This study aims to systematically investigate the influence mechanism of particle size and surface roughness on the shear mechanical behavior of spherical particle materials. Rough glass beads with different particle sizes (2 mm, 3 mm, 4 mm) were prepared using sandblasting technique. Together with smooth glass beads, they were used as test raw materials for indoor triaxial consolidated-drained (CD) tests. Based on the quantitative characterization of particle surface roughness, the differences in the shear mechanical properties of spherical particle materials, including stress-strain curves, strength parameters, critical state characteristics, and stick-slip behavior, etc., were discussed from the aspects of the particle size effect (R), the surface roughness index (Ra), and the normalized roughness effect (Ra/R). The main research results show that: increasing the surface roughness of particles can improve various shear mechanical parameters to a certain extent. This includes effectively increasing the peak deviatoric stress, expanding the range of the strength envelope, and raising the deviatoric stress corresponding to the specimen in the critical failure state. It can significantly increase the peak friction angle phi by approximately 10 %-40 % and the critical state line slope (CSL slope) by about 5 %-23 %. Moreover, the increase becomes more pronounced as the particle size decreases. Meanwhile, as the normalized roughness effect (Ra/R) increases, the friction coefficient becomes larger, which greatly weakens the stick-slip behavior between particles.

Spherical glass beads weaken the influences of particle morphology, surface properties, and microscopic fabric on shear strength, which is significant for revealing the relationship between macroscopic particle friction mechanisms and the particle size distribution of sand. This paper explores the shear mechanical properties of glass beads with different particle size ratios under different confining pressures. It obtains the particle size ratio and fractal dimension D through an optimal mechanical response. Simultaneously, we explore the range of the fractal dimension D under well-graded conditions. The test results show that the strain-softening degree of R-s is more obvious under a highly effective confining pressure, and the strain-softening degree of R-s can reach 0.669 when the average particle size (d) over bar is 0.5 mm. The changes in the normalized modulus ratio E-u/E-u50 indicate that the particle ratio and arrangement are the fundamental reasons for the different macroscopic shear behaviors of particles. The range of the peak effective internal friction angle phi is 23 degrees similar to 35 degrees, and it first increases and then decreases with the increase in the effective confining pressure. As the average particle size increases, the peak stress ratio M-FL and the peak effective internal friction angle phi first increase and then decrease, and both can be expressed using the Gaussian function. The range of the fractal dimension D for well-graded particles is 1.873 to 2.612, and the corresponding average particle size (d) over bar ranges from 0.433 to 0.598. Under the optimal mechanical properties of glass beads, the particle size ratio of 0.25 mm to 0.75 mm is 23:27, and the fractal dimension D is 2.368. The study results provide a reference for exploring friction mechanics mechanisms and the optimal particle size distributions of isotropic sand.