Displacements ahead of an advancing NATM tunnel in the London clay

This paper reports the results of monitoring carried out to study the patterns of ground displacement that develop ahead of an advancing sprayed concrete lined tunnel in soft ground. Extensive instrumentation was installed and monitored during the construction of the Heathrow Express platform tunnels forming part of the underground station at London Heathrow's Terminal 4. The construction of the concourse tunnel, built between the two completed platform tunnels, provided the opportunity to install additional instrumentation ahead of the tunnel drive, and to measure ground movements as the tunnel approached the instrumented sections. The paper describes the ground conditions at Heathrow, and the method of construction used for the concourse tunnel. The performance of the instrumentation and the pattern of ground displacements measured are reported. Finally, a synthesis of ground displacements measured is given, and the results are compared with the volume loss recorded during the concourse tunnel construction.