Understanding BGS GeoSure Hazard Ratings (A to E)
The BGS classifies shrink-swell clay subsidence potential into five statutory hazard classes:
• Class A: Ground conditions predominantly non-plastic. No special foundation design required.
• Class B: Low plasticity. Minimal damage potential except in anomalous drought conditions.
• Class C: Moderate plasticity. Noticeable ground movement possible during extended dry summers.
• Class D: High plasticity. Significant ground contraction; shallow foundations likely to experience seasonal movement.
• Class E: Very High plasticity. Cohesive clay basin capable of major volumetric movement, requiring specialized engineering.
BGS GeoSure Classifications & Recommended Engineering Precautions
| BGS Hazard Rating | Soil / Rock Plasticity | Conveyancing Risk Profile | Recommended Foundation Design |
|---|---|---|---|
| Class A (Very Low) | Sands, gravels, hard bedrock | Negligible structural risk | Standard shallow strip foundations (750mm) |
| Class B (Low) | Low plasticity silts & sandy clays | Minimal insurance risk | Standard concrete strip (900mm) |
| Class C (Moderate) | Chalk with flints, boulder clays | Seasonal superficial movement | Engineered trench fill (1,000mm – 1,200mm) |
| Class D (High) | London Clay, Oxford Clay, Lias Clay | High claims density in droughts | Deep trench fill (1.5m) or engineered piles |
| Class E (Very High) | Pure expansive cohesive clays | Extreme volume change potential | Reinforced piled foundations with void formers |