## 5 Discussion
### Fracture Mechanism of Hard Brittle Rocks  
Spalling failure is a typical failure mode observed in hard brittle rocks in deep underground engineering construction and has been systematically explored, with the most representative ones being studies on the fracture problem of marble in the Jinping II Hydropower Station deep-buried diversion tunnel and LdB granite in the URL of AECL. Although the in situ stress and buried depth of these underground caverns are much higher than those of the underground caverns of the Baihetan Hydropower Station, the universality and severity of the spalling failure in marble or granite are not as common as those in basalt. This contradictory relationship may be due to the structures of the rock itself. Therefore, the results of this study are compared with two typical hard brittle rocks in terms of the failure characteristics, structural characteristics, and failure mechanisms, as shown in Figure 19, to understand the fracture problem of brittle rocks more comprehensively and systematically [50; 51; 52]. A rock is a typical heterogeneous material at the grain scale, typically including geometric heterogeneity caused by changes in the grain spatial distributions, shapes, and sizes; mechanical inhomogeneity due to variations in the grain densities, stiffness, and strength; and initial defect structures such as grain boundaries, cleavages, microcracks, and pores. The grain-scale structural characteristics control the meso-mechanical behavior of the grain itself, such as the crack evolutions, stress fields, and displacement fields, thereby controlling the macro-mechanical behavior of the entire rock, such as the deformation, strength, and failure characteristics [53]. As shown in Figure 19a, for cryptocrystalline basalts with an intergranular-intergranular structure, the main mineral components are feldspar, vitreous, pyroxene, and chlorite; the particle shapes are stripped, columnar, and granular; and the particles are extremely small (\(\leq\)0.1 mm on average), without any microcracks inside or between them, indicative of a good grain-scale uniformity. However, due to the existence of hidden cracks, the homogeneity of basalt at the rock block scale is poor. For granite mainly composed of quartz, feldspar, and mica and for marble mainly composed of dolomite and calcite, with a granular structure, the particle shape is interlocking polygonal, the particles are large (approximately 0.5 mm on average), and there are evident microcracks inside or between the grains, indicative of significant grain-scale heterogeneity. The meso-structural characteristics of various brittle rocks are significantly different and are strongly associated with their diagenetic mechanisms. This structural difference is bound to have an important influence on the rock failure characteristics and mechanism. The splitting failure is a typical failure mode for hard brittle rocks under uniaxial compression, such as marble, granite, and sandstone, showing flaky or plate rock slices  
Figure 19: Structural characteristics and failure modes of several different types of hard brittle rocks, including basalt (**left**), marble (**middle**), and granite (**right**) [50; 51; 52]. (**a**) Polarized microscopy images of a thin section of different types of hard brittle rocks; (**b**) failure modes of different types of hard brittle rocks under uniaxial compression; (**c**) failure modes of different types of hard brittle rock mass on site. after failure, as shown in Figure 19b. This failure mode is related to the grain-scale heterogeneity. Intact basalt is dominated by a fragmentation failure mode under uniaxial compression, showing rock fragments after failure; that is, the fragmentation failure is the most evident feature of cryptocrystalline basalt that makes it different from other brittle rocks. This failure mode is related to its good grain-scale homogeneity.