This work presents a study of the mechanical properties of optical fibre sensors based on fibre Bragg gratings for application in the in situ monitoring of displacements in tunnels. Tests were performed on fibres and gratings in both dry and humid environments. The tests encompassed both dynamic fatigue (constant stress rate) in both two-point bend and in tensile loading and static fatigue (constant stress conditions). The results indicate that the fibres and gratings are expected to have a very long lifetime under the likely mechanical and environmental conditions.
Monitoring the movement of existing tunnels when new tunnel construction or other construction activities occur in close proximity is important to the tunnel owners. Existing manual monitoring systems, although considered most reliable, require access to the tunnel outside of passenger traffic hours and thus only provide measurements during a limited time of the day and under non-operational conditions. Remote monitoring systems, enabling 24 hour monitoring, are currently available based on electrolevels and automatic motorised theodolites. However, it is hoped that the proposed optical fibre sensor system will be more versatile and economic to install and operate.
The underlying concept is based on a fibreglass rod containing optical fibres, with Fibre Bragg Gratings (FBG) written into them, which is fixed at discrete points to the tunnel lining. The movement of the tunnel induces a deformation of the rod, and hence strains the FBGs positioned at different points along the optical fibres. The FBGs work by reflecting narrow bands of light propagating along the optical fibres, the wavelength of the reflected light being a function of the strain at the position of the grating.
This paper will discuss one experimental arrangement designed to explore the challenges of installing the monitoring system and interpreting the results. Results from tests are presented and discussed together with the sensitivity and accuracy achievable with the proposed system. Further, one method of interpreting the strain measurements and hence determining the displacement of the structure will be presented.
The effects of the fabrication procedure for fibre Bragg gratings (FBG) on the mechanical properties of fibres were investigated by dynamic fatigue tests in two-point bending in a “wet” environment. The results show that both chemical stripping and continuous wave UV irradiation reduced the mechanical resistance of the fibre.