## 2 Accurate Identification Method of Fractured Rock Structure
### Concept and Determination Method of Occlusion Degree of Fractured Rock Masses
#### 2.3.5 Calculation Method of Occlusion Degree  
The degree of occlusion of the broken rock body from the geometric point of view depends firstly on the combination relationship between the blocks, and there are usually edge-edge contact, edge-angle contact, embedded contact, and other types between the blocks, in the three-dimensional diagram of the structural surface, by discriminating the contact type of each block with its contact block and assigning values to each block; edge-edge contact is 1, edge-angle contact is 2, and embedded contact (concave-convex contact) is 3 so that we can get the value of the contact type between the blocks. In this paper, the value of the contact type between blocks is called the effective contact number. Using the simulated annealing algorithm, all blocks are traversed, and the previously calculated contact type values are not repeatedly calculated. Assuming that block 1 is in contact with the surrounding blocks 2, 3, and 4, all blocks in contact with it participate in the calculation when block 1 is calculated, but when block 2 is calculated, the number of contacts with block 1 is not calculated. Calculate the occlusion degree T according to Equation (2). \[\text{T}=\frac{\sum_{1}^{m}Y_{2}+\sum_{1}^{m}Y_{3}}{\sum(Y_{1}+Y_{2}+Y_{3})} \times 100\% \tag{2}\]  
where \(Y_{1}\), \(Y_{2}\), and \(Y_{3}\) are the number of edge-edge contact, edge-angle contact, and embedded contact, respectively. Obviously, the larger the effective contact number of blocks, the better the degree of rock occlusion, and the more sides a certain block makes contact with the surrounding blocks, the more broken the rock body is. When a large underground cavern is excavated into an arch, the crushed surrounding rock around the cavern will form an occlusion structure, and the model of the crushed rock body occlusion structure is shown in Figure 4 below. As shown in Figure 4, occlusion is generated between the fractured rock blocks around the cavern to maintain the stability of the structure, and when occlusion failure occurs between the occluded fractured blocks, it leads to blocking destabilization until the occluded stable state is reached again. 3.6 Evaluation of Occlusion Degree and Evaluation of the Risk of Block Occlusion Failure in Each Position  
From the above study, it can be seen that the occlusion degree is one of the important indexes for judging and evaluating the quality of the fractured rock mass, and also for evaluating the ability of the fractured rock mass to maintain self-stability under the action of gravity or with ground stress and engineering forces. It can be seen that under the same ground stress and lithological conditions, the occlusion degree of the fractured rock mass is closely related to the structural characteristics and weak surface characteristics of the fractured rock mass, so the classification evaluation of the strength of the fractured rock mass occlusion degree proposed by combining these two aspects is shown in Table 3.  
After the excavation of the underground chamber, the broken rock forms an occlusion structure, and when there are multiple penetrating structural surfaces, the block will produce a sliding tendency along the structural surface; sometimes the sliding surface is two, and its sliding direction will be consistent with the direction of the combined intersection line of the structural surface, and the sliding inclination is consistent with the inclination of the combined intersection line. Even when the force of the block-producing sliding tendency is gravity, its sliding direction is consistent with the direction of the lowest potential energy sliding surface or the combined intersection line.