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A tunnel boring machine (TBM), also known as mole, is employed for the construction of tunnels in hard or soft rock strata. The cutting process utilizes the rotation of the cutterhead (geared with disc cutters) and the blade pressure for excavation on the face of rock. Mostly, gripper and shield TBMs are ideal for tunneling in difficult rock conditions with medium to high stand-up time.

The cost of constructing tunnels using TBM is higher than the traditional methods. However, the additional expense can be compensated by greater advance rates (coverage). But if the wear rate of the tools increases too much on account of the rock strength or other unfavorable criteria, repair and maintenance of regular cutter would lead to a high downtime.

Tunnel Boring Machine

Due to the high downtime, the working time of the device decreases, resulting in reduced efficiency and increased costs. Therefore, the logistical procedures of TBM should be designed very carefully.

Also, in the case of traditional methods, it is possible to respond to the tunnel-rock interactions by either subdividing the excavated section or by a rapid adaptation of the support to the geological situation. But, this is not possible with TBMs.

Above all, the choice to use a TBM needs a much better geological examination than for drilling and blasting methods. Therefore, a comprehensive preparation of the entire driving and supporting procedure becomes very important.

1. Benefits and Drawbacks of TBM over Traditional Tunneling Methods

The benefits and drawbacks of a TBM drive in contrast to traditional tunneling methods are as follows:

1.1 Benefits

  1. A specific excavation profile can be achieved.
  2. Higher advance rates are achievable.
  3. The work is completely automated and continuous.
  4. Low labor expense.
  5. Better operating conditions.
  6. Complete mechanization and automation of the drive is possible.

1.2 Drawbacks

  1. High financial investment.
  2. Specific and detailed geological information is a must for TBM.
  3. Longer preparation time is required for the design of the machine.
  4. TBM has limitations on curve radii, especially, on tight radius curves; it is difficult to rotate the whole setup of TBM.
  5. Adjustment to different rock types and high-water inflow is limited to a certain level.
  6. The transportation cost of the TBM to the tunnel site is very high.

2. Basic Construction Principle

The fundamental elements of a TBM are the cutterhead, the cutterhead provider with the cutterhead drive motors, the machine frame, and the driving and clamping devices. These fundamental elements form four system groups of TBM.

  1. Boring system
  2. Thrust and clamping system
  3. Muck-removal system
  4. Support system
four structure groups of TBM
Types of TBM structure

2.1 Boring System

The boring system is an essential element as it determines the efficiency of a TBM. It consists of a cutterhead with disc cutters mounted over it.

When the cutterhead rotates, the setup of the discs should be such that the entire cutting face should come in concentric tracks. The selection of cutterhead and discs depends upon the rock type and the ease of cutting.

The rotating cutterhead presses the discs with a high pressure against the face, producing a slicing movement across the face. The pressure at the cutting edge of the disc cutters exceeds the compressive strength of the rock and grinds it locally.

boring system of TBM
Cutterhead and disc cutter of TBM

2.2 Thrust and Clamping System

The thrust and clamping system impacts the advancement rate of a TBM. A TBM usually attains the advancement rate of up to 2.0 m in a day.

This system is responsible for the advance push and the boring process. The thrust system provides the push to the cutterhead through hydraulic cylinders. Thus, this system controls the movement of the cutterhead. The clamping system restricts the possible thrust and withstands the moments developed by the rotation of the cutterhead.

2.3 Muck-Removal System

The slot around the boundary of the cutterhead is known as the cutter bucket. The cutter bucket is connected to the conveyor belts and collects the muck generated near the cutterhead, which ultimately is disposed of outside the tunnel.

In order to remove the muck from the tunnel, an effective system should be used, which should not interfere with the supply of the TBM and the essential support systems.

Issues can occur with both the cutterhead bucket and the conveyor belt if the obstruction is triggered by bigger blocks of rock. This would make it difficult for the TBM to operate.

Muck removal system of TBM.
Conveyor belt installed in tunnel to remove muck

2.4 Support System

It is challenging to use TBM in brittle rock because it would sink in with the increase in tunnel diameter. Therefore, supports are provided to avoid the sinking of TBM.

For smaller diameter tunnels, the supports are provided around the rear carriage behind the TBM. While, in the case of bigger diameter tunnels, in addition to supports around the rear carriage, advancement supports such as rock bolts and umbrella reinforcement are provided.

To protect the fault zones, supports like bolts, pipes, grout injection, and ground freezing should be used over or in front of the cutterhead. However, the roofing shield support system is more useful during advancement in fault zones. This roofing shield support system safeguards the crew members working behind the cutterhead from rockfalls.

Nowadays, mainly two types of TBMs are used. In the first type, the tunnel driving through a gripper TBM forms a single invert sector. This invert sector provides temporary as well as permanent support and is also useful in mounting the rails of rear carriage and rail conveyor.

The second type is the tunnel driving through shield TBM, in which the shield guarantees temporary support to the rock around the periphery of the tunnel. Reinforced concrete segments are mostly used for support. The segments are installed singly by the erector and provide immediate support. Compressed air and earth pressure support are beneficial for the shield TBM during the ingress of water.

3. Types of TBMs

Different kinds of TBMs were used in the past for mechanized tunneling in hard or soft rock. However, two major categories of TBMs are used in the modern-day tunneling process. TBM with full-face excavation and TBM for partial excavation.

3.1 TBM with Full-Face Excavation

The following are the different types of TBMs with full-face excavation:

  1. Gripper TBM
  2. Shield TBM

3.1.1 Gripper TBM

Gripper TBM is the oldest form of TBM. It is mainly useful in hard rock with medium to high stand-up time. In such rocks, the requirement of continuous support such as rock anchors, steel arches, and shotcrete is significantly less. Thus, the higher advancement rates are possible by producing more thrust. In order to produce the thrust behind the cutterhead, the gripper TBM should be braced radially to the tunnel wall by clamping shoes. Such clamping shoes are called as grippers.

Gripper TBMs are further categorized into open TBMs, TBMs with roofing system, with roofing and side steering shoes, and with cutterhead shield.

Gripper TBM tunneling machines
Types of gripper TBM
3.1.1.1 Open TBM

Open TBM is useful only in smaller diameter tunnel section as there is no protection provided behind the cutterhead.

3.1.1.2 TBM with Roof Shield

The construction process of the tunnel using TBM with a roofing shield is the same as the open TBM. However, the roof shield provides protection against rockfalls throughout excavation. Such roofing shields are set up behind the cutterhead.

3.1.1.3 TBM with Roofing Shield and Side Steering Shoes

The side steering shoes provide additional support at the front of the machine when moving the machine and steering throughout boring.

3.1.1.4 TBM with Cutterhead Shield

The cutterhead shield provides protection to the workers working under the location of the cutterhead. When moving the machine, the cutterhead shield liner also contributes to the forward support.

Open gripper TBM
Gripper TBM

3.1.2 Shield TBM

Shield TBM is a recent invention. It is useful in both hard and fractured rock with medium to high stand-up time. Shield TBMs are categorized into single shield TBM, double shield, or telescopic shield TBM.

shield tunneling machines
Types of shield TBM
3.1.2.1 Single Shield TBM

Single shield TBMs are mainly used in hard rock with moderate stand-up time. The cutterhead system and muck-removal system are the same as in gripper TBM. However, this type of TBM is useful in supporting the tunnel for a short duration. The shield extends entirely over the complete machine from the cutterhead portion.

In contrast to the gripper TBM, the machine moves forward with thrust jacks directly against the existing tunnel support.

Type of shield TBM
Single shield TBM
3.1.2.2 Double Shield or Telescopic Shield TBM

The double shield or telescopic shield TBM is similar to the single shield TBM for driving the machine in fractured rock with moderate stand-up time. However, it has the following distinctions from the single shield TBM:

  1. The double shield TBM consists of two primary shields, the front shield and the gripper shield.
  2. The machine can adequately clamp itself radially in the tunnel using the clamping units. Also, where the geology is bad, it can push off the existing lining in the direction of the drive.
  3. The front shield can provide the thrust without affecting the gripper shield.
  4. The basic operation is also possible without the setup of lining.

The double shield TBM also has downsides in comparison to the single shield TBM. When double shield TBM is utilized in fractured rock with high strength, the rear shield can obstruct the material getting into the muck-removal unit.

Type of shield TBM
Double shield or telescopic shield TBM

3.2 TBM for Partial Excavation

For a full-face excavation, the TBMs can excavate a tunnel with a limited diameter. Also, the excavated profile is limited to a circular section. Thus, for constructing bigger diameter and non-circular section tunnels, the TBMs for partial excavation are used. TBMs for partial excavation are categorized into enlarged TBM and unique machines for non-circular section.        

3.2.1 Enlarged TBM

It is used to construct tunnels with a diameter of over 8 meters. Firstly, the section of the tunnel is enlarged with a continuous pilot heading, which is completely driven into the center of the tunnel. After that, the tunnel enlargement is done using enlarged TBM machines.

Enlarged TBM machines
Enlarged TBM

3.2.2 Unique Machines for Non-Circular Sections

All types of machines, which excavate the face in a partial process and thus enable sections varying from circular, are categorized as unique machines for non-circular sections.  

Rectangle TBM Machine
Non-Circular TBM machines.

FAQs

What is TBM?

A tunnel boring machine (TBM), also known as mole, is employed for the construction of tunnels in hard or soft rock strata.

What are the types of TBM?

Two major categories of TBM are used in the modern-day tunneling process. TBM with full-face excavation and TBM for partial excavation. TBM with full-face excavation includes gripper and shield TBMs, and TBM for partial excavation includes enlarged TBM and unique machines for non-circular sections.

Which support systems are provided in TBM during the advancement in fault zones?

To protect the TBM from fault zones, support systems like rock bolts, pipes, grout injection, and freezing are provided over or in front of the cutterhead.

What is the use of roofing shield support system?

Roofing shield support system is useful during advancement in fractured rock. This roofing shield support system safeguards the crew members working behind the cutterhead from rock falls.

What is the importance of shield tunneling?

Shield tunneling is mainly useful in hard rock with moderate stand-up time and in fractured rock because the shield extends entirely over the complete machine from the cutterhead portion and provides safety to the entire structure.

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