LYCOS RETRIEVER 
Channel Tunnel: Service Tunnel
built 629 days ago
The tunnel consists of three parallel tunnels running between the respective portals, or tunnel entrances, at either end. There are two rail tunnels, measuring 7.6 m (25 ft) in diameter and about 30 m (98 ft) apart, which carry trains north-west and south-east. Between the rail tunnels is a service tunnel, 4.8 m (16 ft) in diameter, which is connected by cross-passages to the main tunnels at intervals of approximately 375 m (1,230 ft). The service tunnel, served by narrow rubber-tyred vehicles, gives maintenance workers access to the rail tunnels and provides a route for escape during emergencies.
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In 1960, the group proposed a railway tunnel system comprising two main tunnels and a service tunnel. The project was formally launched in 1973. However, the fuel crisis was to see the project abandoned in 1975.
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There are two single-track rail tunnels, and a third smaller service tunnel as an emergency exit (with frequent cross passages). These were bored through the chalk from either side, and met in the middle. They are lined with concrete panels (on the French side, made from the Marquise quarries).
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The service tunnel is ventilated to keep it at a higher air pressure than the two running tunnels. This normal ventilation system (NVS) runs continuously and means that whenever a cross-passage door is opened, air will flow from the service tunnel into the running tunnel.
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Service Tunnel