Mechanistic Evaluation of the Depth to Bedrock and its Effect on the Response of 3D FE Pavement Models

The results from static load-based backcalculation programs indicate sensitivity to the assumed depth to bedrock. In this study, a mechanistic method has been developed for estimating the apparent depth to the stiff layer using the Falling Weight Deflectometer (FWD) data. This predicted depth can be used in theoretical pavement models to assure that the fixed model boundary has an insignificant effect on the estimated surface deflection. The approach is based on utilizing Three-Dimensional Finite Element (3D FE) to simulate pavement response to the FWD impact load. This approach accounts for the dynamic nature of the FWD load, friction at the layers' interfaces, and the 3D geometrical characteristics of pavement structures. After adjusting the model parameters so that the 3D FE-generated deflection basin matches the experimentally measured one, the decay of displacements and stresses in the subgrade layer thickness is investigated along the axis of the FWD load application. The apparent depth to bedrock is defined as the depth at which the subgrade stress decays to approximately 10 percent of its value measured on the subgrade surface (Analogous with the settlement calculation of shallow foundation). Investigation of the effect of an assumed subgrade layer thickness on the 3D FE pavement model response to the short duration FWD-impact load, reveals that as the subgrade layer thickness is increased from 1.0 m (3.3 ft) to 3.66 m (12.0 ft), the change in surface deflections is less than 3 percent. Thus, for the objective of minimizing computation time and memory requirements of 3D FE pavement models, a subgrade layer thickness of around 1.83m (6 ft) will be sufficient to produce satisfactory results.