Microscopic Analysis of Particle Regularity Effects on Cyclic Shear Behavior at Gravel–Geogrid Interfaces

The apparent shape of particles is a key determinant of the mechanical properties of a gravel–geogrid interface. A quantitative analysis of particle shape was conducted to study the effect of particle regularity on the cyclic shear characteristics of a gravel–geogrid interface. The discrete-element method was utilized to establish particles of varying regularities in particle flow software and to simulate direct shear tests. Changes in porosity, coordination number, shear band proportion, particle rotation angle, and fabric anisotropy were analyzed for varying particle regularities and cycle numbers. The results demonstrate that the vertical displacement and shear stress of the gravel–geogrid interface increase with decreasing particle regularity. The interface exhibits reduced porosity and elevated coordination number for lower-regularity particles. The shear band proportion decreases with increasing particle regularity, with low-regularity particles having 1.4–1.7 times more shear band proportion than high-regularity particles. The rotation angle of a particle with a regularity of 0.707 is only 0.4–0.6 times that of a particle with a regularity of 0.975, showing an opposite trend to the shear band proportion. Simultaneously, particles within the shear band also show a significantly higher rotation angle than those outside. Additionally, the mean rotation angle decreases with increasing cycle number. The principal stress direction of the contact force at the gravel–geogrid interface shifts with the increase in shear displacement. The deflection of the direction of the principal stress axis between high-regularity particles is slightly less than that of low-regularity particles.