## 3 Experiment Results and Analysis
### Nonlinear Evolution Characteristics of Seepage-Erosion Process
The property description of each seepage stage is detailed as following:  
Stage I: Up until the 16.8 th min since beginning, water-outflow rate maintains less than 0.5 m\({}^{3}\)/h\({}_{3}\) sands erosion rate starts rising after being below 2.0 kg/h within the first 12 mins, and then up to nearly 5.5 kg/h until the end of this stage; sands mass concentration increases instantly from the very beginning, and reaches the peak after 12.6 mins; as seen from the grain-size distribution of the eroded  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multicolumn{1}{c}{**Test**} & \multicolumn{1}{c}{**Time (min)**} & \multicolumn{1}{c}{**} & \multicolumn{1}{c}{**} & \multicolumn{1}{c}{**} & \multicolumn{1}{c}{**Time (min)**} & \multicolumn{soil particles collected in this stage, shown in Figure 8, the content of soil particles over 1 mm is only about 10%, while the particles smaller than 0.5 mm and 0.075 mm occupy about 70% and 32% of the total eroded mass, respectively, significantly less than the contents of the same grain-size particles of being about 26% and 5% in the original soil sample. At the initiation stage of the seepage-erosion process, soil particles compact densely with the soil skeleton maintaining stable, resulting in fairly small porosity and permeability, which ensures a small seepage rate through the soil sample. Subsequently, with the advancing of penetration process, the fine and unbounded soil particles start being activated and migrated into the seepage flow even under relatively small dragging force exerted by slow seepage flow. In this stage, with the existence of fine and unbounded particles, the migration rate of fluidized soil particles into the flow is greater than the conveyance rate, which manifests as the increase of sands mass concentration, resulting in a greater erosion ability of seepage flow because of the existence of more free particles and their surfaces. Based on the seepage characteristics obtained, this period can be named as \"slow flow\" stage. Stage 2: After Transition Point I, water-outflow rate starts abruptly increasing, reaching Transition Point II of being about 8.0 m\({}^{3}\)/h after 32.0 mins totally; sands erosion rate also quickly rises to the peak right before Transition Point II, and sands mass concentration starts going down sharply; through observation of the grain-size distribution of soil particles collected in this stage, there are more coarser particles showing up than that in stage 1. The content of particles between 0.5 mm and 1 mm significantly rises to 57% from 20%, and the particles smaller than 0.075 mm are barely seen in this stage. As the fine particles kept losing, the microstructure of soil sample is changed accordingly. Due to the loss of significant fine particles, not only the interspaces and voids grows bigger, but also the lateral supports for soil skeleton reduce too, causing the coarser soil particles becoming less stable and easier being eroded away. This process will increase the porosity and permeability of soil sample, which in return accelerates the flowing rate, increases the eroding ability of the fluid, and further facilitates the formation of seepage channels. In this stage, the eroding ability of seepage flow increases with the rising of flowing rate, inducing a growing number of sands being eroded away. The water-outflow rate also climbs dramatically, resulting in a greater transfer rate than the generation rate of fluidized soil particles, so the variation of sands mass concentration in this stage declines rapidly all along. Considering the seepage characteristics and experimental phenomenon, this stage can be characterized as \"transition flow\" stage. Stage 3: By the end of stage 2, water-outflow rate fluctuates around 7.6 m\({}^{3}\)/h, and the acceleration of water-outflow rate suddenly drops to nearly zero; sands erosion rate starts declining soon after the summit of being 18.79 kg/h, at the meantime, sands mass concentration continues to drop consistently with a smaller gradient after Transition Point II.