Tunnel Boring Machines

Tunnel Boring Machines (TBMs) are large machines that excavate below the ground surface, while simultaneously installing concrete lining units (segments) 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 were disassembled and shipped to Henderson Port. The TBMs were then transported to site, reassembled and lowered into the dive structure at Forrestfield, where they began their two-year journey towards Bayswater in 2017. 

During their 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.


TBM tracker

As at 11.41am, Friday December 14, 2018:

Metres tunnelled Number of tunnel rings installed Status
TBM Grace 3786m 2265 Stopped at Airport West Emergency Egress Shaft for a scheduled atmospheric intervention.
TBM Sandy 3386m 2015 Tunnelling underneath Airport estate.






Naming the TBMs

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.

Our first TBM is 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 was decorated with artwork by Year 6 Walliston Primary School student Georgia Fields.

Our second TBM is named Sandy as 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 was decorated with artwork by Rossmoyne Primary School Year 5 students Faith Brand and Jood Al Jashammi.


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.


Frequently Asked Questions - Ground disturbance

Operation of the two tunnel boring machines for the Forrestfield-Airport Link was ceased temporarily in mid-February and late-March, 2018 respectively. The temporary suspension was to allow for the processes associated with the tunnelling to be independently reviewed and validated.

Upon completion of the review, TBM Grace was restarted on April 17, 2018. TBM Sandy was restarted shortly after on April 24, in order to maintain a safe distance behind TBM Grace. 


Due to some ground disturbance issues, tunnelling work by TBM Grace for the Forrestfield-Airport Link was temporarily suspended on February 14. The temporary suspension was a safe guard to allow for the processes associated with the tunnelling to be independently reviewed and validated. 

TBM Sandy stopped on March 28 when it came within 40m - the minimum distance - of the then stationary TBM Grace.

Though Sandy has not experienced the same ground disturbance issues, it is widely-accepted best practice that two TBMs should not be positioned side-by-side during tunnelling.


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 to 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, an 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.

Furthermore, 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 TBMs 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.   

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.


Yes, the matter has been comprehensively investigated and changes made to reduce the risk of further disturbances. 

The Ascot Formation comprises non-cohesive granular material that makes tunnelling more complex and this presented issues for the project team. 

Since experiencing the minor ground disturbances and subsequent investigations a number of changes have been made including:

  • Changes to the pressure in the excavation chamber when excavating in the Ascot Formation.
  • Continuous operation (no weekend stoppages) especially when tunnelling beneath airport infrastructure. 
  • Slurry and bentonite quality adjustments to reduce potential slurry losses.
Yes. The TBMs 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 while the TBMs are within the airport boundary.
 Have there been any further ground disturbance issues since tunnelling resumed?

Yes. It was always expected that there may be some minor ground disturbance issues as TBMs approach the station box. This is because of changes to the pressure at the face of the TBMs when they reach the concrete station box wall.

As a result, the Perth Transport Authority (PTA) and Salini Impregilo-NRW JV (SI-NRW) had developed a comprehensive contingency and management plan should any issues arise.

No ground disturbance occurred when TBM Grace arrived at the Airport Central Station box on May 8. With TBM Sandy, a minor ground disturbance issue was recorded on Friday May 18 in the early hours of the morning.

Early investigations indicated some of the slurry from the machine ended up in a sub-floor drainage channel creating an inflow of sand from surface level. This issue was quickly fixed.

There was no damage to infrastructure, and the PTA and SI-NRW worked closely with Perth Airport throughout the process.


The diameter of each tunnel


The weight of the TBMs