Bored tunnel for passenger and baggage transfer

As one of BAA’s first-tier integrated suppliers for Heathrow’s Terminal 5, we have been the design engineer for all the £4.3 billion project’s sub-structures and foundations, and provided rail assurance services, tunnelling advice and project and programme management. As our role has drawn to a close on the airside road tunnel, we have entered a busy period on sub-structure design for the terminal’s second satellite building.

Heathrow’s 1.2 km long airside road tunnel (ART) has a dual purpose – to provide road access from Terminals 1, 2 and 3 to Terminal 5, and to serve the remote aircraft stands on the western edge of the airport from the central terminal area. This complex job has involved working under one of the world’s busiest airports with the challenges of tunnelling at extremely shallow depth, and having to pass under existing utilities. At one point the tunnels run only 3m above the Heathrow Express rail link tunnels and cross the existing Piccadilly Line tunnel with only 5m clearance.

Opened in March, the ART – driven by a 9.16 m external diameter dual mode tunnel boring machine – was completed on time and below budget and with no impact on the airport’s busy day-to-day operations during the tunnelling process. Trains on the Heathrow Express operated as normal and all associated work for monitoring was able to be undertaken within the normal maintenance closures.

Mott MacDonald has been responsible for the design of the twin tunnel bores and the approach structures along with highway design, mechanical and electrical definition design, and instrumentation and monitoring of this complex project.

Safety was a major factor of our twin tunnel design. Recognising the need to provide safe egress and access at all times, the decision was taken to form twin single carriageway tunnels – but to ensure they were sufficiently wide to allow vehicles to pass one another in the case of a simple vehicle breakdown. A key innovation was the use of the observational method (OM) on the ART portals. With strong support from BAA, our designers and the civil contractor Laing O’Rourke developed substantial time and cost savings in temporary works through close teamwork. Based on our extensive previous experience in the use of OM, the team saw it as an excellent opportunity for innovation and continuous improvement. The method demands clear communication, commitment, and trust between the parties involved – which is fostered by the partnering ethos BAA’s contractual framework promotes. This led to programme savings of over 30 weeks as well as improved site safety by avoiding the need to handle heavy steelwork within confined spaces.