In the Netherlands nearly all pavements for motorways and highways can be modelled into a three-layer system: 1. A wearing and base course of asphalt concrete. 2. A Base of (un)bound material. 3. A subbase and subgrade of sand.

In situ deflection measurements can be used for the backcalculation of the moduli of the separate pavement layers including the subgrade. Sand is an unbound granular material, and therefore its modulus ought to be stress dependent. This dependency is rarely noticed when falling weight deflection (FWD) measurements are carried out on a pavement at different force levels. Also, within the measurement on the asphalt surfacing (I) of the sections at issue, no changes were obtained in the backcalculated subgrade moduli when the applied FWD load was varied. However, when the wearing course and base course had been removed carefully, a significant decrease was noticed in the backcalculated subgrade moduli from the FWD measurement carried out on the bound base (II) compared to the values gained from the measurement on the asphalt surfacing (I). But when the load was varied again no significant changes were observed in the backcalculated moduli. The decrease in the subgrade moduli of Measurement I and Measurement II can be explained by assuming a stress dependency for the subgrade modulus. The difference in the subgrade modulus found in both measurements is caused by the difference in the dead weight of the overlaying construction layers. The problem is that this stress dependency will not be found when the load is varied and a backcalculation procedure is used which is based on curve fitting.

An explanation is given why this stress dependency of the subgrade modulus will be overlooked in the backcalculation procedure of FWD measurements at different force levels. Also, a backcalculation procedure is proposed, using a simple model for the stress-dependent character of the subgrade modulus, in which the road can still be considered as a linear elastic multilayer system.