## 3 Experiment Results and Analysis
### Effects of Controlling Factors on Seepage-Erosion Process
#### 3.2.2 Effects of Hydraulic Pressure on Seepage-Erosion Process  
As shown in Figures 11a and 12a, the water-outflow rate is also distinctly affected by hydraulic pressure. Firstly, the occurrence of Transition Point I, II are both advanced with the increase of hydraulic pressure. More specifically, Transition Point I appears at the 21.6 th min for the soil sample under hydraulic pressure of 0.2 MPa, and it is delayed to the 16.8 th min and 7.2 th min as the hydraulic pressure rising to 0.4 MPa and 0.6 MPa, which signifies the \"slow flow\" stage is 21.22% and 66.67% delayed, respectively. As for Transition Point II, it appears at the 40.2 th min for the soil sample with hydraulic pressure of 0.2 MPa, and it is delayed to the 32.0 th min and the 20.1 th min as the hydraulic pressure up to 0.4 MPa and 0.6 MPa, which denotes the \"transition flow\" stage is 18.28% and 69.35% delayed, respectively. This phenomenon indicates the \"slow flow\" and \"transition flow\" seepage stage are both distinctly shortened as the rising of hydraulic pressure, and the shortening amplitudes both exponentially increases with the hydraulic pressure.  
Through observation of Table 8 and Figure 11b,c, the occurrence of sands mass concentration's peak and sands erosion rate's peak are both advanced with the increase of hydraulic pressure, highly consistent with the variation of Transition Point I, II, respectively. The peak value of sands erosion rate and sands mass concentration both increase with the rising of hydraulic pressure. It should be noted that, the peak of sands erosion rate appears at 34.2 th min for the soil sample with hydraulic pressure of 0.2 MPa, and it is delayed to the 30.1 th min and 13.1 th min as the hydraulic pressure up to 0.4 MPa and 0.6 MPa, which is 11.99% and 61.70% delayed, respectively. As seen, compared with the hydraulic pressure increasing from 0.2 MPa to 0.4 MPa, the advancing amplitude of sands erosion rate peak's occurrence is significantly greater as hydraulic pressure up to 0.6 MPa from 0.4 MPa, which indicates  
Figure 11: Variation curve of monitoring objects with hydraulic pressure (\(P_{\text{w}}\)). (**a**) Variation of water-outflow rate with hydraulic pressure; (**b**) Variation of sands erosion rate with hydraulic pressure; (**c**) Variation of sands mass concentration with hydraulic pressure.  
the existence of critical hydraulic pressure between 0.4 MPa and 0.6 MPa, and the seepage-erosion process is supposed to be significantly accelerated once the hydraulic pressure over the critical value.  
As demonstrated in Figure 12b,c, the ultimate water-outflow rate increases from 6.40 m\({}^{3}\)/h to 8.35 m\({}^{3}\)/h as the hydraulic pressure up to 0.6 MPa from 0.2 MPa, in the meantime, the mass loss rate significantly increases to 81.13% from 65.30%. This phenomenon shows that, with the same compactness and confining pressure, higher hydraulic pressure results in greater seepage rate and dragging force, which manifests as stronger erosion ability. Therefore, at the initiation stage of seepage-erosion process, more fine and free particles are migrated into the fluid, resulting in higher sands erosion rate and sands mass concentration, which further accelerates the seepage channels' formation, as well as the occurrence of \"pipe flow\" stage.