## 3 Experiment Results and Analysis
### Identification and Forecast of Seepage-Erosion's Evolutionary Process  
Based on the elaboration in last two sections, it's reasonable to conclude that particle loss caused by seepage-erosion is the internal mechanism of water-sand inrush; the variation of water-outflow characteristics are external manifestations correspondingly. In other words, with the acquaintance of external manifestations' variation features, the specific seepage stage where the water-sand inrush is located can be accurately identified, which could theoretically offer possibilities to forecast the occurrence of water-sand inrush.  
As introduced in Section 3.1, the whole seepage-erosion process is visually spit by Transition Point I, II, characterized with distinct non-linear features, thus the identification of seepage-erosion stage can be determined through the location of Transition Point I, II, which can be accessed through the extremum analysis of water-outflow rate. While considering the complexity of the geo-environment in practical geo-conditions, solely relying on the variation of water-outflow rate seems not adequate and reliable. Through further observation of the seepage test results in Table 6 and Figure 15, one crucial phenomenon should be noted, the occurrence of Transition Point I is closely behind the peak of sands mass concentration. For instance, as shown in Figure 15a, the appearance time of Transition Point I is the 10.4 th min, 16.8 th min and 27.2 th min, as the relative density increasing to 0.75 from 0.25, the peak of sands mass concentration appears at the 5.9 th min, 12.6 th min and 25.0 th min correspondingly, with only 4.5 mins, 4.2 mins and 2.2 mins ahead; as for Transition Point II, it appears at the 22.4 th min, 32.0 th min and 52.4 th min, and the peak of sands mass concentration shows up at the 18.2 th min, 30.1 th min and 45.9 th min correspondingly when the relative density up to 0.75 from 0.25, respectively with only 4.2 mins, 1.9 mins and 6.5 mins in advance. This manifestation that the peak of sands mass concentration occurs slightly ahead of Transition Point I, the peak of sands erosion rate occurs slightly ahead of Transition Point II, which also applies to other groups under varying hydraulic pressure and confining pressure as shown in Figure 15b,c.  
It can be briefly summarized that the sequence between sands mass concentration's peak, sands erosion rate's peak, and transition points is confirmed. To be precise, the seepage stage can be theoretically determined through the identification of Transition Point I, II, and further verified by locating the peak of sands mass concentration and sands erosion rate, respectively. Therefore, with the monitoring data of flow characteristics in practical situation, the critical status of water-sand inrush can be near-real-time identified, which offers an early warning to respond in a timely manner and the basis for ultimately forecasting the occurrence of water-sand inrush. It should be emphasized that the employ of polynomial of 7 th order is to precisely fit the variation of sands erosion rate and sands  
Figure 15: Occurrence time of transition points and peak value of monitor objects along with influence factors. **(a)** Occurrence time of transition points and peak value of monitor objects along with relative density; **(b)** Occurrence time of transition points and peak value of monitor objects along with hydraulic pressure; **(c)** Occurrence time of transition points and peak value of monitor objects along with confining pressure.  
mass concentration, which cannot be employed to quantitatively predict their variation tendency even with extremely high correlation. The strict regression equation needs to be developed and validated through more experiment datasets.