# Mechanisms of Tunnel Rockburst Development Under Complex Geostress Conditions in Plateau Regions
## 4 Influence Mechanisms of Key Geostress Parameters on Tunnel Rockburst Characteristics
### Influence Mechanism of Lateral Pressure Coefficient on Rockburst Risk  
When geostress orientation and tunnel burial depth remain constant, the variation trends across different working conditions are generally consistent with changes in the lateral pressure coefficient \(\lambda\). Therefore, a representative case with a tunnel burial depth of 1500 m and \(\alpha=30^{\circ}\) was selected to study the influence patterns. The results are shown in Figures 9-11. As shown in Figures 9-11, when the lateral pressure coefficient \(\lambda\) is equal to 1, the displacement, tangential stress, and elastic strain energy of the surrounding rock exhibit relatively uniform distributions, with values at various locations along the tunnel cross section being nearly the same. Compared to conditions where \(\lambda\) is not equal to 1, the overall magnitudes in the contour maps are smaller. According to the Russense criterion contour map in Figure 10a, the index values under this condition range between 0.2 and 0.3, indicating a relatively low rockburst risk. As \(\lambda\) increases, the displacement in the S\({}_{1}\) direction shown in Figure 9 gradually becomes more pronounced, with the maximum value increasing from 7.5 mm at \(\lambda=1.0\) to 27 mm at \(\lambda=2.5\). Based on the mechanism analysis in Section 4.1, after stress redistribution, tangential stress and elastic strain energy are concentrated in the S\({}_{2}\) direction perpendicular to S\({}_{1}\), which is also confirmed by the results in Figures 10 and 11. As shown in Figure 10b-d, tangential  
Figure 11: Elastic strain energy density of surrounding rock under different lateral pressure coefficients. Figure 9: Displacement of surrounding rock under different lateral pressure coefficients. stress gradually concentrates at the right shoulder and left invert corresponding to the S\({}_{2}\) direction. In the locally concentrated areas in Figures 10b and 11b, although tangential stress and elastic strain energy slightly increase, the increases are still not significant, and the concentration areas remain relatively small. The Russense criterion index values are distributed between 0.4 and 0.5, indicating a moderate rockburst risk. When \(\lambda\) further increases to 2.0 and 2.5, as shown in Figure 10c,d, the tangential stress values in the concentrated areas increase significantly, with some areas even reaching the compressive strength of the rock. The concentrated areas of elastic strain energy exhibit similar trends, as seen in Figure 11c,d, and the range of concentrated regions extends deeper into the surrounding rock and farther away along the S\({}_{2}\) direction. According to the Russense criterion, the index values at this stage reach or even exceed 0.55, indicating a high risk of severe rockburst, with the risk region extending further into the rock mass. Additionally, by comparing Figure 10a,d, it is evident that despite having the same burial depth, the strong tectonic stress results in significant local stress concentration, with Russense criterion index values in the concentrated area reaching up to 1.0. The difference in rockburst risk under these two conditions provides a theoretical explanation for the occurrence of rockbursts in tunnels with shallow burial depths, demonstrating that as long as the tectonic stress is sufficiently large, rockbursts can still occur even when tunnels are not deeply buried. In summary, the above analysis indicates that an increase in the lateral pressure coefficient \(\lambda\) influences the occurrence of rockbursts, their potential intensity, the extent of the rockburst-affected area in the cross section, and the depth of the bursting pit. Under the same minimum principal stress S\({}_{2}\) in the cross section, a higher \(\lambda\) increases the likelihood of rockburst occurrence, the possible intensity of the event, and the extent and depth of the affected area.