Tunnel Monitoring with Optical Sensor Technology

Tunnels are at the core of our infrastructure. They are found in large cities, mountains and even under water. They provide connections and shorten distances. But how safe are they?

Today, modern monitoring systems allow reliable condition monitoring of tunnels using fiber Bragg technology.

Mechanical deformations in a tunnel may present a significant safety hazard particularly during construction work on the tunnel itself or in its immediate surroundings. It is essential to quickly check the stability and reliability of the structure.

When choosing the sensor technology to be used for tunnel monitoring it is necessary to consider the following challenges:

  • Will the sensors be able to reliably provide the required signal throughout the intended time - both in the short term and in the long term over several years?
  • Long cables are usually used in tunnels. Therefore, can you guarantee the quality of the measurement signal even over long distances?
  • Can the monitoring system be installed, without affecting traffic in the tunnel?
  • And, of course, the fundamental question: What measurement quantities and underlying models are suitable, for example, to trigger an alarm in the event of a problem?

For all of the points mentioned above, optical sensor technology - based on fiber Bragg technology - has developed into a powerful alternative to conventional monitoring systems over the past years.

Fiber Bragg Grating technology: the optimal solution

Fiber Bragg sensors measure physical quantities - such as strain - with light.  In addition to its outstanding long-term stability, the technology offers another major advantage:  It enables measured values to be transmitted over long distances, with virtually no loss in measurement quality. Furthermore, a single optical waveguide can accommodate several fiber Bragg sensors, thus reducing the effort needed to set up the monitoring system to a minimum. This is a crucial aspect, particularly when monitoring tunnels that often are many kilometers long. 

"SysTunnel" from HBM FiberSensing is a special system solution enabling users to take full advantage of the benefits of using fiber Bragg sensors for tunnel monitoring.

SysTunnel components

SysTunnel comprises the following components:

  1. Sensors for measuring strain and temperature, installed around the circumference of a rounded metal bar. These "sensor rings" can be installed in different tunnel sections and are connected with one another by fiber optical cables.
  2. Interrogators for processing the light signals
  3. Software for processing the measured data

MEMCOT method for reliably determining geometric deformations

SysTunnel uses the MEMCOT method for measurements and calculations. MEMCOT means Extensometric Method for Monitoring Convergences in Tunnels.

The method which originated from the theory of materials - involves the continuous checking of displacements in the tunnel. This is done by measuring deformations along the tunnel contour as well as their transformation over time. The mathematical model allows the modeling of the radial offsets and displacements that are essential to tunnel monitoring based on the axial strains that are being measured.

This approach offers many advantages in practical use: Since MEMCOT measures absolute motion at every measurement point, geometric variations in every section can be determined. This is in contrast to other monitoring methods assuming that motion in the tunnel is symmetric (which is not always correct). Moreover, SysTunnel users can determine the exact form of the deformation, which is also not possible with conventional monitoring systems.

Fiber Bragg sensors in tunnels: benefits in use

SysTunnel is suited to both long-term monitoring and shorter measurement cycles. The system allows use during regular operation – as illustrated by the examples of the Rossio tunnel and the São Paulo Metro tunnel.

Important note to users: Once installed, SysTunnel is a reliable monitoring system for continuous use. This means that it not "only" acquires measurement data reliably and in the long term, but also enables automatic alarms to be configured for significant changes to the tunnel geometry.

Read the whole article

More information can be found in our Application Note: Optical Monitoring System for Strain and Convergence Measurement (PDF).

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