## 3 Health Monitoring Data of the HZMB Immersed Tunnel
### Description of Sensors and Health Monitoring Data  
Not all monitoring items produce real-time health monitoring data, and some, such as uneven settlement, vehicle load, etc., are obtained through regular inspection methods. Five types of monitoring data, namely ground motion, joint deformation, concrete strain, temperature, and humidity data are adopted to discuss the proposed wavelet denoising method. The details of health monitoring contents, corresponding sensors, and installation locations are listed in Table 2. The sampling frequency of the five types of data is 50 Hz.  
1. Ground motion  
The upper part of the immersed tunnel is designed with a certain thickness of silt and fine sand covering layer, which may liquefy or move to expose the pipe joints or loosen the foundations under seismic excitation. Therefore, it is necessary to monitor the ground motion and its impact on the structure to determine the state of safety after an earthquake. Considering that the nearly 6-km tunnel is located on the seabed of different landforms, the selection of ground motion monitoring points combines factors such as soil quality, foundation scheme and weak connections. Therefore, a total of five key sections of the immersed tunnel are equipped with 3D accelerometer sensors, which can monitor the ground motion of the tunnel along the three directions of XYZ.
2. Strain of elements  
For immersed tunnels, the main body strain of the elements, the shear stress, tensile stress and compressive stress at the joints of the elements are all items that require monitoring. By embedding strain sensors in concrete, the strain and stress conditions of structural representatives or controlled components or parts during operation can be obtained at any time. In this project, ten sections were selected for monitoring, with a total of 57 strain sensors.
3. Joint deformation  
The monitored joint deformation between adjacent elements of the immersed tunnel is the relative displacement along the longitudinal direction of the tunnel. It is related to the dislocation and opening displacement of the rubber waterstop, which reflects the effect and safety reserve of the waterproofing system. Considering the existence of shear key,  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Monitoring item** & **Data** & **Sensors** & **Installation Location** \\ \hline Structural responses & ground motion & 3D accelerometer \\  & strain of element & FBG strain sensor \\  & joint deformation & displacement meter \\ \hline Environmental loads & temperature humidity & thermometer hygrometer \\ \hline \hline \end{tabular}
\end{table}
Table 2: Health monitoring contents and corresponding sensors.  
the pipe joint mainly produces longitudinal dislocation. Therefore, it is only necessary to monitor the longitudinal displacement of the joint.  
There are a total of 34 joint sections in the whole tunnel, and each section is equipped with a pull-rope displacement gauge along the four corners of the tunnel carriageway, and the entire tunnel has a total of 136 displacement meters.  
The changes of the temperature and humidity in the tunnel are directly related to the stress level of the concrete structure and the working environment of the monitoring system. Through the monitoring of air temperature, the influence of the surface temperature of the concrete structure and the ambient temperature in the tunnel on the stress of the concrete structure and the environment where the monitoring equipment is located can be eliminated, so that the identification method based on the static test can more accurately reflect the structural reference state.  
The installation positions of the thermometers and the strain gauges are very close, and the quantity is the same. A total of 57 thermometers are arranged on 10 sections. Among them, 5 sections were selected for the installation of 2 hygrometers, and there are a total of 10 hygrometers in the whole tunnel.