How are seasonal effects taken into account with the FWD?
The back analysis of a deflection bowl provides results for the specific moisture condition at the time of testing. Seasonal variations in moduli must therefore be considered prior to calculation of residual life and overlay requirements. Software packages vary in the way seasonal effects are incorporated. One option is to increase deflections by a multiplier in the range of 1.1 to 1.6 if measurements are not carried out during a wet period. Another approach is to assume an annual sinusoidal variation in moduli between a maximum and minimum value (usually, the subgrade modulus alone would be varied but the factor could be applied to all unbound layers, with similar end result). In a long term study of deflection changes with season in Australia, Rallings & Chowdhury (1995) found a generally sinusoidal variation in peak deflection each year, and concluded that a seasonal adjustment factor of 1.1 would be appropriate for defection measurements made between mid-summer and the end of autum
