## 3 Types and Mechanism of Tunnel Bottom Drum
### Single-Factor Analysis  
Standard group: The tunnel is buried at a depth of 300 m within thin-layered rock consisting of horizontal (0\({}^{\circ}\) dip angle) hard and soft strata. The hard rock layer is 0.5 m thick with an elastic modulus of 13 GPa, while the soft rock layer is 0.05 m thick with an elastic modulus of 0.186 GPa. The lateral pressure coefficient of the thin-layered rock mass is 1.5. According to the lateral pressure coefficient, dip angle of the rock stratum, thickness of the hard rock stratum, thickness of the thin layer, elastic modulus of surrounding rock, buried depth of the tunnel, and different parameters under these six influencing factors (Table 3), numerical models are established, respectively. The influence of parameter variation of a certain influencing factor on tunnel floor heave under the same conditions is discussed. 1. **Tunnel floor heave characteristics under different lateral pressure coefficients** Regarding Figure 9 and the numerical simulation results, it can be concluded that  
Figure 8: Schematic diagram of finite element model. \begin{table} \begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{\begin{tabular}{c} **Lateral** \\ **Pressure** \\ **Coefficient** \\ \end{tabular} } & \begin{tabular}{c} **Indination** \\ **Angle of** \\ **Surrounding** \\ **Rock/(\({}^{\circ}\))** \\ \end{tabular} & \begin{tabular}{c} **Hard Rock** \\ **Thick-** \\ **ness/cm** \\ \end{tabular} & \begin{tabular}{c} **Thin Layer** \\ **Thick-** \\ **ness/cm** \\ \end{tabular} & \
\begin{tabular}{c} **Elastic** \\ **Modulus of** \\ **Hard Rock** \\ **Mass/Gpa** \\ \end{tabular} &
\begin{tabular}{c} **Depth of** \\ **Tunnel/m** \\ **Mass/Gpa** \\ \end{tabular} \\ \hline
0.5 & 0 & 10 & 1 & 13 & 100 \\
1 & 5 & 30 & 3 & 10 & 200 \\
1.5 & 10 & 50 & 5 & 6 & 300 \\
2 & 15 & 70 & 7 & 3.65 & 400 \\
3 & 20 & 90 & 9 & 1.5 & 500 \\ - & 25 & - & - & - & - \\ \hline \hline \end{tabular}   
* 1. **Tunnel floor heave characteristics under different lateral pressure coefficients** Regarding Figure 9 and the numerical simulation results, it can be concluded that1. The tunnel floor heave value increases with the increase in the lateral pressure coefficient, and the growth rate is accelerated. When the lateral pressure coefficient increases from 0.5 to 3.0, the tunnel floor heave gradually increases from 20.42 mm to 50.04 mm, and the displacement increases by 2.45 times. When the lateral pressure coefficient increases from 1.0 to 1.5, the tunnel bottom drum increases from 22.45 mm to 27.43 mm, and the displacement increases by 1.22 times. When the lateral pressure coefficient increases from 1.5 to 2.0, the tunnel bottom drum increases from 27.43 mm to 34.59 mm, and the displacement increases by 1.26 times. When the lateral pressure coefficient increases from 2.0 to 3.0, the tunnel bottom drum increases from 34.59 mm to 50.04 mm, and the displacement increases by 1.44 times. 2. The displacement of the tunnel sidewall increases with the increase in the lateral pressure coefficient, and the left and right sides converge inward. 2. **Tunnel floor heave characteristics under different rock dip angles** From Figure 10 and the numerical simulation results, it can be concluded that  
1. The tunnel floor heave value decreases first, then increases and then decreases with the increase in the rock dip angle, reaching the peak value of 29.58 mm when the rock dip angle is 10\({}^{\circ}\). Figure 10: Tunnel bottom displacement curve with different dip angles. Figure 9: Fitting curves of tunnel bottom displacement under different lateral pressure coefficients. Black line: the connecting line between numerical simulation results and field testing results; red line: the fitted curve based on the formula. 2. When the dip angle of the rock stratum is not 0, the surrounding rock and the initial tunnel support exhibit noticeable asymmetric displacement, with greater displacement on one side of the dip. As the dip angle increases, the asymmetry in the displacement pattern first grows and then diminishes.