## 7 Conclusions  
Based on the Hoek-Brown failure criterion and the upper limit theorem of limit analysis, this paper studies the progressive collapse mechanism of the surrounding rock of large-span caverns by means of theoretical analysis and model tests. The following conclusions are drawn:  
1. The mechanism model of progressive collapse of large-span caverns is established, and the calculation method of the whole process curve of the progressive collapse of large-span caverns' surrounding rock is deduced and compared with the model test results;
2. Using the self-developed geomechanical model device for visualizing large-span caverns with variable angles, the load on the top of the cavern is increased in a \begin{table}
\begin{tabular}{c c c c} \hline \hline
**CAVERN Span (cm)** & **Number of Collapses** &
\begin{tabular}{c} **Half Width of Falling** \\ **Body (cm)** \\ \end{tabular} & **Collapse Height (cm)** \\ \hline
30 & 0 & 0 & 0 \\ \hline
40 & 1 & 15 & 10.9 \\ \hline
50 & 2 & 25 & 29.9 \\ \hline
60 & 3 & 30 & 45.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The relationship between cavern span and collapse curve. rotating manner, and the progressive collapse process of the surrounding rock of large-span caverns is revealed through the method of model tests;
3. Three collapses occurred in the 60 m large-span chamber. Due to the large span of the cavern, with an increase in surrounding rock pressure, the middle part of the vault roof will collapse first. When the overburden increases to a certain extent, the arch foot is the weakest part, and the rock mass will be found along the arch foot. Two shear failures will occur and gradually extend upward. Finally, the half-width of the collapse body will be 30 cm and the height will be 45.06 cm, which is 75.1% of the span of the cavern;
4. Through theoretical analysis and model tests, it was found that the large-span cavern model underwent collapse three times, and the collapse range calculated only once was quite different from the actual one. The theoretical calculation results in this paper are basically consistent with the model test results, verifying the rationality of the theoretical calculation and the model test for predicting the progressive collapse curve of the surrounding rock of the large-span cavern. The experimental method was used to obtain the progressive collapse characteristics and evolution processes of the surrounding rock of the large-span cavern, providing a basis for the selection of construction plans and the optimization of the supporting structure. It has certain guiding significance for the large-span cavern project;
5. Under the condition of rock mass and the shape of the cavern, the larger the span of the cavern, the more times the surrounding rock collapses. No collapse has occurred in the cavern with a span of 30 m, one collapse has occurred in the cavern with a span of 40 m, two collapses have occurred in the cavern with a span of 50 m, and three collapses have occurred in the cavern with a span of 60 m. Simultaneously, the collapse range also increases as the span increases;
6. Statement on Future Studies: The theoretical models and experimental results of this study lay the foundation for the excavation and support engineering of large-span caverns. Future work will focus on verifying these results under industrial conditions, ensuring practical applicability and enhancing the understanding of large span cavern stability. Conceptualization, H.C.; methodology, H.C.; software, H.C.; validation, H.C., J.F. and J.X. ; formal analysis, H.C.; investigation, J.F. ; resources, J.X. ; data curation, H.C.; writing--original draft preparation, H.C.; writing--review and editing, H.C.; visualization, H.C.; supervision, J.X. ; project administration, J.F. ; funding acquisition, J.F. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable.