## Materials and Methods
### Numerical Simulation Model Construction
#### 2.3.2 Hydraulic Parameter Fitting  
In order to ensure that the numerical model can effectively represent the hydraulic behavior of the rock, the rational hydraulic parameters in the model should be determined first, and a new fitting method for hydraulic parameters is presented in this paper. The specific process used was follows. (1) Crack reconstruction technology. The fracture characteristics of triaxial seepage experiment rock samples were determined by the following method. The research group carried out the triaxial seepage experiment on the Laosangou sandstone sample (Figure 1). The experimental parameters and seepage characteristics of the rock sample after the peak are shown in Table 2. The estimated seepage quantity was 4.057 \(\times\) 10\({}^{-8}\) m\({}^{3}\)/s. The post-peak sandstone samples are shown in Figure 8a. The angle between the fracture surface and the horizontal line was 68\({}^{\circ}\), and the rupture surface ran through the bottom surface of the rock sample. A ruptured surface existed in the sandstone sample. The rupture surface was wide open and completely separated into two parts after the sample had been taken out [34]. (2) The macrocrack internal node information was obtained as follows. The macroscopic crack profile of the rock sample was drawn with CAD software. The \"pline\" command in CAD software was used to draw the crack contour. The \"list\" command in CAD software was used to obtain the coordinates of data points sequentially. The 'FISH' program was used to transform the CAD file into a Udec model, and then, the macrocrack internal node information was obtained. (3) To fit the model construction, Udec software was used to establish a two-dimensional numerical model with dimensions of 100 \(\times\) 50 mm (height \(\times\) width). In this process, first, the Voronoi tessellation generator command is used to divide blocks into small polygons and joints by setting the number of seeds, which usually have a relatively uniform size. The cracks in the model are grouped based on the fish language: \"contact\" and \"fracture\". \"Contact\" represents microscopic cracks inside the rock sample, and \"fracture\" represents the macroscopic cracks that occur after the rock sample breaks, as shown in Figure 8b. (2) Hydraulic aperture of a microjoint under zero stress conditions. The rock samples of the Laosangou coal mine were observed using scanning electron microscopy. This method is based on the research foundations of coal petrology and structural geology. After \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Lithology** & **Confining** & **Osmotic** & & \\
**Pressure** & **Difference** & & & \\ \hline \hline griststone & 12 MPa & 2 MPa & 82 MPa & 4.0 \(\times\) 10\({}^{-13}\) cm\({}^{2}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Seepage experimental parameters. Figure 8: Post-peak fracture rock sample and reconstruction model. (**a**) the outline of the crack (**b**) Joint groups (**c**) Seepage calculation model. the chemical and physical changes, the rock mass was characterized by residual traces [35]. The development characteristics of the pore, the fissure morphology, and the size of the rock mass were observed by scanning electron microscopy. The observation results are shown in Figure 6. The fracture widths, as shown in the graph, were 6.81 \(\upmu\)m, 7.42 \(\upmu\)m, 7.74 \(\upmu\)m, and 6.77 \(\upmu\)m, respectively, as shown in Figure 9 and the hydraulic aperture in the model under zero stress state is the average of the above four values, i.e., \(a_{0}=7.02\)\(\upmu\)m.
3. Hydraulic aperture of the macrocrack. In order to ensure the rationality of the fitting parameters, the hydraulic conditions of the reconstructed model boundary were kept consistent with the boundary conditions of the triaxial seepage experiment. An axial pressure of 82 MPa was applied to the upper and lower boundaries of the model. A confining pressure of 12 MPa was exerted on the left and right sides, and the osmotic pressure difference between the upper and lower sides was 2 MPa.