Structural monitoring of tunnels with fiber optic sensors

MII’s SHM system: conception, implementation, installation, data management.

Curated by Eng. Angelo Annunziata
Hardware Engineer – Marini Impianti Industriali

Structural monitoring is essential to ensure the safety, reliability and durability of critical infrastructure in the country.

These systems enable the collection, transmission, and analysis of data on the condition of a facility over time allowing early detection of signs of deterioration. Recognizing anomalies or problems in advance allows action to be taken before they become dangerous and allows maintenance work to be scheduled, reducing operating costs.

Structural monitoring also allows immediate assessment of the impact of natural events such as earthquakes, floods or landslides on the structure, speeding up decision making.

With this purpose, Marini Impianti Industriali S.p.A. designed and installed for RFI an advanced structural monitoring system for two railway tunnels located on the Battipaglia-Reggio Calabria line using innovative fiber-optic sensors: the Barca tunnel in the province of Vibo Valentia and the Lustra tunnel in the province of Salerno (Fig.1).

Essentially, these are devices that exploit the properties of Bragg gratings to detect and measure deformations in structures or objects and offer several advantages:

• ability to measure deformations with high accuracy;
• high immunity to electromagnetic interference;
• ability to deploy sensors along a single optical fiber and thus potentially over long distances;
• real-time monitoring;

Marini Impianti Industriali supported RFI in all individual phases of the project:

• Conception and design of the monitoring system;
• Planning and field deployment;
• Commissioning and validation of the measured data and structural parameters;

The parameters monitored within the two tunnels were:

• Convergence of the tunnel vault section at specific points;
• stress state of the ribs;
• longitudinal displacements of the cable between various sections;
• width of some visible cracks on the interior linings;

All sensors were then connected via a single multi-fiber cable used as a backbone to an optical interrogator, installed in an outdoor box and located near the tunnel mouth. Inside the Outdoor Box is, in addition to the fiber-optic sensor interrogator, an Embedded PC and an LTE Router for data connection (Fig.2).

All the information detected by the monitoring system was then remoteed to a PC located in the Central Post and located at RFI headquarters.
For data visualization, an interactive dashboard was developed to aggregate and represent all the parameters and data collected in real time, to set thresholds, and to display the history of the detected values (Fig.3).