Shrink-swell
Shrink-swell is the cyclic movement of reactive clay as it dries and wets. Measured as the shrink-swell index (Iss) per AS 1289, it drives the AS 2870 site class.
Ask Chalkline about this →Shrink-swell is the cyclic ground movement of reactive clay: the soil shrinks as it dries out and swells as it takes up moisture. Across wet and dry seasons this repeated movement lifts and drops whatever sits on it, which is why footings and slabs on reactive soil have to be designed to cope with it.
How it’s measured: the shrink-swell index (Iss) is a lab value from a swell test and a core shrinkage test on undisturbed soil samples, run under AS 1289.7.1.1-2003 (verified 2026-05-24, Standards Australia). A higher Iss means a more reactive soil.
How it feeds slab design: the Iss values down a soil profile are used to calculate the characteristic surface movement (ys), the expected vertical surface movement. Under AS 2870, ys sets the site classification (A, S, M, H1, H2, E, P), which drives the footing system and beam depths. The chain runs: shrink-swell behaviour, measured as Iss, converted to ys, classified as a site class, used to design the slab.
Builder takeaway: shrink-swell is why two visually identical blocks can need very different (and very differently priced) slabs. The soil report is what tells you which one you are on.
Also known as: reactive movement; shrink-swell index, Iss (the lab measure).
Related
- Reactive soil
- Characteristic surface movement
- Site classification
- AS 2870: residential slabs and footings
See also
Last updated: 2026-05-24. Verified: 2026-05-24. Quarterly review for currency.