The Channel Tunnel (often called the Chunnel) is one of the great engineering achievements of the 20th century.
Two particularly interesting questions:
- Why didn’t they build it in the shortest possible straight line between England and France?
- What happened to those enormous tunnel-boring machines when the British and French crews finally met underneath the sea?
The surprising part is that some of those giant machines were deliberately buried underground!
1. Why wasn’t the tunnel built along the shortest route?
The shortest crossing of the English Channel is between Dover and Cap Gris-Nez, approximately 34 kilometres across.
But the engineers weren’t simply trying to find the shortest distance. They were trying to find the safest rock to tunnel through.
They chose a route between Folkestone, England, and Coquelles, near Calais, France, because:
- Geology: They wanted to stay within a relatively stable, low-permeability layer called chalk marl.
- Water pressure: Some rock layers have fractures that allow seawater to enter the tunnel.
- Railway connections: Both ends needed suitable land for terminals, tracks and connections to existing transport networks.
- Gradients and curves: Trains require carefully controlled slopes, so the tunnel couldn’t simply follow a straight line regardless of the geology.
The undersea portion is approximately 37.9 kilometres long, while the entire tunnel system runs about 50.5 kilometres.
In other words, the safest route through the rock mattered more than saving a few kilometres.
2. What happened to the giant digging machines?
This is the fascinating part, Freddy.
The British and French didn’t each use just one machine. They used 11 enormous tunnel-boring machines altogether — six British and five French.
When the undersea machines approached one another, engineers had a problem: the machines were enormous, and there wasn’t room for them to pass each other.
Here’s how they solved it:
- The British machines: The British undersea machines were directed downward or sideways into specially excavated areas of rock, then buried and sealed in place. Some remain beneath the Channel to this day.
- The French machines: The French machines completed the connections and were dismantled underground. Their components could then be transported out through the completed tunnels.
The service tunnel used a slightly different sequence, but the same basic principle: one machine was buried and the other dismantled.
3. How accurately did the two countries meet?
Horizontal difference
35.8 cm
Vertical difference
5.8 cm
Reported alignment differences at the initial undersea meeting in 1990.
Think about that! They dug from opposite countries, kilometres beneath the seabed, and their alignment was off by less than 36 centimetres horizontally.
The tunnel officially opened in 1994.
And here’s something I think you’ll appreciate: some of those British machines are still entombed beneath the English Channel, like giant steel time capsules.
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