## 2 Stress Release and Energy Dissipation Mechanisms of TBM Tunnel ASRBs
When two SRBs work together, the stress release area overlaps to form a larger release area (Figure 2b). Moreover, after the construction of multiple SRBs, the stress release area will overlap and converge into an unloading zone (Figure 2c). Under different borehole parameter conditions, SRBs modify the original concentrated stress field to different degrees through the damage zones. By optimizing the borehole parameters, the regional stress release effect of SRBs will be improved, the original stress concentration degree will be reduced, and the high-stress-concentration area will be transferred to the deep high-strength and high-energy storage rock mass. However, TBM-constructed tunnels are unique because of their circular cross-section, the difficulty of withdrawing the TBM equipment, and the simultaneous excavation, slag removal, and support processes. Due to the limited space behind the cutter head, the layout of the stress release boreholes is limited to the two sides of the tunnel. As shown in Figure 3, the extension of the drilling equipment accompanying the TBM instrument and the stress concentration areas in the surrounding rock limit the layout of the SRBs. In this case, the release borehole layout is usually chosen based on the extent of the release area and the desired release effect. For example, if the location of a potential rockburst source area is precisely determined and a high-stress-release effect is needed, the parallel deployment shown in Figure 3a will create a denser SRB, which will achieve a higher stress release effect. When the location of the source area is uncertain, or when the proximity to the tunnel surface does not require a high level of stress release effectiveness, the use of the dispersive placement shown in Figure 3b will be more effective for enhanced stress release. Therefore, due to the special characteristics of TBM tunnels, the stress release scheme will be largely different from that of tunnels constructed by drilling and blasting. On the one hand, construction-space-limiting SRBs can only be arranged in advance, and so are also known as advance stress release boreholes (ASRBs). On the other hand, due to the special arrangement of the SRBs, the existing drill-and-blast stress release technology cannot be fully applied to TBM tunnels. Table 4 shows the comparative information of several common stress release techniques, which shows that ASRBs have more prominent advantages over other stress release techniques in deep-buried hard rock TBM-constructed tunnels. Figure 2: _Cont._  
Figure 2: Schematic diagram of the formation of the maximum principal stress release area of SRBs: (**a**) single stress release borehole; (**b**) two stress release boreholes; (**c**) multiple stress release boreholes. (Note: the maximum principal stress direction is in the vertical direction, and compression is negative, unit Pa.) \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline Young’s modulus/GPa & 20.00 \\ Poisson’s ratio & 0.25 \\ Dilation angle/\({}^{\circ}\) & 10.00 \\ Initial cohesion / MPa & 12.60 \\ Plastic strain limit of cohesion & 0.004 \\ Residual cohesion / MPa & 8.52 \\ Initial internal friction angle/\({}^{\circ}\) & 10.00 \\ Plastic strain limit of internal friction & 0.005 \\ Residual internal friction angle/\({}^{\circ}\) & 35.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Model calculation parameters. Therefore, the research on ASRB technology needs to be developed urgently. Considering the complexity of the field construction environment, the following two special stress release strategies for TBM tunnels are ignored to obtain the correlation law between the parameters of ASRBs and the stress release effect under the prevailing conditions. (1) TBMs cannot drill boreholes in the working face of tunnels because there are still some technical difficulties in using TBM equipment that can drill boreholes in the working face.