## 3 Excavation Procedures and In Situ Stress Level
### Rock Anchor Beam Zone  
Rock anchored beam zone refers to the zone that the crane beam structure is arranged in polygonal-shape. When the zone is excavated, the reinforced concrete beam will be firmly anchored to the rock by grouting a long bolt, which means that the load it bears is transmitted to the rock mass through the friction force. Due to the responsibility for the subsequent installation and maintenance of electromechanical equipment of powerhouses, the rock excavation quality of the rock anchor beam zone is directly related to the installation efficiency and safety. Thus, the layout of structure requires a high quality of surrounding rock in this zone. The outline of a rock-anchored beam is presented in Figure 12.  
The existence of the rock anchor beam reduces the excavation span of the powerhouse, which increases the cavern stability and reduces engineering cost. It has been proved that the rock anchor beam is affected by the blasting vibration wave, and the adjustment of blasting construction procedure should be considered carefully [71]. In general, to handle the blasting vibration situation, the excavation of the rock anchor beam zone is carried out after the blasting test in other parts firstly. Now, the main construction method is:'middle-drift excavation firstly, expanding excavation followed by protective cover excavation',  
Figure 11: The relationship between height (H), span (S), H/S of the top arch zone and in situ stress level.  
Figure 12: Rock anchor beam zone of underground powerhouse. (**a**) The excavation diagram of the rock anchor beam zone. (**b**) The engineering scene of the rock anchor beam zone.  
that is the MEP method, as shown in Figure 13. The MEP is the basic excavation procedure method for the rock anchor beam zone while the MEP(P) method is an improved means to control the vibration by presplitting blasting.  
Combined with the powerhouses in Table 1, the procedures and the key design parameter used in the rock anchor beam zone were summarized and plotted in Figure 14.  
Today, the MEP method has become the mainstream excavation procedure for the rock anchor beam excavation of underground powerhouses. By reserving protective cover, the influence of blasting vibration on the rock anchor beam is reduced. Meanwhile, the auxiliary technology, presplitting blasting, plays the role of isolation and crack resistance. It has also become the key choice to ensure the rock platform excavation quality. From Figure 14b, the methods are used at various in situ stress levels. However, the value of the key parameter, protective cover thickness, has not been determined well. To study the influence of in situ stress level on the protective cover thickness of the rock anchor beam zone excavation, the relationship between protective cover thickness and in situ stress level is analyzed, and the results are shown in Figure 15.  
In Figure 15, the basic distribution of protective cover thickness in the rock anchor beam zone is between about 1.0-6.0 m, and the protective cover thickness will increase as in situ stress level rises, which is indicated by the arrow. Therefrom, it is suggested that the protective cover thickness has to be about 1.0-5.0 m under lower in situ stress (<10 MPa) or 3.0-6.0 m under high in situ stress (>20 MPa), to control the excavation disturbance induced by high in situ stress.  
Figure 14: Statistics of excavation procedures at the rock anchor beam zone. (**a**) The usage proportion of excavation procedures. (**b**) In situ stress distribution of excavation procedures.  
Figure 13: MEP and MEP(P) methods of the rock anchor beam zone.