Outstanding result / Post-mining : Saint-Etienne, emergency mitigation of a sinkhole threatening a business

One of the key missions of BRGM, which manages mining sites in France on behalf of the government, is to oversee the risks created by former mines through the DPSM (BRGM Mine Safety and Prevention). In 2021, for example, it carried out emergency work to ensure the safety of a commercial building and the surrounding area, when a mining sinkhole opened. A sinkhole is caused when a cavity beneath the surface begins to cave in and then suddenly collapses, creating a funnel effect. This is what happened on the night of 2 March 2021, under a commercial building in Saint-Étienne, in east-central France. A mining sinkhole of almost 1,000 m³ suddenly appeared in the middle of the night, following the collapse of a non-listed mine gallery, less than 20m below the surface.

The city of Saint-Étienne is built on mines. It has more mine cavities than any other French city of over 150,000 inhabitants, as the result of its coal mining activities in the 19th century. The city was built at the same time as the mines. Géodéris drafted a mine risk prevention plan that clearly locates these risks. However, events such as this may still occur on rare occasions, since the plan does not cover all the galleries.

This project is a good example of one aspect of the DPSM's work: to ensure the safety of former mining sites in an emergency situation where there is a danger to people and/or property. Fortunately, the crater formed in the middle of the night, but the human consequences could have been serious. A building with three shops was seriously damaged. The roof collapsed in part and one of the surrounding walls developed a lean. BRGM began work two weeks after the event. It shored up the structure, plugged the crater with 800 m³ of concrete, rebuilt the paving base and repaired the underground networks, green spaces and car park. The project was completed in late April 2021, a little ahead of schedule, less than two months after the accident!

The lessons to be drawn from this event include: the fast response by BRGM, the efficient measures taken for safety and conservation, and the strong coordination between the DPSM, Géodéris and government services.