Comparative Study of Static and Dynamic Falling Weight Deflectometer Back-Calculations Using Probabilistic Approach

The falling weight deflectometer (FWD) test is the most widely accepted and used technique for nondestructive evaluation of pavements. In this test, the pavement response to the impulse load of a falling weight is analyzed or back-calculated to evaluate the in situ pavement layer moduli. FWD back-calculation is mathematically an inverse problem that can be approached deterministically or probabilistically. In this paper, the probabilistic approach to FWD back-calculation is used as a tool to investigate the reliability of different dynamic and static back-calculation procedures. The probabilistic approach to FWD back-calculation, a relatively new approach, is initially introduced and a brief overview of its numerical implementation is presented. The approach is then applied to back-calculate moduli from a set of synthetic FWD data using different back-calculation procedures, such as static and dynamic deflection bowl or dynamic time history procedures. Based on the probabilistic solution for back-calculated moduli from each procedure, the reliability of each procedure is then compared to the others. It is shown that, although widely used, the static FWD back-calculation procedures fail to capture the essential dynamic nature of the test and consequently cannot be relied upon to correctly back-calculate the layer moduli. It is also demonstrated that there is little redundancy in the dynamic deflection bowl back-calculation procedures, potentially producing relatively large uncertainties in the obtained back-calculation results. Finally, it is shown that the dynamic deflection time history back-calculation is the most reliable approach for FWD back-calculation. Using the back-calculated moduli from different procedures, effect of different back-calculation approaches on the pavement life is illustrated.