Geotechnical aspects of the construction of the Heathrow Transfer Baggage System tunnel

The Heathrow Transfer Baggage System comprises a tunnel from Heathrow Terminal 1 to Terminal 4 to automatically convey luggage between the two terminals' baggage halls. The 1.4km tunnel, 4.5m in diameter, was driven beneath a runway, the Piccadilly Line and airport buildings to connect with pairs of shafts at each end, which rise into the terminals. Junctions at each end were built using shotcrete for both the primary and secondary linings. The route selection, design and construction of the tunnel are described in the paper, together with the settlements measured. The start of construction of the first shotcrete (NATM) junction was imminent when the Heathrow Express tunnels suffered a major collapse elsewhere in the airport in October 1994. As a result the Transfer Baggage System junction was delayed while a submission was made to the Health and Safety Executive for their approval to proceed. In the paper some design issues are described, together with the results of comprehensive ground monitoring carried out during and after construction.