Pavement foundation geomaterials, i.e. unbound aggregates used in untreated base layers and fine-grained soils of subgrade layers, exhibit nonlinear behavior under repeated wheel loads. Stress-dependent resilient modulus models, commonly used to characterize this nonlinear behavior, were programmed in a User Material Subroutine (UMAT) in the general-purpose finite element (FE) program ABAQUS™ to analyze flexible pavements in both axisymmetric and three-dimensional (3D) stress systems with the intention to study predicted pavement responses, i.e., stress, strain and deformation. Different modulus models considering both axisymmetric and 3D stress states were developed from true triaxial test data on unbound granular materials. Pavement response predictions were then compared from different FE analysis cases studied using these nonlinear characterization models. When the intermediate principal stresses were taken into account in the modulus model development, the most accurate pavement response predictions were obtained from 3D nonlinear FE analyses of flexible pavements with unbound aggregate bases.