Falling Weight Deflectometer Interpretation of Pavement Behaviour during Spring Thaw

Farhad Salour

A rural county road consisting of a thin flexible pavement structure in southern Sweden was instrumented in year 2009 in order to monitor changes in pavement characteristics caused by seasonal variations in environmental factors. Subsurface pavement temperatures, moisture phase and moisture contents were constantly measured throughout the study with main focus on the spring thaw and recovery periods. The pavement structural response was surveyed by conducting Falling Weight Deflectometer (FWD) measurements during these periods. Changes in pavement layers moduli were estimated by performing backcalculations on the deflection data using the Evercalc computer program. Furthermore, deflection basin indices were analysed using the FWD data. Pavement stiffness decreased significantly when the base and the subgrade began to thaw. Changes in backcalculated layers moduli showed clear correlation with layers moisture content measurements. The field data showed a considerable decrease in the bearing capacity of the pavement structure when the highest annual moisture in subgrade was also registered. Both deflection basin indices and backcalculated layers moduli indicated that the pavement was weakest when the subgrade completely thawed. Thereafter, the pavement gradually regained its stiffness as the excess water drained out from the layers. Complete recovery of the pavement took more than one month. Backcalculations of the FWD data showed 63 percent loss in the subgrade modulus and 48 percent loss in granular base and subbase moduli respectively during spring thaw compared to the summer values.

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