Falling Weight Deflectometer for Estimating Subgrade Elastic Moduli

Subgrade soil characterization in terms of resilient modulus (MR) is essential for pavement design/evaluation. Due to the complexity of the laboratory MR test procedure, highway agencies have been exploring nondestructive tests, such as the falling weight deflectometer. In this study, 12 test sections with subgrade reflecting a range of soil types (fine- and coarse-grain soils) were tested. Falling weight deflector measurements were conducted directly on the prepared subgrade, followed by thin-wall Shelby tube samples from depths up to 1.5 m for laboratory MR tests. Deflection measurements were repeated at the same locations after completion of pavement construction. Results showed that the backcalculated moduli of the prepared subgrade are in good agreement with the laboratory values. Due primarily to confinement and overburden pressure, the backcalculated subgrade modulus, based on deflection measurements on the finished pavement, increases approximately 40 and 100% for fine- and coarse-grain soil, respectively. The modulus increase is substantiated by complementary studies of 20 Mississippi long-term pavement performance sections. In light of these results, the AASHTO factor of 0.33 to adjust for the backcalculated moduli, computed from deflection measurements on finished pavements, warrants further scrutiny and revision.