Tunnel construction

People build tunnels in many ways.

One way is to dig a shallow ditch. Then, workers cover it up with a roof. This is called cut-and-cover.

Other tunnels are bored deep underground. Huge machines do this work.

These big machines can dig through hard rock. They can even work under water.

Some tunnels go under the sea. These tubes are sunk into the water.

They sit on the bottom of the sea.

Workers often use tall shafts. These holes go straight down to the tunnel. They help people and machines get inside.

Tunnels help us move through the world.

People build tunnels in many different ways. The method depends on the ground. It also depends on the depth of the tunnel.

One way is called cut-and-cover. This is used for shallow tunnels. Workers dig a long trench. Then they build a roof over it. This can be done from the bottom up. Or they can use a top-down method. This lets them fix roads on top quickly.

Other tunnels are bored deep underground. A tunnel boring machine, or TBM, does this work.

These are huge machines. They can dig through hard rock or soft soil. Some TBMs have special parts to work under water. They use pressure to keep water out.

Sometimes, workers use shafts to reach the tunnel. A shaft is a deep hole. It goes straight down to the tunnel level. These shafts help move machines and people. After the work is done, shafts can be used for air. They can also be used as exits in an emergency.

Some tunnels go under the sea. These are often immersed tubes. They are sunk into the water. Then they are laid on the bed of the sea.

Tunnels are amazing paths built through the Earth. People build them in many materials like soft clay or hard rock.

Engineers must choose a method based on the ground. They look at the water, the depth, and the shape needed. The cost can be very high for these big jobs. In 2017, city subway tunnels cost about 500 million EUR per kilometer.

Some tunnels, like the one between Denmark and Germany, are planned to cost 425 million per km.

One common way to build is called cut-and-cover. This is used for shallow tunnels. In the bottom-up method, workers dig a trench and build the tunnel inside it. They might use brickwork or concrete to make the structure. The top-down method works a bit differently. Workers first build side walls and a roof from the ground level. This allows them to fix roads on the surface very quickly.

Large cut-and-cover boxes are often used for metro stations like Canary Wharf in London.

For deeper tunnels, engineers often use a tunnel boring machine, or TBM. These huge machines automate the digging process. They can work in hard rock or soft, wet ground. Some TBMs have pressurized parts to balance water pressure. This keeps the workers safe in normal air pressure behind the machine.

These machines can be very large. The Tuen Mun–Chek Lap Kok TBM in Hong Kong was built by Herrenknecht. It was a massive machine used for a big link project.

Sometimes, workers use special methods for specific jobs. In the United Kingdom, people developed clay-kicking for strong clay soil. A worker lies on a plank and uses a tool to extract soil. This method was very quiet during the First World War. It was used by Royal Engineer companies to place mines.

Another way to support a tunnel is the New Austrian Tunneling Method. This was made in the 1960s. It uses sprayed concrete, called shotcrete, to keep the rock stable.

Many tunnels also use shafts to reach the right depth. A shaft is a deep, circular hole that goes straight down. These shafts act as the main entrance during construction. Once the tunnel is finished, they can serve as exits. They can also be used for ventilation to move air.

Some tunnels even go under the sea. These might be immersed tubes that are sunk onto the sea bed.

You might see these when traveling under water in places like the Marmaray.

Tunnel construction is a specialized branch of underground construction. Engineers must create paths through various materials, ranging from soft clay to extremely hard rock.

The specific method used depends on many different factors. These include ground and water conditions, the depth of the tunnel, and the final shape required. Engineers must also consider the length of the drive and the logistics of supporting the excavation. Choosing the right method is essential for managing risks and controlling high costs.

One common approach for shallow tunnels is the cut-and-cover method. This process involves excavating a large trench and then roofing it over with a support system.

There are two main ways to do this. In the bottom-up method, a trench is dug and the tunnel is built directly inside it. The structure might be made of concrete, steel arches, or even brickwork. In the top-down method, workers first build side support walls and capping beams from the ground level. They then create a shallow roof of precast beams or concrete. This allows the surface, like roads or services, to be fixed much sooner.

For deeper projects, engineers often use a Tunnel Boring Machine, or TBM. These massive machines automate the entire tunneling process to reduce costs.

TBMs are designed to work in many environments, from hard rock to soft, water-bearing ground. Some models, like bentonite slurry or earth-pressure balance machines, use pressurized compartments. These compartments balance the water pressure ahead of the cutter head. This allows operators to work in normal air pressure behind the machine. This is a safer alternative to older methods that required workers to use compressed air and undergo decompression procedures.

Large-scale TBMs have reached incredible sizes. The Tuen Mun–Chek Lap Kok TBM in Hong Kong was built by Herrenknecht and holds a record for its head diameter.

Other notable machines include those used for the Gotthard Base Tunnel in Switzerland. The four TBMs used there had a diameter of about 9 meters. The Green Heart Tunnel in the Netherlands also used a very large machine. These machines are difficult to transport to sites, so they are often assembled on-site within the tunnel itself.

Specialized manual methods also exist for specific soil types. In the United Kingdom, workers developed a technique called clay-kicking for strong clay soil.

A worker lies on a plank at a 45-degree angle. They use a tool with a cup-like end to manually extract sections of soil. This method was very quiet, which was useful during the First World War. Royal Engineer tunneling companies used it to place mines beneath enemy lines without being detected. This technique was also helpful for renewing ancient sewerage systems without destroying buildings.

Modern engineering also uses sprayed concrete to stabilize tunnels. The New Austrian Tunneling Method, or NATM, was developed in the 1960s.

This method uses the natural geological stress of the surrounding rock to help stabilize the tunnel. Workers apply a layer of sprayed concrete, known as shotcrete, to protect the excavation. They may also use steel arches, mesh, or rock bolts. By adding steel or polypropylene fibers to the concrete, the lining becomes even stronger. This creates a load-bearing ring that prevents the rock from deforming.

To reach the correct depth, engineers often dig vertical access shafts. These are usually circular, concrete-walled holes that go straight down.

During construction, these shafts serve as the main entrance for workers and machines. Once the tunnel is finished, they often serve new purposes. They can be used as ventilation shafts to move air or as emergency exits. For long tunnels, multiple shafts are bored at different locations to provide better access to the unexcavated areas.

Tunneling is a massive financial undertaking. In 2017, city subway TBM tunnels cost roughly 500 million EUR per kilometer.

In Switzerland, a kilometer of motorway tunnel was calculated at about 300 million CHF, which was 200 million EUR at the time. A planned undersea tunnel between Denmark and Germany was estimated at 425 million per km in 2015. Even underwater tunnels, like the immersed tube type, require careful planning to sink them into the sea bed.

Whether through the sea or deep under a city, these projects connect the world in complex ways.