AbstractThe aim of this study is to investigate the influence of interface conditions on a backcalculated layer property for a flexible pavement with different base types. The simple friction model was utilized in the finite-element method (FEM) to simulate the interface condition, and four interface conditions were taken into account in analyses. The computed deflection time history indicated that the inadequate interface condition between the surface and base lead to the higher deflection peak for the pavement with a cement-treated base (CTB) and granular base (GB), while the interface condition between the base and subgrade has a significant influence on the deflection only for the pavement with a GB. The deflection time-history was backcalculated through a dynamic approach using the spectral element method (SEM) as a forward analysis engine in which the layers were treated as fully bonded. The results show that the interface condition shows an apparent influence on the backcalculated layer property. Due to the different contributions of each layer to the overall loading carrying capacity, the interface condition influence was different for different pavement scenarios, which should be appropriately considered for backcalculation and pavement evaluation.