Pavement structural behaviour during spring thaw: interpretation of FWD measurements by monitoring environmental data from county road 126 at Torpsbruk

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Farhad Salour

The present research project has been carried out based on the environmental data and Falling Weight Deflectometer (FWD) measurements from the county road 126 in southern Sweden during the year 2010. The county road 126 has a relatively thin flexible pavement structure with unbound aggregate base and subbase layers. The major intention of this study was to investigate the behaviour of the pavement structure during spring thaw. Temperature and moisture content of the pavement structure profile were continuously monitored throughout the year 2010. Layer moduli backcalculation as well as deflection basin analyses were performed using the FWD measurement data. A comprehensive study on the effect of environmental factor variations and pavement structural capacity were carried out during the spring thaw and the recovery period. The result showed a considerable decrease in the bearing capacity of the pavement structure during the spring thaw period when the highest annual moisture content was also registered. Both deflection basin indices and backcalculated layer modulus indicated that the pavement was weakest during the subgrade thawing phase. Backcalculation on the FWD measurements showed a 67% loss in stiffness of the subgrade soil and 43% in the granular base and subbase course during the spring thaw compared to the summer values. In addition, the compatibility of the analysis with a predictive moduli-moisture content model for unbound materials was studied. The measured field data from the test road pavement in Torpsbruk showed promising agreement with the resilient modulus predictive model, both for the granular layer and subgrade material. Similar models could be developed or calibrated for other soils and granular materials if sufficient data become available in the future.

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