Density effect and associated unjamming events on the aging-induced stiffness increase in sand

This paper reports the density effect on the aging-induced increase in soil stiffness based on discrete-element method (DEM) simulations conducted on dense, medium-dense, and loose samples for the study. Like experimental observations, among the three investigated, the medium-dense sample showed the highest aging rate in terms of the increase in the small-strain shear modulus. This finding was the result of competition between two opposite effects that coexist during aging: contact-force homogenization and unjamming events; both effects were relevant to the packing density. During aging, though a looser sample could undergo a greater homogenization of contact forces to give rise to a higher aging rate, it had a greater risk of experiencing unjamming event. This unjamming was accompanied by a sudden increase in the number of sliding contacts, which destroyed the aging-induced homogenization of the contact forces, and in turn, demolished the aginginduced stiffness increase. On the contrary, although a denser sample was less susceptible to the occurrence of unjamming events, it had less room to enable the contact forces to become more homogeneous during aging. As a result, eventually the medium-dense sample was revealed as having the greatest aging rate.