Drained preshearing effects on undrained behaviour and microstructural evolution of sand: a DEM investigation

The influence of preshear-induced fabric anisotropy on the liquefaction resistance of sand is more significant than that caused by variations in the void ratio. Using the discrete element method PFC3D, the effects of initial stress-induced anisotropy on the macro- and micro- mechanical characteristics and microstructural evolution are investigated with specimens prepared at different preshear stress ratios. Results demonstrate that preshear effects enhance liquefaction resistance when the preshear stress ratio remains below 1.22, corresponding to the minimum volumetric strain in drained tests. Peak deviatoric stress during undrained loading increases with preshear stress ratio. Undrained specimens exhibit flow deformation for q/p′ ≤ 0.5, restricted flow deformation for 0.5 < q/p′ < 1.22, and no flow deformation for q/p′ ≥ 1.22. The critical state at both macro- and micro- scales is independent of initial stress-induced anisotropy. Increasing preshear stress ratio intensifies coarse force chains spanning from the top to the bottom of the specimen, with maximum normal and tangential contact forces progressively increasing. The maximum normal contact force direction evolves from vertical to 10° deviation in wind rose diagrams. Preshear modifies the intercontact forces in magnitude and orientation, promoting anisotropic force distribution that governs macro mechanical behaviour.