Diana G. Murro, L. Pelecanos, K. Soga
Jun 30, 2016
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Abstract
The Centre for European Nuclear Research (CERN) uses large and complex scientific instruments to study the basic constituents of matter by operating a network of underground particle accelerators and appurtenant tunnels. Several tensile cracks have been observed within a section of a concrete lined tunnel called TT10. As a result, there were concerns about the safety and structural health of CERN infrastructure and a long-term sensing plan was designed to monitor the behaviour of these tunnels. Additionally, two big challenges had to be met: (a) remote monitoring due to tunnel inaccessibility during the operation of the experiment and, (b) potential instrument malfunction due to high radioactivity. Therefore, there was a clear need for radiation-resistant monitoring instruments that could be operated remotely. Distributed Fibre Optic Sensing (DFOS) proved to be the most appropriate monitoring method as it appeared to satisfy the abovementioned requirements. Eight tunnel circumferential loops and one longitudinal were monitored with distributed fibre optic (FO) cables. In the progress readings taken so far, the circumferential tunnel loops showed some minor values of axial strains in the FO cable (peak absolute strain no more than 150με) and a somewhat consistent profile of the strains for all circumferential loops with high values of (minor tensile) strains at the sides and negative-compressive strains at the crown of the tunnel. This strain pattern would suggest a vertical tunnel elongation mechanism of deformation, i.e. there is compression at the tunnel crest and extension at the lateral two sides of the tunnel. Finally, the values of all observed axial strains seem to be insignificant suggesting no major movement or deformation of the tunnel lining over the relatively short monitoring period.