## 5 Discussion
[21] studied the failure of rock mass with a WIZ surrounding the intersection of the main powerhouse, the omnibus bar cave and the transformer chamber. By in situ MS monitoring, the spatiotemporal evolution of the surrounding rock mass failure during the excavation of these intersecting chambers was demonstrated and the failure mechanism was analyzed. [PERSON] et al. [22] studied the circumferential cracking of the surrounding rock mass of the omnibus bar cave. Via numerical simulation, the stress evolution in the surrounding rock mass caused by the excavation of the omnibus bar cave and the main powerhouse was investigated and the cause of cracking was analyzed. Both studies focus on the surrounding rock mass failure at the intersection of caverns, which reflects the multi-cavern effect. Compared with this study, the multi-free face unloading of surrounding rock mass at the intersection is a difference in these two studies. There are multiple caverns of different sizes and directions at the intersection; these caverns influence each other, thus making the stress evolution and rock mass behavior here more complex. On the other hand, the surrounding rock mass failure in LPL5-1 exhibits time dependence and develops continuously with time or excavation processes, which is not shown in these two studies. The reason is that the stress concentration caused by the powerhouse excavation is intensified with the excavation, causing failures to become increasingly serious. A large-scale underground powerhouse has complex structures compared with other underground openings and this kind of surrounding rock mass stability issue between intersecting caverns is also typical. In transportation tunnels, the interaction between adjacent tunnels during construction has been noticed [23; 24]. In the field of hydroelectric underground powerhouses, the interaction between caverns is also recognized. [PERSON] et al. suggested that the problem of the multi-cavern effect is prominent in the construction process of underground powerhouses due to their great depth, high in-situ stress and complex geologic structures [25]. [PERSON] et al. stated that adjacent excavations could exert an influence on EDZ distribution characteristics around a large-scale underground powerhouse [26]. [PERSON] and [PERSON] held that there was an interaction of the EDZ between adjacent excavations, which was important and should be considered [27]. In previous studies, since the multi-cavern effect is not the key factor causing deformation and failure, the features and mechanism of the multi-cavern effect have not been thoroughly researched and elucidated. In this study, the multi-cavern effect is the main reason for the failures in LPL5-1. Via field investigation and numerical simulation, the failure mechanism of the surrounding rock mass is revealed. In similar large-scale underground powerhouses, especially under high in situ stress, the problem induced by the multi-cavern effect may also occur, to which attention should be paid. In the future, cases on this aspect will be studied, mechanisms will be explicated, and countermeasures in construction and design will be explored.