## 3 Results and Discussion
### Experimental Results for the Four Typical Crack Models
#### 3.3.1 Simulation Results of the \"Type Y\" Crack Model  
The \"type Y\" crack model was chosen as an example to analyze the experimental results, in order to explore the influences of the axial pressure and confining pressure on the microscopic and  
Figure 23: Change curve of the average joint hydraulic aperture value: (**a**) 60-degree crack angle and (**b**) 70 degree crack angle. Figure 24: seepage results of different fracture forms. macroscopic joint seepage and seepage channels. The joint hydraulic aperture and seepage channel were analyzed in detail under two stress boundary conditions: (1) axial stress 5 MPa, confining pressure 30-5 MPa and (2) confining pressure 5 MPa, axial stress 45-0 MPa. (1) An axial pressure of 5 MPa was applied to the upper boundary of the model. The confining pressure was 30 MPa to 5 MPa. The pressure difference between the upper and lower ends was \(\Delta\)p = p2 - p1 = 2 MPa. The hydraulic aperture values of the joints are shown in Figure 25. The joint flow rate and the main seepage channel are shown in Figure 26.  
The variation law of hydraulic aperture of microscopic joints, as shown in Figure 25, is similar to that of Figure 15, with the decrease of confining pressure, a large number of vertical joints and inclined joints changes from closed state to open state. The network structure--consisting of vertical joints, inclined joints, and horizontal joints--contributed to the formation of seepage channels. In the process of reducing the confining pressure, the large influence of the confining pressure on microscopic vertical joints led to a large change in the vertical microscopic seepage channel. The increase in the joint aperture value of the macroscopic crack caused an increase in the seepage flow rate, and the main seepage channel was composed of macroscopic cracks and presented a Y-shape, as shown in Figure 26.  
(2) A confining pressure of 5 MPa was applied to the right and left sides of the model. The axial pressure was 45 MPa to 0 MPa. The difference between the upper and lower ends was \(\Delta\)p = p2 - p1 = 2 MPa. The hydraulic aperture values of the joints are shown in Figure 27.  
The variation law of hydraulic aperture of microscopic joints, as shown in Figure 27, is similar to that of Figure 17, with the decrease of axial pressure, a large number of horizontal joints, and inclined joints changes from closed state to open state. The network structure, consisting of vertical joints, inclined joints, and horizontal joints, contributed to the formation of seepage channels. The main seepage channel was composed of macroscopic cracks and presented a Y-shape (similar to Figure 26). During the process of axial pressure reduction, the large influence of axial stress on the microhorizontal  
Figure 26: Main seepage channel. Figure 25: Joint hydraulic aperture. points led to large changes in the horizontal microscopic flow channel, which had less influence on the vertical microscopic seepage channel. Experimental data collected for the Y-type crack model (Figure 24) were selected for analysis. Figure 28 shows the flow rate change curves of the Y-type crack model with decreases in the axial pressure (Figure 28a) and confining pressure (Figure 28b). The seepage flow rate change can be divided into two increasing stages. The slopes of the two stages--K1 and K2--were calculated, respectively. The positive values of K indicate that the flow value gradually increased. In order to facilitate a comparative analysis, the average values of K1 and K2 were used as the slope values. The lowest slope value was 4 \(\times\) 10\({}^{-8}\) when the confining pressure was 30 MPa, and the highest slope value was 1.90 \(\times\) 10\({}^{-7}\) when the confining pressure was 5 MPa. As the confining pressure decreased, the slope value and seepage flow rate gradually increased, as shown in Figure 28a. The minimum slope value was 9.6 \(\times\) 10\({}^{-7}\) when the axial pressure was 45 MPa, and the maximum slope value was 1.23 \(\times\) 10\({}^{-6}\) when the axial pressure was 0 MPa.