## 3 Excavation Procedures and In Situ Stress Level
### High Side Wall Zone  
The excavation and support of high side wall zone are generally carried out in stratification mode, the characteristic of which are a large section, as shown in Figure 16. Generally, deep-hole bench millisecond blasting is used to excavate the middle rock mass, and smooth blasting and presplitting blasting are used to control the excavation contour. Many construction experiences show that there are three main excavation methods (see Figure 17) for the high side wall zone: 1 MEP method, which is the same as that of the rock anchor beam zone; 2 Contour presplitting followed by middle bench blasting excavation, that is CPHB; 3 Contour presplitting followed by half blasting excavation, that is CPHB. For the zone nearest to the rock anchor beam zone, the MEP method will be replaced by the MEP(P) method to improve the excavation quality by presplitting blasting. The second method (CPMB) continues to emphasize the role of contour presplitting on isolation and crack resistance, and then uses the middle trench excavation to gradually expand the excavation. The third method (CPHB) is two-half excavation combined with deep-hole presplitting blasting at the contour. It aims to reduce the influence of blasting on surrounding rock and ensure the excavation quality. Combined with the powerhouses in Table 1, the procedures and the key design parameter used in the high side wall zone were summarized and plotted in Figure 18.  
According to the statistics, the MEP method has become the main construction method in a high side wall zone, accounting for about 64% while the CPMB method accounts for about 29% and the CPHB method accounts for about 7%. For high in situ stress (>20 MPa), the MEP and CPMB methods are in a dominant position to adjust the excavation disturbance process. The key excavation parameters, such as stratification height and protective cover thickness, play a leading role in the stress adjustment path. Different  
Figure 16: High side wall zone of underground powerhouse. (**a**) The excavation diagram of the high side wall zone. (**b**) The engineering scene of the high side wall zone. Figure 15: The relationship between protective cover thickness and in situ stress level at the rock anchor beam zone. stratification height and protective cover thickness correspond to different stress paths, which will lead to a different stress re-distribution. To study the influence of in situ stress level on the key design parameters of the high side wall zone excavation, the relationship between stratification height, protective cover thickness and in situ stress level is analyzed, and the results are shown in Figure 19.  
As shown in Figure 19, with in situ stress level increasing to about 40 MPa, the stratification height shows a downward trend, and the protective cover thickness increases gradually. Specifically, to control the excavation disturbance effect, when the local stress level is low (<10 MPa), the stratification height is about 5.0-8.0 m, and the protective cover thickness is about 1.0-3.5 m. When the local stress level is high (>20 MPa), the stratification height is about 3.0-7.0 m, and the protective cover thickness is about 3.0-5.0 m. It is basically consistent with the requirements in the specification (NBT35090-2016) [72], where the stratification height is generally 4-8 m.  
Figure 19: The relationship between key design parameters and in situ stress level at the high side wall zone. (**a**) The changing trend of stratification height. (**b**) The changing trend of protective cover thickness. Figure 17: Excavation procedures of the high side wall zone. (**a**) MEP method; (**b**) CPMB method; (**c**) CPHB method. Figure 18: Statistics of excavation procedures at the high side wall zone. (**a**) The usage proportion of excavation procedures. (**b**) In situ stress distribution of excavation procedures. The analyses above reveal the influence of high in situ stress level on excavation procedures and indicate the correlation between the key design parameters and in situ stress.