## 5 Rockburst Warning Based on a Mechanical Model
### Warning Method and Application Cases  
The two-body interaction model and 3S model complement each other in describing the mechanical process and stress evolution cycle of rockburst formation. Combining the two methods, a rockburst warning system based on a mechanical model is proposed. Based on the source parameters, the rock stability stage is determined using the two-body interaction model, and the risk and formation period of the rockburst are obtained. According to the 3S theory, the stress adjustment process of the surrounding rock is analyzed to predict the evolution trend of a rockburst, or to evaluate the effectiveness of rockburst prevention measures. Figure 7 shows the flowchart.  
Figure 7: Flowchart of rockburst warning system based on a mechanical model.  
Taking the Lingbei TBM construction section of the Hanjiang-to-Weihe River Diversion Project as the engineering background, two rockbursts occurred successively on the right wall of the tunnel on 6 and 7 January. Figure 8 shows the evolution characteristics of the number of microseismic events and cumulative energy before and after their occurrence. Before 5 January, the number of microseismic events and microseismic energy were low. On 5 January, the number of microseismic events and cumulative energy increased rapidly, indicating a rapid stress accumulation in the rock mass, which resulted in a large number of fissures. Based on the microfracture activity, the rock mass was judged to be in the second stage of the two-body interaction theory, and the risk of a rockburst was significantly high; therefore, a warning report was issued. On 6 January, a moderate rockburst occurred on the right side of the tunnel. After the rockburst, the microseismic events and cumulative energy continued to increase, indicating that the stress in the surrounding rock had not been effectively released or transferred. The continued accumulation of the stress indicates that the risk of a rockburst remained high; therefore, a warning report was issued again. Subsequently, a strong rockburst occurred on the right side of the tunnel on 7 January. After this rockburst, the number of microseismic events decreased rapidly, and the cumulative energy increased gradually. This showed that the stress in the surrounding rock was effectively released and that the risk of rockburst was alleviated.  
Another aspect is the evaluation of the stress relief boreholes for rockburst prevention. Figure 9 shows the microseismic events and energy characteristics at Chainage K39+210--K39+240 in 4# sub-tunnel of Hanjiang-to-Weihe River Diversion Project from 6:00 AM on 6 September to 6:00 AM on 7 September, 80 microseismic events were distributed at Chainage K39+210--K39+240, among which 21 events had an energy of over 1000 J and the maximum energy was 6848.8 J. The abnormal aggregation of a large number of high energy microseismic events indicates that stress is accumulated in the surrounding rock and that the risk of rockburst is high. Based on the early warning advice, a radial stress relief drilling was applied in the section on 7 September. From 6:00 on 7 September to 6:00 on 12 September, a total of 50 microseismic events were generated in the K39+210-K39+240, and only 11 events had an energy of over 1000 J, and the maximum energy was 3891.4 J. The number and energy of the microseismic events decreased evidently, indicating that stress relief drilling successfully released a part of the elastic strain energy. Finally, this measure effectively prevented the occurrence of rockbursts in the area where microseismic events were concentrated.  
Figure 8: Evolution process of the microseismicities in theLingbei TBM construction section.