## 2 Structural Characteristics of Basalt  
### Engineering Background  
The Baihetan Hydropower Station, which is a major national project of \"West-East Power Transmission\" in China, is located in the lower reaches of the Jinsha River between the provinces of Yunnan and Sichuan. It is the second-largest hydropower station in the world, with a total installation capacity of 16,000 MW [39]. The hydraulic buildings mainly include water diversion and power generation systems, a dam, and flood discharge and energy dissipation systems; the height of the concrete double-curvature arch dam is 289 m. The water diversion and power generation systems on the right and left banks are respectively located in the mountains on both sides of the arch dam upstream, which are composed of four caverns arranged in parallel from upstream to downstream, i.e., the main powerhouse, main transformer cavern, tailrace overhaul gate chamber, and tailrace surge chamber. The excavation size of the underground cavern on the right and left banks is the same, where the main powerhouse is 438 m long, 31-34 m wide, and 88.7 m high; the rock wall thickness between the main transformer cavern, with a height of 39.5 m, a length of 368 m, and a width of 21 m, and the main powerhouse is 60.65 m; and the rock wall thickness between the cylindrical tailrace surge chamber, with a height of 77-93 m and a diameter of 43-48 m, and the main transformer cavern is approximately 85 m. Thus, the entire underground cavern group constitutes the largest underground project in the world, as shown in Figure 1.  
Figure 1: Layout of the underground caverns at Baihetan Hydropower Station, where 1–4 represent the main powerhouse, main transformer chamber, headgate chamber, and tailrace surge chamber, respectively, and I–X represent the excavation sequences of the cross section of the underground powerhouse from top to bottom [39]. (**a**) Location of the study area; (**b**) layout of underground caverns; (**c**) 3D visualization of left-bank underground caverns; (**d**) right-bank underground caverns. Geologically, the Baihetan Hydropower Station belongs to a \"V-shaped\" landform in a deep-cut valley area and is located in the Emeishan basalt stratum, with the main lithologies being cryptocrystalline basalt, amygdala basalt, and oblique basalt in P2\(\beta\)\({}^{2}\)-P2\(\beta\)\({}^{6}\) layers. Basalt is mostly fresh, dense, and hard, with good integrity, belonging to a complete or massive structure, and it is mainly dominated by class III surrounding rock mass. Due to geological tectonics and special diagenesis, the structural characteristics of the massive basalt (different from columnar-jointed basalt) are complex. In addition to the series of geological structures exposed in the basalt rock mass at the engineering scale, including randomly developed hard structural planes with a medium size (i.e., meters to ten meters), such as joints and cracks, and locally developed weak structural planes with a large size (i.e., more than 10 m), such as long fissures, faults, and dislocation zones, there are a large number of small non-interpenetrating hidden cracks (i.e., centimeters to decimeters) in the basalt blocks, belonging to Class-V hard structural planes [40]. Among the different lithologies, cryptocrystalline basalt is the most prominent. That is, the basalt in this region has significant multiscale structural characteristics, as shown in Figure 2.  
The underground caverns of the Baihetan Hydropower Station are under medium-deep buried conditions, with the in situ stress being dominated by the valley tectonic stress, i.e., the horizontal stress is greater than the vertical stress, the first and second principal stresses are horizontal, and the third principal stress is roughly vertical.