Investigation of a pavement structural behaviour during spring thaw using falling weight deflectometer

The response of an instrumented flexible pavement structure in southern Sweden, subjected to seasonal freezethaw cycles, was investigated during the spring thaw and the recovery periods in 2010. The pavement environmental condition was continuously monitored by measuring subsurface temperature and moisture contents. The mechanical behaviour of the pavement structure was investigated by conducting frequent falling weight deflectometer (FWD) measurements throughout the study period. Considerable decrease in the pavement-bearing capacity was observed as thaw penetrated the pavement structure and consequently moisture content increased in all pavement unbound layers. Highest annual moisture content in the subgrade upper section was registered as thaw penetrated the subgrade and pavement exhibited its weakest structural condition. The analyses of the deflection basin indices and back-calculated unbound layer stiffness from the FWD measurements exhibited a clear correlation with the measured subsurface moisture content. Furthermore, the dissipated energy measured from the FWD timehistory data was calculated, exhibiting its highest annual value during the thawing period. Using the back-calculated layer stiffness and moisture measurements in unbound layers, a degree of saturation-based moisture-stiffness model was developed for the granular layer and the subgrade. This model fell on a unique curve showing promising agreement with the laboratory-based model proposed by Mechanistic-Empirical Pavement Design Guide that analytically predicts changes in modulus due to changes in moisture.