Three-dimensional finite element analysis of flexible pavements considering nonlinear pavement foundation behavior

With the current move towards adopting mechanistic-empirical concepts in the design of pavement structures, state-of-the-art mechanistic analysis methodologies are needed to determine accurate pavement responses, such as stress, strain, and deformation. This research has focused on the nonlinear modulus and deformation behavior of pavement foundation geomaterials, i.e., fine-grained subgrade soils and unbound aggregates used in untreated base/subbase layers, due to repeated wheel loading. This nonlinear behavior is commonly characterized by stress dependent resilient modulus material models that need to be incorporated into finite element based mechanistic pavement analysis methods to predict more accurately critical pavement responses. This dissertation describes the development of a finite element mechanistic analysis model for both the axisymmetric and three-dimensional analyses of flexible pavements. To properly characterize the resilient behavior of pavement foundations, nonlinear stress-dependent modulus models have been programmed in a User Material Subroutine (UMAT) in the general-purpose finite element program ABAQUS(TM). The developed UMAT is verified first with the results of a well established axisymmetric nonlinear pavement analysis finite element program, GT-PAVE. Next, the UMAT subroutine performance is also validated with the instrumented full scale pavement test section study results from the Federal Aviation Administration's National Airport Pavement Test Facility. The predicted responses at different locations in the test sections are compared with the field measured responses under different sections and load levels to indicate that proper characterizations of the nonlinear, stress-dependent geomaterials make a significant impact on accurately predicting measured pavement responses from three-dimensional pavement analyses. Different resilient modulus models developed from conventional and true triaxial test data on unbound granular materials are also studied. When the intermediate principal stresses are taken into account in the three-dimensional modulus model development unlike in the axisymmetric models, large discrepancies are obtained in the computed pavement responses when compared to those from the axisymmetric nonlinear finite element analyses. Finally, as an important application of the developed UMAT nonlinear material subroutine in the analysis of flexible pavements subjected to multiple axle/wheel loads, load spreading and nonlinear modulus distributions of pavement layers are found to considerably impact pavement surface deflections and critical pavement responses.