## 2 Numerical Model Construction
### Specim Generation  
The rockfill materials used in the test serve as the main construction material of a water conservancy hub project panel rockfill dam. The lithological units of the material are the middle and lower Carboniferous medium-thick layer of chert and dolomitic chert, which is a medium-hard to hard rock with a measured specific gravity of 2.66 g/cm\({}^{3}\) and a dry density of 2.15 g/cm\({}^{3}\). The maximum size of the rock lumps is 60 mm. This study uses a large-scale multifunctional static-dynamic triaxial tester with a capacity of 2000 kN, designed for large-scale static-dynamic properties of soil, rockfill, and concrete materials. The testing device consists of a triaxial unit, an axial loading device, a unit pressure control device, a digital servo control unit, and a measurement and data acquisition system. The specimen is 300 mm in diameter and 600 mm in height. The vertical and confining pressure servos can be controlled separately or simultaneously and the axial strain is measured using a displacement transducer mounted on the loading punch. The volumetric strain can be measured by the water that flows into or out of the specimen through the volumetric tube. The stress or strain control can be freely converted during the test, and the entire testing process is computer-controlled with automated data collection and real-time visualization. Figure 3 shows the equipment used for this procedure. The consistency of the simulated particles is ensured with the actual particle gradation in the chamber. A gradation generation program was developed in the FISH language, which enables the particle gradation of the generated granular flow model of rockfill materials to align with that of the chamber test. Additionally, it allows the generation of model specimens of rockfill materials with any specified gradation as needed. The gradation curves of the simulated particles can then be compared with the test gradation curves, as shown in Figure 4.  
In indoor conventional triaxial tests, a specimen is often wrapped in a rubber membrane to study its mechanical properties, and stresses are applied through the top and  
Figure 4: Comparison of the gradation curves between the indoor test and simulation test results. Figure 3: Large triaxial test device. (**a**) Large-scale triaxial apparatus; (**b**) specimen. bottom loading plates. In the numerical simulation test, the wall unit can be used to simulate the force on the specimen, but the wall established by the wall unit is rigid, and the specimen cannot undergo lateral deformation or other phenomena that occur during the loading process. The method of replacing the lateral wall with a ball unit to simulate the flexible rubber membrane has a large number of particles, which is difficult to carry out in the calculation because of the high requirements on the arithmetic power of the equipment and the long duration of the calculation process. The shell unit in FLAC3D (6.0) can simulate the lateral deformation in the numerical test, and the numerical model established through the coupling of PFC (6.0) and FLAC3D (6.0) is faster in the calculation process, so this paper establishes the numerical model by this method, as shown in Figure 5. Figure 5a shows that the specimen size is 300 \(\times\) 600 mm, and the stacked rock particles are simulated by the rblock unit because the shape of the stacked rock particles has been obtained by 3D scanning technology and imported into the model. The contact between the stacked rock particles can be simulated directly by the linear model. The contact model between the loading plate and the gravel particles, between the particles and between the particles and the flexible membrane is set as a linear contact model.