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

The study consisted of two phases: a field investigation phaseand a laboratory testing program. The field phase involved instrumentingan in-service pavement section in Alberta with thermal conductivitysensors to measure temperature and soil suction. This data coupledwith Falling Weight Deflectometer deflection tests performed withinthe instrumented section were used to accomplish the researchgoals.

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

Both field and laboratory phases produced novel and practicalapproaches for quantifying the influence of seasonal variationson subgrade strength.