## 2 Engineering Background  
The Xiamen Haicang Evacuate Channel project is located in Haicang District, Xiamen City, as shown in Figure 1a. A Lushu interchange is set up in the Xiamen Haicang Evacuate Channel project, and a total of four ramps are laid at the interchange, which is a semi-interworking form. The research section is the bifurcated small clear distance section between the Lushu left mainline tunnel and the ramp, as shown in Figure 1b,c. The maximum span of the Lushu main line tunnel is 16.17 m, the height is 11.32 m, and the maximum excavation area of the section is 361.43 m\({}^{2}\). The maximum span of ramp A is 12.62 m, the height is 10.40 m, and the limit span is 11.10 m. The maximum excavation area of the section is 370.76 m\({}^{2}\). In the research section, the maximum clear distance of the bifurcated small clear distance section and the maximum excavation width reach 28.0 m and 30.52 m, respectively. The clear distance of the ultrasmall clear distance section of the bifurcated tunnel is 1.2 to 8.0 m, as shown in Figure 1d. The pile numbers of the research section are ZK4 + 090-ZK4 + 490, among which ZK4 + 090-ZK4 + 150 are large sections and ZK4 + 150-ZK4 + 490 are small clear distance sections.  
The Xiamen Haicang Evacuate Channel tunnel runs through a granite stratum and is covered with Quaternary residual and slope deposits. The tunnel site is located in the Damaoshan-Shifengyan fault zone and is characterized by developed tectonics and extensive jointing and cracking of the rock mass due to multiphase tectonic stress. The groundwater at the site is also relatively abundant, mainly stored in the alluvium and some structural water storage zones. It is shallowly buried, with depths generally ranging between 1.0 and 5.0 m, and is affected by seasonal precipitation. The research section of the tunnel passes through the second intrusion granite strata of late Yanshan, which is mainly medium weathered. The surrounding rock grading is III, which belongs to relatively intact rock. Surface water is abundant in the research section, but it has little influence on the tunnel construction activities. Groundwater is developed in the fault tectonic zone. To excavate the tunnel, the design scheme adopts the step method, short step method, CD method, and double side heading method, following the construction principle of the new Austrian method. The composite lining includes primary support using the first bolt-shotcrete steel net and the second bolt-shotcrete steel net, temporary support using an I-steel frame, and secondary lining using a concrete layer of the arch wall and the invert. The retaining structures' parameters are shown in Table 1. The tunnel in the research section has a large section and super small clear distance, complicated construction sequence, asynchronous excavation and support, and construction blasting disturbance, which require high-quality monitoring. Traditional monitoring methods cannot satisfy the requirements and cannot be used in real time. Therefore, there is a need to develop methods that can be monitored automatically and in real time.  
Figure 1: Research section diagram: (**a**) location of Xiamen Haicang; (**b**) location of Xiamen Haicang Evacuate Channel project; (**c**) diagram of Lushu interchange; and (**d**) schematic of research section.