The drought indicator that predicts shrinkage and swelling of clay soils currently shows a sharp increase in the second half of the 21st century, according to IPCC assumptions RCP 4.5 (moderate emissions scenario) and RCP 8.5 (high emissions scenario).
© BRGM - Sophie Barthelemy
The risk of shrinkage and swelling, which is associated with periods of drought, is increasing significantly due to climate change.
A surprisingly little-known natural hazard given its consequences and cost to the community, this phenomenon causes the soil to shrink like a sponge following a period of drought, and then swell when the rains return. These variations in volume can damage structures on the surface.
In 2022 alone, this phenomenon cost the authorities and insurers more than three billion euros. This trend could worsen as a result of climate change, particularly in South-West and North-East France.
Recent research has led to the development of an indicator to predict future risk trends.
A risk affecting more than half of France
At the origin of this phenomenon are clay minerals such as smectite, which are capable of retaining water in their layered structure. Their change in volume causes a millimetric variation in surface level that, while imperceptible to the naked eye, can crack roads, pavements and light structures such as single-storey houses.
This type of soil is found on approximately half of the French territory, where over 10 million individual homes are built. In our current temperate climate, the soil is damp most of the time, and problems arise during droughts.
Development of a model to forecast shrinkage and swelling risks
Recent work carried out jointly by Météo France, BRGM and the French state’s reassurance fund (Caisse Centrale de Réassurance, CCR) has improved our understanding of this phenomenon.
An annual drought indicator has been developed to better characterise the factors contributing to the occurrence of shrinkage and swelling, and to statistically quantify the overall number of claims. The purpose of the tool is not, however, to determine the risk of damage for each individual house.
A final part of the study aimed to identify future trends in droughts conducive to shrinkage and swelling under the influence of climate change, by projecting the indicator under different greenhouse gas emission scenarios.
The evolution of the shrinkage and swelling indicator was calculated up to 2065 using a model of soil moisture based on six different climate models under the IPCC's RCP 4.5 and RCP 8.5 scenarios (respectively, a moderate emissions scenario and a pessimistic scenario where emissions continue to increase at the current rate).
The indicator rises in both scenarios, particularly after 2045, reflecting a future intensification of droughts favourable to the shrinking and swelling of clay soils. North-East and South-West France would be particularly affected by this phenomenon under the RCP 8.5 scenario.
