Heathrow’s 1.3km long airside road tunnel (ART) has a dual purpose – not only providing road access from terminals 1, 2 and 3 to Terminal 5, but also serving the remote aircraft stands on the western edge of the airport from the central terminal area. This complex job involved working under one of the world’s busiest airports with the challenges of extremely shallow depth and having to pass under existing utilities – and at one point running only 3m above the Heathrow Express rail link tunnels.
The ART – driven by a 9.16m external diameter dual mode tunnel boring machine – was completed on time and below budget and with no impact on the airport’s 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.
We were also 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 prestigious project.
Safety was a big focus of our twin tunnel design. Recognising the need to provide safe access at all times the decision was taken to bore twin single carriageway tunnels – but 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 on the ART portals. With strong encouragement from BAA, our designers and the civils contractor Laing O’Rourke developed substantial time and cost savings in temporary works through close teamwork. The method 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.
Terminal 5 Phase 2 involved the design and construction of a second satellite building and 13 aircraft stands and an expansion of the baggage and tracked-transit-system housed in tunnels. The satellite building has four levels above ground and three levels of basements up to 18m below ground. We were involved since the feasibility stage with responsibilities including the design of deep basements and a number of cut-and-cover tunnels.
Stage one of Phase 2 included all the works below ground as well as the apron slab and three new aircraft stands, while stage two delivered the remaining aircraft stands and above-ground building works for live operation. The two stage design and construction process creates some unique challenges and requires careful planning, co-ordination and design integration.
The biggest challenge on the sub-surface works involved the presence of four rail tunnels, three shafts and three cross-passages. We undertook detailed geotechnical analysis to model the behaviour of the rail tunnels and understand the impact of the basement design and construction. We worked within strict limits to ensure that tunnel displacements, gradients, distortions and change in lining loads met with specified guidelines. We also incorporated a deep level chamber for launching a tunnel boring machine to construct a baggage tunnel to the central terminal in the future. We developed the monitoring strategy and provided construction support.