## 4 Analysis of Results
### Fracture Behaviors of Basalt Rock Mass during Excavation  
#### 4.2.1 Failure Features of Basalt Rock Mass Found in Underground Caverns  
Due to the complex conditions of basalt, such as strong brittleness, high initial geostress, significant multiscale structural features, and huge engineering scale, the fracture problem during the excavation of an underground cavern is prominent, exhibiting various typical hard brittle rock failure modes and response characteristics. Examples include flaking, spalling, spray layer cracking, and block shedding, among which spalling is the most common, bringing significant challenges to the stability control of surrounding rock mass in underground engineering. Generally, based on whether the macro-scale structural planes are exposed near the fracture surfaces of surrounding rock mass, the brittle fracture of an underground powerhouse can be roughly classified into two types: spalling failure of the intact rock mass containing only hidden cracks and spalling failure of the rock mass containing hidden cracks and engineering-scale random joints [45]. Approximately 40% of the spalling failure area is located in the intact surrounding rock mass, without evident engineering-scale structural planes, mainly developing hidden cracks that are discrete, irregular, and short, which have a certain influence on the spalling failure of the surrounding rock mass, as shown in Figure 14. The distribution position of this spalling failure is relatively stable and shows evident regularity, mainly located in the upstream spandrel and arch and downstream footing, floor, and sidewall. The area of the spalling failure zone is generally large, the fracture surface is flat and fresh, parallel to the cavern surface, and the thickness of the spalling rock plate is small, generally in the range of 0-5 cm. Spalling failure belongs to the shallow surface fracture of the surrounding rock mass near an excavation face, corresponding to the highly excavation damage zone (HDZ). In addition, a fracture of a certain depth occurs inside the surrounding rock mass, corresponding to the excavation damage zone (EDZ). The spalling failure depth of the surrounding rock mass of the underground powerhouse is generally in the range of 0-0.5 m, partly in the range of 0.5-1.0 m; the relaxation depth of the top arch surrounding rock mass is generally in the range of 1.0-2.5 m, reaching a range of 2.0-4.0 m in local areas; and the relaxation depth of the side-wall surrounding rock mass is generally in the range of 2.0-5.0 m, with a value range of 6.0-8.0 m locally. This type of spalling failure is a macroscopic stress-controlled failure. In addition to the spalling failure of the intact basalt rock mass, the rest of the spalling failure occurs in a relatively intact massive rock mass with random joints. The rock mass with this type of spalling failure contains fewer joints, and the number of joints per unit volume is mostly in the range of 0-6, generally not more than two groups of joints. It is a hard and unfilled Class IV structural plane with intermittent extension, insufficient development, low connectivity, and a length of approximately several meters, and the joints typically intersect with the tunnel axis and the tunnel wall at a large angle, as shown in Figure 15. The regularity of the spatial location of this type of spalling failure is not as strong as that observed in the intact rock mass, generally located in the top arch and spandrel upstream and in the sidewall, footing, and floor downstream, and other local areas may also appear. The fracture surface is relatively fresh, rough, and uneven, roughly parallel to the excavation surface; most of the initial joints do not fail along the joint surface and remain on the fracture surface; and a small number of initial joints constitute the boundary of the spalling failure area. The thickness of the spalling rock plate is greater than that of the intact basalt rock mass, generally greater than 5 cm, and it is in the range of 10-50 cm in some areas.