Long-Term Moisture Conditions under Highway Pavements

Equilibrium moisture beneath highway pavements is critical to pavement design because moisture directly affects the strength and stiffness of pavement systems. Moisture is related to soil suction by means of the soil-water characteristic curve (SWCC). Previous research has indicated a correlation of suction with Thornthwaite Moisture Index (TMI) and soil type; however, these suction correlations exhibited large variability. Under an NCHRP project sponsored by the Federal Highway Administration, soil samples were collected from beneath thirty pavement sections throughout the United States: two from the WesTrack test facility, one from the MnRoad Project, and twenty-seven from the Long Term Pavement Performance sites. SWCCs and index properties were measured on collected samples at Arizona State University. The in-situ degree of saturation was obtained from soil index properties, dry unit weight, and moisture content, and the corresponding in-situ soil suction was obtained from SWCCs. Based on the field and laboratory data, an algorithm was developed to predict suction under the pavement using TMI, percent passing 200 (P₂₀₀), and Plasticity Index. The suction prediction models, named the TMI-P₂₀₀ model and the TMI-P₂₀₀/wPI model, showed good results with variability within acceptable limits.