Our cities are becoming increasingly overcrowded and congested. With land prices rising rapidly, can greater use of underground space enable us to meet the increasing demand for more buildings and more extensive transport systems? Fiona Chow explores some of the possibilities.
Engineering in our cities
We seem to be hemmed in from all sides: the last 50 years have seen unprecedented demands on space. Major world cities such as London are experiencing population growth combined with the trend towards smaller households, placing greater pressure on housing and land prices, while the demand for greater mobility and the distribution of consumer goods has led to new requirements for transport infrastructure. How can we meet these demands? Expansion into green-belt areas is fiercely resisted; construction in flood-plain areas such as the Thames estuary has risks connected with climate change and flood potential; skyscrapers block out natural light, create windy cities and present targets for terrorist attack. And yet, the thirst for city-living remains undiminished. It is predicted that, by 2006, half the world’s population will live in cities. Mega-cities, similar to Tokyo where the population exceeds 26 million, will dominate Africa and Asia, with China planning 40 new cities to house 30 million people.
Where does this leave today’s engineer? We can and are working harder, striving to meet the demands of society, in providing flood protection, advanced weather warning systems, and in designing skyscrapers that will withstand the unthinkable. Under pressure to solve technical challenges, we sometimes forget that it is also our duty to take the lead in informing society and providing a vision of the alternatives that can be achieved with today’s technology. One such alternative is in the improved utilisation of underground space.
The final frontier?
The demands on urban space have not gone unnoticed. In their report ‘Towards an Urban Renaissance’ the UK’s Urban Task Force, led by Lord Rogers, examined ways in which towns and cities can be revitalised and regenerated. Key recommendations included the development of better land-utilisation plans and systems to encourage the regeneration of ‘brownfield’ sites.
While the Government has set a target for 60% of all new developments to be on brownfield sites, it is much harder to set targets for land utilisation. One direction that is often overlooked is underground space and the huge resource that lies beneath our feet. Architects and structural engineers often shy away from underground development since the very fact that the structure is buried underground means that it is hidden from view, leaving no monumental visual impact for the passer-by. Yet, who can fail to be impressed by the monumental caverns created by the Jubilee Line Extension stations at Westminster and Canary Wharf?
Underground space development is regarded as being a high-risk area, yet the enormous advances that have taken place over the last ten years have increased our understanding and capabilities immeasurably. For example, tunnelling and excavation for the Jubilee Line Extension was complemented with careful protection measures such as compensation grouting. Structures such as Big Ben were undamaged, despite their sensitivity and close proximity to the underground works (Figure 1). Advances in numerical analysis allow the improved prediction of ground movements, stress changes and groundwater flows resulting from underground construction and tunnelling. Risks and costs can be minimised through the early involvement of a geotechnical advisor, proper site investigation, value engineering during the design process, well managed construction and through appropriate procurement contracts.
Utilising underground space remains one of the great challenges for the future and it is the responsibility of the engineer to take this message to the town planners, architects, structural engineers, surveyors and to the general public.
Underground space utilisation is not a new concept to the UK. The British Library in London has a 23 m deep basement storing nearly 12 million volumes, there is a new underground lecture hall beneath the Reading Room in the British Museum, we have underground theatres, restaurants, swimming pools as well as underground train and metro systems and where do we store our precious wine collections but in underground cellars.
Yet, in other ways we have moved away from using underground space. Almost all Victorian and Edwardian city homes were built with underground coal cellars or sub-basement living areas. Nowadays, relatively few houses are built with basements, although these represent ideal utility areas or car parking spaces.
There are numerous benefits to utilising underground space which can be summarised as follows:
efficient land use and improvement of the environment – realising the potential of underground space in congested urban areas and releasing surface space for other uses such as parks and recreational areas
aesthetic – removing unattractive structures such as car parks, roads and shopping malls from the horizon.
sustainable development – removing the need for external cladding and finishes, leading to efficient use of materials and cost savings
conservation of energy – using the ground’s natural insulating properties to absorb noise and energy, allowing more efficient heating or cooling systems, or harnessing the ground for energy storage.
Ground works are often perceived as one of the high-risk areas of development, and underground construction is not cheap. However, a study by the Automobile Association (2001) showed that through new technologies the cost of tunnel construction has been falling by around 4% each year. The cost of urban tunnels in the UK starts at around £50 million per kilometre and can be cheaper than surface building, where acquiring land and moving buried utilities is expensive. Imagine, for example, the land acquisition costs that would have been associated with building the Jubilee Line Extension above ground. The initial costs associated with underground construction may be relatively high, but can be offset in the long term through benefits in the surface environment and rising land values. In other countries tunnelling can be cheaper, particularly where there are good rock conditions and previous tunnelling experience. Table 1 compares the construction costs for the Helsinki metro, showing that the deepest option, the rock tunnel, is relatively cheap due to the refinement of good construction techniques and the favourable geotechnical conditions for tunnelling.
Savings can also be made in other areas such as a reduction in need for external building cladding. Cladding and finishes typically account for about 15% of a building’s cost and the market is volatile and often over-heated, with demand outstripping the supply capacity of reputable subcontractors. In comparison, the costs and performance of underground construction are more stable and predictable.
The potential in underground development is best illustrated through some of the successful international examples that follow.
Preserving historic areas:
The Louvre, Paris, France
The new entrance hall to the Louvre is a buried structure built in the courtyard of the historic Louvre Palace. The glass pyramids designed by IM Pei are the only sign above ground of the hall and new galleries, and provide an eye-catching entrance point and natural light to the space below. Locating the entrance hall below ground removed a large potential intrusion from the courtyard and allowed the designers freedom in choosing the best shape and layout for the hall (Figure 2).
Improving mobility, the environment and economic growth:
The ‘Big Dig’, Boston, USA
Boston’s ‘Big Dig’ is the most ambitious tunnelling project in the world, with a cost running at US$12.2 billion. Currently, six lanes of elevated highway run through the city centre, carrying 190 000 vehicles a day. Congestion costs US$500 million a year and accidents are four times the average for an urban interstate highway. The new tunnels will free up traffic, create more than 150 acres of new parks and open up new spaces. Carbon monoxide levels are forecast to drop by 12% across the city.
Innovative road tunnels:
Versailles, Paris, France
The final section of the A86 ring road around Paris is being completed near Versailles. Costs are radically reduced by building the tunnel for light vehicles only. Reduced ceiling heights (2.44 m) enable two road decks, each with two lanes and a hard shoulder, to fit within a single 10.4 m diameter tunnel (Figure 4). The project, is costing nearly c1.6 billion and is being privately funded with toll collection in place when operational. Tunnel boring machines are being used to excavate the soil and, with 90% of the construction underground, noise and vibration disturbance at the surface are imperceptible.
Underground industrial plants, protecting the environment:
Helsinki’s Viikinmaki waste water treatment plant is one of the largest underground spaces in the world, processing all of Helsinki’s waste water (Figure 3). Locating the treatment plant below ground and providing housing and leafy park areas overhead, was considered the only way to obtain planning approval for such a large industrial plant. Burying this structure also reduced gas emissions and noise pollution and allowed the owners to expand the plant with minimum disruption to the community. The housing estate covers an area of 60 hectares, providing accommodation for 3500 people.
Underground cities, protection from the climate:
Montreal has the world’s largest underground city containing 31 km of passageways, 10 metro stations, a railway station, bus terminal, more than 1600 shops, 200 restaurants, 40 banks and 30 cinemas, as well as hotels, offices, swimming pools and theatres. Begun in the 1960s, apparently based on an idea of Leonardo de Vinci’s, the ‘city under the city’ grew as developers realised the importance of linking into the underground network and the metro system. The subterranean world protects citizens from the snow, rain, wind and heat, providing a climate that is ‘eternally spring’ and an environment that is free from traffic and road-related accidents.
Storm-water storage to prevent flooding:
The Snake tunnel, Stockholm, Sweden
‘The Snake’ tunnel beneath centre of Stockholm is 3.7 km in length with a total volume of 35 000 m3. Its purpose is to store storm waters at times of heavy rainfall, preventing the attenuation of water run-off and flooding.
Planning and management for sustainable development
Many European cities suffer from an historic uncoordinated proliferation in underground development, ranging from utility and infrastructure tunnels to building foundations. Everything that is built in the ground puts constraints on future development and can potentially reduce the value of a site in future years as the cost of redevelopment becomes disproportionately high. Therefore, underground space needs to be managed and controlled, and construction records stored safely in order to allow future development.
Helsinki has provided for this with an underground space allocation plan that has enabled the development of over 7 million cubic metres of underground space. A single set of deep multipurpose utility tunnels accommodates electricity, water, heating and communication cables with a roadway for vehicular access, inspection and repair without the need for digging up roads (Figure 5). Information from 200 000 boreholes and 4000 groundwater monitoring points is maintained in a GIS (geographical information system) database with details of existing and planned foundations and tunnels. The information is maintained by the Helsinki City Authority and can be accessed at nominal cost by members of the public.
As city sites go through several generations of development and redevelopment, the foundations that they leave behind will progressively reduce the space available for new foundation construction. The cost of removing pile foundations in London is between two and five times the cost of installing a new pile and is therefore to be avoided wherever possible. The reuse of existing foundations is a trend that is becoming increasingly common in old cities such as London and Dublin, but this relies on good construction records and foundation durability. Steps need to be taken to properly document and store as-built construction information to safeguard future redevelopment. Basements and foundations could be constructed for a longer design life than the superstructure to provide for sustainable development.
Underground thermal energy storage
There are enormous thermal energy resources on Earth, but in many cases they are available at the wrong place and time. The situation is analogous to the availability of fresh water. Efficient transport and energy storage systems are therefore essential for sustainable development.
Underground thermal energy storage (UTES) represents one of the most sustainable and environmentally friendly approaches, with great future potential. Storage systems are required for the utilisation of solar energy and other renewable energy sources. In this context, energy storage is important to cut the peaks of production and distribution. Thermal energy storage saves power, reduces the size of distribution units and hence lowers the cost and environmental impact of energy systems.
Underground energy storage technologies make use of aquifers, rock caverns and ducts in clays and rocks.
The development of UTES has been successful in Europe since the beginning of the 1990s, and in the Netherlands there are now some 200 aquifer storage projects in operation, mainly for cooling. In Sweden there are some 100 different projects for heating and heating/cooling. One of the first uses in the UK of pile foundations for energy storage has recently been undertaken by Keble College Oxford, involving the circulation of saline solution through pipes embedded in the piles and through the building’s floor and walls. The ground temperature remains at about 13°C throughout the year, and through the use of a heat exchanger, can be used as a source of warmth in winter and for air conditioning in summer, reducing potential heating costs by up to two-thirds over the design life of the building (Figure 6).
Provision of geotechnical information
As demand grows for more streamlined planning processes, reduced construction costs and sustainable development, we should be turning towards multi-disciplinary approaches and improved communication between engineers, planners, developers and the construction team.
There have been major advances in the presentation of geotechnical information for planners in thematic maps and non-technical reports. Computer-based geographical information systems (GIS) allow electronic databases and digital maps to be overlaid, providing a series of site plans that can be examined easily and quickly. Internet-based planning portals are bringing these systems to the wider community. As with all database systems, GIS systems need to be well designed for the end-user and continually updated by well trained staff. The early integration of regional and national databases will reduce the need for harmonisation in the future. The responsibility for these activities lies with national governments and regional or local planning authorities.
There are amazing opportunities for underground space development in the UK. We, as engineers, need to reach out and pass this message to the government, planning authorities and developers. More co-ordinated urban planning guidelines are needed that take into account more comprehensively geological, geotechnical and geo-environmental constraints. The benefits of such action can be observed in European cities such as Helsinki, and would include:
better space utilisation in urban areas and protection of the countryside
improvements towards sustainable development
reduced unforeseen financial risks and delays to development
protection of existing properties and improved public safety.
Development of underground space requires a multi-disciplinary approach from urban planners, developers, architects, civil engineers, geotechnical engineers and geologists. Good communication between the professions is vital. Engineers must recognise their obligation to ensure that they communicate their knowledge to spatial planners; planners must recognise their duty to include geotechnical considerations in their planning. Ultimately, greater cooperation and understanding will lead to more cost-effective, safe and sustainable land-use in our towns and cities, creating a better environment in which to live.
Paul, T., Chow, F.C. and Kjekstad, O. (eds) (2002) Hidden Aspects of Urban Planning, Surface and Underground Development, Thomas Telford Ltd, ISBN 0 7277 3101 7, prepared as part of the European COST C7 initiative with input from the Royal Town Planning Institute.
Associate Director, Geotechnical Consulting Group, London
Dr Fiona Chow is a Chartered Civil Engineer and Associate Director of the Geotechnical Consulting Group in London. Her work covers a wide variety of projects ranging from tunnels to buildings in London, Canary Wharf, Venezuela and the Far East. In 2000 she was seconded to the University of Western Australia as part of The Royal Academy of Engineering Foresight Award scheme investigating the application of an innovative method of ground improvement to offshore foundations and railway embankments.