Evaluation of backcalculation layer moduli on three-layer flexible pavement system

In 1943, Burmister introduced an analytical solution to overcome a practical problem in designing an economical pavement with sufficient thickness over a subgrade to provide adequate support for traffic load. Using this solution, a backcalculation algorithm can be developed to obtain the layer elastic moduli. Making use of hypothetical data, this paper evaluates the choice of the optimum configuration of deflection measurements for backcalculating the layer elastic moduli of a flexible pavement system consisted of three layers, i.e. surface layer, subbase layer and subgrade. Two computer programs (FPAVE3 based on closed-form backcalculation algorithm; and EVERCALC based on iterative trial and error backcalculation algorithm) were evaluated based on two types of deflections, exact and random deflections. The assessment of the optimum deflection selection was conducted by comparing the discrepancies between the computed elastic moduli against their true moduli and analyzing the dispersion of the computed moduli calculated based on random deflection. The results showed that the deflection sensors near the load could better estimate the layer moduli, especially for elastic moduli of surface and subbase layer. On the other hand, the estimation of elastic modulus of subgrade is not much dependent on the choice of deflection sensor configuration.