## 4 Discussion and Conclusions  
This experiment study focuses on the water-sand inrush disaster caused by the instability of filling medium in karst cavity. In response to this purpose, a stress-controlled seepage test apparatus in consideration of particle erosion phenomenon was designed, and a series of seepage tests were carried out correspondingly. The water-outflow rate, sands erosion rate and sands mass concentration were adopted as main indicators to analyze the variation characteristics of seepage-erosion process, as well as the impacts of soil compactness, hydraulic pressure and confining pressure. It should be noted that the stability status of filling medium in karst cavities is also affected by the excavation disturbance and soil compaction history [3], which are currently difficult to be considered in model tests. Moreover, besides the impact factors investigated in this paper, the progress characteristics of water-sand inrush is also tightly related to the soil constituents, karst cavity geometry and others. Nevertheless, the main findings concluded in this study still have typical representatives for the evolution characteristics of seepage-erosion process, and offer significant comprehension of water-sand inrush's progress. More critically, it provides important experiment foundations for the early warning and forecast of water-sand inrush in practice. The main conclusions can be briefly summarized as follows. (1): According to the non-linear characteristics of water-outflow pattern, the whole seepage-erosion process can be split into three seepage stages, \"slow flow\" stage, \"transition flow\" stage and \"pipe flow\" stage. It can be concluded that the particle erosion is the internal mechanism of water-sand inrush, and the variation characteristics of water-outflow pattern are the external manifestation of seepage-erosion progress. (2): The seepage-erosion phenomenon is obviously affected by soil compactness, hydraulic pressure and confining pressure. The \"slow flow\" and \"transition flow\" seepage stages are both extended as the higher relative density brings stronger erosion-resistance ability, and the extension amplitudes exponentially increase with the relative density. In contrast, the higher hydraulic pressure causes greater particles erosion ability, which shortens the seepage-erosion process, and the shortening amplitude also increases exponentially with the hydraulic pressure; the increasing confining pressure extends the \"slow flow\" seepage stage by providing more compression, but shortens the duration of \"transition flow\" stage as the compression transforms into shear force. In addition, the existence of critical hydraulic pressure for the seepage-erosion progress is confirmed, and the occurrence of \"pipe flow\" stage is significantly advanced once the hydraulic pressure over the critical value. For practical purposes, the value of critical hydraulic pressure under specific geo-conditions needs to be further systematically explored in the future. (3): The specific seepage stage where the seepage-erosion process is located can be identified by Transition Point I, II, which are obtained through the extremum analysis of water-outflow rate's fitted curve, and confirmed by the occurrence of sands mass concentration's peak and sands erosion rate's peak, respectively. In other words, with the monitoring data of inrush fluid's flow pattern as precursors in practical situation, the critical status of water-sand inrush can be theoretically identified, which offers an early warning to respond in a timely manner and theoretical possibilities to ultimately forecast the occurrence of this type of disaster. Methodology, Q.L. and B.L. ; formal analysis, Q.L. ; writing--original draft preparation, Q.L. ; writing--review and editing, Q.L. and B.L. ; supervision, Q.L. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Natural Science Foundation of China, grant number 41672311 and 51991392. The authors declare no conflict of interest.