## 3 Monitoring Scheme of Large Section Bifurcated Small Clear Distance Tunnel
### Monitoring Items
#### 3.1.1 Advanced Geology Prediction  
Tunnel construction is greatly affected by terrain, hydrogeology, and tunnel burial depth, among other factors. Building tunnels in unfavorable geological conditions is extremely challenging, and local collapse and large deformation are prone to occur during construction. Advanced geological prediction can effectively predict unfavorable geological conditions and ensure the safety of tunnel construction. Common unfavorable geology includes loose formations, karst caves, collapses, faults, groundwater, fracture zones, and gas formations. The methods of tunnel advanced geological prediction mainly include geological methods, geophysical prospecting methods, and drilling methods. The geological method predicts the possible types and scales of unfavorable geological conditions developed in a tunnel based on lithology, occurrence, structure, and other characteristics. The geophysical prospecting method uses the characteristics of seismic waves reflected in the inhomogeneous geological body to predict geological conditions in front of the tunnel. The drilling method identifies the lithology of the stratum in front of the tunnel face through horizontal drilling, determines the grade and thickness of the surrounding rock, and explores the characteristics of the carbonate karst cave and the occurrence conditions of groundwater in the rock stratum. In the research section, advanced geological forecasting work was performed to provide information on the surrounding rock as the basis for excavation and support. Figure 2 shows the advanced geological prediction process. When advanced geological prediction detects fault fracture zones, abundant groundwater, developed rock veins, uneven weathering of tuff, and hard rock bursts, the geological condition can be judged as being unfavorable geology.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Name of the Support** & & **Support Method** \\ \hline \multicolumn{3}{c}{Main tunnel} \\ \hline \multirow{3}{*}{Primary lining support} & Concrete spraying layer & C25 shotcrete, 20 cm \\  & Rock bolt & \(\Phi\)22 mortar bolt, L300 cm \\  & Reinforcing mesh & \(\Phi\)8 reinforcing mesh, 25 cm \(\times\) 25 cm \\ \hline \multirow{3}{*}{Temporary support} & I-steel truss & I18 I steel frame, longitudinal spacing, 50 cm \\  & Concrete spray layer & C25 concrete spray layer, 22 cm \\  & Reinforcing mesh & \(\Phi\)8 reinforcing mesh, 20 cm \(\times\) 20 cm \\ \hline \multirow{3}{*}{Secondary lining support} & Arch wall & C30 waterproof reinforced concrete, 40 cm thick \\  & Inverted arch & C30 waterproof reinforced concrete, 40 cm thick \\ \hline \multicolumn{3}{c}{Ramp section} \\ \hline \multirow{3}{*}{Primary lining support} & Concrete spraying layer & C25 shotcrete, 20 cm \\  & Rock bolt & \(\Phi\)22 mortar bolt, L2500 cm \\  & Reinforcing mesh & \(\Phi\)8 reinforcing mesh, 25 cm \(\times\) 25 cm \\ \hline \multirow{3}{*}{Temporary support} & I-steel truss & I18 I steel frame, longitudinal spacing, 50 cm \\  & Concrete spray layer & C25 concrete spray layer, 22 cm \\  & Reinforcing mesh & \(\Phi\)8 reinforcing mesh, 20 cm \(\times\) 20 cm \\ \hline \multirow{3}{*}{Secondary lining support} & Arch wall & C30 waterproof reinforced concrete, 40 cm thick \\  & Inverted arch & C30 waterproof reinforced concrete, 40 cm thick \\ \hline \hline \end{tabular}
\end{table}
Table 1: Parameters of the retaining structure.