## 3 Types and Mechanism of Tunnel Bottom Drum
### Influence Analysis of Support Mechanical Behavior  
To preliminarily investigate the influence of support mechanical behavior on tunnel floor heave under gently inclined thin-layered surrounding rock, lateral pressure coefficients of 1.0, 2.0, and 3.0 were considered. The dip angles of the surrounding rock were set at \(0^{\circ}\), \(5^{\circ}\), \(10^{\circ}\), \(20^{\circ}\), and \(25^{\circ}\), while the rock grades were classified as IIIb, IVb, and Vb. The support conditions evaluated included the initial support structure alone, the initial support combined with a secondary lining, and the initial support with both a secondary lining and an inverted arch filling layer. This resulted in a total of 162 models (\(3\times 6\times 3\times 3\)). Due to space constraints, the finite element model schematic is not provided, but the numerical simulation results are illustrated in Figure 15.  
From the above statistical and numerical simulation results, it can be obtained that  
1. [label=(0)]
2. Tunnel floor heave predominantly manifests in surrounding rock of grades III and IV. However, in grade V rock, tunnel bottom sinking can occur due to the combined influence of high ground stress, tectonic stress, deviatoric stress, and the timing of support installation. This phenomenon is consistent with previous field investigation findings.
3. As illustrated in Figure 16, the application of secondary lining, in contrast to solely relying on initial support, can significantly mitigate tunnel floor heave. The effectiveness of this mitigation becomes more pronounced with an increase in the lateral pressure coefficient and the deterioration of surrounding rock conditions. For instance, when the lateral pressure coefficient is 1.0, floor heave ranging from 1 to 6 mm can be somewhat reduced. At a lateral pressure coefficient of 2.0, the floor heave in the range of 6 to 12 mm can be curtailed to some extent. When the lateral pressure coefficient reaches 3.0, the reduction in floor heave spans 12 to 22 mm, depending on the degree of the condition. Furthermore, under grade III surrounding rock conditions, floor heave in the range of 1 to 14 mm can be mitigated to varying degrees. This reduction is more pronounced under grade IV surrounding rock conditions, where floor heave ranging from 1 to 30 mm can be diminished to different extents.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Origin** & \begin{tabular}{c} **Square Sum of** \\ **Type III** \\ \end{tabular} & \begin{tabular}{c} **Degree of** \\ **Freedom** \\ \end{tabular} & \begin{tabular}{c} **Mean Square** \\ \end{tabular} & \
\begin{tabular}{c} **F** \\ \end{tabular} &
\begin{tabular}{c} **Significance** \\ \end{tabular} \\ \hline Modified model & 10,190.906 a  & 30 & 339.697 & 11.005 & 0.003 \\ Intercept & 4335.575 & 1 & 4335.575 & 140.458 & 0 \\ Side pressure coefficient & 604.816 & 4 & 151.204 & 4.899 & 0.042 \\ Inclination angle of surrounding rock & 35.976 & 5 & 7.195 & 0.233 & 0.934 \\ Hard rock thickness & 17.084 & 4 & 4.271 & 0.138 & 0.962 \\ Thin layer thickness & 291.86 & 4 & 72.965 & 2.364 & 0.166 \\ Elastic modulus of hard rock mass & 64.544 & 4 & 16.136 & 0.523 & 0.724 \\ Tunnel buried depth & 737.608 & 5 & 184.402 & 5.974 & 0.027 \\ Error & 185.204 & 6 & 30.867 & - & - \\ Total & 37,946.872 & 37 & - & - & - \\ Total after correction & 10,376.11 & 36 & - & - & - \\ \hline \hline \end{tabular}
\end{table}
Table 4: Inter-subject effect test.  
3. While the initial support, secondary lining structure, and inverted arch filling layer all effectively suppress tunnel floor heave, the disparity between the initial support and secondary lining structure is minimal. The difference typically ranges from 0.09 to 6 mm, following a trend similar to the one previously described. In most instances, this difference is within the 0.09 to 1.5 mm range. However, when the lateral pressure coefficient reaches 3.0 or when surrounding rock conditions deteriorate further, the disparity between the two becomes more pronounced.