## 4 Discussion  
Existing studies have indicated [11; 40] that the mechanical properties of rockfill materials are closely related to the degree of compactness and perimeter pressure, and as an important basis for evaluating the structural stability of rockfill materials in engineering, it is necessary to consider the influence of density and confining pressure on the mechanical properties and deformation characteristics of rockfill materials. Some scholars have investigated the macro-mechanical characteristics of rockfill materials under the influence of density and confining pressure and their internal evolution mechanism via physical tests and numerical simulations. Although scholars have explored the microscopic characteristics of rockfill materials under different densities, which are difficult to measure by physical tests, based on the DEM, these studies are still mostly based on the reduction of physical tests, which do not provide a better explanation of the evolution law of microscopic structures. Relying on the coupling technology of discrete element and finite element methods, this paper carries out a large-scale triaxial numerical simulation test of rockfill materials to explore the influence of different densities and confining pressures on the mechanical behavior of rockfill materials and to reveal the mechanism of fine deformation damage, and preliminary results were obtained. Compared with previous studies, this paper further describes the microscopic mechanism under macroscopic characterization, provides a reference for explaining the intrinsic influence mechanism of density and confining pressure on the mechanical properties of rockfill materials, compares the macroscopic and microscopic changes in the initial, shear expansion, peak, and critical states, analyzes the mechanism, and discusses the discovery of a close link between the macroscopic mechanical properties and the microscopic grained group structural evolution of rockfill materials. For the stress-strain softening type curve, the macroscopic performance for the initial state of the specimen is relatively loose. With the application of an axial load, the specimen is compacted with a small magnitude of shear shrinkage deformation, the deviatoric stress gradually increases, and the phase transition state shows a tendency toward shear distension deformation. The peak state of the deviatoric stress to reach the extreme value of the specimen shear distension deformation becomes more significant. Then, the deviatoric stress begins to decrease to the critical state and tends to stabilize. The specimen shows significant shear expansion and softening deformation [13]. Figure 17 shows the internal deformation mechanism of the specimen with a stress-strain softening curve, which is represented by the particle displacement cloud. As shown in Figure 17, in the initial state, the number of pores between the particles is large, the number of contacts is small, and the particles have a small effect on the deformation of the specimen. With increasing loading, the number of pores between the particles decreases due to the axial load, but the energy provided by the load is not sufficient to compact all the pores between the particles; thus, the specimen soon enters a phase transition state, and the particles squeeze each other with misalignments, rotations and other behaviors that lead to some degree of shear deformation. The load between the particles continues to increase, and the particle displacement cloud deforms the middle part of the specimen. With a certain degree of shear deformation, the load between the particles continued to increase, the middle of the specimen particles began to expand outwardly, and the specimen reached the peak state when the shear deformation became more significant.