Tunnel boring machine facts for kids
A tunnel boring machine (TBM) is a giant machine that digs tunnels. People sometimes call them "moles" or "worms" because of how they work underground. TBMs are a modern way to dig tunnels, much faster and safer than older methods like drilling and blasting or digging by hand. They can dig through hard rock, wet soil, dry soil, or sand.
TBMs create tunnels that are usually round. But they can also make square, rectangular, or horseshoe-shaped tunnels. These machines can dig very wide tunnels, some as wide as 17.6 meters (58 ft) (that's about the height of a five-story building!). Smaller tunnels are often made using different methods.
One great thing about TBMs is that they cause less disturbance to the ground above. They also create smooth tunnel walls. This makes it cheaper to finish the inside of the tunnel. It also allows tunnels to be built in busy cities without much disruption. While big TBMs are costly to build and move, they save money on very long tunnels. In the 2000s, TBMs could dig over 700 meters (about 2,300 feet) in rock per week. In softer soil, they could dig more than 200 meters (about 650 feet) per week.
The History of Tunnel Boring Machines
Early Ideas and Inventions
The idea of a machine to help dig tunnels started a long time ago. In 1825, Sir Marc Isambard Brunel invented the first successful tunnelling shield. This shield helped protect workers while they dug the Thames Tunnel in London. It was a big step, but workers still had to dig by hand inside the shield.
The first machine designed to bore a tunnel was called the Mountain Slicer. It was built in 1846 for the King of Sardinia. This machine had over 100 drills at its front. It was meant to dig the Fréjus Rail Tunnel through the Alps between France and Italy. However, funding problems meant the tunnel was finished later using simpler tools.
In the United States, Charles Wilson created a boring machine in 1853. It was used for the Hoosac Tunnel in Massachusetts. This machine used rotating cutting discs, similar to modern TBMs. These discs broke rock by applying high pressure, which was a new idea at the time.
TBMs Start Digging Longer Tunnels
Major Frederick Beaumont and Major Thomas English improved TBM designs in the late 1800s. Their machines were the first to dig significant distances. In 1875, a trial run for a tunnel under the English Channel used English's TBM. It dug over 1,840 meters (about 6,000 feet) through chalk. A similar machine was used to dig a railway ventilation tunnel under the River Mersey in England in 1883.
Building the Uptown Hudson Tubes for the Hudson & Manhattan Railroad in New York also used a special shield. This project, from 1890 to 1908, used compressed air to prevent tunnel collapses. However, workers faced dangers like tunnel collapses and health issues from working under high pressure.
TBMs in the 1900s and 2000s
In the early 1900s, inventors kept trying to make TBMs better. They designed machines for railroads, subways, and water pipes. Many early designs were too expensive or couldn't dig through hard rock. But TBMs continued to improve, especially for softer rock in mines.
A very large TBM, 14.4 m (47 ft 3 in) wide, was built for Canada's Niagara Tunnel Project. This machine, nicknamed "Big Becky," dug a tunnel for a hydroelectric power plant under Niagara Falls.
In the 2000s, TBMs became even bigger and more powerful. One famous TBM was Bertha, built in 2013. It was one of the largest TBMs ever, with a diameter of 17.45 meters (57.3 ft). Bertha dug a highway tunnel in Seattle, Washington. It started in 2013 and finished in 2017, even after needing major repairs.
Another huge TBM, named Martina, was built by Herrenknecht AG. It was used in Italy in 2013 to dig a 2.4 km (1.5 mile) tunnel near Florence. Martina was 15.62 m (51.2 ft) wide and weighed 4,500 tons!
The world's largest TBM, called "Qin Liangyu" or Mixshield S-880, was also built by Herrenknecht. It has an excavation diameter of 17.63 meters (57.8 ft). This giant machine was used to dig an undersea road tunnel to Hong Kong, China, completing its work in 2019.
How Tunnel Boring Machines Work
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TBMs are complex machines with several main parts. At the very front is a spinning cutter head that digs into the ground. Behind it are systems that push the machine forward, remove the dug-out material (called muck), and support the newly formed tunnel walls. The exact design of a TBM changes depending on the type of ground it will dig through and how much water is present.
Supporting Tunnel Walls
TBMs use different ways to support the tunnel walls as they dig.
Concrete Lining
Some TBMs, called shielded TBMs, are often used in soft soil. They install concrete segments right behind the machine to create the permanent tunnel lining. The machine pushes against these new concrete segments to move forward and keep digging.
Ground Support
Other TBMs, especially those used in strong rock, do not install concrete segments immediately. Instead, they use temporary supports like metal rings, rock bolts, or sprayed concrete (shotcrete) to hold the rock in place.
Shields for Protection
Many TBMs have one or more cylindrical shields behind the cutter head. These shields protect the workers and the tunnel walls until permanent support is built. The type of shield also affects how the TBM pushes itself forward.
Open/Gripper TBMs
Gripper TBMs are used in strong rock. They don't use a shield to support the tunnel walls. Instead, they push directly against the sides of the rock tunnel to move forward.
Single Shield TBMs
A single-shield TBM has one protective cylinder. It builds the permanent concrete lining right after the shield. This type of TBM has to stop digging while it builds a new section of the lining.
Double Shield TBMs
Double shield TBMs have two shields that can move separately. This design allows the machine to dig and build the concrete lining at the same time. This makes the tunneling process faster.
Keeping the Tunnel Face Stable
The "tunnel face" is the area where the TBM is actively digging. In hard rock with little water, the rock face can usually support itself. But in soft soil or where there's a lot of groundwater, pressure must be applied to the tunnel face to stop it from collapsing or letting water in.
Earth Pressure Balance (EPB) Machines
EPB machines are used in soft ground. They use the dug-out soil (muck) to create pressure at the tunnel face. By carefully controlling how much muck is removed and how fast the machine moves, the pressure stays balanced. This prevents collapses without needing special liquids. Sometimes, special foams or polymers are added to the soil to make it more stable.
EPB machines have made it possible to dig safely and quickly through soft, wet, or unstable ground. Many modern subway tunnels and parts of the Channel Tunnel were built using this method.
Slurry Shield Machines
Slurry shield machines are used in very wet, soft ground, especially with sand and gravel. Their cutter head is filled with a pressurized liquid called slurry, usually made of bentonite clay. This slurry pushes against the tunnel face to keep it stable. The slurry mixes with the muck and is then pumped out of the tunnel to a special plant that separates the muck from the reusable slurry.
Open Face Soft Ground TBMs
These TBMs are used in ground that can briefly stand up on its own without support. They dig a section, then push forward. Concrete segments are then installed to support the tunnel walls.
Tunnel Size
TBMs come in many sizes, from 1 to 17 meters (3 to 56 ft) in diameter. Smaller TBMs, called micro tunnel shields, dig tunnels from 1 to 1.5 meters (3.3 to 4.9 ft) wide. These tunnels are too small for people to walk inside.
Backup Systems
Behind every TBM, in the part of the tunnel that's already finished, there's a long system of support decks. This "backup system" includes conveyors to remove the muck, pipes for slurry (if used), control rooms for operators, and systems for electricity, ventilation, and dust removal. It also helps transport the pre-cast concrete segments used for the tunnel lining.
Tunnels in Cities
Building tunnels in cities has a special challenge: the ground surface above must not be disturbed. This means avoiding any sinking of the ground. TBMs with good face control, like EPB and slurry shield machines, are perfect for this. They can keep the ground stable during and after construction. When digging in cities, engineers must also consider other tunnels, utility lines, and building foundations to make sure they are not damaged.
See also
- Channel Tunnel
- New Austrian tunneling method
- Roadheader
- Subterrene
- Trenchless technology
