Geotechnical · 7 min read
Brisbane soil types: reactive clay, fill and what your site classification means
Two identical houses on two Brisbane streets can need very different footings. The reason is under the grass. Here is how South East Queensland soils behave, what your site classification letter means, and where the money goes.
What is under Brisbane
The region is a patchwork rather than one soil. Broadly, we deal with these.
- Reactive clays — common through the western and southern suburbs; they swell wet and shrink dry, moving footings up and down
- Alluvial soils near creeks and the river — soft, sometimes deep, prone to settlement
- Sands — coastal and bayside areas, low reactivity but poor at holding a slope
- Fill — old cut and fill housing estates and battle-axe blocks; uncontrolled fill can settle for years
- Rock and weathered rock — excellent bearing, expensive to excavate for pools and basements
Site classifications under AS 2870
A geotechnical report classifies the site by how much the surface can move as moisture changes.
- A — stable sand or rock, minimal movement
- S — slightly reactive clay
- M and M-D — moderately reactive; very common in Brisbane
- H1, H2 and H-D — highly reactive clay; deeper beams, more steel, sometimes piers
- E — extremely reactive
- P — problem site: uncontrolled fill, soft soil, a slope, mine subsidence or landslip risk; the footing must be designed specifically
Why a class P site is not a disaster
A P classification only means standard footing tables do not apply and the footing has to be engineered. Piering to rock or competent material, a stiffened raft, screw piles or a suspended floor all solve it.
What it does mean is that you should not price the build from a generic slab allowance, and you should not skip the geotechnical report.
How soil drives cost
Moving from M to H2 typically adds concrete and reinforcement to a slab. Uncontrolled fill can add piling. Rock adds excavation cost to pools and basements, and reactive clay makes drainage and site grading part of the structural design, not landscaping.
The cheapest lever is information: a soil test lets us design for the ground you have instead of the worst case we would otherwise assume.
Frequently asked questions
- Is all Brisbane soil reactive clay?
- No. Reactive clay is common, especially in western and southern suburbs, but sands, alluvium, fill and rock all occur. A geotechnical report classifies your specific site.
- What does a class P site mean?
- It means the site needs a specifically engineered footing — usually because of fill, soft soil, a slope or landslip risk. It is designed, not standard.
- Do I need a soil test before design?
- It is strongly recommended and usually saves more in footing cost than it costs. Without one, the design has to assume conservative conditions.
Send your soil report for footing design from $800.
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