## 3 Experiment Results and Analysis
### Effects of Controlling Factors on Seepage-Erosion Process
#### 3.2.3 Effects of Confining Pressure on Seepage-Erosion Process  
First, the water-outflow rate displays distinct changes with confining pressure, as shown in Figures 13a and 14b. The ultimate water-outflow rate changes to 8.02 m\({}^{3}\)/h from 6.34 m\({}^{3}\)/h as the confining pressure up to 0.4 MPa from zero. In addition, through the observation of Table 9 and Figure 14a, the occurrence of Transition Point I delays with the increase of confining pressure, while the occurrence of Transition Point II is slightly advanced. More specifically, Transition Point I appears at the 13.6 th min for the soil sample without confining pressure, and it is delayed to the 16.8 th min and 18.3 th min as the confining pressure rising to 0.2 MPa and 0.4 MPa, respectively. As for Transition Point II, it appears at the 33.2 th min for the soil sample without confining pressure, and it is advanced to 32.0 th min and 29.4 th min as the confining pressure up to 0.2 MPa and 0.4 MPa, respectively. It can be determined that the duration of \"slow flow\" stage is 23.53% and 34.56% extended as the confining pressure up to 0.2 MPa and 0.4 MPa; while the \"transition flow\" stage is 22.45% and 56.63% shortened correspondingly. This phenomenon indicates the increasing confining pressure extend the \"slow flow\" seepage stage, but shortens the \"transition flow\" stage at the same time, and the variation amplitude increases with the growth of confining pressure. Figure 12: Variation tendency of monitoring objects’ characteristic value with hydraulic pressure (\(P_{\text{w}}\)). (**a**) Occurrence time of Transition Point I, II along with hydraulic pressure; (**b**) Ultimate water-outflow rate along with hydraulic pressure; (**c**) Mass loss rate along with hydraulic pressure. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Hydraulic Pressure (MPa)** & **Ultimate Water-Outflow Rate (m\({}^{3}\)/h)** & **Soil Sample Mass (kg)** & **Total Mass Loss** & **Mass Loss Rate (\%)** \\ \hline
0.2 & 6.40 & 9.54 & 6.23 & 65.30 \\
0.4 & 7.62 & 9.54 & 6.83 & 71.59 \\
0.6 & 8.35 & 9.54 & 7.74 & 81.13 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Variation of monitoring objects’ characteristic value with hydraulic pressure (\(P_{\text{w}}\)). Secondly, through observation of Figure 13b,c the occurrence of sands mass concentration's peak and sands erosion rate's peak are both consistently matching the variation of Transition Point I, II, respectively. The peak value of sands erosion rate increases with the confining pressure, which denotes higher confining pressure brings stronger erosion ability, as demonstrated in Figure 14c, the mass loss rate significantly increases to 84.07% from 66.46% as confining pressure up to 0.4 MPa from zero. Meanwhile, the peak value of sands mass concentration shows slight fluctuations as the confining pressure rises. The impacting mechanism behind is that, higher confining pressure exerts greater compression on soil particles, which increases the compactness, strength and the erosion-resistance ability [28], thus the \"slow flow\" stage will be extended under higher confining pressure; after entering the \"transition flow\" stage, considerable soil particles eroded away, causing more voids and less lateral supports for soil particles, which transforms the compressing force into shear force, further destabilizes soil particles and ultimately accelerates the \"transition flow\" stage with greater erosion ability. Figure 14: Variation tendency of monitoring objects’ characteristic value with confining pressure (\(P_{c}\)). (a) Occurrence time of Transition Point I, II along with confining pressure; (b) Ultimate water-outflow rate along with confining pressure; (c) Mass loss rate along with confining pressure. Figure 13: Variation curve of monitoring objects with confining pressure (\(P_{c}\)). (a) Variation of water-outflow rate with confining pressure; (b) Variation of sands erosion rate with confining pressure; (c) Variation of sands mass concentration with confining pressure.