Opportunity
London Heathrow is the seventh busiest airport in the world by total passenger traffic. In 2019, it handled a record 80.8 million passengers.
Heathrow's Terminal 5 was opened in 2008. Its main building is the largest free-standing structure in the UK. In 2018, Terminal 5 handled more than 32 million passengers on 211,000 flights. It was the busiest terminal at London Heathrow by both passenger numbers and flight movements, driving the need for expansion and improvement.
Solution
Mott MacDonald is one of the first-tier integrated suppliers for the British Airports Authority (BAA Ltd.), which operates five British airports. Our underground work has supported the development of Terminal 5 for many years.
Airside Road Tunnel
Heathrow’s Airside Road Tunnel is 0.8 miles (1.3 kilometers) long. It provides road access from Terminals 1, 2 and 3 to Terminal 5, and serves the remote aircraft stands on the western edge of the airport from the central terminal area.
Constructing the tunnel required working under one of the world’s busiest airports, at extremely shallow depth, and having to pass under existing utilities. At one point the tunnel ran only 10 feet (3 meters) above the tunnels for the Heathrow Express rail link.
We were 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.
Safety was a big focus of our twin tunnel design. Recognizing the need to provide safe access at all times, the decision was taken to bore twin single carriageway tunnels, ensuring they were wide enough to allow vehicles to pass each another in case of a vehicle breakdown.
A key innovation was the use of the observational method on the tunnel portals. With strong encouragement from BAA, our designers and the contractor Laing O’Rourke achieved substantial time and cost savings in temporary works through close teamwork. The method led to program savings of over 30 weeks and improved site safety by avoiding the need to handle heavy steelwork within confined spaces.
Phase 2 improvements
Phase 2 of the development of Terminal 5 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 59 feet (18 meters) below ground.
Stage one of Phase 2 included all the works below ground as well as the apron slab and three new aircraft stands. Stage two delivered the remaining aircraft stands and above-ground building works for live operation. The two-stage design and construction process created unique challenges and required careful planning, coordination, and design integration.
Beginning at the feasibility stage, our responsibilities included the design of deep basements and a number of cut-and-cover tunnels. We incorporated a deep level chamber in the design that could be used in the future to launch a tunnel boring machine to construct a baggage tunnel to the central terminal. We developed the monitoring strategy and provided construction support.
Outcome
The Airside Road Tunnel, driven by a dual-mode tunnel boring machine, was completed on time, below budget, and with no impact on the airport’s day-to-day operations. Trains on the Heathrow Express operated as normal and all associated work for monitoring was undertaken within the normal maintenance closures.
For the sub-surface works in the Phase 2 improvements, the biggest challenge was the presence of four rail tunnels, three shafts, and three cross-passages.
Our detailed geotechnical analysis modeled the behavior of the rail tunnels and communicated 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.