Effects of Rayleigh Damping on the Subgrade’s Apparent Nonlinearity

Abstract

The effect of the subgrade’s apparent (superficial) nonlinearity captured during falling weight deflectometer (FWD) testing has been a major challenge for pavement engineers in trying to back-calculate reliable layer moduli. The current state of the practice back-calculation software accounts for the subgrade’s apparent nonlinearity by using a nonlinear stress-softening subgrade model or introducing an artificial stiff layer at a certain depth to obtain more realistic moduli. While these models tend to provide relatively acceptable results for typical pavements in many cases, they appear to be ineffective in back-calculating reliable moduli for rigid or thick and stiff flexible pavement structures. Dynamic models, which provide more realistic approximations, tend to focus on the viscoelastic behavior of the asphalt concrete layer and the nonlinear behavior of the unbound layers, knowing that the unbound layers in pavement structures mainly behave as linear stress-independent materials. This paper focuses on identifying the shortcomings of the current models and developing a simple and robust model that approximates the overall pavement behavior. A two-dimensional (2D) axisymmetric finite element (FE) model was developed addressing the limitations of the existing models, and properly incorporating the subgrade’s damping behavior. The subgrade damping was found to produce the superficial subgrade nonlinearity that has puzzled pavement engineers for the past 30 to 40 years. The model was used for the back-calculation of layer moduli for flexible and rigid pavement structures built over the same subgrade at the Federal Aviation Administration (FAA) National Airport Pavement Test Facility (NAPTF). The back-calculation with the new model produced reasonable layer moduli that are consistent with the type of material in each layer, and more importantly, it produced layer moduli that are almost equal for the same subgrade under the different pavement structures and types.