Effect of stress dependency and anisotropy of pavement materials on TSD deflection slopes

The Traffic Speed Deflectometer (TSD) is a moving vehicle equipped with Doppler laser sensors designed for measuring pavement surface deflection slopes and performing pavement management tasks, including layer moduli backcalculation. Many studies have focused on backcalculating pavement layer moduli using TSD deflection slopes. However, a common limitation in prior studies is lack of considering the nonlinear stress-dependency behaviour of the unbound granular materials and fine-grained soil layers. Instead, these studies have mainly assumed linear elastic moduli for aforementioned layers in their backcalculation procedures.

This study investigates the influence of the stress dependency and anisotropy of the unbound granular materials and fine-grained soil layers on  the deflection slopes measured by the TSD using the finite element software ABAQUS. 2D-axisymmetric pavement systems subjected to a uniform tyre loading are simulated, and numerical analyses yield deflection slopes. The results reveal a significant variation of up to 40 percent in deflection slopes when considering linear elasticity and disregarding nonlinear stress dependency. This discrepancy is most pronounced at sensors located further from the loading centre. Furthermore, when compared to the nonlinear isotropic case, the differences in deflection slopes consistently remain below 10 percent when considering an anisotropy ratio of 0.5 or higher. However, considering an anisotropy ratio of less than 0.5 can lead to a discrepancy increase of up to 30 percent.
It becomes evident that neglecting the accurate stress dependency and anisotropy behaviour of unbound granular materials and fine-grained soils during the backcalculation process of pavement layers from TSD deflection slopes is an inadequate approach.