Journey to the center of cities: The underground, a new urban frontier

Putting a stop to urban sprawl, re-densifying the core of cities: environmental, demographic, or economic constraints require a modification in the form of our cities. How can we accelerate housing construction, maintain trade, plan logistical, industrial or agricultural spaces, ensure mobility for all, develop public spaces and green spaces… on territory where non-agricultural land is scarce, and therefore expensive? Verticality, the daughter of electricity, was one of the 20th century’s responses to cities undergoing strong demographic growth. The skylines of large metropolises attest to this, while constructing eternally higher buildings remains a significant trend, notably in large Asian metropolises.

The history of the vertical city does not end with this implicit challenge issued by cities: “the sky is the limit”; Several cities are now turning to their underground. Granted, depths are no terra incognita: our modern cities have stocked the majority of their ancillary functions underground – water and energy networks, waste disposal. They have buried what they deemed unworthy of being seen. In most cities, underground space was never considered as an interesting asset to learn about or a new frontier to conquer, but rather as a commodity. This explains why underground occupation is so rarely coordinated and often deprived of any strategic vision.

However, several pioneer cities, historically conscious of the potential of underground space, are innovating and paving the way for a more virtuous, generalized underground urbanism. Tokyo authorizes the launch of underground construction before the end of expropriation measures. Helsinki provides a maximum authorized depth of construction for owners and, just like Tokyo, follows an underground master development plan defining zones to be preserved, priority underground urbanization zones, and zones destined for long-term development. Singapore has granted itself ownership of all space below 30 meters underground, and allows for the purchase of specific strata of underground land. Montreal has implemented several adjustments and incentives to underground development (such as exemptions for surfaces built below the ground) and pays attention to the cultural and artistic animation of the underground. Toronto has developed an underground pedestrian network, PATH, to go hand in hand with the densification of the business district and to respond to the saturation of outdoor sidewalks, while offering small businesses chased away by office towers with an alternative.

This new urban frontier is not easily conquered. Architect Dominique Perrault, who has long been convinced of the necessity of building the city under the city, reiterates this: “walls can be torn down, but the ground must be fought”. And the first battle to wage is not technological, legal or even economic: it is psychological. Who could imagine living underground? Memories of Fritz Lang’s Metropolis are close by, with its underground reserved to the working masses and its elite relishing in the heights…

Still, the underground city represents a considerable wealth of opportunities for metropolises. The availability of underground spaces, the resources it contains (specifically, materials and water), its excellent thermal properties, the structural solidity and safety it can bring in the face of certain natural phenomena and technological risks, are major assets for cities’ sustainable development. Further, construction technologies allow us to dig into almost any soil in a limited amount of time. But these technologies will remain the prerogative of engineers if we do not first build a new approach to our underground urban space.

It is therefore about adopting a new vision of the underground, which can no longer be seen as a simple repository for networks designed in silos. It is high time to reflect on interrelations between the surface and the underground, both in terms of risks and synergies. Underground planning must become systemic and must be connected to the planning of space above ground. This 3D vertical city can only be made possible by dialogue and collaboration between the various planning stakeholders, based on a shared knowledge of this space made possible by new technologies. Thus, though we may never amble in underground forests in search for the center of the Earth as Jules Verne’s characters once did, we can at least give our cities one more chance to address the formidable challenge of human-scale densification.

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Cécile Maisonneuve, president of La Fabrique de la Cité - Introduction

Thinking user-centric cities

La Fabrique de la Cité

La Fabrique de la Cité is a think tank dedicated to urban foresight, created by the VINCI group, its sponsor, in 2010. La Fabrique de la Cité acts as a forum where urban stakeholders, whether French or international, collaborate to bring forth new ways of building and rebuilding cities.

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