Movements in the Piccadilly Line tunnels due to the Heathrow Express construction

This paper presents the results of analyses of monitoring records taken from inside one of the existing Piccadilly Line tunnels during the construction of three station tunnels at the Heathrow Express Central Terminal Area. The monitoring data allow a detailed picture of the way in which existing segmental concrete-lined tunnels behave as a result of tunnelling works below. Settlement curves at critical construction stages are presented. It is shown that the computed ‘volume loss’ varied between 1·3% and 2·5%. Long-term settlement records indicate that the maximum settlement increased by 27% for a period three years after tunnel completion. This result is compared with earlier findings from the Heathrow trial tunnel. Asymmetry of the settlement troughs caused by the outer tunnels, constructed after the central concourse tunnel, is discussed, and asymmetry parameters are defined from a number of case histories. The relationship of asymmetry parameters to pillar width between tunnels is presented and discus...