Book Description

The American Association of State Highway and Transportation Officials (AASHTO) adopted the Mechanistic-Empirical Pavement Design Guide (MEPDG) as an interim pavement design standard in 2008. In 2015, the Nevada Department of Transportation (NDOT) implemented the MEPDG for the structural design of new and rehabilitated flexible pavements. Resilient modulus of the unbound material 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 evaluated the impact of SWCC and saturated hydraulic conductivity on Nevada Pavement ME Design. An extensive laboratory evaluation was conducted on 23 Nevada unbound materials, which included testing for gradation, Atterberg limits, maximum dry density, optimum water content, specific gravity of solids, SWCC, methylene blue value (MBV), percent fines content (PFC), 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 (ver. 2.3.1), 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 on AC bottom-up fatigue cracking. 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 designs.