# Stability Index of Surrounding Rock during Deep Rock Excavation Considering Energy Release Speed
## 4 Results and Discussing  
Stress balancing was performed before the excavation, and all units' strain energy densities were recorded at every iterative step using a customized variable. The variation process of the energy density of the maximum and minimum principal stress direction depths of 0 m, 0.2 m, 0.5 m, 1 m, 2 m, and 5 m with the simulation process is shown in Figure 5. It can be seen from the figure that stress release occurred in the unit in the direction of maximum principal stress at the moment of excavation due to the change in the stress state. Although some of the energy was restored during the subsequent stress adjustment, the final value was still less than the initial value. The energy release of the unit in the direction of the maximum principal stress occurred earlier, when the step was about 500, the energy release began at the measuring point buried 5 m, and the earlier the energy release occurred, the shallower the measuring point was buried. It can be seen that the change in stress state caused by the excavation induced the release of energy in the direction of minimum principal stress due to the unloading wave transmitted from the excavation to the depth of rock mass. The energy disturbance caused by the transient stress state change caused by the excavation of the unit in the direction of minimum principal stress could be neglected, and the whole process lasted less than 100 steps. Then, the energy rapidly accumulated during the stress redistribution process, with unit energy near the excavation boundary accumulating faster. When the strain energy accumulated to a certain extent, the unit was destroyed, and a portion of the strain energy was released. The numerical results showed that the maximum strain energy density of the unit on the excavation boundary in the direction of large principal stress was 29,059 J/m\({}^{3}\), occurring before the 100 th step (the excavation moment was the 100 th step). The strain energy density after the energy release was 8499 J/m\({}^{3}\) and it occurred at the 115 th step. It can be seen from Figure 5a that the energy release in the direction of maximum principal stress occurred at the moment of excavation. The reason was that the stress state of the surrounding rock changed from three-direction compression to two-direction compression. The main factor was not the failure of the rock mass. Moreover, as the buried depth of the measuring point increased, the energy release process became smoother and the local energy release rate showed a decreasing trend. From Figure 5b, it can be seen that the strain energy density of surrounding rocks in the direction of minimum principal stress was released after the process of accumulation after excavation. The maximum strain energy density on the excavation boundary in the direction of minimum principal stress was 83,708 J/m\({}^{3}\), occurring in the 839 th step. After the energy release, the strain energy density was reduced to 5594 J/m\({}^{3}\), occurring in the 1081 st step. The local energy release rate was 78,114 J/m\({}^{3}\) and the energy release speed was 323 J/m\({}^{3}\) per step, resulting in an energy release effectiveness of 25.2 \(\times\) 10\({}^{6}\) J\({}^{2}\)/(m\({}^{6}\)-step). For the element buried 0.2 m in the direction of minimum principal stress, the maximum strain energy density was 123,482 J/m\({}^{3}\), occurring in the 1191 st step. After energy release, the strain energy density was reduced to 3477 J/m\({}^{3}\), occurring in the 2465 th step. The local energy release rate was 8705 J/m\({}^{3}\), the energy release speed was 70 J/(m\({}^{3}\)-step), and the energy release effectiveness was 6.2 \(\times\) 10\({}^{6}\) J\({}^{2}\)/(m\({}^{6}\)-step). Although the local energy release rate of the element at the excavation face was smaller than that of the 0.2 m buried depth, the energy release effectiveness was higher at the excavation face.