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Eagle country. When the ground threatens to collapse, climate-proof underground cavities

The house of the Louveau family was destroyed after the discovery of an underground cavity, which could be heard even under the bedroom ©Réveil normand

Earth, pebbles and a condemned well, that’s all that remains on this land in Saint-Hilaire-sur-Risle (Orne). ” 50 years of life are gonein no time,” breathes Sandrine Louveau, staring into space. Her parents’ house stretched from the side of the road to the tree in the field. But on this graying, damp day, only the skinny trunk is still standing. The old farmhouse was destroyed after the discovery of an underground cavity on January 23, 2018.

That day, a friend of the family accidentally sees a hole in front of his parents’ garage. The mayor is then notified urgently and within a few hours, the parents must leave their home. “The situation became very complicated, because the cavity extended right under the couple’s bedroom. In agreement with the services of the prefecture, we relocated the elderly to Sapinettes in L’Aigle, ”explains Jean-Guy Grandin, the mayor of Saint-Hilaire-sur-Risle. Due to risk of collapsethe property is declared in danger and it is redeemed by the Barnier fund. This system, financed by the State and insurance companies, is used in particular to pay for the expropriations of victims during natural disasters.

The rain weakens the soil

Like Sandrine’s parents, the underground cavities upset the lives of dozens of families in Normandy every year. If the phenomenon remains marginal, it is likely to multiply with climate change. To understand why, we must look at the composition of the soil. The soil in eastern Normandy is chalky. However, this rock weakens in contact with water and the region is experiencing rain peaks these last years.

“When the chalk is dry, the cavity will be able to withstand a certain vertical pressure, so it will be stable,” explains Baptiste Meire, geologist at the Bureau of Geological and Mining Research (BRGM). The expert worked on the Saint-Hilaire-sur-Risle file. “If you soak this chalk, its resistance is halved. So there are phenomena of collapses and disorders on the surface. »

In Saint-Hilaire-sur-Risle, we still do not know with certainty the nature of these cavities. The BRGM experts put forward two hypotheses. These holes in the basements could be marls. In other words, underground galleries dug by men to collect chalk used in the amendment of agricultural land. But it could also be natural limestone cavities. This is the hypothesis favored by geologists. To validate it, soil studies are necessary.

Closing is useless.

Baptist Meire

These holes have long been known in the village. Here, the inhabitants have become accustomed to fill in with pebbles and earth. These testimonies do not surprise Baptiste Meire, at the BRGM they have “dozens and dozens”. But it is formal, the technique is useless. “Filling the surface does not correct the problem. It’s like putting on a bandage or a patch. The origin of a collapse is deep in the chalk. The cavity can collapse again, as long as we do not proceed to the filling in the rules of the art, that is to say by an injection of concrete under pressure. But this technique is very expensive. expropriation is therefore often privileged, when the collapses present an immediate danger for the inhabitants.

A collapse and cracks on the house

The Lêveque family house cracks due to landslides
The Levêque family house cracks due to landslides ©Archives/September 2021

In Saint-Hilaire-sur-Risle, the departure of the Louveau couple in 2018 worried the other inhabitants, in particular the Levêque family. The couple and their two children have lived in the village since 2014. “When I learned that the neighbors had been expropriated because of an underground cavity, I said to myself that the hole, which we had in the garden, was not not just a small gallery dug by animals,” says Emilie. The mother then contacts the mayor.

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After expertise, she realizes that in addition to the collapse in the garden, her house cracks “everywhere”. The BRGM thinks that it could be a phenomenon of clay swelling removal. “As soon as it rains, the clay will increase in volume and, conversely, when it is very dry, it can shrink. These variations can cause ground movements, which can cause cracks,” explains Baptiste Meire. But to validate this hypothesis, soil studies must be carried out.

The limits of the Barnier fund

The problem is that these expertises must be financed by the owners, because the French law considers that the basements belong to them. Aid exists, such as the Barnier fund, but unlike the Louveau couple, the Levêque family is not eligible. “They are in a very difficult situation,” acknowledges Mayor Jean-Guy Grandin. “The Barnier fund can only be committed if there is a real danger. We are not talking about a crack phenomenon, but about a proven and demonstrated cavity under the house, which is not the case. The only lever we had was to investigate a natural disaster file. After in these files, it is up to the insurance companies to solve the problem. But for the moment, the insurance companies refuse to cover it. So the family has moved at own expense in another village.

I couldn’t let my children play outside, I didn’t have peace of mind.

Emilie Leveque

“If my son drops his ball near the hole and he wants to go get it back, I’m not immune to him getting hit by the ground because he collapses at that moment. “says Emilie. The family is now at an impasse. The house still belongs to them, impossible to live there or sell it.

The case of Saint-Hilaire-sur-Risle remains marginal, but according to the BRGM, these phenomena of collapse and cracks will multiply in the future with increasing rain peaks and dry spells. “It’s crisp and clear. We observe swelling shrinkage phenomena in sectors that had never been affected before”, observes Baptiste Meire. To deal with this, the office is working in particular on the census of these underground cavities. The objective is to predict land movements and adapt land use planning.

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