the ordeal of owners of cracked houses

It all started with a front door that suddenly became strangely difficult to open. It’s the end of summer 2019. It has been particularly dry, even in the Nord department, in Wannehain, where Thierry Paris lives. In addition to a capricious door, this retiree from the banking sector discovers cracks on his facade. A few millimeters at most. Three summers later, they are four centimeters wide.

“The state of natural disaster was recognized in 2020relates Thierry Paris, who had his house built forty years ago. Today, I am still at the stage of discussions with my insurance to take charge of the work ».

10.4 million homes exposed

These cracks are linked to the phenomenon known as “clay shrinkage-swelling”: clay soils contract during periods of drought and swell when they are hydrated. The phenomenon itself is not new: “We saw the first effects of droughts on homes in the 1980sexplains David Moncoulon, director of research and development for the public reinsurer CCR. The phenomenon was integrated into the natural disaster system in 1989”.

This particularly affects individual houses, because of their lighter structure than buildings. In 2021, a study by the General Commission for Sustainable Development estimated the number of individual houses built in moderately or highly exposed areas at 10.4 million. It’s half the park. It is difficult to know how many houses in France have already been affected, but each year since 2015, 2,000 French municipalities have been affected by the phenomenon.

Sequence of droughts

“During a normal year, a house can absorb small movements linked to the shrinkage-swelling of clays, explains Sébastien Gourdier, engineer at the Bureau of Geological and Mining Research (BRGM). But an episode of intense drought or a succession of droughts can create cracks. The extent of the damage will depend on many factors, including the design of the house”.

The sequence of particularly dry summers since 2015 has been accompanied by an intensification of the phenomenon. According to a report by the Court of Auditors published in 2021, the number of requests for recognition of natural disaster – “catnat” in insurance jargon – was four times higher over the period 2015-2020 than over the period 2010 -2015. Concentrated in the past in Occitania, New Aquitaine and Centre-Val de Loire, the shrinkage-swelling extends to areas that were previously relatively spared, such as the North or the Loiret.

Tedious procedures

For those affected, recognition of the state of natural disaster is like a long obstacle course. You must first turn to your municipality, so that it asks for recognition by the State. Then, the victims can turn to their insurer.

But this recognition is not automatic. The natural risks mission estimates that 40% of the rate of classification without follow-up of file for the shrinkage-swelling of clays. “The role of the insurance adjuster is first to establish whether there is a causal link between the drought at the origin of the natural disaster decree and the damage observed, explains Franck Le Vallois, Managing Director of France Insurers. However, the disaster can also be linked to the design of the building, its maintenance or its obsolescence”.

The phenomenon is complex

Only half of the municipalities that request recognition of the state of natural disaster obtain it. In its report on the subject, the Court of Auditors judges that the coverage by the “catnat” scheme proves to be “unsuitable” shrinkage-swelling of clays. To be recognized, the drought episode must be of intensity “abnormal”i.e. the largest or the second largest in the last 50 years.

« However, the disasters (…) result from landslides that undergo a succession of dehydration and swelling over several months, even several years, continues the Court of Auditors. The appearance of the damage cannot easily be linked to a precise date in particular. » The magistrates also criticized the lack of transparency of the procedures. The drought factor is assessed by numerical modeling, by Météo France. « The criteria are complex and are not based on taking measurements on the ground, which leads to misunderstanding by unrecognized victims even though damage is observed”.

Adaptation

The question becomes all the more pressing for the public authorities as global warming will cause an intensification and multiplication of drought phenomena. Owners who wish to build a new house on clay soil have been required since 2018 to carry out a geotechnical study and adapt the work to strengthen the foundations of the house.

But what to do with the 10.4 million homes already at risk? “There is an urgent need to adapt the existing buildings built on clay soilsinsists Lamine Ighil Ameur, soil mechanics researcher at the Public Center for Studies and Expertise on Risks, the Environment, Mobility and Development (Cerema). The first adaptation measure is to carry out a diagnosis of your land to understand if the vegetation or water management around the construction can accentuate the shrinkage-swelling phenomenon, and make the necessary adjustments..

Technologies – such as foundation recovery – exist in parallel to reinforce the building. But the bill can go up to €76,000, which makes it irrelevant for preventive use. On the other hand, the public reinsurer CCR is increasingly interested in so-called “soft” prevention methods: ” Of the geomembranes installed around the house help limit hydration and dehydrationexplains David Moncoulon, of CCR. It is also possible to install runoff water drainage technologies, but this is not widespread on a national scale. »

A cost of one billion euros per year

Cerema is working to develop other adaptation methods and test their reliability in the event of repeated droughts. An experimental house project carried out between 2016 and 2020 has made it possible to develop a technique for rewetting the soil, which stabilizes existing cracks and prevents the appearance of new damage. But the device still needs to be tested on a large scale. “The objective is to complete the experiments in 2025 and to lower the cost, currently €15,000”, explains Lamine Ighil Ameur.

Beyond the risks for the victims and the insurers, the phenomenon goes so far as to raise the question of the sustainability of the compensation system for this natural disaster. The phenomenon is already « one of the costliest impacts of global warming”, noted the High Council for the Climate in its last annual report. Over the period 2016-2020, it cost the public reinsurer one billion euros per year.

——

An increasingly widespread and costly phenomenon

Until 2015, the municipalities where there are cracked houses were concentrated in 70 departments. Now the number is up to 90.

The BRGM offers a websiteto identify if his property is located in an exposed area.

In 80% of cases, the cost of a claim is less than €15,000. On the other hand, for 9.4% of claims, the damage is between €50,000 and €150,000.

According to the projections of France insurersthe cumulative cost of shrinkage-swelling of clays for insurance companies should increase from 13.8 billion euros over the period 1999-2019 to 43 billion over the next thirty years.

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