Tunnelling induced strains and deformations at Central Line

Summary

In October and November 2012, Imperial College installed electrical resistance strain gauge rosettes, potentiometric displacement transducers and tape extensometer eye bolts at selected locations in the Central Line eastbound running tunnel between Lancaster Gate and Marble Arch. The installations were completed before the two Crossrail tunnel boring machines (TBMs) passed below the Central Line running tunnel at this interface.
Automatic continuous measurements of strain in the selected tunnel segments were made before, during and after the passage of the TBMs at this interface. Similarly, automatic continuous measurements were made of any opening and/or closing of horizontal joints of the tunnel rings during this time period. Manual measurements of tunnel convergence and diameter/chord length changes were made during engineering hours.
This paper describes the installation process and presents the monitoring data for each TBM passage as well as the combined effect of both Crossrail tunnels on Central Line. The interpretation of the strain measurements to provide information on the mode of deformation of the instrumented tunnel segments is explained. This information is assessed in conjunction with the manual tape extensometer measurements to ascertain the deformed shape of the instrumented tunnel rings as a result of the Crossrail tunnelling works.
The paper also discusses the use of the field data in guiding the research experiments currently being conducted on a half-scale grey cast iron tunnel ring at Imperial College. Finally, recommendations for future in-tunnel monitoring are proposed based on the wisdom gleaned from this experience.