# Numerical Simulation Analysis Method for Rockburst Control in Deeply Buried Caverns
## 5 Analysis of the Rockburst Control Effect in Deep Tunnel Excavation  
In this paper, the engineering measures to contain rockburst in the Jiulongling Water Conveyance Tunnel Project in the Quanmutang Irrigation Area of Hunan Province are simulated and analyzed by using borehole stress relief and anchor rod support. The calculation program was developed on the basis of the three-dimensional elastoplastic damage finite element program developed by the Underground Engineering Research Group of Wuhan University using Fortran. The three-dimensional elastoplastic damage finite element calculation software (FeaGraphics Version 1.7) was fully validated in more than 100 large hydropower station underground caverns in China and worldwide. The specific calculation method is shown in reference [31]. The analysis results of Quanmutang guided the practice of relevant support measures. According to the lithology of the tunnel, we took the section of the Jiulongling tunnel with a buried depth of about 280-300 m and established a three-dimensional finite element model with a total of 200,480 elements, as shown in Figure 3. The lithology of the surrounding rock is limestone and the physical parameters of the rock mass are as shown in Table 1.  
Figure 3: Finite element calculation grid. 1 is Class II surrounding rock, and 2 is Class III surrounding rock. According to the tunnel topography and burial depth, the initial geostress inversion is carried out. We can obtain the following results: the horizontal direction of the first principal stress at the tunnel is consistent with the direction of the tunnel axis and its value is between \(\sigma_{1}\) = 19-23 MPa. The horizontal direction of the second principal stress is perpendicular to the direction of the tunnel axis and its value is between \(\sigma_{2}\) = 14-18 MPa. The vertical direction of the third principal stress is consistent with the lithostatic stress and its value is between \(\sigma_{3}\) = 10-13 MPa. The geostress belongs to the medium-high stress level. Based on the geological conditions and rock mass characteristics, it is preliminarily determined that medium rockburst may occur during tunnel construction. In this paper, we use the elastoplastic damage finite element to simulate the damage evolution process of the surrounding rock during tunnel construction and excavation. Strength theory and energy theory are used to analyze the effectiveness of borehole stress relief and anchor rod support in controlling rockburst during tunnel excavation. The details are described as follows. We use the incremental variable plastic damage stiffness method to carry out the staged loading iterative calculation for the diversion tunnel excavation and the distribution of rockburst grades is calculated according to the energy theory and the strength theory (see Figure 4). We can see from the figure that the rockburst grades and distribution laws are basically the same and that both are medium rockburst of a large scale on the tunnel face, with a volume of 9.0 m\({}^{3}\). Light rockburst occurs around medium rockburst on the tunnel face and at the entire bottom turning range of the tunnel, with volumes of 10.3 m\({}^{3}\) and 8.4 m\({}^{3}\). The strong and extremely strong rockburst calculated by both theories occur at the corners of the tunnel bottom with high-stress concentration but the volumes do not exceed 0.8 m\({}^{3}\). We can see that when we use energy theory and strength theory to evaluate the occurrence and grade distribution of rockburst in construction and excavation, rockburst is concentrated in the parts with enormous elastic strain energy release and large stress concentration which indicates that the incremental variable plastic damage stiffness method can fully reflect the mechanism and characteristics of rockburst occurrence calculated by energy theory and strength theory.