Mechanisms at the origin of size effects in coarse soils and rockfills

The reliability of small-scaling techniques for coarse soils and rockfill materials have remained a subject of great interest in geotechnical research and practice. However, researchers have often reported contradictory experimental trends of size effects on mechanical behavior. This paper summarizes extensive research conducted by the authors aimed at elucidating the mechanisms underlying size effects in coarse granular materials. Based on comprehensive large-scale drained triaxial compression laboratory tests, the effects of several factors are systematically studied: material intrinsic parameters (grading, particle shape and particle crushing strength), specimen characteristics (sample size, representative element volume) and testing boundary conditions. The analyses identify the sources of size effects in small-scale triaxial specimens, aiming to clarify the limitations of small-scale testing techniques. This understanding is essential to prevent optimistic estimates of soil properties, ensuring safe and reliable geotechnical designs of large structures such as rockfill dams, railway ballast platforms, and mine waste rock piles.