# Structural Health Monitoring of a Road Tunnel Intersecting a Large and Active Landslide
## 3 The Tunnel Monitoring System  
Starting in 2014, following the request of the Italian Road Authority (ANAS), which is responsible for the maintenance and safety of the tunnel, a monitoring system has been running in the tunnel. The main aim of the system is to control the evolution of the cracks on the vault and assist with periodic inspections (Figure 4). Data would also be used to define effective mitigation strategies to reduce stresses on the structure and increase the life span of the tunnel.  
Figure 4: Distribution of tension cracks along the tunnel: the white stars refer to the crackmeters deployed along cracks, the grey stars to the crackmeter deployed between the segments and the black squares to the two-axis clinometers. The area comprises the section of the tunnel between 261 m and 309 m from the eastern entrance.  
Figure 5 represents the layout of the monitoring system. It consists of a datalogger recording the data from four clinometers and 24 crackmeters using 4-20 mA direct current loop transmission protocol. The datalogger digitises the analogue signal with a sampling period of 30 min and 16 bits of accuracy. The crackmeter and clinometer employed in this monitoring system are shown in Figure 6. The crackmeter is a standard resistive bar displacement transducer made of stainless steel. According to the datasheet, it operates from -20 \({}^{\circ}\)C to +80 \({}^{\circ}\)C with a thermal drift <10 ppm. The clinometer, made of aluminium, has a resolution of 0.001\({}^{\circ}\), a measurement range of \(\pm\)2\({}^{\circ}\), and operates from -20 \({}^{\circ}\)C to +60 \({}^{\circ}\)C. The data output from the datalogger is transmitted to an industrial media converter and then routed through a multimode optical cable link to the entrance of the tunnel, where the optical signal is converted back into an electric serial signal using an additional media converter, before being transmitted to the remote web-portal server using a machine-to-machine modem. The choice of an optical cable link, 400 m long, instead of a more common Ethernet link, avoids the use of signal boosters/repeaters that require access to the electric line to be powered. Furthermore, only two of the eight fibres inside the optical cable are used for transmitting the data from the sensors, while the other six are at our disposal for any system upgrade.  
As to the installed sensors, 16 of the 24 crackmeters are installed across some major cracks near the slip surface on the vault and the other eight across the tunnel lining segments. Concerning the clinometers, three are located on the uphill side of the tunnel; the last one is in the downhill part of segment 28 at the tunnel pillar's head. Figure 4 shows the approximate positions of these sensors.  
In addition to these sensors, the underground drainage system of the tunnel is equipped with two thin-plate wears to measure the water discharge. Eventually, the precipitation in the site is measured using a rain gauge installed at the eastern entrance. The aim is to investigate a possible correlation  
Figure 5: Scheme of the sensor sensing system. Each node of the sensing system indicates the need for the power supply and the data protocol used at each input port (2 G M2M: machine-to-machine modem enabling data transfer over a 2 G cellular network; MMF: multimode optical fibre; RS232: serial data protocol). Furthermore, the lengths of each link are reported.  
Figure 6: An extensometer (**A**) and a clinometer (**B**) installed in the tunnel.  
between meteoric events, the hydrogeological behaviour of the rock masses and structure strain. The monitoring system was also used to identify the factors influencing the landslide/tunnel dynamics and to provide tailored options for mitigation works, such as the extension of the drainage tunnel.  
Sensor recording started in April 2014. No malfunctions occurred, except that the water discharge probe deployed in August 2014 failed between 26 March and 14 May 2015.