## 4 Analysis of Results
### Fracture Behaviors of Basalt Rock Mass during Excavation
#### 4.2.3 Failure Mechanism of Basalt Rock Mass  
Based on the failure characteristics and engineering geological conditions of the surrounding rock mass in the underground powerhouse at Baihetan Hydropower Station, it was found that hard brittle lithologies, complex rock mass structures, and high ground stresses are the basic geological environmental factors causing the failure of the surrounding rock mass in the underground cavern. In particular, the fracture evolution laws and failure characteristics of the surrounding rock mass were highly correlated with the rock mass structures and in situ stresses. Therefore, the effects of these two aspects on the failure of the surrounding rock mass in the underground cavern were systematically investigated, further revealing the failure mechanism in hard rock engineering under high geostresses. In terms of the initial in situ stresses of the left-bank underground powerhouse, the maximum principal stress with a value range of 19-23 MPa was horizontal and intersected with the axial direction of the underground powerhouse at a large angle. The intermediate principal stress was also horizontal, with a value range of 13-16 MPa, and was approximately parallel to the axial direction of the underground powerhouse. The third principal stress was approximately vertical to the axial direction of the underground powerhouse, with a value range of 8-12 MPa. Under the influence of this initial stress condition, the stress concentration area of the surrounding rock mass near a tunnel face after underground cavern excavation was mainly located in the upstream roof and downstream floor, with a value in the range of approximately 60-80 MPa. This value had reached or exceeded the UCS of basalt blocks at the REV size, resulting in the spalling failure of the surrounding rock mass mainly located in the upstream crown and spandrel and downstream footing, floor, and side walls, indicating that the distribution position of the spalling failure of the surrounding rock mass in the underground cavern was perpendicular to the maximum principal stress direction. From a mechanical perspective, the above failure phenomenon can be essentially explained as follows: under a specific initial in situ stress field, the stress concentration zone position of the surrounding rock mass after excavation is approximately vertical to the initial maximum principal stress direction, and the stress direction remains unchanged. Once the stress level reaches the critical condition, the surrounding rock mass fails, with the fracture surface being approximately parallel to the maximum principal stress direction. Therefore, the magnitude and direction of the in situ stress field determine whether the surrounding rock mass undergoes damage, as well as the main distribution location of the failure after underground cavern excavation; that is, the in situ stress is the main external factor resulting in the failure of the surrounding rock mass. The main lithology of Baihetan Hydropower Station is cryptocrystalline basalt, which is fresh, dense, and hard with good integrity. In addition to the engineering-scale structural planes, such as random joints, cracks, faults, and staggered zones exposed in the rock mass, small-sized hidden cracks generally exist in rock blocks. The value of the stress concentration area of the surrounding rock mass after underground cavern excavation is approximately in the range of 60-80 MPa, while the average value of the UCS of basalt with a standard size is approximately 200 MPa, indicating that the basalt is difficult to be damaged under this stress condition. However, due to the existence of hidden cracks,the UCS of the basalt blocks was significantly reduced, with a value of approximately 60 MPa at the REV size. The strength was lower than that in the stress concentration area, resulting in significant spalling failure of the surrounding rock mass.