The Channel Tunnel Project is one of the largest ground engineering projects currently being undertaken in the world. The scheme is being designed and built by Transmanche Link (TML), a consortium of British and French contractors for the client Eurotunnel (ET) and will provide a complete transportation system linking the UK with Continental Europe.
The rail link will be provided by two tunnels with their portals located at Castle Hill, north of Folkestone (UK) and Beussingue Farm, near Sangatte in France. Each tunnel is approximately 50 km long, of which 38 km is located under the seabed between Shakespeare Cliff and Sangatte (Fig. 1).
The system comprises two 7.6 m diameter running tunnels, 30 m apart, with a 4.8 m diameter service tunnel located centrally between them. For operational and safety reasons all three tunnels are connected by 3.3 m diameter transverse cross passages located at intervals of 375 m throughout the drive. In addition, 2 m diameter piston relief ducts pass between the running tunnels every 250 m. These will allow dissipation of the air pressure pulse in front of the train and thereby reduce aerodynamic drag.
At two points along the tunnel length i.e. km 27 and 45 approximately, there are large underground crossover chambers. These will allow trains to transfer between the running tunnels, for example whilst maintenance is being carried out in a particular section of tunnel.
The construction of the tunnels, crossover caverns, pumping stations and all ancillary works, involves the excavation, lining and ventilation