In the footsteps of Brunel: tunnelling under the Thames calls for sophisticated handling plant to manage thousands of tonnes of spoil. Lee Hibbert visits the Crossrail site.
But despite this encouraging progress, there is no sign of complacency among those involved, especially as one of the most complex phases is ongoing. For at least the next few weeks, engineering managers will continue to hold their breath as the second of two gigantic machines completes its work boring a total of 2.6km of twin tunnels under the River Thames.
The project is potentially dangerous: tunnelling under water presents multifarious technical challenges, with the threat of inundation always foremost in the mind. And the geology of sand, gravels and chalk encountered on this part of the route has required a bespoke materials-handler to cope with the sloppy nature of the spoil.
So far, one tunnel-boring machine (TBM), named Sophia, has completed her journey. She started at a portal in Plumstead, south-east London, travelled onwards to a custom-built box at Woolwich and then dipped 15m under the river, to emerge at a site close to City Airport in North Woolwich. Over the next month or so a second machine, called Mary, will carry out a similar journey, and is scheduled to complete the twin-bore tunnel in a few months' time.
The project manager for the Thames Tunnel is Gus Scott, a chartered mechanical engineer with two decades of design and construction experience around the world. He says the work has gone well to date, finishing on time with the creation of the minimum amount of settlement. And he is hopeful that the smooth progress will continue.
"Brunel was the first engineer to tunnel under the Thames, so we know we are following history," he says. "So far, it's all been straightforward. It's been a non-event. And that's the way I like it."
Much of the tunnelling work on Crossrail has involved earth pressure balanced TBMs. These have a rotating cutter head at the front and a series of trailers behind, housing all the mechanical and electrical equipment required for excavation. The material loosened by the cutter head is removed by a screw conveyor, which transports it to the back of the TBM and out of the tunnel on a conveyor belt.
But earth pressure balanced machines were deemed unsuitable for use under the Thames because of the geology. Scott says the flintiness of the chalky soil would damaged the cutter heads, requiring frequent maintenance to be carried out under difficult conditions beneath the river.
Instead, Crossrail chose to use more expensive mix-shield slurry TBMs from German maker Herrenknecht, which are specially equipped to deal with chalk, flint and wet ground conditions. Mix-shield slurry machines were used on High Speed One's tunnel, built between 2000 and 2004, which was a similar length to, and with the same kind of geology as, that being encountered on Crossrail.
The two Herrenknecht machines specified for the Thames Tunnel project feature 7.2m-diameter cutting shields with pressurised, air-locked chambers that are filled with a slurry of bentonite clay and water. This design makes for easier cutting of the chalky soil, with the sealed compartment preventing the ingress of water from the surrounding ground.
The bentonite slurry is mixed with the cut material at the face, allowing it to be pumped from the tunnel through a system of slurry tubes, rather than being carried along conveyors, before emerging at a treatment plant in Plumstead.
The state-of-the-art treatment facility takes the excavated slurry through a series of trommels, hyper-cyclones and filter presses, separating it into sand, gravel, water and chalk, which can be reused for beneficial purposes. The chalk comes out in 'cakes' of filtered particles with less than 35% water content, so it can be transported away on trucks.
"It's all about geology--we only use the slurry machines in chalky environments," says Scott.
"You can go through chalk with an earth pressure balanced machine, but other projects that have done so have encountered problems. The flints can cause a lot of damage to the cutter heads. So if we had decided to use an earth pressure balanced machine I think we would have got a lot more wear.
"With the mix-shield slurry TBMs, by adding bentonite we can effectively create a jelly at the front of the machine, so that when the chalk and flint is cut it all comes away in a solution that can be pumped away. The bentonite minimises the wear, and means we do not need to stop the machine for maintenance as much," he says.
"The last thing you want under the Thames is to be dealing with compressed-air intervention and putting people in there to carry out maintenance.
"Also, by creating an air bubble at the front of the machine, we felt we would have better control of the face pressure--making it an all-round safer operation," he says.
The 110m-long, 850-tonne mix-shield slurry boring machines, each costing 8.5 million[pounds sterling], are effectively gigantic underground factories that can operate 24 hours a day. Each machine has installed power of 2,750kW and a working pressure of 4bar, and comprises 44 main thrust cylinders producing an impressive 48,000kN.
The cutter heads turn at three revolutions per minute, with the machine advancing 27m a day on average. Technically, it would be possible to progress faster, but the machines on the Thames Tunnel project are limited by the rate at which the Plumstead treatment plant can handle the vast amounts of slurry that are being produced.
Scott says that the treatment facility is an example of clever engineering in itself. "Earth-balanced pressure machines typically deal with spoil comprising mainly clay, which can be carried out on conveyor and shipped by truck," he says. "But our bentonite slurry comes out with a consistency similar to that of toothpaste, so it needs to go through a chemical engineering process that involves a series of trommels, hyper-cyclones and a filter press to get the moisture out.
"Traditionally, we would have used centrifuges, and over the years this sort of treatment system has worked well. But in our experience it's a struggle to remove enough of the moisture content.
"You end up spinning and spinning but you cannot get to the stage where you can get the muck on to trucks. The centrifuges were often the constraint in the system. Our approach is more impressive and reliable," he says.
With the chalk cakes transported away, much of the water and bentonite can be pumped back into the system and reused. "It's like a large-scale, sophisticated materials recycling facility," says Scott.
The Plumstead treatment plant will remain in use until June, when Mary is expected to finish her drive under the Thames, emerging at the North Woolwich site. When this occurs, the tunnelling activities on the project will officially come to an end.
Much has been achieved already, and Scott puts the smooth progress of the work down to the skill of his engineers and the tried-and-tested techniques being used.
In many ways, he says, the greater challenge on the project has been the need to liaise with a multitude of stakeholders.
The tunnelling work runs close to the strategically important north Kent railway line, with the cavernous Plumstead portal positioned just metres from passing trains. So Crossrail has had to forge a close working relationship with Network Rail to ensure there is no disruption to passenger services.
Local businesses and public bodies have also had to be kept informed of progress. There have been educational outreach activities, too, with Scott and his team going into schools to present their work.
This contact with outside organisations has proved hugely rewarding, he says. "It's been great. Some schoolchildren thought we dug tunnels with our hands. And to other groups, Crossrail was just a row of blue hoardings. So stakeholder engagement has been an important part of what we have been doing."
Joint effort on river job
The C310 Thames Tunnel project is a joint venture between Hochtief of Germany and Murphy Group.
The work includes construction of a 2.6km twin-bore tunnel from a launch portal in Plumstead to a box at Woolwich, and then under the Thames to a site in North Woolwich.
The tunnel will comprise eight segments which form a 6.2m-diameter ring. These segments are being made at Shay Murtagh's precast factory in Ireland before being shipped across and stored on the Crossrail site.
What's in a name?
The Crossrail project will use eight tunnel-boring machines (TBMs) to dig 10 tunnels under London. A gang of 20 men and women works in a tunnel on each shift to keep the machines operating 24 hours a day.
Each TBM has a name reflecting historic interest. Ada is called after Lady Ada Lovelace, one of the first computer programmers, while Phyllis derives her name from Phyllis Pearsall, creator of the London A-Z street maps.
Elizabeth and Victoria acquire their names from Queen Elizabeth II and Queen Victoria, while Sophia was the wife of Marc Isambard Brunel, who built the first tunnel under the Thames. Mary is called after the wife of Isambard Kingdom Brunel, who helped his father Marc on this tunnel.
TBM Jessica derives her name from Olympic champion Jessica Ennis-Hill CBE. And Ellie is called after Paralympic gold medallist Ellie Simmonds QBE.
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|Publication:||Professional Engineering Magazine|
|Date:||Mar 1, 2014|
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