Brillouin optical fiber distributed sensor for settlement monitoring while tunneling the metro line 3 in Cairo, Egypt

Safety while tunneling is one of the main challenges for underground constructions, avoiding confinement losses, which remain an important risk for public works, leading to additional delays and high insurance costs. In such applications, usual surface instrumentations cannot be set up because of high building density in many overcrowded cities. Tunnelling deals with the challenge of requiring ground surface undisturbed. One original concept proposed in the framework of the European Tunconstruct project, consists in very early settlement detection close to the tunnel vault and before any detectable effect on the surface. The adopted solution is to set-up a sensing element inserted into a directional drilling excavated above the foreseen tunnel. The methodology is based on the well known Brillouin Optical Time Domain Reflectometry (B-OTDR) in singlemode optical fibres and a special cable design dedicated to bending measurement. Two cables, based on different industrial manufacturing processes, have been developed taking into account the strain sensitivity required, the flexibility and the robustness for borehole installation, a low power attenuation and storage on a drum. Industrial prototypes have been manufactured and validated with tests in open air where settlement profiles geometry can be accurately controlled. Demonstration on job site took place on The Greater Cairo Metro Line 3 (CML3) at the beginning of 2009.