each tunnel is divided into thirds, the running tunnels come together in single, large caverns housing scissor crossovers. These two crossovers and the switch-and-crossing work near each portal divide the tunnels into six sectors, three in the North Tunnel and three in the South Tunnel. By working single-line traffic on one of these sections, the client (Eurotunnel [ET]) will be able to take possession of one-sixth of the tunnel for maintenance. The maintenance is planned for short periods each night, with slightly longer possessions on weekends.
A third, smaller tunnel (4.8 m in diameter) lies between the running tunnels. This service tunnel carries much of the linear, fixed equipment and is a crucial element of the ventilation system. It will also serve as an access spine for rubber-tired vehicles to cover maintenance and emergencies. At the portals and both the United Kingdom and French crossovers, the service tunnel extends under a running tunnel and then back again to permit the larger tunnels to come together.
There are cross passages between each of the running tunnels and the service tunnel at every 375 m. These can be used in emergencies and are also the route taken by many of the linear, fixed-equipment services. At 250-m intervals there are 2-m-diameter, piston-relief ducts between the running tunnels that pass over the top of the service tunnel but are not connected to it.
After ET starts commercial service, ventilation of the tunnel complex will normally be accomplished by pressurizing the service tunnel with cross-passage doors closed and with doors at the crossovers also closed. The two remaining tunnels will thus be separated from the service tunnel, which should be a safe haven in the case of fire. Should it be necessary to ventilate the running tunnels, a supplementary ventilation system can be used that can operate bidirectionally, with United Kingdom fans blowing and French fans sucking and vice versa.
At each portal, major terminals have been built. The United Kingdom terminal is the smaller of the two due to the geographical restraints of the North Downs to the north of the terminal and a motorway to the south. The French terminal is not as constrained and is many times larger. ET will develop an industrial estate on the French terminal.
The commercial services will be of a number of different types. Principally, there will be shuttle services for heavy goods vehicles and for private cars and coaches. The shuttle vehicles are large in cross section and are, therefore, captive to the project. On each terminal there are toll booths, customs, and immigration facilities for both countries. Tunnel users will clear all these formalities in the country from which they are leaving. Upon arrival across the channel, vehicles will be free to drive from the terminal straight onto the motorway network. Car passengers will drive themselves onto the trains and then ride in their own cars. Lorry drivers, having driven onto their shuttle trains, will ride in an amenity coach on the same train. In addition to the shuttle trains, there will also be