Evaluating Weak Subgrade for Pavement Design and Performance Prediction: A Case Study of US 550

In this study, a weak subgrade with a wide variation in strength and stiffness has been evaluated for its influence on pavement design and performance. Subgrade strength and stiffness were represented by a soil resistance R-value, and the study conducted employing the pavement structure of US 550, a rural highway in Northwest New Mexico. Subgrade R-value was calculated from geotechnical data and compared to the R-value used for actual design of US 550. Using the calculated and actual R-values, pavement design simulations were run using Mechanistic-Empirical Pavement Design Guide (MEPDG) as well as elastic analysis. The MEPDG outputs shows that the existing design of US 550 may fail due to top-down longitudinal cracking, which matches the actual cracking measured during the field investigations. Top-down cracking was less sensitive to subgrade strength, while rutting is shown to be sensitive to low R-value or weak subgrade. From MEPDG and elastic analyses, it was shown that an R-value of 17 could differentiate the good subgrade from the poor based on the sustainability against pavement rutting and roughness degradation. From the elastic analysis, it is shown that the compressive strain at the top of subgrade can be reduced significantly by increasing subgrade R-values. Subgrade treatment is effective in reducing stress and strains in weak subgrade. The study will be useful for designing and predicting performance of pavements constructed on weak subgrade.