Stress and strain monitoring at Tottenham Court Road Station, London, UK

Removal of old tunnel lining segments at Tottenham Court Road underground railway station in London, UK provided an opportunity to trial in-tunnel instrumentation. Mechanical and electrical resistance strain gauges were installed on adjacent tunnel segments to make discrete measurements of changes in strain owing to unloading as the segments were removed from the tunnel rings. Linear variable differential transformer type displacement transducers were installed. This paper describes the installation process and highlights the lessons learnt for future applications. The in situ strain measurements are presented and compared with the expected response based on laboratory tests conducted on grey cast-iron tunnel segments in the 1970s. The changes in strain measured by both types of strain gauges agreed well with the estimated changes assuming full overburden unloading.