DEM insights into shear strength weakening mechanism of granular material under high-frequency vibration load

High-speed trains caused high-frequency vibration may weak the strength of subgrade soil. This study employed the discrete element method (DEM) to explore the dynamic mechanical response characteristics of granular material subjected to high-frequency vibration load. The correlation between microscopic mechanisms including energy dissipation, evolution of the mechanical coordination number, and fabric anisotropy with macroscopic behavior was systematically analyzed to reveal the underlying physical mechanisms. The results indicated that in the pre-peak and critical stages, the relative effective stress ratio attenuation of the granular material is linearly correlated with the change in relative void ratio. The evolution of mechanical coordination number exhibits high synchronization with void ratio changes at the microscopic scale. Meanwhile, during static shear and vibratory loading, the internal energy of the system is dissipated mainly through frictional energy and secondly through damping energy. In addition, there is a linear correlation between the change in the strain energy of the particles and the relative attenuation of the effective stress ratio Δ(q/p′)/(q/p′)_mon. Under high-frequency vibration, the reduction in effective stress ratio can be attributed to the combined effects of contact normal anisotropy (a_n), tangential contact force anisotropy (a_t), and normal contact force anisotropy (a_c), with contact normal anisotropy being the primary contributor. Finally, the limitations of this study and future research directions are summarized.