Frequency and time domain backcalculation of flexible pavement layer parameters

In this study, new algorithms method for backcalculating flexible pavement layer parameters based on dynamic interpretation of FWD deflection time histories using frequency and time-domain solutions have been developed. The backcalculation procedure is based on the modified Newton-Raphson method originally adopted in the MICHBACK program. Singular value decomposition (SVD), in conjunction with scaling techniques is employed in solving for the inverse problem. The frequency-domain method uses real and imaginary deflection basins as the measured quantities, while the time-domain method uses either the peak deflections and corresponding time lags or traces of the deflection time histories as the measured quantities to be matched by the backcalculation procedure.; The new associated program called DYNABACK has been written in the FORTRAN 77 language, and offers two options: (i) frequency-domain analysis, and (ii) time-domain analysis. The new program has been incorporated into the Windows(TM) based MFPDS program, which allows for user-friendly features including interactive input and output screens, and the ability to view and process the deflection data before analyzing it.; The new program was theoretically verified using synthetic data. Numerical examples show that the proposed methods are able to backcalculate layer moduli and thicknesses accurately from synthetically generated FWD data. The applicability of the new program to interpret field tests was evaluated using measured deflection time history data from several FWD tests conducted in Michigan and elsewhere. The analyses included the comparison of backcalculated layer moduli and damping ratios with MICHBACK results for various pavement sections and load levels. The backcalculation was done in both frequency and time domains, where the time-domain solution included backcalculating layer moduli and thicknesses. The data were obtained from tests involving KUAB and Dynatest FWD machines. Most pavement sections were analyzed as three- and four-layer systems with some sections involving a stiff layer at shallow depth. The results indicate that dynamic backcalculation of layer parameters using field data presents some serious challenges. The frequency-domain method can lead to large errors if the measured FWD records are truncated before the motions fully decay in time, and the time-domain method when simultaneously backcalculating layer moduli and thicknesses produces mixed results.