RGA

Shallow clay soils shrink during droughts and swell when rain returns. These slow movements have sufficient amplitude to damage detached houses with shallow foundations in particular. The exposure map updated in 2026 places 55% of mainland France at medium or high exposure, representing 12.1 million detached houses (61.5% of the stock), compared with 48% of the territory and 10.3 million houses in 2020. The cost has become structural for insurance: the natural catastrophe scheme’s average annual cost is around €1 billion, compared with €400 million over 1989-2015, with a record claims cost of €3.5 billion in 2022. Since October 2020, under the ELAN Law, medium and high exposure zones require a preliminary geotechnical study and construction specifications, with foundations at least 0.80 metres deep in medium exposure zones and 1.20 metres in high exposure zones. BRGM produces the susceptibility mapping and has classified more than 2,000 clay formations. This is a hazard whose geology does not change: only the frequency of drought and rewetting cycles increases with climate change.