CHARACTERIZATION OF RESILIENT MODULUS OF COMPACTED SUBGRADE SOILS USING RESONANT COLUMN AND TORSIONAL SHEAR TESTS (WITH DISCUSSION AND CLOSURE)

Both resonant column (RC) and torsional shear (TS) tests were performed at small (below 0.001%) to intermediate (below 0.1%) strain levels to investigate the effects of variables such as strain amplitude, loading frequency, and number of cycles of loading on the resilient modulus (M sub R) of compacted subgrade soils. Plasticity index (PI) was found to be an important variable in evaluating these effects, with the majority of the tests performed on cohesive subgrade soils compacted at optimum moisture content. Resilient moduli determined by RC, TS, and M sub R tests compare well when measurements are compared at the same strain amplitude, time of confinement, and excitation frequency. Elastic threshold strains were determined for the compacted subgrade soils. At cyclic strains below the elastic threshold strain, resilient modulus of a given soil is independent of strain amplitude and is the maximum value measured. At cyclic strains above the elastic threshold strain, M sub R decreases as strain amplitude increases. The elastic threshold strains of compacted cohesive soils ranged from 0.0008 to 0.0048% as PI varied from 4 to 52%. Both RC and TS tests accurately measured resilient moduli below the elastic threshold strain. On the other hand, M sub R equipment could not be used in this strain range because of the lack of resolution. Resilient modulus was found to increase as loading frequency increased, even below the elastic threshold. Moduli obtained from RC, TS, and M sub R tests agreed well at strains above about 0.01% once the effect of frequency was taken into account. Modulus reduction curves showing the variation in normalized modulus, resilient modulus divided by the small-strain resilient modulus, versus strain amplitude correlate well with plasticity index.