## 1 Introduction
Strain softening is most evident under a low confining pressure and high density, while strain hardening is observed under the opposite conditions [15; 16; 17; 18]. As rockfill materials' deformation and strength properties are heavily influenced by their microscopic structure, a detailed study of rockfill materials at this level can effectively explain their macroscopic mechanical phenomena. Scholars have used the discrete element method for numerical simulations to conduct triaxial numerical simulation tests on rockfill materials. This method was first proposed by [PERSON] [19] to study the macroscopic behavior of granular materials at the microscopic scale in rock mechanics. Subsequently, some scholars have extensively analyzed rockfill materials at the microscopic scale using various parameters ([PERSON] H, Zhou W, etc. [20; 21]) to study the effect of particle shape on the mechanical properties of rockfill materials through the establishment of different shapes of rockfill material particles in three-axis numerical simulation tests and found that the more complex the shapes of the particles are, the closer the simulation is to the mechanical properties of real rocks. The mechanical properties of rockfill materials are not only affected by microscopic factors, such as particle morphology and grading, but also closely related to different loading stress paths. [PERSON], [PERSON] et al. [22; 23] investigated the macro- and micromechanical properties of rockfill materials under different stress paths, and the results showed that there was a significant difference in the effect of stress paths on the deviatoric stress-strain, and strain-volume change characteristics of rockfill materials. The three-dimensional (3D) discrete element method (DEM) for triaxial tests has been used to simulate and study the shear behavior of granular materials [24]. DEM simulations of triaxial tests under membrane boundary conditions consisting of membrane particles were carried out, and the DEM model under the membrane boundary conditions suitably reflected the evolution of the sample's irregular deformations and the shear bands during the shear process. This study greatly improves the accuracy of the DEM simulationsof triaxial tests under membrane boundary conditions. A new algorithm was proposed to reproduce flexible film boundaries in triaxial tests [25]. A comparison of triaxial tests with flexible and rigid boundaries was made at the macroscopic scale and the microscopic scale, and it was found that flexible film boundaries better reflect physical test variations. A triaxial test based on the discrete element method (DEM) found that the contact force chains in the microscopic structure can reflect their corresponding macroscopic behavior, and the greater the number of force chains is, the greater the peak strength [26]. DEM simulations under real triaxial conditions were also carried out to investigate the critical state and swelling properties of granular materials at both the macroscopic and microscopic levels [27], and it was found that both the contact direction and the contact force affect the mechanical properties of granular materials. [PERSON], [PERSON] et al. [28; 29] investigated granular materials by discrete element numerical simulation and analyzed the deformation and damage process of a numerical model using a deviatoric stress-strain curve, microcracks, displacement fields, stress fields, and energy fields. It was found that the displacement field can more significantly show the shear damage region and form, and displacement zoning appears. Therefore, in this paper, based on a triaxial test of rockfill materials, a numerical simulation is used to carry out a large-scale triaxial numerical simulation test under different densities.