A geological phenomenon resurfaces in France, especially in certain regions: field sags can appear in gardens, forming visible cavities on the surface.
It is a fact that followed closely by experts in geology and earth sciences: overnight, owners of houses discover cavities several meters deep in their garden. A phenomenon as surprising as it is instrumental. Because these sags can sometimes extend and weaken nearby constructions. In some cases, temporary precautions are taken, such as preventive rehousing, time to secure the premises.
But why do these holes appear without prior sign? These training courses are often linked to old chalk quarries, called Marnières, which are busy over time, especially in connection with weather conditions. In France, there are more than 100,000 abandoned quarries, often dug more than a century ago, and sensitive to natural developments of the soil. At the time, the chalk extracted was used to enrich agricultural land. These cavities, forgotten since, reappears today under the effect of erosion and bad weather, revealing a part unknown to our underground heritage.
The phenomenon is today observed in many areas, to the surprise of the inhabitants who sometimes discover that they live above a void formed by ancient human activity. The most concerned regions are Normandy, Île-de-France and Hauts-de-France, where thousands of Marnières are listed under today urbanized land. In more sensitive cases, the authorities may recommend prudence measures.
To find out if your house is in risk zone, you can consult the official georisques.gouv.fr website, which lists known underground cavities. Town planning services of your town hall can also provide useful information.
Faced with this natural phenomenon, several specialized companies offer soil studies or targeted interventions. Depending on the case, it is possible to fill the cavities or have geotechnical surveys carried out. These solutions make it possible to best support the nature of the terrain, by protecting buildings and arrangements. In some areas, these procedures become common today.
