## 2 Accurate Identification Method of Fractured Rock Structure
### Safety Criteria for the Strength of the Underground Cavern Enclosure of Broken Rock Body
In 3 DEC discrete element software, a fixed displacement boundary is set at the bottom to limit the deformation of the bottom; The displacement boundary and stress boundary are set at the front, back, left, and right of the model, and the initial geo-stress is set inside the model. Under engineering conditions, the initial geo-stress is mainly generated by gravity. The vertical stress is converted by gravity, the horizontal stress is taken as 0.5 times the vertical stress, and the stress value increases linearly with depth. The rock mass model is set as an elastoplastic Mohr-Coulomb model and the structural plane is set as a Coulomb slip model. In the model calculation, 5 monitoring surfaces are arranged in the main powerhouse, 25 monitoring points are arranged in each monitoring surface, 5 monitoring surfaces are arranged in the main transformer room, and 10 monitoring points are arranged in each monitoring surface. The layout of monitoring points is shown in Figure 11 below, and the layout of monitoring surfaces is shown in Tables 10 and 11 below. The key to underground cavern enclosure damage prediction is to calculate the parameters, at present, the physical-mechanical parameters of rock masses are mainly taken by the test method, engineering analogy method, and displacement inverse analysis method, due to the factors of personality and number of sampling points of the test method, it is difficult to completely represent the parameters of different locations, engineering analogy method needs a lot of relevant engineering experience, therefore, displacement inverse analysis is a better method to determine the physical-mechanical parameters of rock masses. According to the structural characteristics of the fractured rock masses in Liyang, the monitoring information of the excavation and support process and the numerical analysis method are used to invert the physical and mechanical parameters of different fractured rock masses. Therefore, it is necessary to compare the actual engineering monitoring situation with the trial calculation results, and the trial calculation results can be used only when they are more in line with the actual situation. The parameters selected for this simulation are determined by combining the actual situation of the area and the corresponding trial calculation results. Three types of surrounding rocks (IV-1, IV-2, and IV-3) are used for this parameter inversion simulation calculation, and three sets of parameters are selected for each surrounding rock for numerical calculation, and the most suitable parameters are selected only after comparing with the actual monitoring values. According to the safety monitoring report of the Liyang underground powerhouse, the monitoring sections C1, C2, F1, C3 and C4 of the Liyang underground powerhouse correspond to the monitoring sections A1, A2, A3, A4 and A5 in the numerical calculation, respectively. The displacement monitoring values are shown in Table 12 below. By comparing the actual monitoring values at the crown of each section with the displacement monitoring values at the crown of each section through numerical simulation, the mechanical parameters of different broken rock masses are obtained as shown in Table 13.  
The evolution characteristics of the plastic zone are analyzed by comparing the supported and unsupported conditions of type III-2 surrounding rock at the A4 monitoring surface; see Figures 12 and 13. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Actual Monitoring Surface** & **C1** & **C2** & **F1** & **C3** & **C4** \\ \hline Displacement value (cm) & 1.80 & 1.10 & 1.72 & 2.30 & 2.67 \\ \hline Simulated monitoring section & A1 & A2 & A3 & A4 & A5 \\ \hline Displacement value (cm) & 1.14 & 1.10 & 1.81 & 2.53 & 4.42 \\ \hline \hline \end{tabular}
\end{table} 
Table 12: Displacement monitoring of monitoring section.