## 4 Analysis of Results
### Fracture Behaviors of Basalt Rock Mass during Excavation
#### 4.2.3 Failure Mechanism of Basalt Rock Mass
From a mechanical perspective, the deformation characteristics of the defect structure, such as hidden cracks and joints, were significantly different from those of intact rocks. It is precisely the existence of uncoordinated deformation caused by the structural heterogeneity that a significant local stress concentration area appeared near the defects after the excavation of underground caverns, causing the local material to fail in advance, ultimately leading to a significant reduction in the rock block strength. Therefore, hidden cracks have a controlling effect on the mechanical behaviors, failure mechanism, and local structures of the surrounding rock mass; that is, the structure itself is the main internal factor resulting in the failure of the surrounding rock mass. In summary, the spalling failure process and failure mechanism of an intact basalt rock mass can be explained as follows: the surrounding rock mass is in an initial 3D compression equilibrium state before the excavation of the underground caverns; that is, the maximum principal stress (i.e., tangential stress) is parallel to the excavation surface, the intermediate principal stress (i.e., axial stress) is parallel to the axial direction of the caverns, and the third principal stress (i.e., radial stress) is perpendicular to the excavation surface, as shown in Figure 17(a-1,b-1). The stress balance of the surrounding rock mass is broken and readjusted after the unloading of the cavern excavation, where the tangential stress increases sharply and the radial stress decreases sharply. In this far-field stress state, the stress concentration degree near the tips of the hidden cracks is more significant, exceeding the ultimate strength of basalt, because of which the tips of the hidden cracks align along the maximum principal stress direction (i.e., tensile crack appearance), and they continue to expand, penetrate, and accumulate to form a local fracture approximately parallel to the excavation surface, as shown in Figure 17(a-2,b-2). With the continuous advancement of the underground cavern excavation surface and further unloading of the surrounding rock mass, the concentration, release, and adjustment of the stress fields, as well as the transformation, accumulation, and dissipation of the energy also vary accordingly. The increase degree in the tangential stress and the decrease degree in the radial stress are more significant, and the radial stress is even reduced to 0, resulting in more local fractures, which continue to expand and coalesce to form splitting rock plates, as shown in Figure 17(a-3,b-3). Under the combined action of the tangential stress and normal supporting stress of the surrounding rock mass, the rock plate gradually bends and deforms toward the free direction, and radial horizontal tensile cracks appear at the point of maximum curvature of the rock plate when the bending develops to a certain extent, as shown in Figure 17(a-4,b-4). With the gradual expansion of the radial tensile cracks, the rock slab breaks and falls off from the parent rock under gravity or blasting disturbance. With the continuous influence of the stress adjustment or nearby blasting disturbance, the rock plate gradually breaks, detaches, and peels off from the surface to the inside. When the stress state of the surrounding rock mass within a certain depth range or the artificial support provide sufficient constraints to inhibit crack propagation, the development of the notch tip stops, leading to a stable V-shaped spalling pit, as shown in Figure 17(a-5,b-5). Therefore, the spalling failure of a seemingly intact basalt rock mass is not a simple tensile fracture mechanism caused by stress concentration after excavation but is rather a tensile fracture mechanism under the coupling of high stress concentration and complex structures.