Report – In Finland and France, underground construction is reinventing the transformation of urban centres

In order to limit urban sprawl, underground construction makes it possible to de-densify the city on the surface, while multiplying the services available to inhabitants. This is a transformation model that Helsinki has particularly developed.

From 14 to 16 November 2022, the urban foresight think tank La Fabrique de la Cité and Leonard, the VINCI Group’s innovation platform, organised a learning expedition to Helsinki to draw inspiration from the practices in force in the Finnish capital. Underground construction was the subject of a round table discussion that brought together the architect and urban planner François Decoster of Agence l’AUC, Jeremy Hammond, co-director of HyperTunnel, a start-up that develops innovative methods for digging tunnels, and Jarmo Roinisto, development director of Rockplan, an underground infrastructure design office.

Helsinki has been a pioneer in the development of underground spaces. While in most large cities, underground spaces are mainly used for technical purposes – sewers, waste treatment, transport services, electricity production – the Finnish capital decided several decades ago to accommodate a large number of infrastructures under its surface.

To date, 10 million square metres of underground space have been developed, which is 10% of the surface area of Paris. Initiated during the Second World War, continued during the Cold War, and developed from the 1980s onwards, this vast underground network includes 400 different sites: a church, an art museum, an Olympic swimming pool, go-kart tracks, sports halls, shopping centres, car parks, as well as several hundred kilometres of tunnels dedicated to pedestrian traffic. A city under the city. A veritable urban mille-feuille carved out of the rock.

What is the strategy for exploiting Helsinki’s underground? Is it a model that can be easily reproduced? What are the advantages?

Planning a second city

In order to achieve the deep doubling of the city, Helsinki has made the underground an integral part of its development strategy. Since 2010, the underground area has had a master plan for development, and any changes are supported by a strong political will.

“For each new project, we first discuss it with all the stakeholders. Then, when the city council makes a decision, it becomes a legal contract. It is like a law,” says Jarmo Roinisto.

This master plan organises current and future construction. It is updated each time the subsoil is used, which gives a precise vision of the spaces that remain free underground. This also makes it possible to orchestrate uses and to manage the underground as a stock.

In Helsinki, this new approach to underground development has made it possible to improve traffic flow on the surface, free up space for new services and make city life more pleasant. In addition, it has led to changes in cadastral and land regulations. A property is now defined both by its location on the ground and by its underground potential. This is a real paradigm shift.

Other examples exist in France and reveal the interest of underground development. Still very limited to technical spaces and urban backdrops (transport, energy, waste), the underground has other assets. In Lyon, the ongoing transformation of the Part-Dieu station is indicative of this new way of thinking about the city. Underneath the historic square, a particularly congested interchange between public transport users and motorists, the architecture firm AUC, co-founded by François Decoster, has installed a second square underground, which accommodates logistics functions and numerous parking spaces.

“The fact that we worked underground made it possible to enlarge the space given to pedestrians, bicycles and public transport on the surface. The space in front of the station will be freed from car traffic,” says François Decoster.

According to the municipality, the other advantages of this project are that it will “de-densify the real estate programme and produce calming public spaces with lots of vegetation, open to all”.

Although building under the surface remains costly and complicated from a technical point of view, this type of development is now facilitated by new methods of digging tunnels that use new technologies to gain in efficiency, precision and speed. hyperTunnel, a startup incubated by Leonard’s CATALYST programme that combines artificial intelligence, 3D printing and swarm robotics is emblematic of this trend.

At a time of extreme densification of urban centres, these projects are representative of the current mutations in urban planning, which aim to bring about a new model of metropolitan manufacturing, more humane, more sustainable and more resilient in order to face systemic crises and global warming.

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