## 3 Experiment Results and Analysis
### Effects of Controlling Factors on Seepage-Erosion Process
#### 3.2.1 Effects of Soil Compactness on Seepage-Erosion Process  
First, the water-outflow rate exhibits distinct changes along with the variation of soil sample's relative density, as shown in Figure 9a. The occurrence of Transition Point I, II are both delayed with the increase of relative density, as illustrated in Figure 10a and Table 7. Specifically, Transition Point I appears at 10.4 th min for the soil sample with relative density as 0.25, and it is delayed to 16.8 th min and 27.2 th min as relative density rising to 0.50 and 0.75, which indicates the \"slow flow\" stage is 61.5% and 165.54% delayed, respectively; as for Transition Point II, it appears at 22.4 th min for the soil sample with relative density as 0.25, and it is delayed to the 32.0 th min and 52.4 th min as the relative  
Figure 8: Grain-size curve of eroded soil particles. Figure 7: Seepage test results of group 2.  
density up to 0.50 and 0.75, it can be calculated the duration of \"transition flow\" stage is 26.67% and 210.00% delayed, respectively. This phenomenon shows the \"slow flow\" and \"transition flow\" seepage stage are both significantly extended as the rising of soil sample's relative density, and the extension amplitudes both exponentially increases with the relative density \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Test Number**} & \multicolumn{3}{c}{**Occurrence Time of Monitoring Object (min)**} \\  & **Transition Point I** & **Transition Point II** & **Peak of Sands Mass Concentration** & **Peak of Sands Erosion Rate** \\ \hline
1 & 10.4 & 22.4 & 5.9 & 18.2 \\
2 & 16.8 & 32.0 & 12.6 & 30.1 \\
3 & 27.2 & 52.4 & 25.0 & 45.9 \\
4 & 21.6 & 40.2 & 18.2 & 34.2 \\
2 & 16.8 & 32.0 & 12.6 & 30.1 \\
5 & 7.2 & 20.1 & 4.9 & 13.1 \\
6 & 13.6 & 33.2 & 11.2 & 31.8 \\
2 & 16.8 & 32.0 & 12.6 & 30.1 \\
7 & 18.3 & 29.4 & 17.1 & 28.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Occurrence time of transition points and peak value of monitor objects. Figure 10: Variation tendency of monitoring objects’ characteristic value with relative density (\(D_{\rm r}\)). (a) Occurrence time of Transition Point I, II along with relative density; (b) Ultimate water-outflow rate along with relative density; (c) Mass loss rate along with relative density. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Relative Density**} & \multicolumn{2}{c}{**Ultimate Water-Outflow Rate (m\({}^{3}\)/h)**} & **Soil Sample Mass** & **Total Mass Loss** & **Mass Loss Rate** \\  & **Rate (m\({}^{3}\)/h)** & **(kg)** & **(kg)** & **(\%)** \\ \hline
0.25 & 8.14 & 8.64 & 7.79 & 90.16 \\
0.50 & 7.62 & 9.54 & 6.83 & 71.59 \\
0.75 & 6.49 & 10.68 & 5.97 & 55.90 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Variation of monitoring objects’ characteristic value with relative density (\(D_{\rm r}\)). Figure 9: Variation curve of monitoring objects with relative density (\(D_{\rm r}\)). (a) Variation of water-outflow rate with relative density; (b) Variation of sands erosion rate with relative density; (c) Variation of sands mass concentration with relative density. Secondly, through observation of Figure 9b,c, the occurrence of sands mass concentration's peak and sands erosion rate's peak are both delayed with the increase of relative density, consistently matching the variation of Transition Point I, II, respectively; the peak of sands erosion rate and sands mass concentration keep declining with the increase of relative density. The reason behind this is the higher soil compactness generally represents the existence of less voids, more lateral restraints and less free particles, causing more stable soil skeleton and stronger erosion-resistance ability [27]. Therefore, for the soil with higher density under the same hydraulic pressure and confining pressure, sands erosion rate and sands mass concentration remain smaller, which further reduces the particle loss, retards the growth of porosity and permeability, and ends up with lower ultimate water-outflow rate and smaller mass loss rate, as illustrated in Figure 10b,c.