## 6 Engineering Applications
### V-Grade Surrounding Rock Step Method Construction  
ZK1 + 358-ZK1 + 523 on the left line of the entrance of the Lianhuashan Tunnel is in the shallow buried section. The surrounding rock of this section is mainly hard plastic red clay, and the clay and weathered limestone are interlaced, as shown in Figure 7. The advanced support of this section adopts a 976 self-propelled advanced large pipe shed, and the specific implementation of the advanced large pipe shed is shown in Table 5. Among them, four sections, ZK1 + 382-ZK1 + 402, ZK1 + 415-ZK1 + 435, ZK1 + 460-ZK1 + 490, and ZK1 + 493-ZK1 + 523 have developed karst caves, and some of the pipe sheds have been lengthened to penetrate the karst cave and reach a depth of 4 m.  
Taking ZK1 + 382-ZK1 + 402 as a typical example, the construction scheme of the two-step method for V-grade surrounding rock is described in detail as follows:  
1. The geological radar is used to detect the range of 30 m in front of the tunnel face in the ZK1 + 382 section. The arch line map shows that the radar wave reflection is strong in the 9-15 m section in front of the tunnel face, which may develop large-scale karst caves, groundwater is not developed, and the surrounding rock level is V-grade.
2. According to the conditions of the surrounding rock and the development of the karst cave, the advanced support adopts the advanced large pipe shed. The design length of the advanced large pipe shed is 20 m, and the circumferential reinforcement arch is 120\({}^{\circ}\). A total of 31 holes were arranged. Through the analysis of the drilling records of the pipe shed, the 11-18 m section of 8-14 holes are filled with karst cave, so the length of 8-14 holes is extended to 22 m. The layout of the advanced large pipe shed is shown in Figure 8.  
\begin{table}
\begin{tabular}{c c c} \hline
**Section** & **Number/Hole \(N\)** & **Length \(L\) (m)** \\ \hline ZK1 + 358–ZK1 + 373 & 41 & 15 \\ ZK1 + 367–ZK1 + 387 & 42 & 20 \\ ZK1 + 382–ZK1 + 402 & 42 & 20/22 \\ ZK1 + 398–ZK1 + 418 & 42 & 20 \\ ZK1 + 415–ZK1 + 435 & 42 & 20/25 \\ ZK1 + 433–ZK1 + 463 & 42 & 20 \\ ZK1 + 460–ZK1 + 490 & 48 & 20/25 \\ ZK1 + 493–ZK1 + 523 & 37 & 20/25 \\ \hline \end{tabular}
\end{table}
Table 5: The construction quantity of the advanced large pipe shed.  
Figure 7: Typical surrounding rock of tunnel face.  
3. The upper steps and lower steps are excavated in parallel. The length of the step is 4 m, the height of the step is 5 m, and the cyclic footage is the spacing of 2 steel frames.
4. After the ventilation and mucking operation, the geological sketch results show that the 3 m \(\times\) 3 m \(\times\) 5 m (length, width, height) semi-filled silty clay karst cave is exposed on the left sidewall of the lower step, and the primary support is closed after cleaning the filling and concrete backfilling. The tunnel face is relatively stable, there is no obvious extrusion deformation, and there is a small amount of falling blocks. The initial support adopts HW200 \(\times\) 200H steel, a 3.5 m-long \(\varphi\)42 mm grouting steel pipe lock foot, C20 thick 29 cm early strength shotcrete, and a 5.0 m-long \(\varphi\)22 mm cartridge bolt.  
During the construction process, 14 monitoring sections were arranged in the ZK1 + 358\(\sim\)ZK1 + 523 section (as shown in Figure 9). The arch settlement of the ZK1 + 400 section was the largest at 74.2 mm, followed by 51.2 mm of the ZK1 + 410 section, and 4.3\(\sim\)17.2 mm in the other sections. The deformation of the tunnel support structure is controllable. The two-step method and advanced large pipe roof measures adopted in this section have a significant effect on tunnel deformation control.  
Figure 8: The layout diagram of the advanced large pipe roof.  
Figure 9: The arch settlement of each monitoring section.