presented. The pavement performance predicted by the proposed models will be compared with those in the current version of Pavement ME Design.  The sixth chapter presents conclusions and suggested further research. The main body of the report overviews the approach and results of this project. More detailed discussions of the topics in the report are contained in Appendices A through M.

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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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