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Evaluation of the Impact of Soil Water Characteristic Curves (SWCC) on Nevada Pavement ME Design
AuthorStolte, Sarah Elizabeth
AdvisorHajj, Elie Y.
Civil and Environmental Engineering
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The Mechanistic Empirical Pavement Design Guide (MEPDG) was developed by the American Association of State Highway and Transportation Officials (AASHTO), in cooperation with the Federal Highway Administration (FHWA), as part of the National Cooperative Highway Research Program (NCHRP), project 1-37A (1). The purpose of this work was to provide a state-of-practice tool that can be used to design new and rehabilitated pavement, and it relies on mechanistic-empirical principles. The mechanistic-empirical design method differs from previous methods by taking into consideration traffic conditions, climatic data, and material properties. Resilient modulus of the unbound layers plays a large role in pavement performance, and this parameter changes with seasonal variation. Currently, the AASHTOWare® Pavement ME software internally calculates this seasonal variation using climatic data and estimated unbound material parameters, including soil water characteristic curves (SWCC) and saturated hydraulic conductivity. This study seeks to evaluate the impact of SWCC and saturated hydraulic conductivity on Nevada Pavement ME Design. An extensive laboratory evaluation was conducted on 24Nevada unbound materials, which included testing for gradation, Atterberg Limits, maximum dry density, optimum water content, specific gravity of solids, SWCC, methylene blue value, percent fines content, r-value, and saturated hydraulic conductivity. The model used to fit the SWCCs, consistent with the MEPDG, was the Fredlund and Xing model, which fit the SWCC data well. A sensitivity analysis was conducted in AASHTOWare® Pavement ME, where directly measured unbound material properties, estimated unbound material properties, and internally estimated unbound material properties were used. It was found that for District 1, the internally estimated properties underestimate the impact of SWCC and saturated hydraulic conductivity, which is seen in an under-prediction of AC bottom-up fatigue cracking. For District 2, the internally estimated properties overestimate the impact of SWCC and saturated hydraulic conductivity, which is seen in an over-prediction of AC bottom-up fatigue cracking. In District 3, there was little impact from SWCC and saturated hydraulic conductivity. Additionally, historical records were collected from recent NDOT pavement projects and summarized in an electronic format. Combined with the laboratory evaluation, a comprehensive database for Nevada unbound material properties was produced. This database was used to make recommendations for unbound material properties for use in Nevada Pavement ME Design.