I am an avid consumer of podcasts. If you know me personally, you’re definitely going to hear me say ‘I was listening to this podcast the other day…’, followed by whatever the cool fact or story was that I heard. One of my favourite podcasts is Every Little Thing, which answers listener questions, covering a wide range of topics; from the history of cheerleading to the origins of cheese, no topic is too big or too small. Recently, a listener wanted to know on how underwater tunnels are made. I’ve actually had a fascination with heavy machinery and construction for a few years, and the explanation of how tunnels are built is pretty nifty.
In a lot of cases, tunnels are made using a huge worm-like machine, imaginatively named the tunnel boring machine (TBM). This monolithic cylinder inches through the earth, slowly grinding through rock and soil while simultaneously reinforcing the space, leaving behind the new tunnel’s skeleton.
The basic layout of a TBM is quite simple: you’ve got a cutting shield at the front, which performs the grunt work, excavated debris is then removed by a conveyor belt, and finally, concrete segments are laid to form the supporting walls of the tunnel. TBMs can be used to excavate tunnels large enough for cars to drive through, so their internal space is actually pretty big, providing an office, kitchen, and toilets for workers! I can’t help but imagine being inside one of these colossal machines while it creeps slowly through the earth at ten metres a day. You probably wouldn’t even realise it was moving, but then several weeks into the whole process you’d get out one day and be shocked to find that you’d created a hole in the ground that spanned kilometres!
As you might expect, there are different types of TBM, which are used for tunnelling through various geological conditions. For example, you might be tunnelling into soft ground that contains water, or into very hard rock, two different situations which require specialised cutting shields. In the case where there’s water in the soil, a pressure difference will exist between the outside and the inside of the TBM. This is undesirable because it can lead to flooding, so the TBM is engineered such that it can deliberately maintain its internal pressure at the same level as what’s outside. However, if you’re tunnelling through very hard rock, the most important feature is cutting ability, so in this case, the cutting shield combines forward thrust with rotation to achieve maximum destructive power.
In recent years there has also been an effort to engineer TBMs that can withstand wear and tear. Traditional designs use direct contact between the cutting shield and the rock to perform the excavation, however, new work has shown that digging aided by a water jet can help extend the life of the TBM.
Something I did not expect when I first started looking into this topic, was how old the concept of the TBM is. Being such a large and impressive machine, I would’ve guessed that TBMs are a recent development, certainly within the last 50 years, however, the first cutting shield was created in 1825!
Successful tunnelling using a TBM did not occur for a while, not until the channel tunnel between France and England was being made. During its construction, a TBM managed to dig 1.84 km from the English side and 1.67 km from the French side. An impressive feat, and one which cemented the place of TBMs in the construction industry.
Improvements to TBMs have continued to the present day; now there are a multitude of companies that make TBMs. The field has even attracted the interest of Elon Musk, who started ‘The Boring Company‘ as a hobby, with the intent of excavating train tunnels under various cities in America.
I live in Melbourne where there is currently large-scale construction of a new rail tunnel to connect various parts of the city, including a major stop at The University of Melbourne, my current home! However, construction is set to conclude in 2025, meaning there’s a good chance I won’t experience the full benefits of the upgrade. But progress doesn’t happen overnight!
While it will be a real asset for the city, the main reason The Metro Tunnel Project is of interest, is because they’ve brought in four TBMs to get the job done. They’re being deployed at either end of the city, and will first travel away from the CBD to create the ends of the tunnel. After this initial deployment, they will return to their starting points, and move slowly under the buildings of Melbourne to meet the current rail network. What’s more, they’ve been named after important women in Australia’s history: Joan Kirner, Victoria’s first female Premier; Meg Lanning, captain of the Australian cricket team; Alice Appleford, who was a military nurse during both World Wars; and Millie Peacock, Victoria’s first female MP.
Finally, if you’d like to learn more about TBMs or just tunnels in general, because they’re pretty cool, I would recommend Tunnel Insider. As an introduction, check out their article on the longest road tunnel in the world. Situated in Norway, it’s 24.51 km long; a bit too claustrophobia-inducing for my tastes, but undoubtedly a remarkable engineering achievement!