In this paper, experimental studies were conducted using a true triaxial apparatus with a bender element system to examine the mechanisms of aging-induced, small-strain shear modulus changes in sand samples under isotropic and anisotropic loading. Numerical simulations based on the discrete element method (DEM) were also carried out in parallel. In the isotropic loading cases, the three measured shear moduli, G(xy), G(yz), and G(zx), and associated aging rates, in terms of the modulus changes, are similar in every loading stage. DEM simulations reproduced the experimental findings and suggested a general trend. A sample with a lower shear modulus before aging, because of a greater percentage of weak forces, allows more forces to be redistributed from the strong force network to the weak force network through the process of contact force homogenization during aging and therefore can have a higher aging rate. In the anisotropic loading cases where sigma(z) > sigma(x) = sigma(y), the measured modulus increase (i.e., the aging rate) is greater in G(yz) (or G(zx)) than in G(xy). This behavior can be attributed to the increase in both the strong and weak forces in the z-direction during aging, because of arching breakdowns. DOI: 10.1061/(ASCE)GT.1943-5606.0000772. (C) 2013 American Society of Civil Engineers.
