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 METRONET Forrestfield-Airport Link by German company Herrenknecht, the world’s leading supplier of 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.

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 High Wycombe, where they tunnelled between High Wycombe and Bayswater between July 2017 and April 2020.

During their 8km journeys the TBMs excavated 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 or the TBM animation.

TBM tracker

As at 9am, Wednesday April 22, 2020:

	Metres tunnelled	Number of tunnel rings installed	Status
TBM Grace	7458m	4468	Tunnelling complete
TBM Sandy	7456m	4461	Tunnelling complete

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 was 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 was 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.

Yes. 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. 

It was anticipated that there might be some minor ground disturbance issues as the TBMs entered and exited Airport Central Station. 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, comprehensive contingency and management plans were in place should any issues arise. No ground disturbance occurred when TBM Grace arrived at the Airport Central Station box on May 8, 2018. With TBM Sandy, a minor ground disturbance issue was recorded on Friday May 18, 2018.

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

In September 2018, a leak developed at the first tunnel-to-tunnel cross passage located approximately 200m north of the High Wycombe Station site. As a result, water and silt entered Tunnel One (TBM Grace’s tunnel) leading to the formation of a sinkhole at the surface alongside Dundas Road. In this instance the ground disturbance did not occur as a result of TBM-powered tunnelling, which had been completed some 12 months prior. Cross passage construction works were the catalyst to the incident and future cross passage construction methodologies have since been comprehensively reviewed.

Minor ground disturbances were recorded when the TBMs entered and exited the Redcliffe Station box. As with the breakthrough into Airport Central’s station box, this outcome was expected and all ground disturbances were contained within the dry box areas outside the stations.

The same occurred when the TBMs arrived at Bayswater Junction, where all ground disturbances were safely contained within the dry box.

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.

The tunnels are regularly surveyed to confirm no movement of the segments has occurred. Strain gauges are also installed at each cross passage to monitor the tunnels' stability.

The tunnels are designed and built to have 120-year durability, and are constructed to Australian and international standards. They undergo quality assurance and independent verification checks at every step of construction and fit-out.