It is considered necessary to undertake in-tunnel monitoring when new tunnels are constructed near to important existing tunnels. The records from monitoring on the Heathrow Express, in London, constructed by BAA between 1994 and 1996 have been examined with a view to seeking improvements for future projects. On the project a number of large diameter tunnels passed beneath the existing Piccadilly Line running tunnels with limited vertical clearance. Extensive monitoring was carried out inside the existing tunnels. A description is given of the instruments used, which included electrolevels, precise levels and tape extensometers. The relative merits of manual methods (such as precise levelling and tape extensometer readings) and remote reading methods (such as electrolevel strings) are discussed. When using manual methods in rail tunnels, access can only be gained for the period nightly when trains are not running. If new nearby tunnelling is undertaken rapidly, this means that unless the manual monitoring is supplemented by remote reading methods, large settlements may occur between successive monitoring readings. This may provide insufficient control. Comparisons are briefly made between the monitoring arrays used at Heathrow and those on other tunnelling projects in London.
When new tunnelling is undertaken in soft ground in close proximity to an existing tunnel there is concern about the possible effects on the existing tunnel and monitoring of the existing tunnels is normally undertaken during construction. This paper deals with the monitoring within the Piccadilly Line tunnels during construction of the Heathrow Express tunnels constructed in close proximity at Central Terminal Area (CTA) Station, Heathrow, London, completed in 1996. Reference is also made to monitoring at the Angel Station Reconstruction completed in 1993 (Moriarty and Cooper 1993) and Old Street Reconstruction completed in 1996 (Cooper et al 2000).
