## 4 Stability Control Measures for Step Method Construction and Analysis of Design Parameters
### Calculation Conditions  
Based on the above research results and the case statistics of karst caves prone to mud inrush disasters, according to the stability of the surrounding rock and the development of karst caves, the advanced support measures and design parameters of IV- and V-grade surrounding rock, the design parameters of bench excavation, the initial support measures, and the treatment measures of karst caves, the construction of super large cross-section tunnels through karst caves is determined. Advanced support measures and design parameters of IV-grade surrounding rock:  
(1) The advanced small pipe with a length of at least 4.5 m is used to improve the stability of the surrounding rock. (2) When the karst cave is developed in front of the tunnel arch [21], the partition advanced large pipe shed measure (only arranging the advanced large pipe shed in the range of the karst cave development in front of the tunnel arch) is adopted to improve the stability of the karst cave. The length of the advanced large pipe shed should pass through the karst cave and ensure that the depth of the rock is at least 2.5 m. The criterion for the development of filled karst caves in front of the tunnel arch is that the karst cave is within 120\({}^{\circ}\) of the tunnel arches, the size of the karst cave is greater than or equal to 50 m\({}^{3}\), and the distance between the karst cave and the tunnel face is not more than 4 m.  
Advanced support measures and design parameters of V-grade surrounding rock:  
(3) Advanced pipe sheds should be adopted to improve the stability of the surrounding rock or karst caves. The length of the advanced pipe shed should be 20 m and the circumferential reinforcement range should be within 120\({}^{\circ}\) of the arches. The length of the advanced large pipe roof should be optimized according to the size of the karst cave, the location of the karst cave, and the type of filling material, according to the drilling records of the pipe roof. The length of the advanced large pipe roof should pass through the karst cave, and the depth of rock entry should be at least 2.5 m.  
Design parameters of the step excavation:  
(4) The step design parameters include step height, step length, and cycle footage. The step length should be 3-5 m, the step height should be 4-6 m, and the cycle footage should be 1-2 steel frame spacing. (5) To avoid hanging the initial support of the upper step at the same time, the lower step should be excavated by a staggered platform. When the karst cave is exposed on the side of the upper step, the side of the karst cave should be excavated first when the \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Surrounding Rock**} & \multicolumn{3}{c}{**Supporting Structure**} & \\ \cline{3-6}
**Project** & \multicolumn{1}{c}{\begin{tabular}{c} **Arch** \\ **Settlement** \(\mathbf{D_{\text{V}}}\) **(mm)** \\ \end{tabular} } & \begin{tabular}{c} **Horizontal** \\ **Convergence** \\ **(mm)** \\ \end{tabular} & \begin{tabular}{c} **Arch** \\ **Settlement** \(\mathbf{D_{\text{V}}}\) **(mm)** \\ \end{tabular} & \
\begin{tabular}{c} **Horizontal** \\ **Convergence** \\ **(mm)** \\ \end{tabular} &
\begin{tabular}{c} **Displacement** \\ **of Tunnel Face** \\ **(mm)** \\ \end{tabular} \\ \hline Cycle footage & \(0.5\) & \(-103\) & \(107\) & \(-58\) & \(56\) & \(-161\) \\ \(z\) (m) & \(1.0\) & \(-115\) & \(117\) & \(-65\) & \(61\) & \(-145\) \\  & \(1.5\) & \(-140\) & \(124\) & \(-90\) & \(62\) & \(-137\) \\ Step length & \(4.0\) & \(-93\) & \(99\) & \(-47\) & \(57\) & \(-157\) \\  & \(6.0\) & \(-115\) & \(117\) & \(-65\) & \(61\) & \(-145\) \\ \(l\) (m) & \(8.0\) & \(-143\) & \(133\) & \(-93\) & \(73\) & \(-134\) \\  & \(4.0\) & \(-98\) & \(80\) & \(-57\) & \(20\) & \(-126\) \\ Step height & \(5.0\) & \(-102\) & \(86\) & \(-58\) & \(55\) & \(-132\) \\ \(h\) (m) & \(6.0\) & \(-115\) & \(117\) & \(-65\) & \(61\) & \(-145\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Calculation results of different step design parameters. lower step is staggered.