by Christopher Allan, Journalist, Infrastructure magazine

Don’t let the name fool you: Tunnel Boring Machines (TBMs) are fascinating machines, carving out the underground road and rail tunnels of Australia’s future while leaving life at the surface undisturbed. Here we break down how exactly these giant excavators work, and look at their sizable value across the national infrastructure pipeline.

Tunnel Boring Machines (TBMs) are gigantic, worm-like machines – weighing up to 4,000 tonnes and often 90 metres long – but you probably won’t ever spot one on your local construction site.

TBMs never come cheap, are typically reserved for the most ambitious tunnelling projects in Australia’s infrastructure schedule – and did we mention that these machines love to hide up to 60m below ground?

With Australia’s major cities locked into waves of tunnel infrastructure builds – from the Sydney Metro to Melbourne’s West Gate Tunnel – it pays to learn a bit more about how TBMs work and consider their value in an expanding infrastructure sector.

Tunnel boring machines: the essentials

Tunnel Boring Machines (TBMs) are like moving factories – methodically cutting through rock, dirt, or sand, transporting the excavated materials away, then laying down the foundational concrete segments of a new tunnel.

Most TBMs comprise a rotating cutting wheel or cutterhead, a main bearing, and trailing support mechanisms. TBMs come in many different shapes and sizes: the two machines used in Melbourne’s West Gate Tunnel project are some of the largest in the Southern Hemisphere, weighing 4,000 tonnes, stretching 90m long and 15.6m high.

All Tunnel Boring Machines are typically given female names, a long-standing tradition in recognition of Saint Barbara, the patron saint of miners who is said to protect those working underground.

The TBM process

When constructing a new tunnel, a Tunnel Boring Machine starts carving through rock and soil with its rotating cutterhead, which can be 15m wide. A conveyor belt or pump will send excavated material to the rear of the TBM, before the material is sent back through the tunnel to the surface.

As the machine moves forward, a concrete lining is installed by a specialised rotating machine. Once this machine has finished the tunnel’s walls, roof and base, specialised crews behind the TBM will build the road surface, installing electrics, ventilation, and safety systems.

Crews will often inspect the TBM cutterhead in teams of up to four, entering a pressurised air lock that protects them from groundwater and allows them to perform routine maintenance.

The physical effects of this airlock can be equivalent to scuba diving at a depth of up to 35m, so inspection workers will typically spend up to two hours depressurising before returning to the surface.

When TBMs are deployed to bore two parallel tunnels, crews will often use traditional mining techniques to add cross passages that link the two structures, an important safety feature in the event of an emergency: Melbourne’s Metro Tunnel project will feature 26 cross passages, located every 230m along the tunnel alignment.

TBMs are typically controlled by an operator and a crew of up to 20 people. Because TBMs lay an average of just ten metres of tunnel every 24 hours, adequate staffing arrangements are critical to optimise continuous operation.

A common solution to overcome this ten-metre daily limit is to source multiple TBMs to work on the same tunnel route in a kind of ‘meet-in-the-middle’ approach. In fact, London’s Crossrail project recently called on a total of eight TBMs to build a massive 42km of new rail tunnel, in just over three years.

TBMS down under: challenges and projects

Tunnel Boring Machines are found across infrastructure projects nationwide, servicing some of the most ambitious road and rail tunnel projects in our country’s history.

Major transport projects are integral in many ongoing federal and state government infrastructure plans, to the extent that some industry leaders have speculated that future projects may struggle to run concurrently, given the COVID-19 pandemic’s strain on human resources in the sector.

Indeed, because TBMs are both expensive and difficult to move, state and federal governments need to plan carefully around the national distribution of these major infrastructure assets.

For example, two TBMs that worked on Sydney’s new Metro tunnel have now been procured for Brisbane’s Cross River Rail, a win for both sustainability and the national infrastructure schedule.

Here are some of the most ambitious tunnel projects leveraging TBMs to deliver future-ready infrastructure without the short-term inconvenience.

Sydney metro

The Sydney Metro project has already utilised five Tunnel Boring Machines in the City and Southwest project, tunnels stretching from Chatswood to Sydenham.

One specialised TBM – named Kathleen – was tasked with digging a historic rail crossing underneath Sydney Harbour, cutting through 175,000 tonnes of sandstone, clay, and marine sediments.

TBM Kathleen takes its name from Kathleen Butler, a woman who was fittingly nicknamed ‘Godmother of Sydney Harbour Bridge’ for her work as technical advisor to John Bradfield.

Other TBMs of the Sydney Metro project include TBM Nancy, named after Nancy Bird Walton OBE, and TBM Mum Shirl, in honour of Colleen Shirley Perry.

Metro tunnel (Melbourne)

In May 2021, it was announced that the four TBMs of Melbourne’s Metro Tunnel project had finished digging the twin 9km tunnels of the Metro Tunnel between South Yarra and Kensington.

The Metro Tunnel project utilised mix shield TBMs, which are perfectly suited to local ground conditions. Mix shield TBMs typically blend excavated materials with slurry, allowing the raw material to be easily transferred above ground to a slurry treatment plant.

The four Metro Tunnel TBMs were named as follows:

♦ TBM Joan, in recognition of the first female Premier of Victoria, Joan Kirner

♦ TBM Meg, in recognition of the Australian cricket captain, Meg Lanning

♦ TBM Alice, in recognition of wartime medical hero of both World Wars, Alice Appleford

♦ TBM Millie, in recognition of Victoria’s first female member of Parliament, Millie Peacock

Cross river rail project (Brisbane)

The Queensland Government has procured and launched two TBMs from the Sydney Metro project for Brisbane’s ongoing Cross River Rail project. The Cross River Rail is a 10.2km rail from Bowen Hills to Dutton Park, set to feature 5.9km of twin tunnels beneath the Brisbane CBD and the Brisbane River.

One of the state’s most important job-generating projects, the Cross River Rail hopes to resolve a major bottleneck in rail converging on Brisbane River and city centre.

The two TBMs currently underway in the Cross River Rail project are:

♦ TBM Else, in recognition of Else Shepherd AM, an engineer and academic who was the first woman to graduate with an electrical engineering degree in Queensland

♦ TBM Merle, in recognition of Merle Thornton AM, an academic and feminist activist who famously chained herself to a bar rail in 1965 to protest the exclusion of women from public bars

Tunnel boring machine cutter head

The west gate tunnel project

While the West Gate Tunnel Project has already secured two of the largest TBMs in the southern hemisphere, named Bella and Vida, tunnelling is yet to start due to delays surrounding the project.

For over a year, the West Gate Tunnel Project had stalled due to an inability for stakeholders to agree on where contaminated soil from the tunnel’s predicted location would be transferred to.

Indeed, what began as a funding stand-off between Transurban, John Holland CPB and the State Government further disintegrated, when Victoria’s Environment Protection Authority (EPA) reversed an initial decision on the soil’s fate.

But in June this year, the West Gate Tunnel’s soil finally found a home: Hi-Quality’s landfill site in Bulla. With construction of the West Gate Tunnel Project back on track, it is likely that TBMs Bella and Vida will be put to good use soon.

TBMs Bella and Vida are named in honour of Bella Guerin, the first woman to graduate from a university in Australia, and Vida Goldstein, a ground-breaking campaigner for women’s rights and suffrage.

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