How the world's longest underwater tunnel was built

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How the world's longest underwater tunnel was built

Lesson created by Simply English

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Flanked

two powerful European nations, the English Channel has long been one

the world’s most important maritime passages. Yet for most of its history, the channel’s rocky shores and stormy weather made crossing a dangerous prospect.

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Engineers of the early 1800's proposed numerous plans for spanning the 33 kilometre gap. Their designs included artificial islands linked by bridges, submerged tubes suspended from floating platforms, and an underwater

more than twice the length of any existing tunnel.

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By the end of the century, this last proposal had captured European imagination. The invention of the tunnel boring machine and the discovery of a stable layer of chalk marl below the seabed made this fantastic tunnel more

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But the project’s most urgent obstacles were ones no engineer could solve.

At the time, Britons viewed their geographic isolation as a strategic advantage,

and fears about French invasion

down plans for the tunnel.

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The rise of aerial warfare rendered these worries obsolete, but new economic concerns arose to replace them. Finally,

years after the initial excavation, the two countries reached an agreement, the tunnel would proceed with private funding.

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In 1985, a group of French and British companies invested the modern equivalent of 14 billion pounds, making the tunnel the

expensive infrastructure project to date.

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The design called for three separate tunnels, one for trains to France, one for trains to England, and one service tunnel between

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Alongside crossover chambers, emergency passages, and air ducts, this amounted to over 200 kilometres of tunnels. In

, workers began excavating from both sides, planning to meet in the middle. Early surveys of the French coast revealed the site was full of fault lines.

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These small cracks let water seep into the rock, so engineers had to develop waterproof boring machines. The British anticipated drier conditions, and forged ahead with regular borers. But only

into the work, water flooded in through undetected fissures.

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To drill in this wet chalk, the British had to use grout to seal the cracks created in the borer’s wake, and even work ahead

main borer to reinforce the chalk about to be drilled.

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With these obstacles behind them, both teams began drilling at full speed.

Boring machines

up to 1,300 tons drilled at nearly 3.5 meters per hour.

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As they dug, they installed lining rings to stabilise the tunnel behind them, making way for support wagons following each machine.

at top speed, work had to proceed carefully.

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The chalk layer followed a winding path between unstable rock and clay, punctured by over 100 boring holes made by previous surveyors. Furthermore, both teams had to constantly check their coordinates to ensure they were on track to meet within

centimetres of each other.

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To maintain this delicate trajectory, the borers employed satellite positioning systems, as well as palaeontologists who used excavated fossils to confirm they were at the right depth. During

, the project employed over 13,000 people and cost the lives of ten workers.

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But after two and a half years of

, the two sides finally made contact. British worker Graham Fagg emerged on the French side, becoming the first human to cross the channel by land since the Ice Age.

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There was still work to be done, from installing crossover chambers and

stations, too laying over a hundred miles of tracks, cables, and sensors. But on May 6, 1994, an opening ceremony marked the tunnel’s completion.

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Full public service began 16 months later, with trains for passengers and rail

for cars and trucks. Today, the Channel Tunnel services over 20 million passengers a year, transporting riders across the channel in just 35 minutes.

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Unfortunately, not everyone has the privilege of making this trip legally. Thousands of refugees have tried to enter

through the tunnel in sometimes fatal attempts.

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These tragedies have transformed the tunnel’s southern entrance into an ongoing site of conflict. Hopefully, the structure’s history

serve as a reminder that humanity is at their best when breaking down barriers.

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How the world's longest underwater tunnel was built

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