Evaluation of In-Situ Stiffness of Subgrade by Resilient and FWD Modulus

Abstract

Resilient modulus has been used for characterizing the stress-strain behavior of subgrade soils subjected to traffic loadings in the design of pavements. With the recent release of the Mechanistic-Empirical Pavement Design Guide (MEPDG), highway agencies are further encouraged to implement the resilient modulus test to improve subgrade design. A laboratory resilient modus test and a falling weight deflectometer (FWD) test are usually used to obtain the resilient modulus of subgrade. However, the difference in the resilient modulus obtained from these two methods is considerably large due to the fact that these tests are conducted under different conditions. This difference gives engineers a significant confusion about how they input appropriately the resilient modulus in the MEPDG software. In the present study, FWD tests, resilient modulus (Mr) tests and physical property tests were conducted to develop the relationship between the modulus from the FWD test and the resilient modulus from the lab resilient modulus test by comparing the results obtained from the FWD test on subgrade and the laboratory repeated triaxial load test on subgrade soil samples molded at OMC in Indiana varying over different climatic conditions. Based on the results of FWD tests and laboratory tests on some Indiana subgrades, the following conclusions can be drawn: (1) on average, the FWD modulus is about 75% lower than the lab resilient modulus of the soil compacted at OMC; (2) winter FWD modulus is about 40% higher than early summer FWD modulus; and (3) when inputting the resilient modulus of subgrade in the MEPDG software, this relationship can be implemented.