In Situ Stress Strain Measurements at Tottenham Court Road Platform Tunnel

Summary

The redevelopment of Tottenham Court Road Underground Station started in 2011 as part of the Tube Upgrade Plan to improve and increase the capacity of the existing facility. The plan is to upgrade the station by 2016 to meet an estimated demand of more than 200,000 journeys per day once Crossrail is built. During April to November 2011, major structural work was carried out on the Northern Line platform tunnels as part of the station upgrade. This included removing grey cast iron tunnel lining segments on the platform side to allow for construction of new cross passages to improve access to the platforms. The upgrade works provided an opportunity to trial in-tunnel instrumentation prior to implementation in other London Underground (LUL) tunnels which interface with the Crossrail project. Mechanical and electrical resistance strain gauges were installed on tunnel segments to make discrete measurements of changes in strain due to unloading as the segments were removed from the tunnel rings. Linear variable differential transformer type displacement transducers were installed to make continuous measurements of the opening and closing of circumferential and longitudinal joints on trackside segments which are left insitu and affected by adjacent excavations. This paper describes the installation process and highlights the lessons learnt for future applications. The insitu strain measurements are presented and compared to 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.

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