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IStock / City Presse
Before settling in a given place, it is better to know where you are setting foot and, in this case, its foundations.
On the basis of this principle, the State has recently strengthened the legislation relating to the prevention of the risk of land movement of clay soils.
The goal: to reduce the number of claims, far too high at the present time.
A natural phenomenon involved
Today it is possible to build buildings almost anywhere.
But the techniques and methods vary depending on the soil structure.
Clay has, in particular, an annoying tendency to change according to variations in the water in the ground.
In times of drought, the soil settles irregularly and causes deformation on the surface.
Conversely, when the rain returns, this clay soil begins to swell.
This is called the “shrinkage-swelling” phenomenon.
Thanks to its temperate climate, France is relatively unaffected by the swelling effect, since the clay areas are often saturated with water.
On the other hand, the increase in periods of drought in recent years is undermining unsuitable constructions.
Cracks in facades, distortion of doors and windows, dislocations of pavements and partitions, rupture of buried pipes, even detachment and subsidence of terraces or porches, the damage can be colossal.
And it is essentially single-family homes that bear the brunt, since their foundations are shallower than those of collective buildings and individual owners do not have the expertise of professionals.
This phenomenon is, moreover, the first item of compensation under the structural damage insurance for this type of dwelling.
Prevent to build better
Faced with a major risk, the Élan law of 23 November 2018 took matters into hand to reduce the number of claims and limit the compensation bill in the process.
The solution ?
Require the completion of a geotechnical study for any construction in sensitive areas.
This obligation entered into force on January 1, 2020, although the necessary implementing decree did not actually appear until August 6.
With this in mind, a new map has been drawn up in order to determine which areas are most exposed to the phenomenon of clay shrinkage-swelling.
In the latter, the seller of building land must henceforth provide a preliminary geotechnical study to identify the risks of the site.
This document, which is valid for thirty years, must remain attached to the title deed for all successive transfers.
Likewise, before the start of any construction site, the client must have a so-called design geotechnical study carried out, taking into account the location and characteristics of the building.
It must then be given to the builder so that he can choose the appropriate construction methods.
Please note, this obligation is aimed at promoters of small real estate projects (buildings with no more than two dwellings), but also private owners who have their single-family house built or carry out major construction work there.
The addition of a veranda or a garage of less than 20 m2 however escapes this regulation.
How to build on clay soil?
Different construction techniques make it possible to secure a building built on clay soil, but all of them follow basic principles.
Thus, it is necessary to move away as far as possible from trees or provide anti-root screens, make a waterproof belt around the house, waterproof the buried pipes, bet on a good drainage system, provide a crawl space or a sub ground, stiffen the building structure, adapt the foundations or separate the various elements of the structure.
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Is your municipality concerned?
The clay shrinkage-swelling exposure mapping is available on the official Géorisques.gouv.fr website.
You just need to indicate your department and your municipality of residence to obtain its classification.
Only the areas undergoing a strong (in red) or medium (in orange) hazard are concerned by the geotechnical study obligation.
At the top of the list, a large part of the Occitanie, Nouvelle-Aquitaine and Paca regions are stamped with strong hazard, while Center-Val-de-Loire, Bourgogne-Franche-Comté, Ile-de-France and some Northern and Eastern departments have an exposure considered average.
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