Evaluation of Seasonal Variability in Cohesive Subgrades Using Backcalculation

Abstract

Seasonal variation in pavement strength is an important concern for engineers and transportation authorities. In 1990, an investigation was initiated that aimed at quantifying seasonal variations within a typical cohesive subgrade soil in Alberta. The study consisted of two phases: a field investigation phase and a laboratory testing program. The field phase involved instrumenting a pavement section that is representative of the primary highway system in Alberta with thermal conductivity suction sensors. Falling weight deflectometer (FWD) deflection tests were conducted at regular time intervals over a period of 2 years and at various locations within this section. Effective pavement layer moduli were backcalculated using measured deflections, and the results were plotted versus time to quantify seasonal changes occurring in pavement structural strength. Only the research findings obtained from the FWD deflection tests that deal primarily with subgrade moduli are discussed. Remaining field data represent a valuable resource to researchers investigating other pavement structural factors. Use of the backcalculation approach for quantifying seasonal variations in subgrade stiffness seems promising. Following a significant reduction in subgrade stiffness upon thawing, a long period of relatively constant strength prevails. After that a period of gradual recovery and subsequent increase in strength is experienced as freezing approaches. Backcalculated subgrade fill moduli were found to be 65 percent greater than their counterparts in cut areas. Furthermore, the subgrade fill moduli were observed to experience less seasonal fluctuation than the cut moduli. Finally, the backcalculated subgrade moduli were found to be insensitive to asphalt concrete surface temperature influences.