Date of Graduation

1999

Document Type

Thesis

Degree Type

MS

College

Statler College of Engineering and Mineral Resources

Department

Civil and Environmental Engineering

Committee Chair

Samir N. Shoukry

Committee Member

David R. Martinelli

Committee Member

W. J. Head

Abstract

After reviewing the existing literature on FWD testing and backcalculation algorithms, a new backcalculation approach based on Three Dimensional Finite Element Modeling (3D FEM) was developed. This approach accounts for the transient dynamic nature of FWD load, the three dimensional geometry of the pavement structure, and the friction and bonding characteristics of pavement layers interfaces. The 3D FEM backcalculation approach was used to backcalculate the layers moduli of flexible, rigid, and composite pavement sites located in West Virginia. The layers moduli of each site were also evaluated using three widely used backcalculation algorithms: MODULUS, EVERCALC, and MODCOMP. Comparison of their results with those obtained using 3D FEM revealed that the former should be multiplied by correction factors in order to match the latter. Using 3D FEM backcalculation results as reference values, correction factors were developed for each program and pavement type. The mechanistically evaluated correction factors were found to be in close agreement with the experience-based factors recommended for flexible and rigid pavements in the American Association of State Highway and Transportation Officials (AASHTO) Pavement Design Guide. The 3D FEM approach was also used to predict the apparent depth to bedrock. The decay of vertical stress and displacement in the subgrade layer were examined and used to predict the apparent depths to bedrock for four pavement sites located in Texas. The 3D FEM results were found to be in good agreement with the measured values provided by Texas DOT. A parametric study was conducted to evaluate the effect of the subgrade layer thickness (assumed in the finite element pavement structural model) on the stress and deformation obtained on top of the subgrade layer and on the 3D FEM-generated deflection basin. It was found that a subgrade layer thickness of 6 ft. would produce satisfactory results. The effect of concrete slab length on the deflection basin was examined for both doweled and undoweled concrete slabs. For doweled concrete pavements, slab length has no effect on the deflection basin. For broken slabs or undoweled ones, the minimum slab length required to produce an acceptable deflection basin was found to be 10 ft.

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