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How Are Tunnels Built?

Whether underwater, above ground, or through a mountain, tunnels have been around for centuries. In the early 20th century, as cities boomed, tunnels were big-business: They carried trains, drinking water, and automobiles — they were, in other words, the new lifeblood of society. Here’s a deeper look into how they got there.

Early Tunnels Were Built From Caves

Derinkuyu cave city in Cappadocia, Turkey.
Credit: Pakhnyushchy/ Shutterstock

People have been building tunnels for centuries. Millennia ago, most tunnels were excavated from existing caves. Turkey, for instance, is home to a maze of tunnels and underground cities. One of them, called Derinkuyu, is 18 stories deep and was once home to 20,000 people. When caves weren’t available as starting points, ancient tunnels were dug the old-fashioned way: by hand. The Pausilippo tunnel, near Naples, connects two hard-to-reach fields. The longest and largest tunnel in the ancient world, the hand-carved passage extends nearly a mile.

Tunnel-building technology would improve, but slowly. By the 16th century, engineers were using gunpowder. Two centuries later, they replaced that with dynamite. But when the industrial revolution rolled around, steam-powered drills stole the show.

“Cut-and-Cover” Tunnels Make Their Debut

Workers inside the construction area of the Eurasia Tunnel Project.
Credit: Burak Akbulut/ Anadolu Agency via Getty Images

Many early modern tunnels employed a “cut-and-cover” technique, with engineers digging a shallow trench, installing the tunnel walls, and then covering the tunnel with soil. This tried-and-true method is known for being economical and efficient and is still regularly used today. In 2016, the Eurasia Tunnel in Turkey — which connects the Asian part of Istanbul with its European side — was built with help from the cut-and-cover technique.

Of course, the cut-and-cover technique doesn’t work if you’re trying to drill deep into a mountain. Even under normal conditions, like on a city street, the technique can be extremely disruptive, since it involves significant above-ground construction. And it’s impossible when trying to tunnel underwater.

For underwater, or “subaqueous” tunnels, tunnel-building would require extra insight. And it would all start with a mollusk.

The “Shield Method” Helped Make Underwater Tunnels Possible

Interior of the Gotthard Base Tunnel.
Credit: Pier Marco Tacca via Getty Images

Teredo navalis is a species of saltwater clam, a mollusk more commonly called the “naval shipworm.” The clam uses a pair of plates to safely burrow into wood. One day, in the early 19th century, engineer Marc Brunel was walking along the River Thames in London, England, when he saw one of the critters biting into a plank of wood. Suddenly, he had an idea.

It was 1825. At the time, underwater tunnels were considered impossible. But Brunel felt otherwise. Inspired by the shipworm, he developed a shield system that could be used to protect miners as they dug the world’s first underwater tunnel. As David Bressan writes at Forbes, “Miners would dig at the front of the tunnel, protected by a movable wooden framework. Immediately behind the miners, workers would secure the tunnel, covering the walls with bricks and concrete.” Today, this is called the “shield method.”

Brunel’s first attempt was rough. His tunnel routinely flooded and six workers drowned. But the Wapping-Rotherhithe Tunnel was finally completed after 16 years of work in 1841. The shield system would see several improvements over the years, thanks in part to the introduction of the “greathead shield,” a method that compresses air to prevent exterior water pressure from flooding the tunnel.

Today, that system is still in use. But instead of employing steam-powered tools, modern tunnelers often use a Tube Boring Machine (TBM), a giant mechanical worm with a rotating head covered in blades. This machine cuts through rock and clay and funnels the crushed rock, called “spoil,” out the back of the tunnel.  

TBMs are wildly productive. They were used to build the 35-mile Gotthard Base Tunnel in Switzerland, the world’s longest and deepest traffic tunnel. Thanks to the TBMs that built the Gotthard, high-speed trains now zoom through the Swiss Alps in record time.

The “Immersed Tube Method” Changed the Game

The traffic on Chesapeake Bay Bridge Tunnel in the summer.
Credit: Khairil Azhar Junos / Alamy Stock Photo

While TBMs and the shield method are still widely used, a new system would take over the world of underwater tunnel building in the 1950s: the Immersed Tube Method.

In this method, tunnels are prefabricated above ground, engineers dredge the riverbed, and the tunnels are floated onto the water. When they’re set in the correct location, a system of weights is used to carefully sink the tunnel into location. Then, dirt and rock are filled over the submerged tunnel.

The method, which was used for tunnels such as the Chesapeake Bay Bridge-Tunnel, has several advantages, including that the tunnel can be constructed without any of the risks associated with dangerous air pressure — a total game changer.

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