EFFECT OF DEPTH TO BEDROCK ON DEFLECTION BASINS OBTAINED WITH DYNAFLECT AND FALLING WEIGHT DEFLECTOMETER TESTS

The dynaflect and falling weight deflectometer (FWD) are commonly used for nondestructive testing of pavements. In both cases a dynamic load is imparted on the surface of the pavement, and deflections are measured at various points along the surface. Evaluation of the moduli of the surface layer, base, and subgrade is normally performed by comparing the experimental deflections with the results of static analyses. The moduli of the layers in the static model are then varied in an iterative procedure until a reasonable match between experimental and theoretical deflections is obtained. This solution ignores the dynamic nature of these nondestructive tests. In this paper the effect of depth to bedrock on the amplitude of the deflections and the shape of the deflection basins obtained with the dynaflect and the FWD tests is investigated analytically. Dynamic and static deflections at four different pavement profiles are compared. The results show that the range of bedrock depths over which dynamic effects are important differs between the two nondestructive tests because of the excitation frequencies and depends mainly on the stiffness of the subgrade. The results also show that, when dynamic effects occur in the measurements but are not taken into account in the analysis, the modulus of the subgrade is generally underestimated, sometimes by 50% or more, and the moduli of the base and surface layer are overestimated. Finally, a simple method is suggested for the FWD that makes it possible to estimate the depth to bedrock by recording the free vibration of the pavement system.