When you engage a structural engineer to design footings for a new home or extension, one of the first documents we ask for is the geotechnical (soil) report. Under AS 2870, sites are classified according to how much the surface is expected to move with seasonal moisture changes.
Class A (mostly sand and rock) sites move very little, while Class M (moderately reactive), H1/H2 (highly reactive) and E (extremely reactive) clay sites can experience significant ground movement as clays swell when wet and shrink when dry. Class P covers problem sites — soft soils, fill, trees or abnormal moisture conditions.
The more reactive the site, the stiffer the footing system needs to be. On a Class A or S site a conventional slab may be perfectly adequate, while an H2 or E site will typically require a stiffened or waffle raft with deeper edge beams and more reinforcement — which costs more to build.
Practical tips for owners: keep site drainage working, avoid planting large trees close to the house, and repair leaking plumbing promptly. Stable moisture means a stable slab.
If you're planning a build and want to understand what your soil report means for your budget, get in touch — we're happy to talk it through before you commit.