Isolated effects of three-dimensional particle shape on the critical state behaviour of granular soils under constant mean stress shearing

In this study, true triaxial simulations under constant mean stress are employed to investigate the isolated effect of three-dimensional (3D) particle shape on the critical state behaviour of granular soils, with emphasis on stress π-plane and contact fabric characteristics. Superellipsoids with systematically varied aspect ratio, squareness exponent and sharpness exponent represent a wide range of particle geometries, while gradation and surface roughness are kept constant. A 3D sphericity descriptor (S3d) is adopted instead of the conventional two-dimensional measure (S2d). The S2d of the maximum cross-section of the particle provides a good approximation of S3d. Isotropic consolidation and true triaxial tests with varying intermediate principal stress ratios (b) are conducted on 15 assemblies of distinct shapes. Particle shape markedly affects packing density, with cubic particles exhibiting the highest compactivity and spherical ones the lowest. The critical state stress ratio on the π-plane follows a conventional Lode-angle interpolation function across different particle shapes. Phase diagrams relating the critical stress ratio at compression (Mc) and the material constant c to superellipsoidal parameters are established, together with an empirical relationship between Mc, c and S3d. Fabric analysis shows that the squareness exponent dominates contact orientation, while the influences of aspect ratio and sharpness exponent are secondary.