Effect of Gradation on Nonlinear Stress-Dependent Behavior of a Sandy Flexible Pavement Subgrade

The development of a simple method to estimate in situ resilient modulus properties of sandy subgrade soils is described in this paper based on gradation properties, granular base to asphalt concrete pavement thickness ratios, and the applied surface loading. A total of 75 nondestructive testing (NDT) data sets were collected from conventional flexible pavements at sites consisting of sandy subgrade soils. The NDT was performed using a falling weight deflectometer at three different load levels and the NDT data from the various pavement areas were utilized to back-calculate subgrade modulus using the WESDEF program. Cores of the asphalt concrete pavements and the particle size distributions of subgrade soils at the NDT locations were obtained concurrently. The data generated such as the applied load levels, asphalt concrete and aggregate base thicknesses and their ratios, and particle size distributions were in turn related to in situ subgrade modulus properties. The effects of particle size distribution and nonlinear stress dependency of sandy subgrade modulus to the pavement responses were analyzed using the finite-element program TTI-PAVE. A horizontal to vertical modulus ratio of approximately 0.3 was assumed for the unbound aggregate base to account for its anisotropic stiffness behavior (Kim 2004). The results demonstrated that the gradation of granular soils and thicknesses of the asphalt concrete and aggregate base pavement layers influenced the subgrade modulus, which in turn has a substantial effect on the pavement response predictions that impact pavement overlay design.