## 2 Rock Tunnel Deformation Monitoring Scheme  
### Sensor for Tunnel Deformation Monitoring  
In rock tunnel construction, the lining and face deformations of the surrounding rocks are important indicators for site engineers. A typical monitoring scheme includes the horizontal convergence and the crown settlement of a transverse section in front of the rock tunnel excavation. Based on successful monitoring applications in the metro tunnel maintenance, the Omni WTDSs produced by the WISENMESHNET(r) were adopted in this study to achieve high-granularity monitoring data.  
The WTDS is designed to work in a harsh outdoor environment with an IP66-rated enclosure, which has the highest dust-tight protection and the third-highest level of water protection. The WTDS also has high precision during sampling and solid immunity to radio interference. The accuracy and resolution of the sensor are summarized in Table 1. The high-precision laser distance module can accurately measure the distance change between two points of the tunnel structure. The high-precision MEMS Omni tilt sensing module can accurately measure the tilt attached to tunnel linings.  
This sensor can monitor tunnel deformation with high-granularity data and low power consumption. It can operate for more than 12 months with a monitoring frequency of 5 min. Therefore, high-granularity data can be obtained using this type of sensor, and the monitoring and data transmission frequency can be remotely controlled in the cloud-based online platform.  
Two sensors are located on one side of the tunnel to monitor the rock tunnel deformation of a typical tunnel transverse section. The other is located on the opposite side (Figure 1). The horizontal convergence and the tunnel crown settlement are measured  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Monitoring item** & **Accuracy** & **Resolution** \\ \hline Inclination & 0.01\({}^{\circ}\) & 0.0025\({}^{\circ}\) \\ Distance & 0.5 mm & 0.1 mm \\ \hline \hline \end{tabular}
\end{table}
Table 1: The accuracy and resolution of the Omni tilt and distance sensor.  
by interpreting the tilt of the tunnel surface and the distance change between the sensor and the targeting points in the space of the tunnel structure. To monitor the tunnel face deformation, a sensor is placed slightly further from the excavation face, and the laser points at the tunnel face's middle section. The tunnel face deformation can be derived based on the changing distance and the initial shorting angle between the tunnel face plane and the laser beam line.  
In this study, a reading was taken every five minutes to achieve high granularity of the data whilst ensuring a reasonable life span and power consumption. During the installation, all the WTDS nodes should face the same direction, and clear notes must be taken so that a monitored structure's laser distance sensor direction can be correctly interpreted.  
It should be noted that movement of the sensor relative to the attached tunnel lining can affect the accuracy of the measurements. Therefore, fixing the sensor to the primary tunnel lining should be relatively rigid to avoid significant movement during the rock tunnel construction. The monitoring data should be regularly analysed to check if the sensor node has considerable movement. During the blasting stage of the tunnel excavation, flying rock masses are likely to hit and destroy the installed sensors. The heavy dust generated during this construction process can also block the laser beam and affect the distance measurement. Therefore, protection measures are required for the sensors, and notable signs should be implemented at the sensor location.