# Mechanisms of Tunnel Rockburst Development Under Complex Geostress Conditions in Plateau Regions
## 4 Influence Mechanisms of Key Geostress Parameters on Tunnel Rockburst Characteristics
As the geostress orientation gradually changes to \(\alpha=90^{\circ}\), the vertical stress becomes dominant, shifting the stress concentration areas to the haunches on both sides. During this transition, the magnitude and range of \(\sigma_{\theta}\) remain relatively constant from \(0^{\circ}\) to \(90^{\circ}\), while the concentration location changes consistently with the variation in the direction of the minimum principal stress S\({}_{2}\) of the tunnel cross section. According to the Russense rockburst criterion, the rockburst risk areas align with the concentration areas of \(\sigma_{\theta}\), indicating that rockbursts in the tunnel cross section will occur at the locations indicated by the minimum principal stress S\({}_{2}\). Therefore, in rockburst prediction under complex stress environments, particular attention should be paid to this region of the surrounding rock within the cross-section. Figure 8: Elastic strain energy density of surrounding rock under different ground stress directions. Elastic strain energy results from the integration of rock deformation and stress and can comprehensively reflect the combined effects of both factors. Local concentration of strain energy is considered the fundamental cause of rockburst formation [29; 34]. Therefore, Figure 8 illustrates the contour maps of elastic strain energy density of the tunnel surrounding rock under different geostress orientations. Comparing Figures 7 and 8 reveals that the locations of elastic strain energy concentration in the tunnel cross section are essentially the same as those of tangential stress concentration. A high concentration of elastic strain energy indicates significant energy storage in these regions. Since elastic strain energy is a form of potential energy, the more it is locally stored, the more violent its release will be under certain conditions, thereby increasing the risk of rockbursts. Conversely, comparing Figures 6 and 8 shows that, macroscopically, regions with greater convergence deformation in the tunnel cross section exhibit less concentration of elastic strain energy, whereas regions with smaller convergence deformation show relatively obvious energy concentration. This phenomenon may be attributed to two reasons. Firstly, as shown in Figure 6, areas with larger surrounding rock displacement have greater convergence deformation, and the excavation-induced free surface provides space for this convergence deformation, preventing strain concentration. The surrounding rock in these areas is relatively \"relaxed,\" and the insufficiently concentrated tangential stress cannot produce significant circumferential compressive deformation, resulting in lower elastic strain energy. Secondly, since the tangential stress \(\sigma_{\Theta}\) is compressive, the surrounding rock around the tunnel is under compressive stress. The greater the circumferential stress in a region, the more severe the compression of the surrounding rock, leading to greater local compressive deformation. Due to the circumferential distribution of \(\sigma_{\Theta}\), the compressive deformation of the surrounding rock is not directed toward the tunnel center but is oriented perpendicularly in the circumferential direction, thus differing from the concept of macroscopic convergence deformation. As a result, in regions with concentrated tangential stress, the combination of large tangential stress and the resulting greater circumferential compressive deformation of the surrounding rock leads to a simultaneous increase in elastic strain energy, thereby significantly increasing the risk of rockburst. In summary, after tunnel excavation creates a free surface, the surrounding rock in the direction of the maximum principal stress \(S_{1}\) experiences greater convergence, allowing strain to be released and preventing strain and local stress concentration. As a result, the redistributed stress in this area is not significant.