Size Effect on the Strength Behavior of Cohesionless Soil Under Triaxial Stress State

Many underground projects are built in cohesionless soil regions, where soil strength is crucial for stability. Particle size greatly influences the mechanical behavior of cohesionless soil. To investigate the relationship between particle size (as a single internal variable) and the strength behavior of cohesionless soil, this study employed idealized spherical glass beads of varying sizes as an experimental material. A series of consolidated-drained triaxial compression tests, including both conventional and large-scale tests, were conducted on specimens with different particle sizes. The correlation between particle size and stress-strain behavior, as well as strength characteristics, was analyzed. Additionally, the influence of particle size variations on the macroscopic strength characteristics was investigated. Results indicated that for both small-sized (2 mm–6 mm) or large-sized (10 mm–30 mm) granular materials, the peak shear stress and internal friction angle increased with increased particle size. The strength of large-sized granular materials was significantly higher than that of small-sized ones. During the shear process of large-sized particles, the particle breakage rate initially increased and then decreased with increasing particle size. The internal friction angle rose monotonically with particle size, but showed insensitivity in the 4 mm–5 mm and 20 mm–25 mm particle size ranges. This insensitivity reflects a macroscopic effect resulting from the interplay between the number of inter-particle contacts and the micro-area of their surface, which reaches an extremum. These findings provide valuable insights into the micromechanical interactions governing the strength of behavior of cohesionless soils and highlight the importance considering particle size effects in geotechnical analysis. The derived particle-interaction framework provides theoretical underpinnings for optimizing design methodologies in underground infrastructure projects involving granular media.