Mechanisms of Aging-Induced Modulus Changes in Sand with Inherent Fabric Anisotropy

In this paper, experiments and discrete element method (DEM) simulations were conducted in parallel to examine the underlying mechanisms of aging effects on the stiffness changes of sand with inherent fabric anisotropy. The true triaxial apparatus, equipped with a bender-element system, was used to monitor the evolution of the small strain shear moduli of sand samples, i.e., $G_{\mathit{xy}}$ (or ${\text{G}}_{\mathit{hh}}$), $G_{\mathit{yz}}$ (or ${\text{G}}_{\mathit{hv}}$), and $G_{\mathit{zx}}$ (or ${\text{G}}_{\mathit{vh}}$), during the aging process. DEM simulations where clumped particles were used to mimic fabric anisotropy replicated the experimental observations closely and provided insights from the micromechanical perspective. The inherent fabric anisotropy gave rise not only to the stiffness anisotropy, i.e., $G_{\mathit{xy}}>G_{\mathit{yz}}\approx G_{\mathit{zx}}$, but also to a higher aging rate for $G_{\mathit{xy}}$ than for $G_{\mathit{yz}}$ (or $G_{\mathit{zx}}$), i.e., $G_{\mathit{xy}}$ increased more than $G_{\mathit{yz}}$ (or $G_{\mathit{zx}}$) during aging, thereby increasing the stiffness anisotropy. As the particle aspect ratio or fabric anisotropy increased, the distribution of contact forces became more inhomogeneous and more large forces, in particular contact tangential forces, appeared in the $x\text{-}$ and $y\text{-directions}$ than in the $z\text{-}\text{direction}$. Larger tangential forces in the $x\text{-}$ and $y\text{-directions}$ can induce a higher sliding creep in the same direction during aging. As a result, a greater force redistribution took place in the $x\text{-}$ and $y\text{-directions}$ than in the $z\text{-}\text{direction}$ during the process of contact force homogenization induced by aging. This in turn created a greater stiffness gain, i.e., a higher aging rate for $G_{\mathit{xy}}$ than for $G_{\mathit{yz}}$ (or $G_{\mathit{zx}}$) because of a larger enhancement of weak forces in the $x\text{-}$ and $y\text{-}\text{directions}$ to strengthen the soil structure.