## 2 Structural Characteristics of Basalt
### Structural Characteristics of Basalt Blocks
#### 2.2.2 Statistical Characteristics of Hidden Cracks in basalt  
The CT scanning test conducted on the standard-sized basalt core samples provides valuable insights into the distribution characteristics of hidden joints in basalt to a certain extent. However, due to the high CT scanning cost, scanning a large number of basalt samples with a standard size is unrealistic, resulting in limited CT scanning data. Moreover, due to the limited penetration ability of CT scanning and the compactness of basalt itself, CT scanning can only be performed on standard-sized basalt samples, whereas large basalt blocks with a size in the tens of centimeter range are unsuitable, leading to a lack of real and comprehensive data. Therefore, it is difficult to accurately obtain the distribution characteristics of hidden cracks inside basalt by solely relying on the CT scanning results of a limited number of small samples. The following statistical analysis of the superficial hidden cracks in several large basalt blocks is more meaningful.  
Figure 3: Meso-structural characteristics of cryptocrystalline basalt. (**a**) Polarizing microscopy image of a thin section; (**b**) CT scanning images.  
A series of operations, such as digital photography, image processing, and statistical analysis, was performed on a large number of basalt blocks, generally tens of centimeters in size, taken from underground caverns. The distribution characteristics of the geometric parameters of the hidden cracks were obtained, mainly including the crack length, crack density, and crack orientation (which is defined as the angle between the rock block length direction and the crack direction). In this work, 28 basalt blocks containing approximately 800 hidden cracks with lengths greater than 10 mm were selected and analyzed, as shown in Figures 4 and 5. Irregular hidden cracks are generally developed in basalt blocks, with a hidden distribution, short closure, intermittent extension, and low connectivity. The crack size ranges from a few centimeters to tens of centimeters, and the crack orientation has no evident regularities, which may be closely related to its formation mechanism. The crack orientation conforms to a uniform distribution, and the crack length conforms to a logarithmic normal distribution with \(\sigma=0.58\) and \(\mu=3.68\), in which \(\sigma\) represents the standard deviation and \(\mu\) refers to the average value. Moreover, the areal density \(P_{21}\) is 0.014 mm/mm\({}^{2}\), and the areal density \(P_{20}\) is 0.0003 pieces/mm\({}^{2}\), where the areal density \(P_{21}\) is the total crack length per unit sample area, and the areal density \(P_{20}\) is defined as the total crack number per unit area of the sample. Hidden cracks are primary tensile fracture structures formed by the rapid condensation and contraction of magma during the diagenesis of basalt. They belong to the Class-V hard structural plane, mainly affecting the mechanical properties of the rock block and, in turn, having a certain influence on the stability of the surrounding rock mass.  
Figure 4: Physical forms and sketches of basalt blocks with hidden cracks obtained from the underground caverns at Baihetan Hydropower Station.  
Figure 5: Statistical distribution characteristics of the geometric parameters of hidden cracks in a basalt block. (**a**) Crack length; (**b**) crack orientation.