# Analysis of Collapse Mechanism and Treatment Evaluation of a Deeply Buried Hard Rock Tunnel
## 4 Collapse Treatment Plan  
According to the collapse mechanism and characteristics, the treatment method of \"first protect, then fill and then dig\" is adopted, following the principle of \"stable, safe and reliable, economical and applicable\". The specific treatment process is shown in Figure 6.  
* Safe evacuation  
After the tunnel collapsed, all construction was halted, and concrete was sprayed to cover the collapse surface. Dangerous rocks were removed in a timely manner to prevent secondary collapse. Figure 6: Flow chart of tunnel collapse treatment. * Advance bolt support According to the scale and characteristics of the collapse, a \(\Phi\)25 hollow grouting anchor bolt was installed immediately for advance support after the formation of a safe working face. The length of the anchor bolt was 3 m, and the spacing between the anchor bolts was 1.0 m. According to the plum staggered layout, more bolts were installed around the collapse chamber. * Steel arch support The initial support was strengthened, and the original surrounding rock support design of grade III was changed to that of grade IV (see Table 3). No.16 I-shaped steel was adopted, with a spacing of 0.8 m. Upon completion of the erection of the arch frame, longitudinal connecting steel bars and anchor bolts shall be installed immediately to fix the arch frame. * Cavity filling The surface of the collapsed body shall be closed by hanging steel mesh and shotcrete, and I16 steel arch frame shall be installed after curing. Steel plate bolts shall be used for circumferential connection and \(\Phi\)25 steel bars shall be used for longitudinal connection. After the installation, the system anchor bolt and locking anchor bolt shall be drilled immediately and fixed with the steel arch frame. \(\Phi\)25 hollow grouting anchor bolt was drilled obliquely around the collapse position to reinforce the loose collapse body in the tunnel section. When the strength of the collapsed body meets the requirements, concrete backfill shall be carried out to ensure that the arch back and surrounding rock would be bond together. * Step method construction According to the excavation principle of \"weak blasting, short footage, early support\", blasting was strictly controlled, with each cycle footage about 1-2 m after excavation, concrete was sprayed to seal the surrounding rock in time to ensure the timely follow-up of support measures. The basic process was: blasting and excavation of upper stage\(\rightarrow\)mechanical hazard check\(\rightarrow\)installation of steel mesh\(\rightarrow\)erection of steel arch frame\(\rightarrow\)installation of system anchor bolt and locking anchor bolt\(\rightarrow\)spraying concrete to complete the construction of upper stage. When the upper step is 5-10 m ahead of the lower step, the left and right sides of the lower step should be excavated alternately. Each excavation should be controlled at the spacing between 2-3 steel arches. Meanwhile, the foot locking anchor shall be strengthened to ensure the arch at the foot is stable. * Monitoring and tracking The implementation should be carried out in strict accordance with the monitoring plan. The monitoring for special sections should be strengthened. The monitoring frequency should be improved, and the dynamic changes of surrounding rock should be closely monitored. Abnormalities should be reported immediately once they occur. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{2}{c}{**Grade of**} & \multirow{2}{*}{**Bolt**} & \multirow{2}{*}{**Steel Mesh**} & \multirow{2}{*}{**Shotcrete**} & \multirow{2}{*}{**Steel Arch**} \\ \cline{3-3} \cline{5-5} \multicolumn{1}{c}{**Surrounding Rock**} & & & & \\ \hline \multirow{4}{*}{IV} & \(\Phi\)25 bolt, & & 20 cm & I16 steel arch, \\  & spacing 1.0 x 1.0 m, & & C25 concrete & spacing 0.8 m \\ \cline{1-1}  & Length 3 m & & & \\ \cline{1-1}  & \(\Phi\)25 bolt, & & & \\ \cline{1-1}  & spacing 1.5 x 1.5 m, & & C25 concrete & / \\ \cline{1-1}  & Length 2.5 m & & & \\ \hline \hline \end{tabular}
\end{table}