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05 Dec 2008
For those working on projects that are beneath our feet, life is never boring. There are plenty of challenges involved in the fast-paced and frequently high-pressure world of tunnelling.
Tunnelling is a complicated business and the stakes can be high. Tunnel construction works are, by their nature, exposed to relatively high risks. At the same time, the engineering involved in ensuring underground space is properly usable and safe is very complex.
Nevertheless, organisations all over the world place increasing value in developing underground space:
For example, the Crossrail project under way in the UK will build major new railway connections under central London within the next 10 years, and tunnelling is at the heart of the initiative.
“Over the past two decades, some of the most complex and challenging construction projects in the UK and abroad have involved tunnelling. The Channel Tunnel and Jubilee Line Extension, to name but two, relied on tunnelling expertise,” says Douglas Oakervee, executive chairman of Cross London Rail Links (CLRL). The company was established in 2001 to promote and develop efficient, direct rail links across London.
“All projects, and Crossrail is no different, are judged on their contribution to society. By linking east and west London through 22km of twin bore tunnels, for example, Crossrail will play an enormous part in encouraging commuters and leisure travellers on to the railway and reduce road traffic.
“At the same time, Crossrail aims to leave a legacy of a trained workforce so that once construction of what is Europe’s largest civil engineering project is complete, the skills and expert knowledge will be available for the next generation of major projects.”
As the Crossrail example illustrates, tunnel solutions offer significant benefits with respect to infrastructure sustainability, lifecycle costs and environmental impact. At the same time, disruption to the public and property owners is limited, with most tunnel projects being conducted largely unseen and unnoticed.
According to Paul Groves, Tunnelling Chair for Atkins, “For our clients, the ability to deliver tunnel solutions is integral to being able to solve their wider problems. For us, being good at tunnelling has given us access to a wide range of large, prestigious projects, most notably in the transportation and water sectors. In fact, we are working on many such tunnel projects around the world – including 13km of tunnels for the Dubai Metro Red and Green Lines, and Hong Kong’s West Island Line project with station caverns that span 25 metres.”
Groves heads up a network of technical tunnelling experts that runs across the entire Atkins operation.
Indeed, from the inner-city mined station complexes of London’s Crossrail project and the tunnels of Bahrain’s chilled water distribution network, to the twin 16km tunnels of Hong Kong’s Express Rail Link into mainland China, tunnel engineers from Atkins are very busy people. They form one of the world’s largest resources of tunnelling professionals centred in the UK, the Middle East and China.
One current Atkins project is the Belfast Sewers Project, a major stormwater management project launched by Northern Ireland Water in 2003. The scheme will transform water quality in the River Lagan and its tributaries, and help alleviate flooding in Belfast. Atkins was brought in to undertake the project management and planning, including the preliminary design associated with the tunnel system.
The Belfast Sewers Project is one of the largest civil engineering infrastructure projects in Ireland. It includes more than 9.4km of new tunnels ranging from 1.5 to 4m in diameter, and will cost around £140m to deliver. By the time the project is complete, half a million tonnes of material will have been excavated from the tunnels and access shafts.
Belfast’s 19th-century brick sewers are a contrast to the modern sewers found elsewhere in the city. But there is plenty of useful life left in them, which is why the Belfast Sewers Project is also seeking to renovate rather than replace these veteran systems.
“Overall, the quality of the original sewers is excellent,” observes Dr Alan Skates, project manager with Atkins on the Belfast scheme. “But what has happened is that, with increased vehicle loading, many of these sewer systems have suffered from stress. They were never designed to carry the sorts of heavy traffic they now have to endure.”
The new tunnels will help to alleviate some of that burden. However, building them presents a number of challenges and the complex geology of Northern Ireland is one of them. “Belfast is built on a mixed bag of materials that were deposited when the glaciers retreated,” explains Skates. “We have everything from rock to boulder clays, sand, gravel – and overlying the whole lot is a material known as ‘Sleech’, a soft, estuarine clay.”
This kind of mixed ground can be a problem, because the machines work best when cutting through uniform material. When this changes from relatively soft clay to much harder sandstone, for example, work has to stop so that the picks and tunnel cutters can be changed.
“We kept the depth of the tunnel to 30 metres,” says Skates. “The aim was to end up in reasonable tunnelling strata as much as possible. Mostly, we’re in either boulder clay or sandstone, but occasionally we intercept areas that are mixed – these present the biggest challenges.”
No stone is left unturned in the quest to avert problems. In the case of Belfast’s stormwater system, that meant spending a year planning the project carrying out site investigations, drilling boreholes, selecting sites for access shafts and choosing the best route.
It’s a painstaking process. But even with modern ground investigation techniques, there’s always scope for the unexpected. “A lot of investigative work will have been carried out, so you’ll have a pretty good idea of what you’re facing, but there are still challenges because of the inherent variability of the ground conditions,” adds Skates.
Tunnelling remains one of the toughest jobs in construction. But, despite the heat and the noise of the machinery – which carries away a tonne of material every seven seconds – it is now safer and faster. This is thanks in part to sophisticated tunnel boring machines, as well as the much improved and highly developed approach to risk management within the industry.
“A huge amount of nursing goes on with the machines, coaxing them through Belfast’s different strata,” explains Skates. “But if everything is going well, they can romp along quite happily at 40 metres a day. That’s above the average that the contractors had originally anticipated.”
Schemes like the Belfast Sewers Project deliver significant social, economic and environmental benefits. But because the work is rarely seen by the public, it doesn’t attract the attention it deserves. Railway schemes, on the other hand, tend to enjoy a rather higher public profile.
The Gautrain Rapid Rail Link project in South Africa is one such scheme. Currently under construction, it will provide a high-speed link between the capital, Pretoria, and Johannesburg. It will be a totally new transport option for people moving between these two major commercial hubs. The 80km railway also promises to cut congestion and offer an environmentally friendly alternative to road transport in time for the 2010 World Cup football tournament.
Atkins is playing a key role in delivery of the project, undertaking detailed design for the contractor, Bombela Civil Joint Venture, including the construction of three under-ground stations. Atkins’ locally-based design team draws on expertise from the UK, South Africa, Hong Kong, India and Scandinavia.
The landscape of the Veldt, through which the project passes, includes hard-rock granite and volcanic lava, but weathering – the chemical and mechanical erosion of the rock – can still reduce it to a soft soil over time, with various stages in between. The ground at Park station, Gautrain’s city-centre terminus in Johannesburg, lies somewhere in between. “At Park, where the excavation depth is in the order of 25 metres, the ground conditions are essentially soil-like,” explains Dr David French, Chair of Atkins’ Geotechnical Network, another source of technical expertise to which clients have access. “There, we can use a construction technique called diaphragm walling.”
This involves marking out the station box – in this case, about 200 metres in length and up to 20 metres wide – then using specialised equipment to cut deep, narrow slots down to solid rock. The slots are filled with concrete to create a wall in situ. The soil between the walls is then removed to create a huge void in which the station is constructed. “It’s basically cut-and-cover construction,” says French. Further up the line, ground conditions are quite different. “At Sandton, the upper part of the profile is soil, but the lower is solid rock,” says French. “At platform level, where the trains come in, it’s an underground cavern, dug into rock.”
Creating caverns, shafts and tunnels is a specialised business – rock presents a challenge because it’s often far from stable. There’s a risk that wedges of rock will slide out of position with potentially disastrous consequences.
“It’s an area of rock mechanics called kinematic feasibility,” explains French. “Could one of these massive wedges of rock be released when we excavate down? If it could, we have to put in rock bolts and tie the whole thing back so that doesn’t happen, because these wedges can weigh as much as a house.”
Despite the sophisticated modelling software available to tunnel engineers, experience still counts for a lot.
“We sometimes talk about geotechnical engineering as a fusion of art and science,” says French. “Often we have a relatively small amount of borehole information, so the skill is to take that data and create a model that can be applied safely and reliably to the project as a whole.”
There is no question that tunnelling has changed, but there remain challenges to be overcome. In many cases, it’s a question of perception versus reality.
“Even within the last 10 years, improved technology and risk management processes have allowed us to extend the feasibility of tunnel options into increasingly more difficult ground conditions and to handle more onerous constraints,” explains Atkins’ Groves.
“Despite such improvements, the world’s insurers still consider tunnelling as risky as deep-sea construction, a product of inherent uncertainties with respect to the ground and the sheer cost of recovery works when things go wrong. Atkins tries very hard to help clients, be they financiers, project owners or contractors, mitigate risk and to understand the degree of risk that must be managed.”
These views must also be balanced against the public perception of tunnels and tunnelling. Despite millions of daily tunnel users, particularly urban metro tunnels, some people feel distinctly uncomfortable about being underground.
“To compensate, we try and make tunnels not feel like confined spaces. A recent and very welcome trend has been to provide pleasant, airy spaces for passengers, with great emphasis on fire and life safety design,” says Groves.
The same kind of fresh thinking is needed to attract new talent to this field. With a worldwide boom in urban rail construction and growing pressure on the infrastructure of cities, demand for tunnelling skills has never been greater. Yet engineering numbers are down, leaving the industry working hard to find new talent.
“Crossrail is committed to working with government and industry to address the current shortfall in engineering skills in the UK,” says Crossrail’s Oakervee. “A key part of our approach is ensuring that jobs are made available to Londoners – and that Londoners have the skills required. Crossrail’s tunnelling works will be at the heart of the project. But Crossrail is not the only proposed tunnelling project in the region so we are working with the various authorities, other employer organisations, and the industry, to establish a Tunnelling Academy offering training and recognised qualifications.”
Martin Knights, president of the International Tunnelling Association (ITA), agrees that industry and academia must attract the next generation of engineers.
“We’re all chasing the same resource pool,” says Knights. “We need to rethink university courses, get more graduates into the business and think of a way to engage them much earlier.”
It isn’t only new engineering expertise that is needed. Craft skills are also in short supply – and time is running out to save some of them.
“Some of the traditional artisan skills need to be recorded and preserved,” Knights stresses. “Everybody thinks about the high-tech side of tunnelling, but there are lots of junctions and interfaces where traditional skills such as close-timbering are needed – and those skills are disappearing.”
Raising the profile of tunnelling remains a challenge. “There are still some negative, old-fashioned images about it,” says Knights. “But when you inform the public about what you do and they see the gigantic machinery, they’re agog that the underground world has such a level of sophistication about it.”
Groves adds that “organisations like the ITA and Atkins need to get the right message out when it comes to attracting new talent to this field. Tunnelling is an important part of infrastructure around the world. It will continue to play a vital role, whether keeping our urban landscape functioning under a growing population or contributing to a low carbon economy by helping mass transit systems get from A to B.
“It also offers a brilliant career opportunity. Whether creating railway stations under Central London that relieve congestion or tunnelling to drain polluted flood waters in Bangkok, we’re making a positive contribution to society, and that feels good.”
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