## 4 Analysis of Results
### Fracture Behaviors of Basalt Rock Mass during Excavation
#### 4.2.2 Fracture Evolution Laws of Basalt Rock Mass during Excavation  
The left-bank underground powerhouse was excavated in 10 layers due to its large size. For the first layer (I), the scheme of first excavating the middle pilot tunnel and then expanding the excavation on two sides in sequence was adopted. On this basis, each remaining layer (II-X) was excavated step by step. Based on the focus of this paper, in the numerical simulation, only a systematic analysis and elaboration of the intact basalt rock mass was conducted, in which the excavation sequence was the same as the actual situation of the underground powerhouse. After the numerical simulation calculations, the fracture evolution process parameters of the basalt rock mass during the excavation of each layer of the left-bank underground powerhouse, such as the stress fields, deformation fields, and crack propagation, were quantitatively analyzed, as shown in Figure 16, and they were compared with the project site monitoring results or existing results reported in the literature. Thus, the progressive fracture laws of the basalt rock mass in the underground  
Figure 14: Typical failure characteristics of a intact basalt rock mass of the underground powerhouse during excavation at Baihetan Hydropower Station [45]. cavern under high ground stresses and strong excavation unloading could be thoroughly revealed [46]. As shown in Figure 16a, during the middle pilot tunnel excavation stage of the first layer, the initial in situ stress balance was broken to form a secondary stress with continuous excavation, mainly including a stress release area and a stress concentration area. The stress concentration area was mainly distributed in the upstream roof and the downstream floor, and some local corners, such as the upstream footing and downstream spandrel, were also in the stress concentration area, with the stress magnitude ranging from 30 to 60 MPa, while the stresses of the sidewalls in the upstream and downstream were significantly reduced. The displacement concentration area was mainly located at the sidewalls of the upstream and downstream and the downstream roof, which was roughly opposite to the distribution characteristics of the stress concentration area, i.e., an approximately vertical relationship, and the overall deformation value was small, generally in the range of 5-10 mm, which was mainly the elastic deformation due to excavation unloading. With the continuous excavation advancement and stress adjustment, cracks started to initiate along the maximum principal stress direction once the stress concentration exceeded the ultimate strength of the basalt rock mass, and continued to expand, penetrate, and coalesce to form local plate or layered spalling failure, with the fracture surface being approximately parallel to the excavation-free surface. Spalling failure was mainly located at four corner areas; in particular, the spalling failure in the upstream spandrel or top arch was significant, corresponding to the distribution features of the stress concentration area. The depth of  
Figure 15: Typical failure characteristics of a jointed basalt rock mass of the underground powerhouse during excavation at Baihetan Hydropower Station [45]. (**a**) Spatial form of spalling failure; (**b**) Sketch of spalling failure; (**c**) Enlarged diagram of failure modes; (**d**) Thin slab after spalling failure.