## 6 Engineering Applications
### IV-Grade Surrounding Rock Bench Method Construction  
The surrounding rock of the ZK1 + 950-ZK2 + 850 (YK1 + 900-YK2 + 850) section of the Lianhuashan Tunnel is mainly composed of limestone and siliceous limestone with mudstone, and the surrounding rock grade is IV. There may be hidden karst caves in the bedrock, and mud inrush and other disasters may occur. The TSP is used for detection every 100 m in this section. After roughly determining the distribution of bad geology in front of the tunnel face, a geological radar is used for detection once it is close to approximately 30 m from the bad geological body to further verify and understand the distribution of bad geology. Taking the ZK2 + 155 section as a typical example, the construction scheme of the two-bench method for IV-grade surrounding rock is described in detail.  
1. In the range of 30 m in front of the tunnel face, the geological radar is used for detection. The arch line map shows that there is a strong radar reflection section in the 3-9 m section in front of the tunnel face, so it is speculated that there is a large-scale cavity in this section. The sidewall survey line map shows that there is a strong radar reflection section in the front 2-5 m section, so it is speculated that there may be a weak interlayer or cavity in this section. The surrounding rock is IV-grade.
2. According to the condition of the surrounding rock and the development of the karst cave, it is judged that the advanced support adopts \(\varphi\)42 advanced small pipe and \(\varphi\)76 partition advanced large pipe shed. The design length of the advanced small pipe is 4.5 m, the circumferential spacing is 40 cm, the extrapolation angle is 10\({}^{\circ}\), the arch is 120\({}^{\circ}\), and the lap length is greater than 1.0 m. The preliminary determination range of the partition advanced large pipe roof layout is 30\({}^{\circ}\) on the left side of the arch. Then, according to the analysis of the pipe roof drilling records, the 4-11 m section of 10-15 holes is determined as the filling karst cave. The final partition advanced large pipe roof layout scheme is 10-15 holes with a length of 15 m. The partition advanced large pipe roof layout is shown in Figure 10.
3. The length of the step is 5 m, the height of the step is 5 m, and the cyclic footage is two steel frames. The upper steps and lower steps are excavated in parallel.
4. After the ventilation and mucking operation, the geological sketch results show that the tunnel face is unstable, the extrusion deformation of the tunnel face is obvious, and there is a local collapse phenomenon. The tunnel face is closed by 20 cm-thick shotcrete. The initial support adopted I20 b I-shaped steel, a 3.5 m-long \(\varphi\)42 mm grouting steel pipe locking foot, C20 thick 25 cm early strength shotcrete, 4.0 m long \(\varphi\)22 mm cartridge anchor rods. The 2 m \(\times\) 2 cm \(\times\) 3 cm (length, width, height) semi-filled silty clay karst cave was exposed on the left arch foot of the upper step, and the primary support was closed after cleaning the filling and concrete backfilling.
5. According to the exposed conditions of the karst cave and the state of the tunnel face, the length and height of the next cycle step are adjusted according to the longitudinal and vertical ranges of the karst cave. The adjusted step length can be greater than 5 m, and the step height should be less than or equal to 6 m. In the next cycle, the leading small pipe is adjusted to a \(\varphi\)42 double-layer length of 6.0 m, circumferential spacing of 40 cm, extrapolation angle of 10\({}^{\circ}\), and arch range of 120\({}^{\circ}\).