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Fibre bragg grating strain sensor

The fibre optic strain-cable measures displacement or strain between several fixation points along a borehole in order to monitor strain and /or settlement in rock, concrete and soils with a very high accuracy and long-term stability. This cable interconnects in series and in the borehole with several fibre optic displacement cells (the transducers). The diameter of the displacement cells being as small as 20mm, the borehole diameter can be reduced, thus decreasing the cost of drilling.

But the displacement cells can be implemented in two other ways:

1) In a bundle of anchored displacement sensors or rod-extensometers. The fibre optic rod-extensometers perform basically the same measurements as the strain cable along a borehole but the displacement fibre optic cells are mounted externally of the structure, allowing them to be retrieved and repaired in case of damage.

2) In a chain of surface displacement sensors, the fibre optic surface displacement chain measures displacements between different anchors mounted into a rock or concrete wall. This configuration is particularly suited to the convergence monitoring of a tunnel or a cavity.

The fibre optic displacement cell can be dimensioned to fulfil the end-user requirements. In addition, the cells are multiplexed in the optical wavelength domain. The measurements are thus centralised by a remote interrogating unit and the monitoring is flexible and user friendly.

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