## 4 Mechanism Analysis  
LPL5-1 is situated in relatively good geological condition. The surrounding rock mass is hard and relatively intact and there is no geological structure that can threaten the surrounding rock mass stability. Judging from the failure pattern and location, shotcrete and floor concrete failures in LPL5-1 belong to the stress-dominated type; the geological structure is not a major affecting factor. The in situ maximum principal stress here is 19-23 MPa. When LPL5-1 was excavated, only mild spalling occurred and the surrounding rock mass could have remained stable after systematic support. Why did the failures develop continuously after support? The failures are likely to derive from other nearby caverns, not from LPL5-1 itself. As reflected in Figure 4a, the caverns around LPL5-1 are LPL4-1, LPL6-1, pressure piping and the main powerhouse. LPL4-1 is above LPL5-1 and has similar excavation dimensions to LPL5-1. The distance between LPL4-1 and LPL5-1 is over 30 m, which exceeds five times the section radius, and the stress adjustment by LPL4-1 excavation was unlikely to have so wide a range that LPL5-1 could be affected. LPL6-1 is also far away and the excavation disturbance has difficulty influencing LPL5-1. The pressure piping is at the upstream side of LPL5-1 and is 20 m away from it, with a section size larger than that of the drainage gallery. However, it is improbable that the pressure piping excavation brought about the failures in LPL5-1. The reason for this inference is as follows. As shown in Figure 9, the pressure piping is also at the upstream side of LPL6-1, with a distance of 20 m. At the lower horizontal section, the vertical distance between the top arch of pressure piping and the floor of LPL6-1 is about 15 m. LPL6-1 is adjacent to the pressure piping at the upstream and lower side and has a smaller distance than LPL5-1. Hence LPL6-1 is closer to the pressure piping than LPL5-1. Generally, the interaction with respect to rock mass behavior caused by cavern excavation and stress adjustment occurs between adjacent or intersecting caverns and the interaction becomes more significant if the caverns are closer. This has been proven in engineering practice and numerical simulations. If the excavation of pressure piping had an impact on the adjacent caverns, the most obviously affected cavern was LPL6-1. Namely, LPL6-1 should have encountered more serious surrounding rock mass and shotcrete failures than LPL4-1 and LPL5-1. The justification is that the stress adjustment induced by pressure piping excavation will first affect a closer cavern. However, the actual severity of shotcrete cracking in LPL6-1 is much lower than that in LPL5-1. Therefore, the pressure piping excavation is not the controlling factor for the failures in LPL5-1. The main powerhouse is 28.5 m apart from LPL5-1. Although the distance is also large, the stress adjustment from the main powerhouse excavation has a large range due to the large dimensions and long-lasting excavation. So LPL5-1 can be affected by it. For other large caverns, the longer distance makes it less likely for them to influence LPL5-1. Figure 9: A cross-section of the underground powerhouse. When the shotcrete in LPL5-1 cracked, the sidewalls of the main powerhouse were excavated. The surrounding rock mass stress was in dynamic adjustment and had the potential for influencing adjacent caverns. In terms of failure evolution, there is a connection between the failure development in LPL5-1 and the excavation progress of the main powerhouse. Hence, we can preliminarily conclude that the failures in LPL5-1 resulted from the stress adjustment induced by the main powerhouse excavation. In the following section, the correlation between the failures in LPL5-1 and the main powerhouse excavation will be deeply analyzed in four aspects: failure pattern, failure time, stress adjustment and failure evolution. 1. Correlation in failure pattern  
Failure forms in LPL5-1 cover cracking, ballooning and peeling of shotcrete, as well as rebar bending toward the free face.