Tunnel Boring Machines

Tunnel Boring Machines (TBMs) are large machines that excavate below the ground surface, while simultaneously installing concrete lining units to build a tunnel. Two TBMs have been specifically designed for the Forrestfield-Airport Link by German company Herrenknecht, the world’s leading supplier of TBMs.

There are various types of TBMs to cater for different ground conditions and project requirements. For this project, the TBMs are Mixshield, which use the latest dual-mode technology capable of adapting to variable ground conditions (such as sand, rock and clay) as the machine progresses.

Key components for our TBMs were manufactured in various places around the world before an in-depth nine-month assembly and testing program was conducted in China. Once testing was finished, they are disassembled and shipped to Henderson Port. The TBMs were then transported to site and lowered into the dive structure at Forrestfield, where they began their two-year journey towards Bayswater. During this 8km journey, the TBMs will excavate under Perth Airport and the Swan River reaching up to 26m depth below the surface.

To find out more about how these machines operate, view the TBM fact sheet.

Frequently Asked Questions - TBM Grace ceases tunnelling temporarily

Operation of the first of two tunnel boring machines for the Forrestfield-Airport Link has ceased temporarily.

Naming the TBMs

The two tunnel boring machines that will dig the tunnels for the Forrestfield-Airport Link have been officially named.

Like ships, TBMs are named before they begin work to bring good luck. Traditionally, a TBM cannot start work until it is given a name. TBMs are generally given female names as underground workers look to Saint Barbara for protection.

The first TBM has been named Grace, in honour of pre-primary student Grace McPhee who was nominated by her classmates at Edney Primary School in High Wycombe. The students said Grace, who is undergoing treatment for leukaemia, was the toughest person they knew - a toughness the TBM would need to bore through the earth. This TBM is decorated with artwork by Year 6 Walliston Primary School student Georgia Fields.

The second TBM has been named Sandy - suggested by High Wycombe Primary School Year 4 student Sarah Spratt. Sarah was inspired after finding a sandgroper in her backyard, as the local insect (which is also a colloquial name for Western Australians) is 'excellent at tunnelling, just like the TBM'. This TBM is decorated with artwork by Rossmoyne Primary School Year 5 students Faith Brand and Jood Al Jashammi.

More than 100 students entered a competition for the chance to be a part of the project.

Where are the TBMs now? 


Arrive in Henderson

Start at Forrestfield

Finish at Bayswater


Mid-May 2017

July 2017

April 2019


Mid-July 2017

September 2017

June 2019

Estimated tunnelling timeframes (subject to change).


Will I hear the TBMs?

Despite their huge electric and hydraulic-powered motor drives, TBMs create little noise at the surface and cause only minor vibrations as they cut through the soil and rock in their path. If you are living near the tunnel route, you’ll be given plenty of notice and information before the tunnels are bored, but most people will not notice when the boring machines are close by.


Due to some ground disturbance issues, tunnelling work by TBM Grace for the Forrestfield-Airport Link – which has not yet reached any critical airport infrastructure - has been temporarily suspended.

The temporary suspension is a safe guard and will allow for the processes associated with the tunnelling to be independently reviewed and validated. 


Perth presents challenging geological conditions for tunnelling, with some elements of uncertainty to be expected in adapting the TBMs to varying ground conditions.

The Ascot Formation, through which the TBMs were tunnelling when disturbance occurred, comprises non-cohesive granular material that makes tunnelling more complex.

When the contract for the project was signed all parties involved were well informed about the potential challenges involved in tunnelling. 


Like any underground work, tunnelling comes with some level of risk. However, the machines purchased for the Forrestfield-Airport Link use advanced technology that help ensure high performance tunnelling.

Experienced tunnelling staff have been flown in from across the globe to operate the highly specialised TBMs. An extensive network of ground monitoring equipment has also been installed along the rail route to provide updates in near real time regarding any ground movement at the surface. 


Yes. The TBM is designed to protect the tunnelling crew from any ground movement around the machine.

The TBMs have been manufactured by one of the best manufacturers in the world. On board they have a refuge chamber, fire suppression equipment, gas detection facilities, emergency communication system and a first aider is rostered onto every shift. Radio-frequency identification (RFID) tracking is also used for all workers so that their position in the tunnel can be quickly identified in case of an emergency.

Further, strict international procedures are in place for all work performed under hyperbaric conditions. The TBMs have a man lock on board and another medical lock is on standby at the surface. The closest hospital is also advised when works are to occur under hyperbaric conditions. 

Yes. An extensive network of ground monitoring equipment has also been installed along the rail route to provide updates in near real time regarding any ground movement at the surface. If anomalies occur in the TBM’s readings a precautionary spotter is deployed to the surface to survey the area. As an added precaution the area may be cordoned off until it can be confirmed that no ground disturbance will occur.   

An independent review of the tunnelling operations is underway and the parties will work cooperatively through this review process, with appropriate recommendations to be incorporated into any future tunnelling operations. All parties are committed to working together to progress the review in a timely manner.


Yes. The reinforced tunnelling segments have been specifically designed to withstand external ground pressure along the Forrestfield-Airport Link alignment. The concrete used for the segments has also undergone a series of tests to ensure it meets the required standards for strength and durability. 

Once formed, the tunnels are regularly surveyed to confirm no movement of the segments has occurred. Strain gauges are also being installed at each cross-passage to monitor the tunnels stability.   


PTA was aware of this issue and conducted extensive geotechnical investigations prior to contract award. The findings from these investigations were shared with SI-NRW who conducted further geotechnical investigations to confirm and add to PTA’s findings.

These investigations conducted prior to tunnelling commencing revealed the ground conditions, in particular tunnelling through the Ascot Formation, would be challenging. The TBMs were designed specifically for Perth’s challenging ground conditions and are deemed suitable to complete the job safely. However, there is always some residual risk when carrying out tunnelling operations.  


The Public Transport Authority and Perth Airport have engaged independent experts as part of the review of tunnelling operations. Appropriate recommendations provided will be incorporated into the process for future tunnelling operations. Salini Impregilo-NRW is running detailed risk workshops to determine any lessons that can be incorporated into tunnelling operations. A specialist advisor from the TBM manufacturer has reviewed operations and provided recommendations for improvement.

Yes. The TBMs have not yet reached the runways but when they do, will pass approximately 22m below the runways without impact to airport operations. The Public Transport Authority will work closely with Perth Airport to ensure adequate controls are in place.


The diameter of each tunnel



The weight of the TBMs