Multi-Scale Study of Specimen Size Effect on Shear Strength of Polydisperse Granular Materials Using DEM

In soil shear strength characterization, particle size is usually much smaller than the testing device size. However, when particle size is similar to the size of the apparatus (e.g., for coarse granular materials), the mechanical response becomes less reliable. To address this, international standards prescribe minimum sample scales based on maximal particle and device sizes. Nevertheless, the influence of the sample scale on the mechanical response is still not well understood. This topic is studied through simple shear simulations in the frame of the discrete-element method, covering a wide range of sample scales and particle size distributions. Micromechanical analyses of force and contact configurations reveal that the stability of parameters is linked to the formation of local rigid structures that carry forces significantly higher than the average. When the height of these structures becomes comparable to that of the sample, macroscopic and microscopic parameters deviate from those found under larger sample scales. Although these findings require further validation, this work suggests that the standards may need to be re-evaluated for an effective material characterization.