Crossrail: The monster tunnelling under London

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.

In the belly of the beast

But beneath the surface, it’s not even immediately obvious where the tunnel ends and the machine starts. And it’s something of a surprise when – after a circuitous journey involving ladders, lifts, and a stop-start trip aboard a cramped, rattling parody of the trains that will one day ply this route – our host, site manager David Shepherd of engineering and construction giant Bechtel, announces that we’re standing in the belly of the beast.

The machine is a remarkable and unique piece of equipment. Purpose-built for Crossrail by German firm Herrenknecht – one of a handful of TBM manufacturers in the world – it cost around £10m ($15m), weighs close to 1,000 tonnes, has an external diameter of 7.1 metres (23ft) and from cutting-face-to-end stretches 150m (500ft).

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.

This pressure balance is constantly monitored by the TBM’s driver, who can regulate it with careful adjustment of both the machine’s advance rate (or thrust) and the speed of the screw conveyor that’s used to take spoil away from the cutter head. “By controlling the amount of thrust and coordinating with the extraction of the muck via the screw, we can maintain the face pressure which mitigates against settlement on the surface,” says Shepherd.

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.

Once a ring is complete, high-pressure grout is injected around the segments to lock them in place, and the machine is thrust forward by hydraulic propulsion rams that push against the leading edge of the last ring built. 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.

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!”

Tiny movements

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. While stops are kept to a bare minimum, the machine’s cutting tools can only take so much and, says Shepherd, approximately once every kilometre it’s necessary to inspect the cutter-head and see whether it needs replacing.

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. During this process, which Shepherd says has only happened on a handful of occasions, mechanics enter an airlock at the front of the TBM. The machine is slowly filled with compressed air until the pressure equals that of the tunnel face, which happens typically around 2 bar: the same pressure a diver would experience at around 10m (33ft) below the surface.

After a short period of acclimatisation, they can leave the TBM through a small hatch in the cutter face and return to their work. Decompression takes much longer and – as with scuba divers – varies according to the amount of time the mechanics have spent at the tunnel face. 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.

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.

By carefully following the route and paying constant attention to the pressure balance between the machine and the ground around it, the driver plays a key role in minimising ground movement around the tunnel – also called settlement – which can have detrimental effects on surrounding tunnels or infrastructure. 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.

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.

After almost three years, some 250,000 cement segments, and enough debris to create a new island, the circuit that forms Crossrail’s tunnels is almost complete. Now all they need to do is build a railway.

Jon Excell