Density Effects on the Aging Behavior of Sands and the Anisotropy of Aging-Induced Stiffness Increases

This study investigates the density effect on the aging behavior of sands and explore whether the aging-induced stiffness gain is isotropic by nature. The results of resonant column tests on Ottawa sand samples reveal that the medium-dense sample had the highest aging rate, as determined by its shear-modulus increase, followed by the dense and loose sand samples. This result might be attributed to two competing processes, the tendency toward force-chain homogenization and the susceptibility to local structure collapse. This attribution does not apply to Toyoura sand samples, which suggests that the effect of packing density on the aging behavior of sand indeed varies with the type of sand. Bender element tests in the true triaxial apparatus demonstrate that aging behavior is not isotropic by nature because the increase of shear wave velocities in different polarization planes varies under isotropic stress states. Increasing the stress in one direction leads to more significant increases in the velocities that are either polarized or propagating in that direction.