Capital gains: digging into Crossrail22 October 2013
It’s the construction project that’s set to drastically reduce journey times across London and the south-east of the UK, but behind Crossrail’s vast publicity lies a meticulous operation with an emphasis on recruiting the best talent. Ross Davies meets chief engineer Chris Dulake to learn about excavation techniques and negotiating potential hurdles under the capital’s streets.
Through the window of Crossrail's large office in Canada Square, North Greenwich, a cable car can be seen making its leisurely passage across the Thames to the Royal Docks. At the same time, Chris Dulake, chief engineer of Europe's biggest infrastructure project to date, is enthusiastically sifting through a sheaf of documents.
The bundle consists of various photos, cross-section blueprints, renderings, equations and graphs. The first he seizes upon depicts an enormous cavern, 40m beneath Stepney Green station, and one of the largest spray concrete lining (SCL) cavities in Europe; the huddle of Crossrail engineers featured in the image appear miniscule.
Another blow-up follows. This time it is Farringdon, another part of Crossrail to successfully employ the SCL technique.
"I took this only yesterday, when I visited the Farringdon site," says Dulake with discernible pride. "The guys are doing really well. They've just broken out of the circular shaft, turned 90° and are now coming back on themselves. That really demonstrates the agility you have with the SCL method to build some of these more complex stations."
SCL is one of the two tunnelling techniques being used by Crossrail; the other uses seven giant tunnel-boring machines (TBMs), with an eighth to be added at the start of next year, as the project takes greater shape.
However, unlike TBMs, which excavate tunnels through repetitive cycles of digging forward, SCL is able to allow greater variation in tunnel shape and diameter, while eschewing the need for precast concrete. This is achieved by rapidly spraying the excavated ground with concrete to stabilise it and form a permanent tunnel lining.
The technique will be used to build 12km of station platform tunnels, passages, and access and grout tunnels along the routes where smaller, relatively short tunnels are needed.
"The reason we use SCL is its agility," says Dulake. "When you are building varying section sizes and pairing the configurations of these tunnels, this technique is vital. In addition, most of the stations and platforms aren't particularly straight or linear, which also means you need to deploy an alternative to traditional TBMs."
Crossrail's completion is tabled for 2018. And while it will require excavation and mining on an unprecedented scale - "it's more an urban metro than subway," notes Dulake, who has close to 30 years' civil engineering experience - navigating through the network of existing features deep below London requires large amounts of preliminary research and desk study.
"Well, firstly, we need to make sure that we miss existing tunnels," he says. "That's a difficult thing to get right, as we do come close to the likes of the Piccadilly, Central and Northern lines. Of course, you also want to try and bring these stations as close to the surface as possible, so as to reduce the time of passengers entering and exiting, and improve the efficiency of the station. Missing those tunnels - it's a bit like battleships."
What lies beneath
Crossrail has been understandably scrupulous in its approach; it has even used magnetometer testing to vet for unexploded ordnance dating back to the Second World War; it has yet to uncover any threats.
Instead, a more palpable challenge has presented itself - that of trying to avoid the capital's water mains network, which has seen Dulake and his colleagues collaborate extensively with Thames Water in doing some "very good detective work".
"We have worked very closely with them to ensure we protect their assets, some of which are very old," says Dulake. "I'd hate to tempt fate, but so far, we've yet to encounter any of them, and the biggest challenges are now behind rather than in front of us.
"While we don't have X-ray vision, we are a lot more confident now. We've also done a lot of enabling work in advance with Thames Water on sewers, and have strengthened and relined operations, which had allowed us to tunnel under these sewers."
Fortunately, most of London's natural geology lends itself to tunnelling and excavation; as well as being devoid of cross-passages, Crossrail hasn't had to contend with bedrock. Instead, the city's geologic underbelly mainly consists of easily-penetrable London Clay - a stiff, bluish marine deposit - and sand and gravels.
Nonetheless, Dulake admits that the Lambeth Group rock strata, which is found at lower levels, and comprises consolidated clays and lenses of fine sand containing water, has proven to be problematic at times.
"We can tunnel through most things, but one thing we don't really like is water, as found with Lambeth Group materials," he explains. "So, for us, the task is ensuring that we manage that water, both by dewatering from the surface, and within the structure. We take a very cautious approach, and it can be challenging, but on the whole, London is blessed by having some good tunnelling conditions."
The danger of ground subsidence - commonly known as settlement - is also at the forefront of engineers' minds - and is of particular importance in such a densely populated urban metropolis as London. Crossrail has managed to offset any potential risks through the use of compensation grouting, in which cement is injected into the ground to firm up the area likely to be affected by settlement.
"Through using compensation grouting, we've had very few green and amber triggers on any of the buildings we've passed under," says Dulake. "I would say that the best way of defending against settlement is to put a lot of early effort into specifying the technique."
Engineering projects of the scale of Crossrail also need to ensure that safety measures are rigorous and watertight. What protocol is being followed?
"It's an area of several facets," explains Dulake. "Every aspect of construction is itemised, so we can make sure that the teams are prepped, comfortable and ready to go. It's a very honest process, and I think everyone appreciates it.
"Needless to say, workers on site also have to have certain safety accreditation, whether it's a CCSC [construction skills certification scheme], tunnel safety card or higher accreditations."
Sustainable construction and human capital
As with all modern design initiatives, keeping green is an overriding touchstone. One of Crossrail's most notable efforts in this area has been its employment of Wallasea Island, a man-made jetty on the nearby Essex coast where refuse materials are transported and deposited. Presently, 85% of spoils are being transferred from construction sites by either rail or water.
"We're very pleased with the figures, in terms of protecting London during construction," says Dulake. "Aside from transportation and water, we also have materials being taken by lorry from some of our central London sites."
Further efforts are also being made in driving sustainability, energy efficiency and long-term performance.
"We've spent a lot of time looking at carbon footprint, and how we can actively reduce our energy consumption," says Dulake. "For example, we've designed with flood levels and temperature rise in mind to accommodate climate change. We've thought about how the facilities and stations need to be ventilated and temperature-controlled, and the overall impact on the systems and the railway."
For all the cutting edge technology at its disposal, Crossrail has also been successful in its recruitment drive and investments in human capital. In September 2011, on the back of committing roughly £13 million, it opened its Tunnelling and Underground Construction Academy. Located in Ilford, east London, the centre offers various practical courses to engineers on the project.
"Civil engineers need to be trained properly, particularly when it comes to a complicated technique like SCL," says Dulake. "Our facility can help in this regard; apprentices have the opportunity to practise using underground construction equipment in a real-life environment [there is full-sized tunnel mock-up on the premises]. It's a really unique facility."
And the scheme is set to grow; Crossrail is hoping to have 400 apprentices signed up within the next couple of years (it presently has 220). The aim, according to Dulake, is to both expand and diversify the scale of its labour resources.
"There's a human side to this project," he explains. "For all the guys that have been in tunnelling for years [many engineers previously worked on the Channel tunnel], we have a lot of new, young engineers entering the fold. I think one of the things the civil engineering industry realises - and this doesn't necessarily relate just to tunnelling - is that training tomorrow's leaders and supervisors is vital."
Our discussion draws to a close. Another cable car can be seen beginning its journey; while many feet underground, across and within the substrata of the London Basin, an expedition of a considerably larger and far-reaching scale, continues to make significant headway.