Subterranean London is a crowded environment, home to a tangled tapestry of tunnels, sewers, foundations, power lines and abandoned stations that excite archaeologists and send a shiver down the spine of anyone who has to actually build anything down there.
Which makes it all the more remarkable that engineers have managed to construct 26 miles of fresh tunnels for the underground section of one of the world’s biggest engineering schemes: London’s showcase public transport project, Crossrail.
The quiet tunnels are in stark contrast to the busy streets above (Credit: Crossrail)
The boring machines have tunnelled some 26 miles underneath London (Credit: Crossrail)
Shortly before the final breakthrough, we visited Elizabeth 40m (130ft) beneath the streets of central London, at a point midway between Liverpool Street and the finishing line at Farringdon, to witness this enormous underground factory in action.
It’s far easier to get a sense of a TBM’s size and scale above ground – where its giant teeth and hulking mass instantly evoke the kind of mechanical diggers seen in science fiction films.
The giant-toothed diggers resemble something from science fiction films (Credit: Crossrail)
The Crossrail project is one of the world’s most ambitious engineering feats (Credit: Crossrail)
How can you keep the tunnel from collapsing?
The TBM Elizabeth, which began its journey in November 2012, is typical of the so-called earth pressure balance machines used on the majority of Crossrail’s tunnels. Pioneered in Japan in the late 1970s, this technique works by carefully compacting extracted spoil to a pressure which equals that of the surrounding soil and groundwater – and therefore reduces the risk of the tunnel face collapsing as the TBM carves out its route.
Each of the tunnelling machines weighs up to 1,000 tonnes (Credit: Crossrail)
The concrete segments lining the tunnel can weigh up to 3,000kg (Credit: Crossrail)
As the machine moves through the ground, pre-cast concrete segments – each weighing 3,000kg – are ferried along a conveyor toward the front, where they’re picked up by giant hydraulic arms and moved into place to form the tunnel lining. Eight of these segments are used to form a 1.6m-long (5.2ft) ring; a total of 250,000 segments will be used across the network.
The machines’ advance has to be carefully monitored to ensure they remain on target (Credit: Crossrail)
This process continues 24 hours a day, seven days a week, with a rolling crew of 15 swapping shifts every 12 hours and taking short breaks beneath ground to use the on-board toilet (we weren’t allowed in) or grab a drink.
The tunnelling machine has an onboard toilet
In tunnelling terms, the machine’s rate of progress is impressive. Shepherd, a veteran of projects including the Channel Tunnel and the Jubilee Line Extension, says that on a typical shift without any stoppages he would build around 16m of tunnel. And sometimes, if the conditions are right, it’s possible to do much more. “We’ve had some record shifts….where we’ve built over 20 rings in a 12-hour shift,” he says, “but that was exceptional. It was a short drive, it was perfect ground, we had the best teams on there, and they were going for it!”
Nevertheless, at around one metre an hour, there’s little sense that you’re aboard a vehicle. The incessant rumble of the cutter and the sedate movement of the tiny wheels supporting the TBM’s rolling gantries are the only cues that you’re actually moving.
Each crew of 15 works a 12-hour shift under the tunnels (Credit: Crossrail)
Sometimes, an extreme procedure becomes necessary
Although this can usually be done in an unpressurised environment, concerns over ground stability mean that sometimes the pressure on the tunneling face cannot be reduced. In that case, it’s necessary to perform an extreme procedure known as a hyperbaric intervention.
Every kilometre, the cutting teeth are inspected to make sure they’re still sharp (Credit: Crossrail)
The route that the TBM takes is exceptionally carefully pre-planned. The ground it passes through has been meticulously probed and mapped and an advanced laser guiding system is used to help ensure the machine stays on track. Talking to Shepherd, it’s clear that there’s not even the faintest glimmer of concern that the ends won’t meet up.
The tunnel atmosphere is sometimes equivalent to 10m (33ft) underwater (Credit: Crossrail)
Despite the precautions, there are still unknowns
But despite all of this, there are, he says, still unknowns, such as minor local variations in the soil density that aren’t discovered until the TBM arrives – and which require decisive action from the driver to ensure that the machine constantly remains within 50mm of its pre-planned position. “You might encounter hard ground on one side of the face which tends to tilt the machine,” explains Shepherd, “or hard ground on the bottom which makes the machine want to come up. So the driver has to steer.”
To do this, the driver adjusts the pressure on the propulsion rams used to push the TBM forward. These rams run around the circumference of the machine and can be individually adjusted to subtly alter the machine’s direction.
The machine has to be kept within 50mm of its planned position (Credit: Crossrail)
But the drivers alone don’t have sole responsibility for ensuring that the excavations don’t start causing problems. One method used widely to stabilise the ground along the length of Crossrail’s sub-surface is known as compensation grouting. This involves the excavation of grouting shafts: 10-20m deep (33-66ft), 5m-diameter (16ft) holes from which engineers can inject high-pressure grout into the ground around the excavations.
The grout is injected into the ground via a series of small-diameter horizontal underground pipes known as Tubes-a-Manchette (TAM) that radiate from the base of the shaft and can be up to 80m (264ft) long.
These vertical shafts, which aren’t as deep as the running tunnels, have yielded some of Crossrail’s most startling and grisly archaeological discoveries, including a 14th Century pit close to Farringdon, which holds the remains of plague victims.
The remains of 14th Century plague victims is one of the grislier discoveries (Credit: Crossrail)
Back beneath the surface, as we stand aboard Elizabeth, it’s easy to forget the bustling metropolis above. In some ways, we’re forgotten, too: Londoners have managed to tune out the massive project over the past few years.
But the TBM’s bustling crew have no time to take stock of the project yet: there’s a tunnel to finish, and there’s a palpable sense that the final breakthrough is in sight.
The tunnels will soon be complete – but now a railway line will have to be built (Credit: Crossrail)
Jon Excell is the Editor of The Engineer
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