# Rock Mass Structure Classification of Caves Based on the 3D Rock Block Index
## 1 Introduction  
A rock's mass structure is the basis and important control factor for rock mass quality evaluation and stability analyses [1; 2; 3; 4; 5]. The quantitative indexes of rock structure classification mainly include structural plane spacing, integrity coefficient, volume nodule number, rock quality index, etc. [6; 7; 8; 9; 10]. At present, the most commonly used index of rock structure classification is the rock quality index (RQD) [11; 12; 13; 14; 10]. The rock quality index is a quantitative parameter that responds to the degree of engineering rock integrity and rock structurecharacteristics [15], but it suffers from the defects of directional one-dimensionality and threshold singularity [16; 17; 18]. However, these quantitative indexes cannot directly reflect the structure type and block composition of a rock's mass. In order to comprehensively characterize the size of a rock's mass and its structural characteristics to reflect the integrity of its mass and the change characteristics of its corresponding mechanical properties, [PERSON] proposed the concept of rock block index (RBI). RBI is defined as the cumulative value of the product of the respective coefficients obtained by weighting the measured core lengths in flat caves or boreholes according to the core acquisition rates of 3 to 10 cm, 10 to 30 cm, 30 to 50 cm, 50 to 100 cm, and greater than 100 cm. It has been successfully used in Ertan, Jinping I, and other large hydropower projects [19]. Based on the Xiluoud hydropower project, [PERSON] proposed a quantitative RBI value method [20; 21]. [PERSON] analyzed and studied the quantitative value of the rock block index and established critical values of rock block index classification for the overall structure, blocky structure, secondary block structure, mosaic structure, cataclastic texture, and loose structure [22]. In order to express the three-dimensional structural characteristics of the dam foundation's rock mass, [PERSON] proposed the three-dimensional equivalent rock block index. It is defined as the arithmetic mean of the RBI in orthogonal planes in three different directions of the rock mass, but it has not been applied to underground engineering [23; 24].  
This paper takes the long exploratory cave CPD1 in the water transmission and power generation system of the Qingtian pumped storage power station in Zhejiang Province as the research object. According to the distribution characteristics of joint groups, the probabilistic models of joint geometric parameters were obtained statistically. The Monte Carlo method was used to carry out random simulations and generate three-dimensional joint network models. On this basis, virtual survey lines representing boreholes were arranged on the front, side, and top surfaces of the 3D network model at the same angles (\(5^{\circ}\)) as the center, respectively, and the rock mass block index of 108 virtual survey lines on the three planes was obtained statistically. Then, the concept of the 3D rock block index was utilized to finely classify the peripheral rock structure of the flat cave. Using the above method to classify the structure of the rock mass in flat caves, this paper realizes the application of the three-dimensional rock mass index in underground engineering. This approach solves the shortcoming of the traditional classification index RQD, which cannot accurately assess the structural integrity of rock masses. Additionally, it overcomes the spatial dimension limitations of RBI in classifying rock mass structures, allowing it to more truly reflect the anisotropy of rock masses.