Notes · Site & structure
Building on Adelaide’s reactive clay
Much of Adelaide sits on reactive clay that swells when wet and shrinks as it dries. Here is what that means for your footings, slab and drainage, and why it should be settled during design rather than discovered on site.
What reactive clay does
Reactive clay takes on water and swells through the wet months, then dries and shrinks through summer. The ground under a house lifts and settles on an annual cycle, and the drier the summer, the deeper the movement reaches.
The movement is rarely even. A slab edge under a wide eave stays drier than one exposed to rain; a garden bed, a downpipe or a leaking pipe wets the soil in one spot and not another. That uneven movement is what damages buildings. Typical signs are cracked masonry, doors and windows that stick, cracked tiles, and gaps opening where two materials meet.
How sites are classified
In Australia, residential footings and slabs are designed to AS 2870, Residential slabs and footings. A geotechnical investigation tests the soil, and the site is given a class based on how much the ground is expected to move:
- A — stable, non-reactive
- S — slightly reactive
- M — moderately reactive
- H1 and H2 — highly reactive
- E — extremely reactive
- P — problem sites, such as filled ground or soft soils
An engineer then designs the footing and slab to suit that class.
What it means for your design
The site class affects more than the footings. It shapes the slab thickness and stiffness, finished floor levels, how stormwater and plumbing are routed away from the house, and where trees, garden beds and paving can go. These decisions change the cost and shape of the building, so they belong in the design from the start.
When the soil is only confronted after the drawings are finished, the usual result is redesign: footings upsized against a fixed budget, levels changed on site, and details reopened.
How we handle it
We test the soil early, have the footings engineered to the site class while the house is still being designed, and plan levels and drainage alongside the floor plan. Then the same team builds it, so what was designed for the ground is what goes into it.
Questions
Why is reactive clay a problem for buildings in Adelaide?
Reactive clay swells as it takes on moisture and shrinks as it dries. Because that movement is uneven across a site, it loads footings and slabs unequally, and if the footing isn't designed for it the movement shows up as cracked walls, sticking doors and windows, and split tiles. It is managed by classifying the site and designing the footings and drainage to suit.
What is AS 2870?
AS 2870, Residential slabs and footings, is the Australian Standard for founding houses. It requires the site to be classified by soil reactivity, from stable (A) to extremely reactive (E) or problem sites (P), and the footing and slab to be designed for that class.
Sources and further reading
- AS 2870 — Residential slabs and footings (Standards Australia)
- National Construction Code, Volume Two — footings and slabs, site classification (Australian Building Codes Board)
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