Behaviour of an old masonry tunnel due to tunnelling-induced ground settlement

A novel technique of distributed strain sensing is introduced using Brillouin optical time-domain reflectometry to examine the performance of an old masonry tunnel during the construction of a new tunnel beneath it in London. Optical fibres attached along the intrados of the tunnel lining at five circumferential sections recorded the relative strains of tunnel deformation between the fixing points as well as detecting localised strain such as cracking in the masonry. The results indicated where cracking might have occurred and were confirmed with visual inspection. Measurements were compared with the data obtained by surveying using total station units and the results show close agreement between the two systems. Flexural behaviour along the longitudinal section of the tunnel was also examined by measuring strain along the springlines and crown.