# 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 Tunnel Burial Depth on Rockburst Risk in Cross Sections  
When geostress orientation and lateral pressure coefficient remain constant, the variation trends across different working conditions are generally consistent with changes in tunnel burial depth d. Therefore, a representative case with a lateral pressure coefficient of 1.5 and \(\alpha=60^{\circ}\) was selected to analyze the influence patterns of tunnel burial depth. The calculated contour maps of surrounding rock displacement, tangential stress, stress-strength ratio \(\sigma_{0}/\sigma_{c}\), and elastic strain energy are presented in Figures 12-14, respectively.  
Figure 12: Displacement of surrounding rock under different buried depth conditions.  
As tunnel burial depth increases, the overall behavior of the surrounding rock undergoes significant changes. At a burial depth of 500 m, due to the relatively shallow depth, the concentration characteristics of displacement, stress, and elastic strain energy are not pronounced. According to the Russense criterion, the index values of the surrounding rock are mostly around 0.2, indicating essentially no rockbursts, with at most weak rockbursts occurring in very small areas.  
As burial depth increases, the displacement at the left shoulder and right invert positions in the S1 direction becomes more pronounced, increasing from 4 mm at 500 m depth in Figure 12a to 14 mm at 2000 m depth in Figure 12d. Meanwhile, stress and elastic strain energy gradually concentrate in the S2 direction, particularly at the right shoulder and left invert of the tunnel cross section, with both their magnitudes and extents expanding continuously.  
At a burial depth of 1000 m (Figure 13b), the Russense criterion index ranges between 0.2 and 0.4, indicating a risk of low-intensity rockbursts. At 1500 m depth (Figure 13c), areas with index values exceeding 0.55 begin to appear, indicating a risk of severe rockburst, although the regions of high-intensity risk remain limited in size. When the burial depth further increases to 2000 m (Figure 13d), the areas with index values greater than 0.55 expand significantly, indicating a considerable increase in the regions at high risk of severe rockbursts.  
The above analysis indicates that, under otherwise constant conditions, increasing tunnel burial depth promotes the occurrence of rockbursts and increases both their intensity and the extent of rockburst-affected areas within the cross section. This observation aligns with the conventional understanding that deeper tunnels are more prone to rockbursts. Essentially, the increase in geostress magnitude with depth leads to higher secondary stresses in the surrounding rock after excavation. When there is a difference between the magnitudes of the two principal stresses, S1 and S2, within the cross section, local stress concentration and energy accumulation occur, thereby creating the necessary conditions for rockburst occurrence.  
Figure 14: Elastic strain energy density of surrounding rock under different buried depths.  
Figure 13: \(\sigma_{\theta}\) and \(\sigma_{\theta}/\sigma_{c}\) (Russense rockburst criterion) of surrounding rock under different burial depth conditions.