Client : SNCF
Location : First phase of the project in the Nerthe tunnel near Marseille, second phase of the project in the Encombrouze tunnel in the Alps.
Fiber optic instrumentation and conventional sensors of the Encombrouze tunnel
The Encombrouze tunnel is an old tunnel built in 1882. Monitoring of this type of structure is important as it ensures that no suspicious movement is detected. Thus, this explains the need to install such sensors there.
Objective of the project :
This project is divided into two phases : the first one is dedicated to testing the resistance of optical fibers to shotcrete and the second one to realize the instrumentation of the Encombrouze tunnel.
The first phase of the Encombrouze tunnel monitoring project was carried out on June 24 and 25. This phase consists of resistance tests of the optical fiber to the shotcrete. To do this, we carried out various measurements before and after spraying the concrete in order to be able to choose which fiber would be the most suitable for monitoring the tunnel. Once these tests were carried out and the data processed, we were able to choose which fiber to use according to several criteria (such as ease of installation, resistance of the fiber or ease of handling of the fiber).
The second phase of the project will be carried out in October and will include the installation of all measurement tools as well as the performance of the first tests.
First tests :
In order to carry out the first tests for the resistance to concrete of the fibers, 4 types of fibers were selected to undergo the resistance tests to shotcrete. The selected fibers were therefore placed in a test box before receiving shotcrete. The selected fibers were more or less reinforced and we were able to verify the resistance that is necessary for the future instrumentation of the tunnel.
Various tests: red laser, OTDR (optical time domain Reflectometer) or Brillouin were carried out in order to ensure better stress resistance of the fibers. First of all, these tests allow us to check the integrity of the fiber and ensure that the concrete has not destroyed it. Secondly, we can check stresses undergone by the fiber after spraying the concrete. We therefore carried out measurements before and after the spraying of concrete in order to clearly visualize the effect of the latter on the fibers.
Second phase of the project :
The second phase of this project will consist of the installation of the fiber in the tunnel as well as the installation of more traditional sensors.
During this phase, we will also carry out the first measurements on the elements installed in order to calibrate those to come. This part of the project is crucial for assessing the evolution of the tunnel.