Micromechanical investigation of the aging mechanism in sand

The current mechanisms and factors influencing soil aging remain inconclusive and incomplete. Discrete element method (DEM) is adopted to investigate the responses of irregularly shaped granular assemblies subjected to creep and post-creep triaxial shear. The simulated creep and aging behaviours are generally in line with those observed in existing laboratory tests and numerical investigations. In addition to an increase in soil stiffness, the DEM analyses predict an enhanced soil strength as a result of the prolonged interlocking between sand particles during the relatively long creep duration. It is demonstrated that the creep-induced interlocking is directly manifested as a shear resistance above the minimum energy line. The development of interlocking explains the overshooting behaviour observed in post-creep shear. The microscopic responses do not support the aging mechanism based on the buckling of strong force chains. A new hypothesis, termed ‘microstructure evolution by stability selection’, is proposed to explain soil aging. This hypothesis suggests that interlocking between particles develops naturally during the creep process because interlocked particle clusters generally have a greater chance to survive the particle rearrangement induced by creep, leading to an increased number of interlocked particle clusters in aged soils.