Verification of the Load-Thickness Design Curves in the Modified Triaxial Design Method (0-4519-1)

The Texas Department of Transportation (TxDOT) uses the modified triaxial design procedure to check pavement designs from the flexible pavement system program. Since its original development more than 50 years ago, little modification has been made to the original triaxial design method. There is a need to verify the existing load-thickness design chart to assess its applicability for the range in pavement materials used by the districts, and the range in service conditions encountered in practice. Additionally, there is a conservatism in the current method, which assumes the worst condition in characterizing the strength properties of the subgrade. While this approach may be applicable for certain areas of the state such as east Texas, it can lead to unduly conservative assessments of pavement load capacity in districts where the climate is drier, or where the soils are not as moisture susceptible. Clearly, there is a need to consider regional differences to come up with a more realistic assessment of pavement thickness requirements for the given local conditions. To verify the existing triaxial design method, researchers executed a comprehensive work plan that included a literature review of the current method, load bearing tests on full-scale field sections, laboratory tests on small-scale pavement specimens at various moisture conditions, and comparisons of load bearing capacity estimates from the existing method with corresponding estimates determined from analyses of test data. This report documents the verification of the modified triaxial design method implemented by TxDOT.