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Building the city under the city

PILING & GROUND ENGINEERING

As urban spaces become increasingly crowded, the UK's planners should start thinking about the potential beneath their feet. Geotechnical consultant Fiona Chow talks to Emma Crates about going underground

THE PROBLEM facing geotechnical experts is that their brilliant successes are hidden from view. It is only when things go wrong that the results become apparent above ground.

'Everything you do has some interaction with the ground. The foundations are arguably the most important part of the building, ' says Fiona Chow, associate director of London-based Geotechnical Consulting Group.

As co-editor of Hidden Aspects of Urban Planning, Surface and Underground Development, Ms Chow is keen to raise the profile of geotechnical engineering to the wider world of construction. She says bringing in geotechnical experts earlier rather than later in a project can save money and reduce risks, and she would also like to enlighten the wider public to the possibilities offered by building underground.

Ms Chow has a frightening collection of photographs to illustrate what can go wrong when developers and planners do not take heed. One shows a bus disappearing into a 'swallow hole' in a road. Another depicts the infamous Field Road development in Reading where houses were built above a 19th-century mine working. In January 2000 a crater 8 m wide and 4 m deep opened up, damaging two houses and prompting the evacuation of 17 homes. A further 18 homes were affected.

Even when projects do bring in ground engineering experts, Ms Chow believes many could save substantial sums by seeking expert advice in the early design stages.

She cites a high-profile millennium funded project.

When geotechnical experts were called to the site planning permission had already been given for the structure, which included basements at varying depths.

'The ground was old clay, which can swell or heave under different weights. It would have been moving at different levels in different parts of the building. To top it all they had a very sensitive barrel vaulted glass roof.'

Changing the roof to a more robust design at this stage was not an option, so the consultants suggested a single-level basement of uniform depth across the whole site.

'These were complicated ground conditions. The cost of foundations was more than double the original estimate, ' says Ms Chow.

She argues that accurate geotechnical information, presented in a responsible way, can also play a major role in reassuring the public about the methods used on a project.

The Dublin Port Tunnel, which was started in 2000, saw its costs double to £270 million when the New Austrian Tunnelling Method was rejected in favour of tunnel boring machines. Local residents strongly objected to the NATM method, fearing a repeat of the Heathrow Tunnel Collapse in 1994.

'TBMs put up the cost phenomenally and I don't think residents' fears were really justified', says Ms Chow, adding that 'the human factor plays a very important role'.

In contrast, Ms Chow was encouraged by a project she visited in Zurich, where tunnelling was being carried out 5 metres below a block of eight-storey apartments in poor ground conditions.

'The project manager was offering residents overnight accommodation if they felt worried by the noise and vibration. But he was surprised at the reaction. The residents said they had complete confidence in the contractors and were prepared to put up with a bit of noise.'

Ms Chow believes the engineering culture that has grown up in Switzerland has had a positive effect on attitudes. As the Swiss public has to vote on whether large projects should go ahead, engineers are keen to present information as accurately as possible. As a result they are more trusted by the public.

Ms Chow is particularly eloquent when addressing the more inspirational possibilities offered by ground engineering.

'Geotechnical engineers are very good at talking to each other. We need to start spreading the message to the wider construction industry, ' she says.

The concept of the 'city under the city', once envisaged by Leonardo da Vinci, has been growing in different guises around the world. By 2006 more than half of the world's population is expected to be living in cities. As space becomes increasingly rare, the need for creativity becomes even greater.

London's population is predicted to surge by 750,000 to 8.15 million by 2016. Experts estimate that 400,000 new houses, 130 new schools and three new rail lines will be needed to meet the rise.

The city's planners could take inspiration from Canadian or Scandinavian cities where extreme winters have led to shops, offices, swimming pools, libraries and even waste treatment plants being placed below ground.

As Ms Chow points out, this has significant advantages for sustainability. It removes the need for cladding - typically 15 per cent of the cost of a building.

Even buildings above ground could benefit from underground thermal energy storage, where renewable energy sources can be stored in aquifers or ducts below ground.

To get the best out of the ground, Ms Chow urges designers and planners to take a long-term view about what they put into it.

Key recommendations of Towards an Urban Renaissance, published in 1999 by Lord Rogers' Urban Task Force, were the use of brownfield sites and marginal land. But by their very nature, brownfield sites tend to have higher risks attached and some are extremely congested.

'In London, removing a pile costs between two to five times the cost of putting it in, ' says Ms Chow.

In very crowded ground, reusing old foundations is sometimes the only sensible option. In recognition of this trend, Ms Chow believes engineers should design foundations to outlast the life of the building or be easy to deal with later.

'Whoever inherits the site in 40 years will have to deal with whatever has been put in the ground. So think how piles could be built around, removed or reused in the future.

You are protecting your land value, ' she advises.

She suggests developers and contractors should keep comprehensive records of ground foundations, the length of piles and amount of reinforcement used.

This would help those working on a site in the future.

She argues that this philosophy should be extended to whole cities and it would help reduce risks at the start of the planning process.

Helsinki City Authority has set an excellent precedent in this area. It has kept an extensive geotechnical database since 1956, with information on more than 200,000 boreholes, 4,000 ground water monitoring points and details of building foundations and tunnels. The information is available to the general public for a nominal fee.

UK cities lag behind Helsinki, although the British Geological Survey has done a survey on a limited basis. As Ms Chow says, there is much more to do: 'The potential for planning is enormous, particularly if the information is available on an internet planning portal.'

Hidden Aspects of Urban Planning, published by Thomas Telford, is produced under the panEuropean Cooperation in Science & Technology (COST) initiative, group C7, for soil structure interaction in urban civil engineering.

Cities moving underground

MONTREAL Montreal has the world's largest underground city with 31 km of passageways, 10 metro stations, a railway station and bus terminal. It also has more than 1,600 underground shops, 200 restaurants, as well as offices, hotels, swimming pools and theatres.

HELSINKI Helsinki's Viikinmaki waste water treatment plant is one of the largest underground spaces in the world, processing all of Helsinki's waste water. Burying the structure is said to reduce gas emissions and noise pollution. A 60 ha housing estate has been constructed above the plant.

PARIS Around 50,000 underground parking spaces have been added to Paris over a 10-year period - at a rate of 10 new car parks a year. Many are very close to historic buildings such as the Palais de Justice. The entrance hall to the Louvre has also been built underground to preserve the courtyard.

BOSTON Boston's $12.2 billion 'big dig' is underway to replace six lanes of elevated highway, which carry 190,000 vehicles a day. The new tunnels will create 150 acres of new parks and open up new spaces. Carbon monoxide levels are forecast to drop by 12 per cent across the city.

OXFORD Pile foundations are being used for energy storage at Keeble College in Oxford (right). A saline solution circulates through pipes embedded in the piles, floor and walls. A heat exchanger allows the system to be used for heating in the winter and air conditioning in the summer. This reduces potential heating costs by up to two thirds over the building's life.