How Are Underwater Tunnels Built? What You Need To Know

Most engineering marvels, such as bridges and skyscrapers, are plain to see, but underwater tunnels are far more elusive. These tunnels, which bridge gaps between cities, states, countries, and continents, run far beneath oceans and rivers, connecting us all.

Underwater tunnels are built using tunneling shields, tunnel boring machines, or immersed tubes. They are dug deep into the earth or sit on the ocean or river floor. The tunnels are usually buried beneath stone to prevent collapse. Underwater tunnels are used as an alternate means of transportation.

People have been making underwater tunnels for over 200 years, and the methods and machinery used in the process have changed significantly. So, let’s talk about underwater tunnel-building and trace the many inventions that continue to make it possible today.

Most Underwater Tunnels Are Dug Into the Earth

When you think of an underwater tunnel, an image of a long tunnel suspended in water might come to mind. However, most underwater tunnels are dug into the firm soil and rock bed below bodies of water.

Placing underwater tunnels deep in the earth or burying them many feet beneath oceans or rivers protects the passageways from flooding and collapse.

In addition, the materials used to make underwater tunnels are usually steel or concrete. These materials are strong enough to prevent collapse, but they degrade when submerged directly into the water. So, embedding the tunnels in soil or stone is crucial for safety.

The First Underwater Tunnel Was Made With a Tunneling Shield

Underwater tunnels were first invented in early 19th-century London.

London has long been a famous port city and international trading hub, but managing the traffic was always a challenge.

During the 17 and 1800s, so many cargo ships sailed along the River Thames that the waters and streets were constantly packed.

Part of the issue in London was that merchants on land received the cargo from ships, and then, the merchants had to cart the goods across the London Bridge.

Thus, the bridge was packed with hundreds of merchants, creating some of the vastest traffic jams of all time.

So, London’s engineers and architects went to the drawing board to come up with a solution for the crowded wharves. Building another bridge would have made it more difficult for ships to come into port, so the issue required an ingenious and brand new solution.

Problems Faced When Building The First Underwater Tunnels

Eventually, it was decided. Britain would build the first underwater tunnel underneath the River Thames to help traffic move along.

Still, the muddy, sandy river made digging tunnels difficult, and the first attempts at construction were far from successful, as miners attempted to construct the tunnel the same way they would have chiseled out a mine by hand.

Flooding, lack of air, and decompression sickness resulted in many deaths during this first attempt, forcing the engineers to put the project on hold for years.

Innovations in Underwater Tunnel Building

In 1818, French engineer Marc Brunel developed a machine that would make digging deep into the earth underneath large bodies of water possible, even in high-pressure environments.

This machine, called a tunneling shield, is still used for bored-hole tunnels today. It is a large wooden frame, covered in moveable iron plates, that miners can stand on as they chisel away the earth to make the tunnel. Overhead iron plates protect miners from flooding and cave-ins, and the entire frame can be inched forward as the miners advance.

This machine made all the difference, although it took many years of trial and error– and several deaths– for the Thames Tunnel to reach completion. Finally, though, in 1841, the tunnel made it to the other side of the Thames.

In the 1860s, a railway was installed in the tunnel, and it is still in use today.

This tunnel marks a critical moment for engineering, and during its construction, it inspired many people around the globe to build underground tunnels in highly trafficked ports, harbors, and piers.

As the popularity of massive undertakings like the Thames Tunnel grew, engineers worldwide used Brunel’s tunneling shield to invent new ways of constructing underwater tunnels.

The Tunnel Boring Machine Revolutionized Tunnel-Building

The boring machine, designed after Brunel’s tunneling shield, created a more efficient way to bore holes into the earth.

The boring machine, often called “the mole,” can mechanically drill tunnels, even in the most tricky or rocky places.

This invention of 1845 consists of an engine attached to a gigantic wheel called a cutting head. Many rock-shattering disks are mounted on the cutting head, which work like vast, rock-cutting food processors.

Once powered up, the cutting head and discs spin rapidly, slicing through long sections of rock and earth within hours.

As the machine cuts through the earth, all debris is deposited onto a conveyor screw and conveyor belt, expelling it from the tunnel.

Some boring machines even have an erector attached to the back of the cutting head. The erector can lift and install pre-made concrete lining for the tunnel, cutting out any need for manual labor.

Engineers can also control boring machines from the land’s surface, so no one needs to go down into the tunnel until it is time for an inspection.

To see what a tunnel boring machine looks like when it is in action, check out this short animated YouTube video from NYCEDC:

This machine replaces the hundreds of miners and construction workers needed to dig underwater tunnels, making construction significantly safer and cheaper.

Although the boring machine was first used to make tunnels underneath mountain ranges such as the Alps and the Hoosac Mountains in Massachusetts, the English government later used it to make the Mersey River Tunnel in 1925.

Boring machines are still used today to bore out tunnels in all sorts of places. One of the most famous underwater tunnels made with boring machines is the English Channel Tunnel, more commonly called the ‘Chunnel’.

This 31-mile (50-km) railroad tunnel brought about the invention of the first waterproof boring machine, and it was officially opened in 1994, over 100 years after the project began.

In the 100 years it took for the government to construct the Chunnel successfully, tunnel boring technology significantly advanced.

Overall, the Chunnel is a testament to what we can do with ingenuity, determination, and international cooperation, and it set a new precedent for underwater tunneling.

To learn more about the construction of the Chunnel, check out this fantastic video from Ted-Ed on YouTube:

Immersed Tube Tunnels Are the Most Common Tunnels Today

Although all of the earliest underwater tunnels are bored-hole tunnels, in the early 1900s, another method of connecting land through underwater passageways came to light.

Immersed tube tunnels were invented in the early 1900s by W.J. Wilgus. Wilgus was an American engineer who tested his invention while working on the Michigan Central Railway Tunnel, which connects Detroit, Michigan, and Windsor, Ontario via the Detroit River.

In 1906, Wilgus began work on the very first immersed tube tunnel. These tunnels are made with prefabricated segments that are eventually submerged and connected under the water.

This is how an immersed tube tunnel is constructed:

The first step is to build the tunnel segments. For the Michigan Central Railway Tunnel, these segments were constructed with concrete, but they can also be made from steel or a combination of steel and concrete.

All of the necessary components are pre-installed into the tubes. Depending on the tunnel’s function, paved asphalt roads, rail lines, or utility machinery can be placed into each segment while still on land.

After the segments are made, a boat called an immersion pontoon drags them out into the water. Once they reach the right spot, each tube is sunk to the bottom and nestled in pre-dug trenches lined with gravel.

Divers connect the tubes using rubber gaskets and steel plates. This construction is crucial since the rubber will allow the tubing to bend and shift without breaking during an earthquake or natural disaster.

After the tubes are lined up, an artificial floor is placed above them to protect them from water and damage. Often, this floor is made of layered stone and gravel, and it is usually at least five feet (1.5 m) thick.

If you want to get a better picture of what it takes to make and install an immersed tube tunnel, check out this short animated YouTube video from Trelleborg Group:

These tunnels, when made correctly, have stood up to the test of time. Indeed, the Michigan Central Railway Tunnel still stands today, and more than 400,000 trains pass through it every year.

Because immersed tube tunnels take less time and labor to make than bored-hole tunnels, they are the most commonly produced type of underwater tunnels today. The installation process is also much safer for the construction workers, which is a significant benefit of this tunnel type.

There are over 150 immersed tube tunnels worldwide, and most of them are used as roads or railroad passages. They are growing in popularity, so if you’re traveling in a tunnel made within the past 20 years, it’s probably an immersed tube tunnel.

When Are Underwater Tunnels the Right Choice?

Tunnels have tons of appeal, and the science behind them is fascinating. However, there are pros and cons to everything.

Underwater tunnels are usually expensive and dangerous to build. Many things can go wrong during the project, and deaths are frequent among the workers. Still, they provide reliable transportation routes and can cost less than bridges in some circumstances.

Underwater tunnels are the right choice in areas with heavy vehicle or ship traffic, as it provides an alternate route. Areas that experience inclement weather benefit as well, as the covered tunnel protects passengers. Finally, they prevent trains from interfering with city planning and traffic.

Let’s take a deeper look at the advantages and disadvantages of underwater tunnels.

The Advantages of Underwater Tunnels

Many things can make underwater tunnels advantageous, especially for cargo transportation and traffic.

Tunnels are advantageous when installed in areas that attract hundreds of cargo and passenger ships every day. The cargo and people can instead pass through an underwater tunnel. Bridges are expensive, as they need to be tall enough to let ships pass underneath. Often, engineers must raise the entire landscape along the banks or coast to ensure the proper bridge height. This is incredibly expensive.

Tunnels also protect passengers from bad weather. Even when storms roll in, and when ships cannot safely sail on the water, tunnels are open and covered. So, coastal locations where ferries are often used might benefit from an underwater tunnel.

Tunnels help in improving car traffic. In other areas, where car traffic is too heavy for narrow streets or urban conditions, tunnels can provide an alternative route, allowing traffic to pass below busy streets.

Tunnels are perfect for trains. Above-ground railroad lines may interfere with urban planning or car traffic.

The Disadvantages of Underwater Tunnels

Still, the geographical conditions must be suitable for a tunnel to be built, and tunnel construction is always very pricey.

For a bored-hole tunnel, the soil quality and fault lines in the area can make or break a tunnel plan. Clay-based, rigid, or rocky soil holds tunnels very well. However, silty, soft, or muddy soil often results in cave-ins and flooding. In addition, when boring into underwater earth, fault lines can leak water into the tunnel, causing flooding.

The ocean or river floor can also affect whether a tunnel is feasible when it comes to immersed tube tunnels. In some locations where the bed is made mainly of sand or silt, the tunnel might be washed away or become unstable.

Immersed tube tunnels could interfere with wildlife and aquatic vegetation. So, areas with an intricate ecosystem or endangered species might not be the ideal location for a tunnel.

Tunnel building projects cost millions of dollars. For example, the Michigan Central Railway Tunnel, which is only 1.4 miles (2.25 km) long, cost $8,500,000 to build. The 34-mile (55 km) Hong Kong–Zhuhai–Macau Bridge, completed in 2018, cost nearly $19 billion in total.

Finding funding for these projects isn’t always possible, and many tunnels experience setbacks because of their budgets. In some cases, project cancellation or private funding is the only option.

So, overall, geography and the budget have to line up to build an underwater tunnel.

Final Thoughts

Underwater tunnels are a marvel of engineering. They can either be made by:

Drilling long tunnels deep into the earth beneath bodies of water
Sinking prefabricated tunnels to the floor of an ocean or river

Over the years, new methods of building tunnels have evolved upon each other, resulting in today’s reliable, unbelievable underwater tunnels.