How to Assign Nonlinear Anisotropic Modulus Model Parameters for Mechanistic Analyses of Unbound Aggregate Pavement Layers

Granular base materials used in pavement systems exhibit anisotropic and nonlinear stress-dependent behavior, characterized through the Mechanistic-Empirical Pavement Design Guide (MEPDG) or Uzan-type models using nine nonlinear model parameters (K ₁ through K ₉ ) related to the horizontal, vertical, and shear resilient moduli. As part of the development of the U.S. Army Corps of Engineers’ flexible pavement analysis program (C-FLEX) based on the finite element method, the MEPDG and Uzan models have been incorporated into the Joint Evaluation and Design Integrated (JEDI) software framework to characterize granular base and subbase layers. However, deriving these model parameters from standard resilient modulus testing remains challenging, especially those for horizontal and shear resilient moduli. This study introduces a methodology to establish nonlinear anisotropic resilient modulus parameters for the MEPDG model using triaxial test data generated with an advanced test apparatus with multi-directional pulsing capabilities. This method builds on previous work and provides for the MEPDG model regression-based relationships between material properties and anisotropic resilient moduli. Multiple datasets from various data sources were used to develop the guidelines presented in this paper for deriving horizontal (K ₁ to K ₃ ) and shear (K ₇ to K ₉ ) modulus model parameters based on vertical modulus model parameters (K ₄ to K ₆ ). The outcome includes recommended K ₁ through K ₉ for aggregate materials of varying quality, intended for implementation in the JEDI software to support nonlinear anisotropic stiffness characterization of unbound aggregate layers for mechanistic-empirical pavement design.