# Experimental Study on Permeability Characteristics of
## 3 Results and Discussion  
### Staged Pressure Bleeding Characteristics of Gangue Slurry  
The pressure bleeding rate of gangue slurry refers to the mass percentage of the water secreted from the slurry under the compression compared to the total mass of water. In the preparation stage of the CBB, the pressure bleeding rate of gangue slurry under different conditions can be obtained by measuring the mass of bleeding at the lower end of the material loading container. The calculation function is shown in Equation (2). \[\varphi=\frac{m_{b}}{S\times H_{0}\times\rho_{g}\times w_{g}}\times 100\% \tag{2}\]  
where \(m_{b}\) is the bleeding quality of the gangue slurry; \(S\) is the cross-sectional area of the material loading container; \(H_{0}\) is the initial height of the gangue slurry loading; \(\rho_{g}\) is the density of the gangue slurry; \(w_{g}\) is the solid mass concentration of the gangue slurry. The mass of water bleeding at the lower end of the material loading container was monitored every 10 min, and the time was recorded. After calculation in Equation (2), the relationship curve between the pressure water leakage rate \(\varphi\) of the gangue slurry and the axial loading time t is drawn. It is found that the curve exhibits obvious phased characteristics. Figure 5 shows the changes in the pressure bleeding rate of gangue slurry in S1-S4 groups. As shown in Figure 5, during the axial stress loading process of the gangue slurry, as the loading time increases, the pressure bleeding increases rapidly; then, its growth rate gradually decreases and, finally, gradually stabilizes. This indicates that the gangue slurry is no longer bleeding. The pressure bleeding changes in gangue slurry exhibit obvious phased characteristics, and the duration of the second stage is significantly longer than the first and third stages. 1. Phase 1. The pressure bleeding rate shows a linear increasing trend with loading time. At this stage, the gangue slurry is in a supersaturated state, with large particle spacing and more free water in the slurry. Under pressure, free water flows out uniformly  
Figure 5: Changes in pressure bleeding rate of gangue slurry. from the seepage channel. However, when the gangue particles are in contact with each other, the first stage of gangue slurry pressure seepage ends immediately, so the time is shorter than the second stage. By comparing the slope of the first stage of the S1-S4 curve, it can be found that the pressure bleeding rate of the gangue slurry increases almost uniformly at the beginning of compression and rarely changes with the axial pressure. However, as shown in Figure 5, the greater the axial pressure on the slurry of the same proportion, the shorter the duration of the first stage and the earlier the transition to the second stage. Figure 6 shows the variation curves of the pressure bleeding rate of gangue slurry in the S4 and S8 groups. Under the same axial pressure, the linear growth rate of gangue slurry with particle sizes ranging from 0 to 5 mm in the first stage is significantly higher than that of the gangue slurry with particle sizes ranging from 0 to 2 mm. This is because there are more small-sized gangues in the gangue slurry with particle sizes ranging from 0 to 2 mm, and some of them follow the bleeding of the free water, thereby affecting the water bleeding rate. 2. Phase 2. At this stage, the pressure bleeding rate gradually slows down with the increase in loading time. Under axial stress loading, the distance between gangue particles gradually decreases. Through the processes of sliding, rotation, and fragmentation, gangue particles gradually assume a bearing role. The compression effect exerted on the gangue particles and the container's side walls can lead to an increased number of smaller-sized gangue. Since the small-sized gangue has a larger specific surface and can adsorb more bound water, the pressure bleeding rate gradually decreases. The second stage is the main stage of gangue slurry bleeding and has the longest duration. 3. Phase 3.