Terminal 5 tunnels at Heathrow Airport

Building more than 14 kilometres of tunnels below one of the world’s busiest airports is a challenging objective that we successfully achieved. To be precise, we were mandated to design and build 7 tunnels of a total length of 14,160 metres. To meet the site specific constraints and the requirements of an airport environment, the project team had to develop customised techniques, including the use of a dual-mode air/earth pressure balance TBM and implementation of sprayed concrete lining.

BACKGROUND

Heathrow is a leading international airport. Prior to the expansion project, it had an annual capacity of 50 million passengers. In 1998, it served nearly 60 million. That is why an expansion project to increase capacity was launched. The new terminal – Terminal 5 – was designed to receive 30 million passengers a year.
So, to connect the airport and the new terminal, we were mandated to create 2 “Tube” tunnels, 2 rail tunnels, 2 road tunnels reserved for internal airport traffic connecting the airport, new terminal, and existing terminals, and a tunnel to evacuate stormwater. All of these tunnels were excavated in London’s clay soil, which provides stability (which is why London was the first city to build an underground public transport network.

TECHNICAL OVERVIEW

To ensure face confinement and make the best use of London’s clay soil, the best option was to implement the compressed-air method. Unfortunately, compressed air was not an appropriate solution for start and end zones where the arch of the tunnel might reach the layer of gravel. That is why we decided to use a dual-mode TBM, combining both air and earth pressure balance. Given the shallow depth of the main tunnels (only 20 metres below the surface) and their small diameter, we decided to use the conventional excavation method with an open shield and wedge-block lining (behind the shield). This method consists in placing a concrete segment ring at the tail end of the TBM in direct contact with the soil.
The structures associated with the main tunnels (ventilation and access shafts, interconnection branches) are very complex with the main challenge being the many section and connection changes between the vertical shafts and the horizontal branches and between the horizontal branches and the main tunnels. Excavation of these structures could only be carried out using so-called conventional methods, namely, a hydraulic excavator and in-situ lining, with sprayed concrete providing the permanent lining.
To build approximately 104,000 concrete segments, the consortium developed its own prefabrication plant. Having one’s own plant ensured the level of safety and supply flexibility that the project required.

As a result of a distressing event for our client – the collapse of a tunnel during its construction in 1995 for the Heathrow Express Rail Link – the worksite of the new terminal was subject to rigorous external control. Soil settlements were monitored daily at 75 points along the tunnel pathway, allowing the TBM team to adjust confinement parameters as finely as possible.

IMPACT

Beyond this mandate’s technical difficulty, logistics was also a permanent challenge for the engineers in charge of this project. Expanding Heathrow Airport is certainly both one of the most ambitious large-scale projects in the United Kingdom and one of the world’s most impressive airport-infrastructure upgrade project. This is an exemplary achievement for VINCI Construction Grands Projets.
This infrastructure enables Heathrow to remain among the world’s leading airports.
fThe project was designed to endow the airport with a 5th terminal and 42 additional aircraft stands, thereby enabling it to increase capacity from 60 million to 80 million passengers annually and to service next-generation large aircraft, for which the A380 is a figurehead.

Project participants

Client
British Airports Authority

Project management
Mott Mac Donald

Key figures

Implementation dates
August 2000 to September 2006

Earthworks
167,567 m3

Tunnel rings
1,501

Precast concrete
23,600 m3

Awards

Building Magazine Health & Safety Awards:
Worker of the year, Best Occupational Health Initiative, Best Site Facilities, Client with Best Commitment to Health and Safety, Health & Safety Organisation of the Year (2005)

British Tunnelling Society:
Project of the Year – Airside Road Tunnel, T5, Heathrow Airport (2005)

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