Evaluation of Proposed Backcalculation Procedure in Indonesia Overlay Design Guide

Indonesia Directorate General of Highways’ overlay design guide Pd T-1-2002-B was developed by adopting the design procedure from 1993 AASHTO Design Guide. One evaluation in the guide was an evaluation of subgrade condition, in terms of elastic moduli, using backcalculation procedure from field deflection data. One main drawback of the backcalculation procedure in the guide is the use of adjustment factor (C) to make the calculated moduli conform to the measured ones. In addition, the guide also did not provide the evaluation of remained capacity of surface layer that could be the important key for deciding the overall condition of the existing pavement. In this paper, a backcalculation procedure in the guide for determining remaining capacity of surface layer was proposed. For the sake of comparison, another backcalculation procedure based on best-fit trial-and-error approach was employed, and the results of both algorithm procedures were validated using measured pavement properties pavement properties from Long-Term Pavement Performance (LTPP) database. The results of the study indicated that proposed procedure failed to produce the expected results. Besides, the high deviation between calculated moduli (from both backcalculation procedure) and measured ones leads to a recommendation of developing a better backcalculation procedure.