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Around ten of us were there in person and twenty online to listen to Michel Gérard and Charles Edouard Delpierre 's talk on multi-network galleries.

The concept is well known and a priori obvious: it's better to group together the various urban networks in underground galleries, rather than burying them separately in the greatest disorder under the sidewalks or sometimes the roadway: the advantages in terms of safety, operation, maintenance, durability, avoidance of surface works, less occupation of the subsoil... are obvious. Yet, despite the historic example of the Belgrand sewer system in Paris, these galleries are rarely used in France: they are considered too expensive, complicated and even dangerous. All these criticisms are false and easily refuted, but they are deeply rooted in many people's minds. The fact remains that the main difficulty lies in financing a long-term investment that relies on those who benefit from it, i.e. the occupying operators (including the city, for its own networks, the longer life of its pavements and the elimination of nuisances) and not on the player providing the opportunity (ZAC, Sem Tramway etc.).

Michel and Charles-Edouard present maps showing the widespread development of these galleries around the world, and how France is currently lagging behind.

They then clearly set out the results of a nationwide research project known as "Clé de Sol", which dealt with the subject between 1999 and 2005; and which shows a practical way of making these structures a success in development projects, and of finding a fair distribution of funding shares to which the operators concerned can adhere.

What's more, the ecological transition, with the massive extension of the electricity grid, and the growing need for district heating and cooling networks, in particular, should multiply the opportunities for these structures...

The participants were clearly convinced by the concept and the proposed implementation approach: at the end of the session, they looked for the best ways to update, publicize and develop the concept in France: more presentations at trade fairs and forums, in urban planning training courses... To be continued...