Abstract
A simple and direct method for backcalculation of the dynamic subgrade stiffness and damping coefficients from falling weight deflectometer (FWD) deflection basins is presented. The method can also be used to detect a stiff layer underneath the pavement system. The method consists first of decomposition of the transient deflection signal of each sensor into a series of harmonic motions by the fast Fourier transform algorithm. Then, for each frequency of interest, the real and imaginary components of the displaced volume underneath the slab are calculated from the complex deflection basin. The dynamic force-displacement relationship is decomposed into real and imaginary parts, leading to a simple system of equations that can easily be solved for the coefficient of the subgrade reaction (k) and the radiation damping coefficient (c). The method was verified by using FWD test data from six different sites in Michigan. In addition, the backcalculated k-values were compared with those obtained by the AASHTO-recommended (static) backcalculation method. The results obtained by the new procedure showed very good agreement and consistency with those obtained by the AASHTO method.
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