## 4 Results and Analysis
### Seepage Simulation and Calculation of Absolute Permeability Coefficient Tensor  
#### 4.3.1 Experimental Simulation of Absolute Permeability  
In order to simulate the seepage process from the initial moment (a) to the arrival of the water flow at the loess surface (b) in Figure 5, we simulated the seepage by observing the state of (b) in Figure 5 when the seepage process lasted 480 s. By combining the reconstructed 3D pore model, a rectangular computational unit with a length of 5.5 cm, a width of 4.1 cm, and a height of 10 cm was selected for the seepage simulation. As shown in Figure 11, the coordinates of corner point A were (0.5.3.3.2); the coordinates of corner point B were (10.9.4.8.5).  
Figure 10: Coordinates of the center of gravity in the Z-direction.  
Figure 9: Coordinates of the center of gravity in the Y-direction.  
After creating the seepage simulation area, the seepage simulation boundary was set, as shown in Figure 12. Figure 12a shows the two-dimensional schematic diagram of the seepage simulation, where the water entered from the bottom of the funnel IV and then entered the sample at the bottom interface III of the specimen. Using the upward air discharge method, the water flowing in the specimen finally reached surface I of the sample; the four face boundaries II in the lateral direction of the seepage flow were all water barrier boundaries. Figure 12b shows the 3D model of the 3D seepage boundary; the model boundaries I', II', and III' correspond to I, II, and III in the 2D model Figure 12a. A pixel-wide solid-phase plane was added to the surface of the boundary III' that was not perpendicular to the mainstream direction X during the seepage simulation setup, so that the selected sample area was allowed to be isolated from the outside world and no liquid flowed out from the system, thus setting the interface III' as a water-isolation boundary. The pressure head was set to 0 Pa at the top surface boundary I' and 10,000 Pa at the bottom inlet boundary III'. The pressure head at interface III' was quasi-static and the water flow was free to diffuse over the input surface III' of the sample. The viscosity coefficient of water (fluid viscosity) was set to 0.1 Pa-s during the simulation. In order to make the model converge during the calculation, the maximum number of iterations was set to 100,000 and the convergence accuracy was set to 0.0001 \(\upmu\)m\({}^{2}\).  
After the boundary conditions were set, the absolute permeability experiment simulation module in AVIZO software was run, as shown in Figure 13. After 25,547 iterations to achieve iterative accuracy, the absolute permeability coefficient k of the loess in the simulated area was finally calculated to be 0.3482 \(\upmu\)m\({}^{2}\). The results of the seepage simulation are shown in Figure 14. The water flow entered from the seepage boundary set at the bottom and first passed through the connected pores in the loess sample in the vertical direction. Seepage in the horizontal direction occurred during the seepage process as the volume of water entering the sample increased. The seepage simulation process was highly consistent with the actual water infiltration process.  
Figure 11: Determination of seepage model boundary.  
Figure 14: Seepage simulation results process (The green box is the simulation area).  
Figure 12: Seepage simulation model. (**a**) is the two-dimensional schematic diagram of the seepage simulation. (**b**)is the 3D model of the 3D seepage boundary.  
Figure 13: Seepage iterative process.