# Mechanisms of Tunnel Rockburst Development Under Complex Geostress Conditions in Plateau Regions
## 4 Influence Mechanisms of Key Geostress Parameters on Tunnel Rockburst Characteristics
### Differences in the Effects of Key Geostress Parameter Variations on Tunnel Cross-Sectional Areas  
The analyses in Sections 4.1-4.3 indicate that geostress orientation, lateral pressure coefficient, and tunnel burial depth all have significant impacts on the occurrence, intensity, and even the extent of rockbursts. These three factors respectively reflect the influence of geostress direction, differences in cross-sectional stress components, and geostress magnitude on rockburst development. Although all three factors affect rockburst characteristics, their impacts on the extent of influence across the entire tunnel cross section and on specific rockburst features differ. To investigate these differences, Figure 15 illustrates the influence patterns of these three parameters on elastic strain energy at five monitoring points--crown, shoulder, haunch, invert, and bottom--within the tunnel cross section, as defined in Figure 2.  
Figure 15a illustrates the influence of geostress orientation on different parts of the tunnel surrounding rock cross section, showing the curves obtained from monitoring points in Figure 8 as the angle changes. During the 90\({}^{\circ}\) clockwise rotation of S\({}_{1}\) and S\({}_{2}\), the values at the crown and invert gradually decrease, while those at the haunch increase. This is due to the interchange of S\({}_{1}\) and S\({}_{2}\) at these locations after rotation, resulting in significant variations at these points. The shoulder initially increases and then decreases; when \(\alpha\) is 0\({}^{\circ}\), the elastic strain energy density is 132 kJ/m\({}^{3}\), and after rotation to \(\alpha\) = 90\({}^{\circ}\), it is 126 kJ/m\({}^{3}\), showing little overall change. The inverse exhibits a similar trend, decreasing initially and then increasing back to nearly its original value of 132 kJ/m\({}^{3}\). These differences indicate that changes in geostress orientation mainly affect local areas corresponding to S\({}_{1}\) and S\({}_{2}\) within the cross section, suggesting that the influence of geostress orientation is a localized effect under stress concentration. For complex geostress orientations in plateau regions, attention should focus on the local areas of the tunnel cross section corresponding to the minimum principal stress (S\({}_{2}\)). Figure 15b presents the curves for the monitoring points corresponding to Figure 11. Similar to the influence of geostress orientation, the lateral pressure coefficient has the greatest impact on the shoulder corresponding to S\({}_{2}\), with an increase of nearly 400 kJ/m\({}^{3}\). Significant impacts are also observed at the crown and invert due to symmetrical effects, with increases of approximately 300 kJ/m\({}^{3}\). In contrast, the haunch and foot corresponding to S\({}_{1}\) show minimal changes, with maximum variations not exceeding 50 kJ/m\({}^{3}\), and values stabilizing or even slightly decreasing after the increase. This indicates that the lateral pressure coefficient mainly has a significant impact on local areas corresponding to S\({}_{2}\), causing local stress and energy concentration. Under the strong tectonic stress environments of plateau regions, attention should likewise focus on the local areas of the tunnel cross section corresponding to the minimum principal stress (S\({}_{2}\)). Figure 15c presents the curves for the monitoring points corresponding to Figure 14. Although the stress concentration area is around the shoulder, corresponding to S\({}_{2}\), where the strain energy increases sharply with greater burial depth, the trend of the curves indicates that adjacent locations such as the haunch, as well as more distant locations  
Figure 15: Influence curve of typical ground stress parameters on strain energy density of surrounding rock. (**a**) Effect of geostress orientation. (**b**) Effect of lateral pressure coefficient. (**c**) Effect of burial depth.