Reactive soil
Reactive clay shrinks when dry and swells when wet, lifting or cracking slabs. AS 2870 site classes M, H1, H2, and E indicate increasing reactivity levels.
Ask Chalkline about this →Reactive soil (also called reactive clay or shrink-swell soil) is clay-dominant soil that changes volume with moisture content: it swells when wet and shrinks when dry. The resulting ground surface movement places differential stress on slab and footing systems, which can cause cracking, heave, or settlement if the footing design does not account for the site’s reactivity level.
AS 2870:2011 quantifies reactivity through site classification. Site classes M (moderately reactive), H1 and H2 (highly reactive), and E (extremely reactive) reflect increasing expected surface movement measured in millimetres. Class A and S sites have negligible or slight reactivity. Class P is a problem site that may include reactive conditions alongside other ground issues.
Inner Melbourne, parts of Brisbane’s clay plains, the Darling Downs, and inland NSW are known reactive clay areas. Perth sands and coastal NSW sandy soils are typically low-reactivity. A geotechnical soil report is the only way to confirm the site class; visual inspection of soil colour or texture is not a substitute.
Also known as: reactive clay, expansive soil, shrink-swell soil.
Category: Structural.
Related
- AS 2870:2011 residential slabs and footings
- Pad footings: construction and sizing for residential builds
- Site classification
See also
Last updated: 2026-05-10. Verified: 2026-05-10. Quarterly review for currency.