Triaxial numerical simulation study on the mechanical properties of rockfill materials under different boundary conditions

Considering the critical role of boundary conditions in influencing the mechanical properties and deformation behaviors of rockfill materials, large-scale triaxial numerical simulation experiments are conducted under various boundary conditions using discrete element and finite-difference coupling (FDM-DEM) technology. This study aims to examine the effects of boundary conditions on the mechanical behavior of rockfill materials and to clarify the discrepancies in their deformation and failure mechanisms at both macroscopic and microscopic scales. The results indicate that the FDM-DEM coupling boundary provides distinct advantages in characterizing the mechanical response of rockfill materials. Under the rigid boundary, both peak strength and volumetric deformation capacity are significantly enhanced. The stress–strain curves observed with the bonded particles boundary consistently show a hardening trend and exhibit a relatively limited capacity for volume change. The “x”-shaped shear band and waist-drum failure mode of rockfill materials are more accurately depicted using the FDM-DEM coupling boundary. In contrast, the rigid boundary samples display collapse failure, an unconventional “k”-shaped shear zone, pronounced stress concentration, and the appearance of stress blind zones, along with a marked reduction in the mechanical coordination number. The bonded particles boundary demonstrates greater potential in capturing the final morphology of the shear band, with the conical dead zone showing increased freedom in its evolution, more uniform stress distribution, and a gradual decline in the mechanical coordination number. The FDM-DEM coupling boundary proves advantageous in maintaining stable confining pressure and effectively simulating continuous elastic deformation, while significant additional stresses are observed at both the rigid boundary and the bonded particles boundary.