## 3 Excavation Procedures and In Situ Stress Level
### Top Arch Zone  
The top arch zone, self-stability structure, has been widely used in underground excavation development [68, 69], as shown in Figure 8. Today, the height of the top arch zone in powerhouses is generally about 7-8.5 m [53] and the arch design of large underground caverns tends to have a higher rise-span ratio, generally 0.30-0.35 [70]. Besides, the three-center circle was used to replace the single-center circle in order to reduce the stress concentration failure around the arch abutment. To control the excavation effect under high in situ stress, [PERSON] et al. [42] also suggested that the rise-span ratio should be adjusted according to the stress level or strength-stress ratio. Due to the large section of the top arch zone, the excavation method of once-forming construction is generally not adopted. The more conventional method is to divide the zone into three parts: middle and both sides. Based on different excavation sequence of the parts, there are generally two main kinds of excavation procedures described in the following. Figure 9 shows the detailed design and sequence of two main excavation procedures. In Figure 9a, the method of middle-drift connection followed by both-sides excavation (MDBS) is presented. The middle-drift connection is to solve the difficulty of ventilation and to diagnose the rock state, the requirements of which on contour are not strict. The forming of the middle-drift connection is helpful to restrict the deformation of the middle of the top arch early. Moreover, the zone of 1-1 and 1-2 in Figure 9a can be a combination, which is the most commonly used and simple without repeated support procedures. Especially under high in situ stress, it started to carry out the reserved protective cover near the excavation contour, that is the MDBS(P) method. The reserved protective cover is beneficial to control the excavation damage and vibration of surrounding rock. Moreover, the method of both sides heading followed by middle pier excavation (BSMP) is provided in Figure 9b. It is suitable for large-span underground caverns (>15 m), where only simple support is needed. The BSMP method needs to carry out undercut blasting twice in 1-1 and 1-2 separately,  
Figure 8: Top arch zone of an underground powerhouse. (**a**) The excavation diagram of the top arch zone. (**b**) The engineering scene of the top arch zone. which has high excavation cost and slow progress. Yet, it can make the stress concentration zones reinforced, firstly, on both sides, to protect the spandrel, which has also become a construction option. Combined with powerhouses in Table 1, the excavation procedures and the key design parameters used in the top arch zone were further summarized and plotted in Figure 10.  
Obviously, the MDBS method is the most common excavation procedure used in the top arch zone of underground powerhouses, about 90% (see Figure 10), including Ertan, Houziyan, etc. Among it, there is a new trend to set a protection cover (generally 0.5-2.0 m) near the blasting contour, about 27%, that is the MDBS(P) method. For the large section, BSMP is in common use, accounting for 10%, such as Longtan station. Furthermore, in situ stress distribution of methods are presented in Figure 10. It can be found that the MDBS and MDBS(P) methods are the most common at each stress level while the BSMP is mainly used under in situ stress of 20 MPa. The initial stress level has an important influence on the method selection, just as [PERSON] et al. [69] found that the influence of in situ stress on cavern excavation stability was obviously greater than that of GSI and rise-span ratio. To study the influence of in situ stress level on the key design parameters of top arch zone excavation, the relationship between height (H), span (S), H/S of the top arch zone and in situ stress level is analyzed, and the results are plotted in Figure 11.  
Evidently, H and S of the top arch zone will increase as in situ stress level rises, which also reflects the current trend of an increasing powerhouse scale.