When colossal tunnel boring machines complete their monumental task, what becomes of these underground giants? It's a question that sparks curiosity about the lifecycle of the incredible engineering feats that shape our cities.
Imagine machines so massive they rival the weight of 171 elephants, stretching an astonishing 170 metres in length, and capable of excavating a staggering 1.58 million tonnes of rock. These weren't just any machines; they were Betty and Dorothy, the formidable tunnel boring machines (TBMs) that carved out the Sydney Metro West tunnels. Once their demanding work was done, these mechanical titans didn't simply vanish. Instead, they underwent a complex dismantling process deep underground, with their individual components meticulously raised to the surface.
These engineering marvels were disassembled within the cavern of the Westmead station. Following their deconstruction, each piece, some weighing as much as 110 tonnes, was carefully lifted by crane to the surface. The main drive units, along with their intricate electrical systems and other vital moving parts, have been returned to their manufacturer, Herrenknecht. This allows for essential repairs and refurbishment, ensuring these components can be reused in future projects – a testament to sustainable engineering.
But here's where it gets controversial... The cutterheads, the formidable circular saws at the very front of these machines, are a different story. A metro spokesperson explained that these crucial components have exceeded their wear limits and cannot be reused. These massive saw blades are dismantled into sections on the surface and then sent for recycling. Is it a shame that such advanced parts can't be given a second life, or is this a necessary step in the lifecycle of such high-stress equipment?
Betty and Dorothy were pioneers in Australia, being the first TBMs to utilize artificial intelligence software for steering and digging. This advanced technology, however, still required a dedicated onboard crew of 15 people for each machine to operate effectively. As the spokesperson elaborated, "Behind the cutterhead, each TBM includes a long back-up section… of connected gantries that house the equipment and facilities needed to excavate the tunnel and install its lining." For Betty and Dorothy, this extensive backup included sophisticated mechanical, electrical, ventilation, and segment handling systems, alongside essential crew amenities.
These workhorses operated tirelessly, running 24 hours a day, five days a week, achieving an impressive rate of around 800 metres of tunnel per month. Over their tenure, Betty and Dorothy were responsible for installing more than 57,900 concrete tunnel lining segments, a truly colossal undertaking.
It's a global tradition to bestow female names upon underground tunnelling machines. Dorothy was honored in recognition of human rights advocate Dorothy Buckland-Fuller, the founder of the Australian Migrant Women’s Association in 1974. Betty, on the other hand, was named after the legendary Olympian Betty Cuthbert, a celebrated figure who grew up in western Sydney and achieved iconic status by winning four gold medals for Australia, including three at the 1956 Melbourne Games.
As the tunnelling for the Metro West network nears its conclusion, other TBMs, Jessie and Ruby, are also wrapping up their assignments. Ruby impressively broke through into the Hunter Street station cave on December 17th of last year, while Jessie is expected to complete its breakthrough within weeks, according to a Sydney Metro spokesperson.
Jessie and Ruby are a different breed of TBM, known as mixed shield (slurry) machines. These are specifically engineered to tackle the demanding geological conditions found beneath Sydney Harbour and within the Central Business District (CBD). Their operation involves mixing a slurry fluid with the excavated rock and soil. This process is crucial for maintaining stable pressure at the cutterhead, thereby enabling safe and effective digging under the harbour. Consequently, they required additional, complex systems for slurry circulation and treatment.
Their digging pace was more measured, averaging about 90 metres per week, and together they excavated a total of 460,000 tonnes of rock. While Jessie and Ruby were more costly to build and maintain compared to Betty and Dorothy, the overall investment for all six TBMs used for Metro West was substantial, costing tens of millions of dollars for their design, manufacture, and commissioning.
And this is the part most people miss... A complex and challenging overnight retrieval operation is anticipated once Jessie reaches its final destination within the confined spaces of Sydney's CBD. This will undoubtedly be a significant logistical feat.
What are your thoughts on the reuse and recycling of such massive pieces of machinery? Do you think the naming tradition adds a special touch to these engineering projects? Share your opinions below!
