Estimation of Subgrade Resilient Modulus for Virginia Soil

In 2004, the Mechanistic-Empirical Pavement Design Guide (MEPDG) was developed under NCHRP Project 1-37A to replace the currently used 1993 AASHTO guide. Implementing the MEPDG requires the mechanistic characterization of pavement materials and the calibration of performance prediction models by the user agencies. The Virginia Department of Transportation required a guideline for the mechanistic characterization of subgrade soils in its MEPDG implementation. More than 100 soil samples representing every physiographic region in Virginia were collected for resilient modulus testing, including quick shear and soil index properties, standard Proctor values, and California bearing ratio. Resilient modulus values and regression coefficients of constitutive models for the resilient modulus of typical Virginia soils were successfully computed. They could be used for the MEPDG Level 2 or Level 3 design and analysis of smaller projects for which costly and complex resilient modulus testing is not justified. Correlations between resilient modulus values and all other test results were not statistically significant except for the quick shear test on fine soil, which was strongly correlated (R-2 = 0.98). A simple version of the quick shear test, the unconfined compression test, is recommended for investigation in developing a predictive model for estimating the resilient modulus.