###### Abstract
Consequently, the large scale and complex structure gives the underground powerhouse unique surrounding rock mass stability issues, which are relatively rare in other engineering structures such as traffic tunnels. Among these issues, the deformation and failure induced by the multi-cavern effect is typical. The multi-cavern effect is the interaction between adjacent or intersecting caverns during the construction period. When a cavern is excavated, the stress in rock mass close to the free face is unloaded and the stress within a certain range around the cavern is adjusted. With the adjustment and redistribution of stress, the surrounding rock mass exhibits some mechanical behaviors, including deformation, damage and even failure. If there is a cavern adjacent to this cavern, the excavation, stress adjustment and rock mass responses of the two caverns may affect each other. When the interaction is obvious, the rock mass surrounding the caverns is apt to be damaged or to fail. For the large-scale underground powerhouse built in a limited space, extensive excavation blasting and multi-procedure construction will inevitably increase the possibility of interactions between the caverns, thus leading to deformation and failure. The deformation and failure of surrounding rock mass induced by the multi-cavern effect in an underground powerhouse can be divided into three types: between large caverns (type LL), between a large cavern and a small cavern (type LS) and between small caverns (type SS). The deformation and failure of a rock pillar between the main powerhouse and the transformer chamber belongs to type LL (Figure 1b). The deformation and failure of a small cavern affected by a large cavern such as the main powerhouse belongs to type LS (Figure 1c). Deformation and failure under the impact of adjacent or intersecting small cavern belongs to type SS (Figure 1d). The underground powerhouse usually has many caverns and a compact layout, and the multi-cavern effect is likely to occur during construction. Importance should be attached to the deformation and failure of surrounding rock mass induced by the multi-cavern effect; its reasons and mechanisms should be studied and explained and countermeasures be put forward to ensure the construction safety and long-term stability of the powerhouse. However, previous studies have rarely focused on failure issues triggered by the multi-cavern effect; this is a research gap at present. This issue usually arises in the underground powerhouse with a large scale and complex spatial structure, which is different from a single cavern. Few cases have been reported so far and not much attention has been paid to this issue. This issue has occurred in a few projects, while previous studies have rarely conducted in-depth analysis on it. The main patterns, characteristics and mechanisms of the multi-cavern effect have not been systematically summarized. For the underground powerhouse of a hydropower station or a pumped storage power station, the deformation and failure of surrounding rock mass caused by the multi-cavern effect is likely to be prominent during construction. With the scale and burial depth of underground powerhouse increasing and the geological conditions becoming more complex, the importance of this issue will be further highlighted. Hence the multi-cavern effect in underground powerhouses is of great practical significance and is worth further study. The surrounding rock mass failure of the drainage gallery LPL5-1 in the underground powerhouse of the Baihetan hydropower station is a typical case. After excavation and systematic support, the surrounding rock mass of LPL5-1 cracked and failed constantly; this has drawn much attention. The remarkable failure problems in LPL5-1 challenged the surrounding rock mass stability and the builders were worried about the construction and long-term operation safety of the drainage gallery.