Seasonal and Spatial Variation of Subgrade Response

Temperature, subgrade moisture content, and Falling Weight Deflectometer (FWD)-deflection data were collected monthly on four asphalt pavement test sections in Kansas for a year. The subgrade moduli were backcalculated using the elastic layer theory. It was found that for almost all sites, the monthly variation in subgrade moisture content was not very significant over the seasons. The patterns of subgrade response, in terms of subgrade moduli versus time, simulated sine-shaped forms signifying a possible temperature effect. Higher variabilities across the site were associated with the extreme temperature conditions, usually very low or high average pavement temperatures. In all cases, the measured precipitation was nominal thereby excluding this climatic variable as a major factor. Extreme test temperatures, both high and low, result in higher variation of measured deflections and subsequently, backcalculated subgrade moduli across a site. Thus, some variabilities in backcalculated subgrade moduli can be minimized by conducting FWD tests in a moderate temperature regime. The analysis of variance (ANOVA) indicated that both seasonal and site variabilities can be significant. After correction for temperature, variations in deflections and moduli become approximately equal to the site variabilities which was also confirmed by ANOVA.