Ever wondered how engineers know whether a soil is stable enough to build on, or how much it will move as it wets and dries? A big part of the answer comes from a deceptively simple lab test: the Atterberg limits.
What It Measures
The Atterberg limit test determines three moisture-content thresholds that mark how a fine-grained soil (clay or silt) changes state as water is added or removed:
- Liquid limit (LL) — the moisture content at which the soil starts behaving like a liquid rather than a plastic solid.
- Plastic limit (PL) — the moisture content at which the soil stops being mouldable and starts to crumble.
- Plasticity index (PI) — the difference between the two (LL minus PL), which indicates how much the soil can change moisture content while still behaving plastically — and is the figure most directly linked to shrink-swell risk.
Why It Matters for Construction
A high plasticity index signals a soil that will shrink significantly when it dries and swell when it gets wet — movement that can crack slabs, distort footings and damage pavements if it isn’t designed for. This is exactly the behaviour that drives Australia’s AS 2870 site classification system, where a site’s expected ground movement (and therefore its classification, from Class A through to Class P) depends heavily on the plasticity of the clay present.
How the Test Is Performed
Liquid limit is typically measured using a cone penetrometer or the traditional Casagrande cup method, recording the moisture content at which the soil closes a standard groove under set conditions. Plastic limit is found by rolling the soil into a thread by hand until it crumbles at a 3mm diameter — a simple technique, but one that needs to be done consistently to give a reliable result. Both are run in accordance with AS 1289, the Australian standard for soil testing methods.
What the Results Feed Into
Atterberg results don’t stand alone — they’re combined with other data from a site investigation (such as borehole logs and in-situ testing) to classify the site and inform footing design. For residential builds, this is often part of the broader site classification process required before a certifier will sign off on footing design.
If you need site classification or geotechnical testing that includes Atterberg limits for a build, extension or subdivision, see our site classification service.