Consideration of Regional Variations in Climatic and Soil Conditions in the Modified Triaxial Design Method

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 apply to certain areas of the state such as east Texas, it can lead to unduly conservative assessments of pavement load bearing 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 address this need, researchers characterized the variation of climatic and soil conditions across Texas to develop a procedure that accounts for moisture effects and differences in moisture susceptibilities among different soils. Researchers incorporated this procedure in a computer program for triaxial design analysis that offers greater versatility in modeling pavement systems compared to the limited range of approximate layered elastic solutions represented in the existing modified triaxial design curves. This program permits engineers to correct soil strength parameters to values considered representative of expected in-service conditions when such corrections are deemed appropriate for the given local climatic and soil conditions.