## 4 Mechanism Analysis
These phenomena indicate that the surrounding rock mass here was subjected to high tangential stress. Similarly, cracking, ballooning and peeling of shotcrete also occurred at the upstream spandrel of the main powerhouse. In terms of form and characteristics, this shotcrete failure is similar to the failures in LPL5-1. The shotcrete failure is distributed at stake ZCO-71.6 m to ZCO+350 m. Shotcrete cracks concentrate on the upstream side of the arch and extend along the main powerhouse axis, as shown in Figure 10. The total length of cracks is about 390 m, accounting for 86.3% of the total length of powerhouse. The width of cracks is generally 3-6 cm, with a local maximum of 16 cm. Shotcrete cracking occurred during the excavation of layer III to VI2 and the evolution of cracks gradually became less obvious later on. The most serious cracking in LPL5-1 is at stake ZCO-026 m to ZCO+130 m and the shotcrete cracks at this section in the main powerhouse are also dense. The similarities in these aspects suggest that the failures in LPL5-1 and the main powerhouse are likely to be caused by the same trigger, i.e., the high tangential stress near the free face at the upstream arch. 2. Correlation in failure time  
With respect to failure time, the failures in LPL5-1 are related to those at the upstream spandrel of the main powerhouse. The shotcrete cracking in LPL5-1 started between June 2015 and April 2016; the shotcrete cracking at the upstream spandrel of the main powerhouse began in May 2015. The failures in the main powerhouse and LPL5-1 are successive in time. LPL5-1 is 28.5 m away from the upstream arch foot of the main powerhouse and it takes some time for the SCZ to expand and transfer from the upstream spandrel to LPL5-1. Therefore, the failures in LPL5-1 occur later than those in the main powerhouse. Furthermore, the failures in LPL5-1 are also related to the excavation of the powerhouse sidewalls, that is, the failures run through the sidewall excavation process. Shotcrete failure in LPL5-1 started when the main powerhouse sidewall was excavated at layers III-IV and it developed continuously as the excavation progressed. When the sidewall excavation finished, the change and the development of failures decelerated remarkably. This indicates that the stress adjustment caused by sidewall excavation affected the failures in LPL5-1 as well as the shotcrete cracking at the upstream spandrel of the main powerhouse. 3. Correlation in stress adjustment  
According to the numerical simulation results, SCZ forms at the upstream spandrel during the excavation of the main powerhouse. In the SCZ, the compressive stress is high and leads to surrounding rock mass spalling, splitting and shotcrete cracking [17]. The impact scope of SCZ is large and expands upstream with the excavation progress. As a result, LPL5-1 is affected and subjected to high stress. On the other hand, the stress adjustment caused by the excavation of LPL5-1 generates a SCZ at the upstream arch. Since the in situ maximum principal stress inclines slightly to the upstream side, compressive stress concentration occurs at the upstream arch of the main powerhouse and the corridor-shaped caverns parallel to the powerhouse during excavation. This is confirmed by the numerical simulation results and the field spalling in LPL5-1. The SCZ induced by the excavation of the main powerhouse, in combination with the SCZ induced by the excavation of LPL5-1, puts the surrounding rock mass at the upstream arch of LPL5-1 under high compressive stress, which provides the prerequisite for the failures here. The field investigation found that there is core discing on the upstream side, while there is none on the downstream side, which proves the higher stress at the upstream arch. 4 Correlation in failure evolution  
The extent and evolution of failures in LPL5-1 is closely linked with the excavation progress of the main powerhouse. As the excavation progressed, the failures of shotcrete and floor concrete in LPL5-1 developed.