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The measurement of shear modulus at small strains in unsaturated soil is critical to evaluating the deformation behavior of soils at working loads. However, little has been done to determine if the small-strain shear modulus surface is unique during variation in either net mean stress or matric suction. In this context, uniqueness is defined as similar samples subjected to different stress paths that converge to the same stress state. Research is presented herein in which a unique constitutive shear modulus surface was established by independently controlling mean net and matric suction stress in a modified resonant column apparatus. Uniqueness was also investigated using data extracted from literature studies. Uniqueness of the shear modulus surface was shown for single-direction stress and hydraulic paths, by the convergence of modulus and by the agreement between predicted and measured modulus. The presence of a unique shear modulus surface allows for the use of a constitutive relationship based on soil type that describes the small-strain shear modulus in terms of the stress state variables. [ABSTRACT FROM AUTHOR]

Traditionally, deflection data from a falling weight deflectometer (FWD) test and thickness data from pavement coring are used to backcalculate layer moduli. In this study, instead of coring, a neural network (NN) model is developed to determine layer thickness from FWD time-deflection histories. Using the NN predicted thicknesses, layer moduli are backcalculated using a commercially available backcalculation software. For validation, backcalculated moduli are compared with the laboratory determined moduli. Results show that backcalculated moduli are nearly equal to the laboratory moduli. Thus, the inclusion of NN-based thickness data has the potential to replace pavement coring, which is very expensive, and/or to enable a backcalculation method to run with a reasonable assumption of thickness whenever coring information is not available. [ABSTRACT FROM AUTHOR]

The falling weight deflectometer (FWD) testing develops a deflection basin on the pavement surface. Depths of this deflection basin from the center of the falling weight are measured at different radial offsets. These deflections are used for the backcalculation of the pavement layer moduli. Most of the available backcalculation software uses the layered elastic theory and static load to calculate moduli from known pavement surface deflections. However, the FWD test load is dynamic, and layer materials may show nonelastic behavior. Layered elastic theory in these types of software cannot characterize dynamic response of the pavement. Also, elastic theory is unable to accurately predict the surface deflection whenever stress developed in any pavement layer exceeds the yield point. For this reason, this study has performed a finite-element analysis of the airport pavement under the FWD test considering the dynamic load and materials plasticity. The analysis presented here includes elastoplastic behavior of pavement layer materials. Both axisymmetric and quarter cube models have been developed in ABAQUS. Time-deflection histories are simulated to match the FWD test data. A comparison is made between the dynamic, static, and field deflection basins. Contours of vertical deflection and strain are also plotted to observe their distribution on both the axisymmetric and quarter cube models. Analysis results show that the time-deflection histories are in close agreement with the field data. The axisymmetric model yields better results than the quarter cube model. Deflections from the static analysis are greater than the dynamic analysis for an identical set of the layer modulus of elasticity. A uniform distribution of strain is observed from the static analysis in both of the geometries. However, the dynamic analysis does not show similar distribution because of the time-dependent response. [ABSTRACT FROM AUTHOR]