Large scale modeling of clay shrink-swell risk for current and future climate scenarios.

In recent years, the risk of clay shrinkage-swelling has emerged as a significant concern for land use planning and for insurance companies. These superficial clay soils exhibit vertical movement (contraction and expansion), linked to meteorological conditions. Despite the slow pace of these fluctuations, they can reach an amplitude large enough to damage buildings located on these soils. In France, this hazard appears in second rank in terms of losses with events that can generate more than one billion euros in losses .

To better mitigate this risk, the French geological and mining risks office (BRGM) conducted a detailed mapping of exposure to clay shrink-swell across France. This departmental-scale analysis is based on the lithological nature of the soil, the mineralogical composition, geotechnical behavior, but also the loss experience observed. However, the susceptibility of a soil to swell has not been studied at global scale but rather over some territories. Given the current climate change, it is also necessary to understand the conditions linked to the occurrence of these events as well as the inherent impacts. This study focus on these two aspects: exposure and impact.

To estimate whether a soil might be prone to swelling, we developed a machine learning model based on exposure maps published for France and the USA with a set of pedological parameters (CEC of clay, soil texture, bulk density, etc. coming from Soilgrids & Harmonized World Soil Database models) and geological parameters; associated with the presence of clayey soils with swelling capacity. We achieved a prediction accuracy of nearly 70% on our test set in these 2 countries. For France, this approach allows us to estimate that 52% of the territory presents a medium or high exposure to this peril, which is consistent with the BRGM analysis of 49%. With this approach we also estimated that 52% of Germany’s territory is exposed to medium to high swelling susceptibility.

The impact analysis of this hazard is performed on France based on the publication of the CatNat decrees by the French Central Reinsurance fund, the loss ratio observed by AXA and climate indicators such as the Standardized Precipitation-Evapotranspiration Index (SPEI). The SPEI is a climatic indicator that is sensitive to water-balance variations, calculated over different time scales, allowing for the assessment of both short-term and long-term climatic conditions. The SPEI is particularly useful in regions where evapotranspiration plays a significant role in moisture availability. By analyzing the SPEI in conjunction with the CatNat decrees and the loss ratio observed by AXA, we can gain a comprehensive understanding of the current clay shrink-swell risk. This multi-faceted approach allows us to not only assess the current state of the hazard but also predict future trends.