Alternative Method of Determining Resilient Modulus of Compacted Subgrade Soils Using Free-Free Resonant Column Test

An alternative procedure of determining the resilient modulus ( MR) of compacted subgrade soils using a free-free resonant column (FF-RC) test is proposed. The FF-RC test was used to determine a small-strain Young’s modulus ( Emax) and Poisson’s ratio on the basis of the elastic wave propagation theory. Resonant column (RC) and torsional shear (TS) tests were performed to investigate the effects of loading frequency and strain amplitude on the resilient modulus of subgrade soils. By performing FF-RC, RC/TS, and MR tests on the synthetic specimens of known stiffnesses, the feasibility of using the FF-RC test to measure the MR was evaluated and the MR testing equipment was calibrated. Moduli of subgrade soils determined by FF-RC tests match well with values obtained from RC and TS tests at small strains, indicating that the FF-RC test can provide a reliable estimation of Emax, if the effects of loading frequency are considered. In the proposed method, Emax obtained from the FF-RC test can be combined with the effect of loading frequency and the modulus reduction curve determined by the data base of the RC and TS tests for the compacted subgrade soils of various plasticity indexes. Moduli obtained from the proposed method overlapped nicely with MR values determined by standard MR testing, showing the capability of the proposed method being used in determining MR values, provided that the effects of loading frequency and modulus reduction curve are considered.