What Is a Tunnel Boring Machine?

Tunnel Boring Machine

Did you ever drive through a tunnel in a long minivan and wonder to yourself how the tunnel boring machine made that?

Or maybe you took a train in the subway and were amazed at the network of tunnels that has made commuting easy.

Tunnels have become an essential part of our existence. They provide shorter passage through mountains and hills and safe installation of utilities like drainage and water.

In essence, all this began with the development of a specialized machine.

A tunnel boring machine (TBM) is a machine invented to excavate tunnels through different soil and rock compositions.

The machine, sometimes referred to as the mole, cuts the hole with a round cross-section.

It is a machine that has the power to push through rocks, sand, and almost any formation.

Micro-TBMs excavate small diameter holes in the range of a meter. Similarly, the bigger TBMs can go up to 16 meters in diameter.

Despite the availability of this technology, drillers often prefer horizontal directional drilling or thrust boring for small diameter holes.

When an alternative to drilling and blasting is required, the tunnel boring machine comes to the rescue.

Forbes news regards tunnel boring machines as essential tools for urbanization.

Today, this technology has made it possible to pursue the building of the world’s first ship tunnel in Norway.

Brief History of Tunnel Boring

As civilization progressed, it became clear that cutting rail tracts through mountains was needed for improved efficiency.

The transportation sector was the first to witness the extensive use of tunnel boring machines.

In the 1680s, gunpowder enjoyed dominance in creating tunnels, and the French used it to blast their way through rocks.

Subsequently, the invention of dynamite in the mid-19th century saw a decline in the use of gunpowder.

Steam and compressed air-driven drills made the planting of explosives easier.

The industrial workers used them to make small holes. Nevertheless, man did not rest until shield techniques were developed.

The shield technique involved using a mobile enclosure to support the earth above as workers dug underground.

It was an effective technique hence its use in constructing tunnels under the Thanes river in London.

Today, self-contained tunnel boring machines open holes for utilities and transportation, and they can have the design to bore through anything between hard rock and sand.

Furthermore, the mining industry has contributed to these underground construction machines.

Comparison between tunnel boring and drilling

Unlike drilling and blasting, tunnel boring machines do not disturb the surrounding earth. They are known to produce smooth tunnel walls, which leads to a reduced cost in tunnel lining.

Due to this, they have enjoyed extensive use in heavily urbanized areas.

The downside to the use of TBMs is the huge upfront cost. Manufacturing companies spend a lot of money to construct and transport these machines.

However, the running price for using these machines in longer tunnels is not as high as drilling and blasting.

Conclusively, the TBMs are more efficient in project execution.

Components of the tunnel boring machine

The TBM is an incredible piece of engineering assembled for the sole purpose of excavating a tunnel. The components listed below may sound simple, but they are complex, requiring expert knowledge.

The cutter Head

The cutter head is an essential part of the TBM performing various functions. First, it carries the excavation tools like the disc cutter. The muck bucket collects muck generated by the boring through the rotation of the cutter head.

Furthermore, the cutter head plays a vital role in supporting the face during interruptions. The TBM uses it to stall cave-ins until the fractured rock is secured.

Muck buckets

Once boring operations have begun, the excavation is continuous. The buckets scoop up the muck and transport them away intermittently. An adequate number of muck trucks is required to ensure smooth operations.

Also, engineers have to design the muck buckets properly to prevent loss of efficiency.

Cutting tools

The primary cutting tools used are the disc cutters, and they are mounted on a roller assembly specially sealed with tapered roller bearings. Swinging motion may occur under high loading if the blades do not firmly fix the cuter head.

Manufactures position the disc cutters in easy to reach places for swapping. Workers can change some blades from behind for cutter heads above 4m.

  1. Power supply systems
  2. Bracing system
  3. Ground support equipment
  4. Shielding equipment
  5. Steering system

Operation of the tunnel boring machine

The TBM consists of dozens of steel blades installed to chip away the soil as it rotates. The forward movement comes when the cutter head spins and digs away the earth, maintaining a steady progression.

You can find the cutter head at the front of the TBM with the following section consisting of the tunnel shield and the concrete panels.

The machine installs these panels behind the roof shield. Subsequently, a rotating vacuum-powered lift picks up the concrete panels and sets them in place.

The TBM’s trailing gear can sometimes contain over 300 feet of equipment. This critical part supports the machine as it pushes deeper into the rock. The trailing gear includes a conveyor system that removes the soil excavated by the disc cutters.

The operating crew keeps the machine moving while equipping the trailing gear with supplies. The working principle of the TBM is simple but requires genuine work to maintain progress.

Types of Tunnel Boring Machine

There are three major types of tunnel boring machine namely:

  1. Hard Rock TBM
  2. Soft Ground TBM and
  3. Micro-tunnel Shield method

Hard Rock TBM

The disc cutters mounted in the cutter head pierce into the rock to create compressive stress fractures. It is a progressive process that chips the stone away, creating a tunnel as it goes. Remember, these machines dig holes in some of the most rigid rocks on earth.

The excavated rocks move through openings in the cutter head to the belt conveyor as they flow out of the tunnel. Hard rock TBMs employ multiple conveyors to ensure the smooth flow of rock fragments.

Engineers use the Shielded hard rock TBMs in unstable rock strata. The machine puts up concrete segments to support the walls as the boring continues. Similarly, double shield TBMs grip the tunnel’s walls while shifting the thrust to the cylinders to push against the tunnel.

Hard rock TBMs can function in the unstable and fractured ground to create secure tunnels for workers.

Soft Ground TBMs

Soft ground boring machines further break into a slurry shield, earth pressure balance, and open-face type. The Slurry shield machines perform on granular soils with high pressure. Workers fill the cutter head with pressurized slurry.

Hydrostatic pressure plays a significant role in excavating with slurry shield machines. The slurry also helps remove excavated material from the tunnel to ensure smooth forward movement. In addition, a slurry separation plant separates the soil particles from the slurry.

Multi-stage filtration systems provide pure slurry for re-use in the cutter head. Slurry shield machines are not suitable for silts and clayey soil, and this limitation is due to the smaller size of the soil particles compared to the bentonite used in making the slurry.

Soft grounds with less than 7 bar of pressure do not require slurry machines; instead, earth pressure balance machines function better. The cutter head comes with tungsten carbide cutting bits, carbide disc cutters, and drag picks.

What happens when the required size is smaller than the hard rock and soft ground TBMs? Read on to find out more.

Micro-Tunnel Shield Method

The micro tunnel shield method gains an advantage over the other types of TBM when small tunnels are required. These machines share several similarities with the big brothers but differ in the digging technique.

Types of Hard Rock TBMs

Today’s construction industry has seen several innovations in mechanized tunneling. Several hard rock boring machines have come with unique designs for specific operations.

The list below discusses the different types of this machine:

Tunnel Boring Machines with Full-Face Excavation

Full-Face tunnel boring machines include the Gripper TBM and the Shielded TBM. These are the two main types of boring machines that give Full-Face excavation.

In addition, subsequent sections will expand on them.

The gripper TBM serves as the classic form of tunnel boring machine. Often referred to as open TBMs, they find extensive usage in hard rocks with high to medium stand-up time, and this machine is the most economical for stones that do not require constant support.

Gripper TBMs benefit from the absence of rock anchors, steel arches, and shotcretes. Engineers designed this machine to brace radially against the tunnel wall through hydraulically-moved clamping shoes, and this action provides the thrust needed behind the cutter head.

New technology brought the development of two different clamping systems, single clamping (Robbins) and double clamping (Wirth and Jarva). Gripper TBMs further divided into open TBMS and TBMs with roofs, partial shield, and cutter head shield.

Open TBMs have no static protection units behind the cutter head, and small diameter holes are the only holes that see extensive use of this boring machine. Roof Shield TBMs have a similar construction to the open TBMs.

Isolated rockfalls can be deadly if left unchecked. The roof shield TBMs have static protection roofs installed behind the cutter head to protect the workers. Furthermore, manufacturers can install side steering shoes to provide support at the front and steer the machine during boring.

Operators can choose to drive the side surfaces radially against the walls. Cutter head shield machines protect the crew members working at the cutter head, and it also forms forward support when boring is in operation.

Shielded TBM

Single Shield TBM

Single Shield TBMs are suitable for challenging and fractured rocks with short stand up-times. The cutter head is similar to the one used in the gripper TBM, and it still serves the additional purpose of transporting the muck away.

The machine comes with a shield to temporarily support and protect the crew during boring operations. It uses a large shield that extends to cover the entire machine, including the cutter head. A tunnel lining comes under the tail of the protective shield.

Boring engineers commonly use reinforced concrete segments to provide support. These segments could be installed either directly as the final lining or temporarily with an in-situ concrete inner skin added later.

Geology refers to the above methods as single-shell construction and double-shell lining. Contrary to gripper TBMs, thrust jacks provide the forward motion directly against the existing tunnel support. The next type of shielded TBM is the double shield TBM.

Double Shield or Telescopic Shield TBM

This variant of the shield TBM allows boring operations in fracture rock with a short stand-up time. However, it is markedly different from the single shield TBM. The first difference is in the addition of two main components.

Double shield TBM comes with a front shield and a gripper or central shield. Telescopic jacks connect both shields to ensure synchronized movements. This technology enables the machine to radially clamp itself in the tunnel using clamping units in the gripper shield.

On the other hand, the machine can push off the lining in the direction of motion. This motion will allow it to thrust the front shield forward without moving the gripper shield, ensuring continuous operation.

Subsequently, the machine can achieve an independent movement without affecting the installation of the lining.

The double shield TBM is not free of disadvantages absent in the single shield TBM. Fractured rock with high strength can block the rear shield by getting into the telescopic joint, and people mistakenly refer to this as shield jamming.

A different agent causes blocking compared to jamming, and it is wrong for people to regard them as the same condition. This jamming of the telescopic joint only occurs when there is rapid advance with a single shell segment lining.

The installation time per ring required for the lining is about thirty to forty minutes. Double shield TBM’s purchase price and repair cost become economical when it involves double-shell lining with installation time per ring of ten to fifteen minutes. 

Closed Systems

This type of TBM combines system solutions for applications under the water table. Compressed air prevents the inflow of water from ensuring smooth and continuous operations.

Also, manufacturers can support the cutting face according to the slurry or EPB principle. Contractors use this type of boring system in hard and fractured rock.


Manufacturers have equipped micromachines with components to allow usage in hard rock.

The machine uses the same principles as larger machines in the cutter head design for excavation. Furthermore, they could come equipped with shields.

Tunnel Boring Machines for Partial Boring

Tunnel Boring Machine

Enlargement Machines

This machine is a specially modified gripper TBM used in tunnels of over 8 meters in diameter.

It enlarges the section to the required diameter beginning from a continuous pilot heading, and the machine is driven to the tunnel’s center before enlargement operations start.

Operators can carry out the enlargement operation in one or more stages depending on the soil formation. The pilot heading in the front of the cutter head gets clamped during the enlargement stage.

Special Machines for non-circular Sections

In the previous sections, I have described tunnel boring machines as circular-cross-section machines. Nonetheless, there are special machines developed for non-circular sectional boring operations.

Machines like Mobile Miner (from Robbins), Continuous Miner (from Wirth), and the Mini-Fullfacer (from AtlasCopco) are some of the machines at the frontlines of this development.

Performance Evaluation for Tunnel Boring Machines

Engineers employ several parameters to evaluate the performance of a TBM system, and they must be clearly defined and applied consistently for comparative applications.

Shift Time

Contractors may decide to use 12hr or 24hr shifting to operate and maintain the equipment as needed.

For those who don’t know, shift time refers to all the working hours, including hours set aside for machine maintenance.

Therefore, shift time refers to the active time (the TBM is in operation) or downtime during repairs and maintenance.

Penetration rate

A clock installed in the TBM records the machine’s operating time during operations.

A set level of propelling pressure, torque, or movement of the cutter head activates the clock. Engineers use this operating time to calculate the penetration rate.

The penetration rate often features an average hourly value over a specified time: an hour, a shift, a day, or the entire project duration.


This parameter is the percentage of the shift time during mining operations. Engineers usually evaluate it as an average over a specified time. These parameters are necessary to assess the efficiency of the boring machine.

Advance Rate (AR)

The advance rate refers to the distance covered in a specified time, and this rate varies based on the changes in the penetration rate.

Complex rock formation could cause a reduction in torque which will lower the advance rate.

Cutting Rate (CR)

This parameter means the volume of fractured rock excavated in a specified mining time. Typical cutting rates range from 20 to 200m3 per TBM mining hour.

Advantages of using a Tunnel Boring Machine

TBMs bring several advantages to stakeholders in mining and tunneling operations. It has become an essential piece of technology used in creating tunnels for different purposes.

The benefits of using this machine include the following:

  1. They have higher advance rates than any other kind of tunnel machines
  2. Operators can operate them continuously
  3. The damage to the rock and surrounding strata is minimal
  4. It requires less support during boring operations
  5. The characteristics of the muck are uniform
  6. Workers are safer with this technology
  7. As technology advances, there is a possibility for crewless operations.


Tunnels have become essential amenities in the life of urban civilization. With them, developers can seamlessly connect facilities such as drainage, water, gas, and sewage.

Also, transportation has become heavily dependent on tunnels for a smoother and shorter commute.

Tunnel boring machines have become development agents, driving the world to a bright future. These machines can open holes in the ground to build roads, rail tracks, and even shipping lines.

The modern boring technology has reduced the running cost of operating amenities despite the enormous initial cost.

These facilities can be safely tucked away underground, leaving the population above to enjoy free movement.

Today, engineers are putting in efforts towards building unmanned tunnel boring machines. When this is achieved, it will enhance the safety of workers and reduce operational costs.

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