World’s longest undersea tunnel link

Mott MacDonald was principal design consultant to contractor Transmanche Link (TML) for all civil and geotechnical engineering on the UK section of the Channel Tunnel, plus key mechanical and electrical services throughout the entire 50km tunnel between Folkestone in England and Sangatte in France – the world's longest undersea crossing.

Our role on Eurotunnel’s project covered the tunnels and associated underground structures, the ventilation and cooling systems, railway trackwork within the UK tunnels and Folkestone terminal, road links to the terminal from the M20 motorway, major tunnel and bridge structures within the terminal and a 1.5km seawall at Shakespeare Cliff to contain tunnel spoil.

Some 500 technical personnel were directly employed by or under the design management of Mott MacDonald at the peak period (1988-1989). Design input was in excess of 1.3 million man hours.

Running tunnels and caverns
The Channel Tunnel comprises two 7.6m diameter bored running tunnels with a central 4.8m diameter service tunnel, connected at regular intervals by cross passages. The service tunnel carries the ventilation system, provides access for maintenance and acts as an emergency escape route with its own transportation system. Also connected to the running tunnels are 2m diameter pressure relief ducts which reduce aerodynamic drag from the high-speed trains.

In two vast undersea caverns – one in the UK section, one in the French – scissor cross-overs linking the main running tunnels allow operations to continue should a section of tunnel need to be closed. The UK cross-over cavern – the world's largest subsea excavation at 156m long by 18m wide and 10m high internally – was constructed using the New Austrian Tunnelling Method. Movement of the cavern excavation was monitored by means of extensive instrumentation at some 200 stations.

UK cut-and-cover
The UK land sections of the Channel Tunnel involved various cut-and-cover construction techniques including embedded cantilever diaphragm walls, multi-propped walls, piled chambers and deep open cut. At Castle Hill near the portal, an extensive section of cut-and-cover had to be constructed wholly within an existing landslip, calling for close control of groundwater movements and levels plus a carefully phased sequence of top-down construction, including use of heave reducing piles.

Folkestone terminal
Where the tunnel emerges to the west of Castle Hill high-speed turnouts connect the tunnel tracks to the new terminal at Folkestone which houses all the services and interchange facilities for the Channel Tunnel trains. To avoid interference between incoming and outgoing trains within the terminal, the railway layout takes the form of a loop, with three arrival lines and twin departure lines initially serving ten parallel platform tracks where vehicles drive on and off the shuttles. The track layout has been designed to permit future expansion to 16 platforms. Nearly 54km of ballasted and non-ballasted track has been provided with more than 100 turnouts, including stabling and maintenance sidings.

Road infrastructure includes new links with the M20 London-Dover motorway and the re-routed A20 trunk road which was also designed by Mott MacDonald. Seven new bridges and viaducts were required, each designed to keep motorway traffic flows moving during construction.

The alignment and configuration for the Channel Tunnel – opened in 1994 – echo a 1975 proposal for which we were also principal designer. Our studies for the earlier scheme were taken into account in the Channel Tunnel design, along with our subsequent investigations and tests in 1986-87 covering aerodynamics and ventilation, alignment, drainage, track form, tunnel lining and geotechnics. Our involvement in schemes to build a tunnel under the English Channel goes back still further to a feasibility study undertaken in 1929.