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The monster tunnelling under London streets

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 canada goose has to actually build anything down there.

Which makes it all the more remarkable that engineers have managed to construct 26 canada goose black friday sale miles of fresh tunnels for the underground section of one of the world biggest engineering schemes: London showcase public transport project, Crossrail.

For years, these gargantuan machines have been painstakingly threading their way beneath London. Canada Goose Coats On Sale Today, three years after its May 2012 start, canada goose store tunnelling is almost complete. Deep beneath the streets of East London, the project final two gargantuan tunnel boring machines (TBMs), Victoria and Elizabeth, are approaching their final canada goose outlet toronto factory destination. When they do, they will complete a process that has seen eight separate machines painstakingly thread their way beneath the heart of one of the world busiest cities.

Shortly before the Canada Goose Online 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 far easier to get a sense of a TBM size canada goose clearance sale and scale above ground where its giant teeth and hulking Canada Goose Outlet mass instantly evoke the kind of mechanical diggers seen in science fiction films.

In the belly of the beast

Canada Goose online But beneath the surface, it not even immediately obvious where the tunnel ends and the machine starts. And it something of a surprise when after a canada goose deals circuitous journey involving ladders, lifts, and a buy canada goose jacket cheap 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 standing in the belly of the beast.

The machine is a remarkable and unique piece of Canada Goose online equipment. Purpose built for Crossrail by German firm Herrenknecht one of a handful of TBM manufacturers in the world it cost around ($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).

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 Canada Goose Jackets of Crossrail tunnels. Pioneered in Japan in the late 1970s, this technique works by carefully compacting extracted spoil to a pressure which equals canada goose coats on sale 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 cheap Canada Goose monitored by the TBM driver, who can regulate it with careful adjustment of both the machine advance rate (or thrust) and the speed of the screw conveyor Canada Goose sale that used to take spoil away from the cutter head. 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 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 canada goose replica is injected around the segments to lock them in place, and the machine is thrust forward by hydraulic propulsion 2018 canada goose outlet 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 canada goose coats hours and taking short breaks beneath ground to use the on board toilet (we weren allowed in) or grab a drink.

The tunnelling machine has an onboard toilet

In tunnelling terms, the machine rate of progress canada goose outlet store locations 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 possible to do much more. had some record shifts we built over 20 rings in a 12 hour shift, he says, that was exceptional. It was a short drive, it was perfect ground, we had the best teams on buy canada goose jacket there, and they were going for it! movements

getcanadagooseoutlet Nevertheless, at around one metre an hour, there little sense that you aboard a vehicle. The incessant rumble of the cutter and the sedate movement of the tiny wheels supporting the TBM rolling gantries are the only cues that you actually moving.

While stops are kept to a bare minimum, the machine cutting tools can only take so much and, says Shepherd, approximately once every kilometre it necessary Canada Goose Parka to canada goose clearance inspect the cutter head and see whether it needs replacing.

Sometimes, an extreme procedure becomes necessary

Although this can usually be done in an unpressurised environment, concerns over ground stability canadian goose jacket mean that sometimes the pressure on the tunneling face cannot be reduced. In that case, it necessary to perform an extreme procedure known as a hyperbaric intervention.