## 4 Analysis and Discussion
### Analysis of Cavern Group Excavation Scheme
#### 4.1.2 Crack Propagation Depth Analysis  
After the excavations were over, the surrounding rock crack propagation of the cavern group under the three schemes are shown in Figures 51, 6e and 7e. It can be seen that the surrounding rocks of the cavern group are damaged to different degrees after the completion of the excavation. According to the measurement results, the depth comparison of the rock fracture zone of the cave group in the three schemes is shown in Figures 8-11. Figure 7: Failure process of SR of the underground cavern group during excavation in Scheme 3: (**a**) The excavation began; (**b**) the 1 st step of excavation is completed; (**c**) the 2 nd step of excavation is completed; (**d**) the 3 rd step of excavation is completed; (**e**) the 4 th step of excavation is completed. Figure 8: Crack depth (m) of SR after excavation of main powerhouse. Figure 10: Crack depth (m) of SR after excavation of tailrace surge tank. Figure 9: Crack depth (m) of SR after excavation of main transformer chamber. After the excavation, the following conclusions can be drawn by comparing the failure patterns of the surrounding rocks in Figures 5l, 6e and 7e and the depth of crack propagation in Figures 8-11. * In Scheme 3, there are long cracks in the arch, the left and right side walls and bottom floor of the MP cavern, and the long cracks at the bottom floor are connected with fault f2, as shown in Figure 7e. There are dense cracks in the surrounding rocks between the right wall of the MP cavern and the left wall of the MTC cavern in Scheme 3, forming a broken zone, as shown in Figure 11. Figure 8 shows the crack propagation depth of each part of the MP cavern after excavation in three schemes; as shown in Figure 8, Scheme 3 has the largest crack propagation depth in the arch, left wall and bottom floor, and Scheme 1 has the largest crack propagation depth in the right wall. Scheme 2 has the smallest propagation depth of arch crack, left wall crack and right wall crack, while Scheme 1 has the smallest propagation depth of bottom floor crack. In conclusion, for the MP cavern, Scheme 2 is the optimal scheme. * The damage degree of surrounding rocks in the MTC cavern of Scheme 1 and Scheme 3 is relatively high. Scheme 1 has the largest crack propagation depth in the MTC cavern, as shown in Figure 5l. In Scheme 3, dense cracks appear on the left wall of the MTC cavern, forming a broken zone, as shown in Figure 11. Figure 9 shows the crack propagation depth of each part after the excavation of the MTC cavern in three schemes. According to the data in Figure 9, it can be seen that Scheme 1 has the largest crack propagation in the arch and bottom floor of the MTC cavern, and Scheme 3 has the largest crack propagation in the left and right side walls. Scheme 2 has the smallest crack propagation depth in the MTC cavern, the left wall, the right wall and the bottom floor. In conclusion, for the MTC cavern, Scheme 2 is the optimal scheme. * Schemed 2 and 3 of the TST cavern of rock damage are mainly concentrated on the right side of the arches, on the right side walls and on the bottom floors; the rock damage on the left side of the walls is not apparent, as shown in Figures 6e and 7e, and the destruction of the TST cavern of Scheme 1 is mainly concentrated in the arch and bottom floor, as shown in Figure 5l. Comparing the crack area, it can be seen that Scheme 2 has the lowest degree of damage. Figure 10 shows the crack propagation depth of each part after the excavation of the TST cavern in three schemes. According to the data in Figure 10, Scheme 3 has the largest crack propagation depth in the arch and the right wall of the TST cavern, and Scheme 1 has the largest crack propagation  
Figure 11: Broken zone after completion of excavation in Scheme 3.  
depth in the bottom floor. The crack propagation depth of the left side wall is 0 in all three schemes.