## 5 Discussion  
The drainage gallery LPL5-1 is an auxiliary cavern located at the upstream side of the main powerhouse and is 28.5 m away from the upstream arch foot. During the main powerhouse excavation, failures took place at the upstream arch of LPL5-1 covering cracking, ballooning and peeling of shotcrete and rebar bending. These failures reflect the deformation and failure of the surrounding rock mass. To find out the cause of failures, field investigation and numerical simulation are conducted. Based on the results, the reason for failures is the multi-cavern effect. The excavation and stress adjustment of the main powerhouse affects the adjacent LPL5-1, makes it subjected to high redistributed stress and leads to the surrounding rock mass failure. For a large-scale underground powerhouse, the multi-cavern effect is an issue that cannot be ignored. The multi-cavern effect can arise between large caverns and small caverns, such as the case in this study, or between large caverns, such as the main powerhouse and the transformer chamber. The main powerhouse and the transformer chamber both have a large span and high sidewalls and are usually excavated simultaneously. If the two caverns are too close, each excavation will affect the other and the rock pillar between them is prone to deformation and failure. The rock pillar has two free faces and is unloaded in two directions. The cracking, deformation and the progressive expansion of EDZ induced by excavation and unloading will occur on both sides of the rock pillar. The results of laboratory tests and field observations suggest that the cracks are denser in the shallow rock mass and a few are distributed in the deep rock mass [19]. If the rock pillar is thin, unloading-induced cracking is more liable to affect the other side, thus making EDZ expand or even connect with each other. This synergy or superposition of adjacent excavations that leads to the deformation and failure of the rock pillar is the multi-cavern effect. Compared with the excavation of a single cavern, this multi-cavern effect under bidirectional unloading can trigger more serious deformation and failure. A large deformation of rock pillar caused by the multi-cavern effect occurred in the Houziyan underground powerhouse (HUP). With respect to the engineering structures, both the BUP and the HUP are large-scale underground powerhouses, although the dimensions of BUP are larger. The distance between the main powerhouse and the transformer chamber of HUP is 45 m, about 1.88 times the average span of the two caverns, while the multiple of BUP is 2.15. Thus, it can be considered that these two caverns of BUP are further apart than those of HUP. In terms of geological conditions, both BUP and HUP are subjected to high in situ stress and complex geological structures. The maximum principal stress at BUP is 19-23 MPa; at HUP it is 22-36 MPa. Faults, weak interlayer zones (WIZs) and joints are developed in both powerhouse areas [20]. Similar to the surrounding rock mass failure in this study, the deformation and failure of HUP is caused by the excavation and unloading of adjacent caverns and the main failure mode is the stress-dominated type. The difference is that the deformation and failure of HUP occurs between two large caverns and the excavation of the two caverns influences each other. The deformation and failure of LPL5-1 occurs between a large cavern and a small cavern, with the former affecting the latter to some extent. In addition, since the main powerhouse and the transformer chamber at BUP are relatively far apart, the interaction of excavation is mitigated. Thus, the rock pillar at BUP did not encounter deformation and failure induced by the multi-cavern effect that HUP experienced. Furthermore, the multi-cavern effect may also occur between intersecting caverns in the underground powerhouse, such as the omnibus bar cave that intersects with the powerhouse sidewall belonging to type LS. [PERSON] et al.