All About Tunnel Boring Machine (TBM) & Parameters

Tunnel Boring Machine (TBM)

A TBM is a massive set of complex equipment assembled together to excavate a tunnel, often called as “Mole”. Major components of this Tunnel Boring Machine includes

  1. Cutter head, with cutting discs/tools and
  2. Muck buckets to carry and dispose excavated muck
  3. Power supply Systems
  4. Cutter head rotation & thrust
  5. Bracing system for the TBM during mining
  6. Equipment for ground support installation
  7. Shielding to protect workers
  8. Steeling system

Back-up equipment systems provide muck transport, personnel and material conveyance, ventilation, and utilities.

Construction of the A5 highway east bypass of Biel; 12.6 m diameter drill head of TBM © Chriusha (Хрюша) CC-BY-SA-3.0
Construction of the A5 highway east bypass of Biel; 12.6 m diameter drill head of TBM © Chriusha (Хрюша) CC-BY-SA-3.0

The advantages of using a TBM include the following:

  1. Higher advance rates.
  2. Continuous operations.
  3. Less rock damage.
  4. Less support requirements
  5. Uniform muck characteristics.
  6. Greater worker safety.
  7. Potential for remote, automated operation.
Typical Diagram for Single Shield TBM
Typical Diagram for Single Shield TBM

Disadvantages of a TMB are the fixed circular geometry, limited flexibility in response to extremes of geologic conditions, longer mobilization time, and higher capital costs

TBM system Design and Operation

A TBM is a system that provides thrust, torque, rotational stability, muck transport, steering, ventilation, and ground support. In most cases, these functions can be accomplished continuously during each mining cycle. Figure shown is a sketch of a typical open or unshielded TBM designed for operation in hard rock. The T BM cutter head is rotated and thrust into the rock surface, causing the cutting disc tools to penetrate and break the rock at the tunnel face. Reaction to applied thrust and torque forces may be developed by anchoring with braces (grippers) extended to the tunnel wall, friction between the cutter head/shield and the tunnel walls, or bracing against support installed behind the TBM.

TBM Operational Parts
TBM’s Operational Parts

TBM Performance Parameters

TBM system performance is evaluated using several parameters that must be defined clearly and used consistently for comparative applications.

Shift time

Some contractors will use 24-hr shifting and maintain equipment as needed “on the fly.” As used here, the shift time on a project is all working hours, including time set aside solely for maintenance purposes. All shift time on a project is therefore either mining time when the TBM operates or downtime when repairs and maintenance occur. Therefore,

Shift time = TBM mining time + Downtime

Penetration rate

When the T BM is operating, a clock on the T BM will record all operating time. The TBM clock is activated by some minimum level of propel pressure and/or by a minimum torque and the start of cutter head rotation. This operating time is used to calculate the penetration rate (PR), as a measure of the cutter head advance per unit mining time. Therefore,

PR = distance mined /TBM mining time

PR is often calculated as an average hourly value over a specified basis of time (i.e., instantaneous, hour, shift, day, month, year, or the entire project), and the basis for calculation should be clearly defined. When averaged over an hour or a shift, PR values can be on the order of 2 to 10 m per hour. The PR can also be calculated on the basis of distance mined per cutter head revolution and expressed as an instantaneous penetration or as averaged over each thrust cylinder cycle or other time period listed above. The particular case of penetration per cutter head revolution is useful for the study of the mechanics of rock cutting and is here given the notation PRev (penetration per revolution). Typical values of PRev can be 2 to 15 mm per revolution.

Model of the tunnel boring machine (TBM) Åsa used to dig and line the Hallandsås tunnel.
Model of the tunnel boring machine (TBM) Åsa used to dig and line the Hallandsås tunnel.


The percentage of shift time during which mining occurs is the Utilization, U, and is usually evaluated as an average over a specified time period.

U (%) = TBM mining time Shift Time x 100

It is particularly important that U is reported together with the basis for calculation—whole project (including start-up), after start-up “production” average, or U over some other subset of the job. On a shift basis, U varies from nearly 100 percent to zero. When evaluated on a whole project basis, values of 35 to 50 percent are typical. There is no clear evidence that projects using a reconditioned machine have a lower U than projects completed with a new machine. Utilization depends more on rock quality, equipment condition, commitment to maintenance, contractor capabilities, project conditions (ent1Y/access, alignment curves, surface space constraints on operations), and human factors (remoteness, underground temperature, and environment).

Advance rate (AR)

AR is defined on the basis of shift time as:

AR = Distance mined /Shift time

If U and PR are expressed on a common time basis, then the AR can be equated to.

AR =  PR U (%)/100

Advance rate can be varied by changes in either PR (such as encountering very hard rock or reduced torque capacity when T BM drive motors fail) or in U changes (such as encountering very poor rock, unstable invert causing train derailments, or highly abrasive rock that results in fast cutter wear).

Cutting rate (CR)

Modell große Tunnelbohrmaschine Herrenknecht TBM Model
Herrenknecht TBM Model
CR is defined as the volume of intact rock excavated per Limit TBM mining time. Again, the averaging time unit must be defined clearly, and typical values of CR range from 20 to 200 m3 per TBM mining hour.

General considerations for TBM application.

Important project features that indicate use of T BM include low grades (cc-3 percent preferable for tunnel mucking and groundwater management) and driving up hill. A minimum grade of 0.2 percent is required for gravity drainage of water inflow. Horizontal curves in an alignment can be negotiated by an open TBM with precision and little delay if curve radii are on the order of 40 to 80 m. Most shielded TBMs and back-up systems are less flexible however, so that a minimum radius of 150 to 400 m should generally be used for design purposes. Tighter curves should be avoided or planned in conjunction with a shaft to facilitate equipment positioning.

Experience indicates that tunnel depth has little Impact on advance rates in civil projects, providing that the contractor has installed adequate mucking capacity for no-delay operation. Therefore, tunnel depth should be chosen primarily by location of good rock. Portal access, as opposed to shafts, will facilitate mucking and material supply, but more important is that the staging area for either shaft or portal be adequate for contractor staging.

TBM used for Metro bus Brescia
TBM used for Metro bus Brescia
Confined surface space can have a severe impact on project schedule and costs. For long tunnels, intermediate access points can be considered for ventilation and mucking exits. However, assuming the contractor has made appropriate plans for the project, a lack of intermediate access may not have a significant impact on project schedule.

Matilda TBM
Matilda TBM
In planning a project schedule, the lead time needed to get a TBM onsite varies from perhaps 9 to 12 months for a new machine from the time of order, to perhaps 3 to 6 months for a refurbished machine, and to nearly no time required for a direct re-use without significant repairs or maintenance. With proper maintenance, used T BMs can be applied reliably, and there is little need to consider specifying new equipment for a particular project. T BM cutter heads can be redesigned to cut excavated diameters different by I to 2 m, but the thrust and torque systems should also be modified accordingly.

With delivery of a TBM onsite, about 3 to 6 weeks will be required for assembly, cluing which time a starter tunnel should be completed. The start of mining rarely occurs with the full back-up system in place. Decreased advance rates on the order of 50 percent less than for production mining should be expected for the first 4 to 8 weeks of mining, as the back-up system is installed and the crew teams the ropes of system operation.


  1. Engineer manual 1110-2-2901
  2. Engineering and design
  3. Tunnels and shafts in rock