Civil infrastructures all over the world often make use of underground development being a perfect choice when it comes to crossing mountain ranges, connecting islands to continents, managing urban and extra-urban roads, rail and road traffic, running large hydroelectric power plants, etc.

Europe and the US have an extensive in-service tunnelling heritage. Many of these tunnels have been in operation for decades already and are therefore prone to deterioration and degradation, though they are still key elements in community life. Their decommissioning might have a strong impact on the economy, logistics, sustainable development (especially in “green” terms) and social activity.

Thus, maintaining existing tunnels and proper construction of new underground infrastructures are crucial. And here come structural health monitoring systems that can be installed in tunnels, providing status information, and helping to manage maintenance.

Fiber optic sensors is a key part in DAS, DSS, DTS and other fully distributed systems. DFOS allows monitoring of deformations and temperatures along the whole length of a tunnel offering unique features: measures of physical parameters (acoustic, temperature, strain, etc.) can be done of thousands of points from a single read-out unit. The sensing cable is not sensitive to electromagnetic disturbances, withstands hostile environments and does not require power supply. The whole length of the sensing cable can reach up to 30 km though tunnel disturbance events such as concrete cracks, deformations, water ingress, fire, and others can be localized quite accurate at the point of damage.

The monitoring system allows a fully automatic and continuous monitoring of the tunnel integrity, and rapid alerts can be generated based on predefined criteria. DFOS provides timely data that help maintain tunnels effectively before any major damage and increase the safety of the structure.

Wires&Bytes MultiSense cable can be a perfect choice for SHM of in-service tunnels. Despite its small size and low weight, it supports all types of DFOS and deliver multiple measurements.

The design combines 4 tight buffer stranded fibers in a stranding layer and 3 or 4 fibers in a central loose tube. Stranded fibers are in a tight physical connection to the outer plastic jacket, ensuring quality transfer of measurands to the optical fibers such as for DAS and DSS measurements.

Since 4 fibers are provided, a careful combination of

• lay length (helix factor)
• cable geometry
• interrogation instrument/technology
• correct calculation method (data processing algorithm)

can provide information on distributed curvature and bending of the cable, as well as compensation of ripples in the Brillouin measurements resulting in profiling plain longitudinal strain measurement apart from other induces stresses.

Loose fibers in the central tube can be both single-mode and multimode type and can be used for building different measurement configurations, such as loops for more precise stimulated Brillouin measurement, or distributed temperature measurements for temperature compensation purposes. Its configuration, multifiber design and versatility were the reasons for MultiSense to be selected and used in several structural health monitoring projects including tunnel monitoring in Italy and France. Once deployed in the structure with secured access to both ends, the cable leaves it up to the operator and the expert to decide how it will be interrogated.

Get in touch today and we will be able to advise, supply, install and support your optimal solution!