Shear Modulus: A Time-Dependent Soil Property

Dynamic shear moduli determined at low shearing strain amplitudes (<10^-3 percent) during sustained-pressure, resonant-column tests are shown to increase with time of confinement. The time-dependent modulus increase is characterized by two phases: (1) an initial phase which results from primary consolidation, and (2) a second phase which occurs after completion of primary consolidation, called the “long-term time effect.” The duration of primary consolidation and the magnitude of the long-term time effect vary with factors such as soil type and stress conditions. Although shapes of the modulus-time relationships differ depending on whether the soil is primarily fine- or coarse-grained, all soils exhibit a long-term time effect. The long-term time effect is also shown to occur at higher shearing strains (0.001 to 0.1 percent) for moduli determined at the onset of cyclic loading. On the basis of these results, confinement time is shown to be an important parameter which must be properly accounted for in the laboratory measurement of shear moduli and which must be considered when interpreting laboratory moduli in terms of in situ response.