In 1990, an investigation was initiated to measure seasonal variations in subgrade stiffness of cohesive soils in Alberta. The main objective of this study was to identify and select suitable strength indices for measuring seasonal variations in pavement response.

The study consisted of two phases: a field investigation phase and a laboratory testing program. The field phase involved instrumenting an in-service pavement section in Alberta with thermal conductivity sensors to measure temperature and soil suction. This data coupled with Falling Weight Deflectometer deflection tests performed within the instrumented section were used to accomplish the research goals.

The laboratory program consisted of extensive repeated load testing on remolded samples of the subgrade material taken from the instrumented site. Results obtained from these tests were used to develop predictive resilient moduli relationships.

Both field and laboratory phases produced novel and practical approaches for quantifying the influence of seasonal variations on subgrade strength.