The Genius Geometry Of the Channel Tunnel

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The Channel Tunnel project required three tunnels—two main tunnels and one service tunnel. We already know the role of the piston relief duct valves between the main tunnels.

Please review these cross passages. Maintenance staff enter the main tunnel from the service tunnel by walking through them. We will see more details of the cross passages by the end of this article.

Why does the Channel tunnel have two crossovers?

If you think this is the geometry of the Channel Tunnel, you are wrong.

The engineers choose a more complex design with two crossovers. Why did they do this? The crossovers effectively split the 50.5 kilometer tunnel into six manageable sections as shown.

What happens in the event of technical failure?

This complex design allows for maintenance work to be carried out in one section of a tunnel while trains are diverted through the crossover to the other tunnel. It enables a significant portion of the system to remain operational. In the event of an incident or obstruction in one of the running tunnels, the crossovers allow for traffic to be rerouted, minimizing disruption to services as shown.

Using the tunnels for effective ventilation

Do you ever wonder how engineers were able to provide fresh air to this tunnel? The large axial fans were installed at the Shakespeare Cliff of the Channel Tunnel. The Channel Tunnel's ventilation system can circulate up to 100 m³/s of air—enough to fill two Olympic swimming pools every minute—ensuring a breathable atmosphere throughout the 50 km tunnel. Fresh air is supplied to the service tunnel at both ends of the tunnel. The service tunnel is maintained at a higher air pressure than the main tunnels. This fresh air then flows into the main running tunnels through controlled louvers and doors in the cross passages as shown.

The cross-passages connect the service tunnel to the main running tunnels at regular intervals, every 375 meters. These passages are equipped with doors that can be opened or closed to control airflow.

A train journey through the channel tunnel

The train enters the Channel Tunnel on the French side at Coquelles. This location is approximately 6 kilometers inland from the English Channel coast.

The train follows a complex path through the Chalk Marl layer. The train will encounter the first rail crossover after traveling 12 kilometers from the tunnel entrance. This region is in fact a massive undersea cavern.

Once the train emerges from the Channel Tunnel at Folkestone, Kent, on the English side.

The train eventually reaches the UK terminal. To start the next service, the train has to turn around. The train is then ready for its return service to France.

How safe is the tunnel?

If a train has a technical problem, service will continue thanks to the two crossovers that divide the tunnel into six sections. The specialized maintenance vehicle enters the tunnel as the service-tunnel gate opens, allowing personnel to access the running tunnel via cross-passages. The service tunnel is kept at a higher pressure than the main tunnel.

If a fire occurs in the running tunnel, higher pressure prevents smoke and flames from entering the service tunnel, allowing it to serve as a safe evacuation route.