# Mechanisms of Tunnel Rockburst Development Under Complex Geostress Conditions in Plateau Regions
## 4 Influence Mechanisms of Key Geostress Parameters on Tunnel Rockburst Characteristics  
The well-established rockburst instability theory [31] suggests that before the occurrence of impact ground pressure and rockbursts, deformation localization takes place, forming a generalized deformation concentration zone. From the perspective of rock mechanics systems, this deformation concentration zone, together with the surrounding rock mass that remains elastic, constitutes a deformation system. Rockburst is thus considered a dynamic instability phenomenon of this system under external forces. During instability, the deformation concentration zone undergoes ejection, forming the subsequent rockburst body. This theory has been corroborated by the studies of [PERSON], [PERSON], [PERSON], and others [12; 32; 33]. Therefore, this section focuses on this region to discuss the influence of key geostress parameters on rockbursts from multiple perspectives, including displacement, tangential stress \(\sigma_{\theta}\), stress-strength ratio \(\sigma_{\theta}/\sigma_{c}\) (Russense rockburst criterion), and elastic strain energy density. In this study, since \(\sigma_{c}\) = 127 MPa is constant, the distribution characteristics and variation patterns of tangential stress \(\sigma_{\theta}\) are consistent with those of the stress-strength ratio \(\sigma_{\theta}/\sigma_{c}\). Figure 4: The monitoring points of numerical solution. As shown in Figures 6-8, the surrounding rock of the tunnel is significantly influenced by the geostress orientation. After tunnel excavation, the contour maps of surrounding rock displacement, tangential stress, stress-strength ratio, and elastic strain energy density all exhibit distinct regional concentration characteristics. The concentration directions display a certain regular variation with changes in geostress orientation, indicating that the direction of geostress and the relative magnitudes of the two principal stresses play a decisive role in controlling the distribution regions of displacement and stress in the tunnel surrounding rock. Specifically, Figure 6 presents the displacement contour maps under different geostress orientations. When \(\alpha=0^{\circ}\), the maximum displacement is primarily located at the haunches on both sides. As \(\alpha\) changes, the location of maximum displacement rotates synchronously with the same angle, while the maximum displacement value remains constant at 12 mm throughout the process. It is evident that the regions with larger displacement correspond to the direction of the maximum principal stress S\({}_{1}\) in the cross section. The main reason is that the stress magnitude in the S\({}_{1}\) direction is large, being 1.5 times that of S\({}_{2}\). After excavation, a free surface forms at the tunnel, and under the substantial thrust of S\({}_{1}\), the free surface in the S\({}_{1}\) direction naturally exhibits greater displacement. This greater displacement allows deformation in this region to be partially released towards the free surface, resulting in significant convergence deformation. In contrast, in the direction perpendicular to S\({}_{1}\) (i.e., S\({}_{2}\)), the surrounding rock is subjected to obvious lateral compression from S\({}_{1}\) on both sides, leading to concentrated compressive strain and deformation-induced compressive stress in the S\({}_{2}\) direction. Figure 7 further confirms this inference. In Figure 7, the surrounding rock experiences a significant compressive force in the S\({}_{1}\) direction, causing stress concentration in the perpendicular S\({}_{2}\) direction. For instance, when \(\alpha=0^{\circ}\), the strong horizontal tectonic stress becomes dominant, and the tangential stress \(\sigma_{\theta}\) is mainly concentrated at the crown and invert, both exhibiting compressive stress significantly higher than at other locations along the tunnel cross section.