DEVELOPMENT OF A BACKCALCULATION PROGRAM AND ITS VERIFICATION . THIRD INTERNATIONAL CONFERENCE ON BEARING CAPACITY OF ROADS AND AIRFIELDS. PROCEEDINGS, NORWEGIAN INSTITUTE OF TECHNOLOGY, TRONDHEIM, NORWAY, JULY 3-5 1990. VOLUMES 1-2

A newly developed backcalculation program (BOUSDEF) for determining the structural properties of existing pavements from nondestructive test data is described in this paper. This program uses the method of equivalent thicknesses and Boussinesq theory as its basis for calculating the surface deflections. An iterative procedure is used to backcalculate pavement layer moduli. The program also considers the overburden stresses as well as nonlinearity of base and subgrade materials. Verification of the program was performed using three approaches: (1) comparing the backcalculated pavement layer moduli with hypothesized theoretical moduli, (2) comparing backcalculated layer moduli with results from other developed backcalculation programs, and (3) comparing backcalculated results with those from laboratory tests. The verification indicated that the moduli backcalculated using the BOUSDEF program compare very well with the theoretical moduli and also are very comparable with results from other developed programs. Comparison between backcalculated and laboratory tested moduli was performed on actual pavements structure. The comparison show that for asphalt concrete surface layer, some differences in moduli exist. However, the relationship of backcalculated moduli versus stress for the base layer appears to be compatible with those determined in the laboratory tests. The BOUSDEF program runs on IBM compatible microcomputers. Because of its extremely fast computing speed, the program can be used as an efficient tool for initial evaluations of deflection test data to determine pavement layer moduli. (TRRL)