How the world’s longest underwater tunnel was built – Alex Gendler

Flanked by two powerful European nations,
the English Channel has long been one of 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. Engineers of the early 1800’s
proposed numerous plans for spanning the 33 kilometer gap. Their designs included artificial islands
linked by bridges, submerged tubes suspended
from floating platforms, and an underwater passage more than twice
the length of any existing tunnel. 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 feasible. 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
shut down plans for the tunnel. The rise of aerial warfare rendered
these worries obsolete, but new economic concerns
arose to replace them. Finally, 100 years after
the initial excavation, the two countries
reached an agreement— the tunnel would proceed
with private funding. In 1985, a group
of French and British companies invested the modern equivalent
of 14 billion pounds, making the tunnel the most expensive
infrastructure project to date. The design called
for three separate tunnels— one for trains to France,
one for trains to England, and one service tunnel between them. Alongside crossover chambers,
emergency passages, and air ducts, this amounted to over 200 kilometers
of tunnels. In 1988, 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. 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 months into the work, water
flooded in through undetected fissures. 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 of the main borer to reinforce the chalk
about to be drilled. With these obstacles behind them,
both teams began drilling at full speed. Boring machines weighing up to 1,300 tons
drilled at nearly 3.5 meters per hour. As they dug, they installed lining rings
to stabilize the tunnel behind them, making way for support wagons
following each machine. Even at top speed,
work had to proceed carefully. 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 2 centimeters of each other. To maintain this delicate trajectory, the borers employed
satellite positioning systems, as well as paleontologists
who used excavated fossils to confirm they were at the right depth. During construction,
the project employed over 13,000 people and cost the lives of ten workers. But after two and a half years
of tunneling, 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. There was still work to be done— from installing crossover chambers
and pumping stations, to laying over a hundred miles of tracks,
cables, and sensors. But on May 6, 1994, an opening ceremony
marked the tunnel’s completion. Full public service began
16 months later, with trains for passengers
and rail shuttles for cars and trucks. Today, the Channel Tunnel services
over 20 million passengers a year, transporting riders across the channel
in just 35 minutes. Unfortunately, not everyone has
the privilege of making this trip legally. Thousands of refugees have tried
to enter Britain through the tunnel in sometimes fatal attempts. These tragedies have transformed
the tunnel’s southern entrance into an ongoing site of conflict. Hopefully, the structure’s history
can serve as a reminder that humanity is at their best
when breaking down barriers.

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