Clay Swelling and Shrinking Risk (France)

Source: BRGM / Géorisques (retrait-gonflement des argiles) Year: 2019 Category: Physical risks · Mitigating services · Geological Coverage: Country (France) Format: Boundary polygons Used in risk analysis: No (contextual layer; not used in materiality scoring)

What it shows

This layer maps susceptibility to clay swelling and shrinking across France. The phenomenon occurs in clay-rich soils that expand when wet and contract when dry, and these movements can cause structural damage to buildings and roads. The layer highlights where soils are most prone to this behaviour, helping flag sites where ground movement is a material consideration for infrastructure.

How it is built

The data is provided by the French geological survey (BRGM) through the Géorisques platform, as an exposure and hazard assessment of clay shrink-swell behaviour (retrait-gonflement des argiles). The territory is classified into susceptibility zones according to soil composition and the assessed hazard, and these zones are mapped as areas across France.

How to read it

Areas are classified into three susceptibility levels: High (Fort) — significant risk of structural damage; Medium (Moyen) — moderate risk, where specific construction standards may apply; and Low (Faible) — minimal risk of soil-related movement. A site falling within a higher-susceptibility zone warrants closer geotechnical attention.

Class thresholds

This is a categorical hazard-susceptibility layer, not a continuous physical flux: it expresses BRGM's assessed susceptibility to clay shrink-swell (retrait-gonflement des argiles) as a three-level qualitative class (the ALEA field) rather than a measured quantity in physical units. There is therefore no numeric break in a single unit; the zones are mapped polygons carrying one of three ordered classes:

This is a contextual layer and is not used in materiality scoring, so it carries no risk-flag threshold of its own. The "classes" are zone membership: a site is informative on the Mitigating services dimension when it falls within a Medium or, especially, a High (Fort) susceptibility zone.

How these thresholds were set. The susceptibility zoning is taken directly from the BRGM / Géorisques retrait-gonflement des argiles (RGA) exposure-and-hazard assessment, which classifies the French territory by soil composition and assessed hazard. The Low/Medium/High boundaries come from BRGM's own methodology and are not a Darwin classification.

Reference: Géorisques / BRGM — retrait-gonflement des argiles.

Source

Géorisques / BRGM — clay shrink-swell (retrait-gonflement des argiles) exposure and hazard database.

Comparison with the WWF Risk Filter Suite

There is no equivalent indicator in the WWF Risk Filter Suite. Clay shrink-swell is a geophysical ground hazard rather than a biodiversity or water risk, so it falls outside the WWF methodology and is a Darwin extension.

Legend

Symbolised field: Risk level

Generated from darwin/layers/layer-clay-swelling-and-shrinking-risk-france.toml and risk_indicator_pairs.toml (develop).