Micromechanical signature of representative volume conditions in granular materials: Particle-shape effects and column-like load-bearing structures

Representative volume conditions in coarse granular materials are investigated using three-dimensional Contact Dynamics simulations of drained triaxial compression tests. Dense assemblies of spherical particles and ballast-like polyhedral grains reconstructed from digitized railway ballast particles are compared to assess how particle shape influences the transition from finite-size-dominated behavior to representative bulk response. The nominal system-size ratio 𝛼 n o m = 𝑊 0 / 𝑑 m a x is varied from 3 to 16. The results show that small systems exhibit pronounced stress fluctuations, lower internal coordination, broader normal force distributions, and strong sensitivity of the force-bearing network to specimen size. As 𝛼 n o m increases, macroscopic quantities and micromechanical descriptors progressively stabilize. For both particle shapes, the macroscopic response becomes only weakly dependent on specimen size for 𝛼 n o m ≥ 8 . Particle shape strongly modifies the mechanisms of stress transmission. Polyhedral grains develop higher critical-state shear strength, larger contact anisotropy, and stronger tangential force anisotropy than spheres, reflecting enhanced interlocking and frictional mobilization. However, the apparent transition toward representative behavior occurs over a comparable nominal size range for both particle shapes. A dedicated filtering procedure is introduced to identify column-like structures, defined as unbranched chains of strongly loaded particles. Small systems contain long and frequently percolating structures spanning a large fraction of the specimen height, whereas larger systems exhibit shorter and more spatially distributed load-bearing paths. The progressive loss of systematically percolating column-like structures occurs over the broad size range in which several macroscopic and micromechanical descriptors become less sensitive to specimen size. These results show that the organization of the force-bearing microstructure provides a complementary physical interpretation of the progressive reduction of finite-size effects.