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
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.