Stressdependency of the (sand)subgrade modulus by the dead weight of the pavement layers : fieldexperiments with the falling weight deflectometer

In situ deflection measurements can be used for the backcalculation of the moduli of the distinguished pavement layers. 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. However, measurements on pavement sections (with a bound base), where the wearing course and base course had been removed carefully, showed a significant decrease in the subgrade modulus. The only explanation for this decrease is a stress dependency of 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. 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.