By QED Consulting Engineers — structural & civil engineers, Notting Hill, Melbourne
A soil test (geotechnical investigation) determines your site classification under the Australian Standard AS 2870 — from stable Class A through to problem Class P. That classification tells your structural engineer how reactive your soil is and, in turn, how to design your footings and slab. It's one of the first steps in any Melbourne build.
Here's what the classes mean and what a soil test involves.
What is site classification?
Site classification describes how much your soil is expected to move as its moisture changes — its "reactivity." Reactive clay swells when wet and shrinks when dry, and that movement is what foundations have to resist. The classification is determined by a geotechnical engineer and reported as a site class.
The AS 2870 site classes
- Class A — stable; little or no ground movement (sand, rock)
- Class S — slightly reactive clay; minor movement
- Class M — moderately reactive clay or silt
- Class H1 / H2 — highly reactive clay; significant movement
- Class E — extremely reactive clay
- Class P — problem site (soft soils, fill, poor drainage, trees, or other factors)
For the moderately-to-extremely reactive classes, the expected depth of moisture movement is also considered. The more reactive the site, the more robust the footing and slab design needs to be.
Why it matters in Melbourne
Large parts of Melbourne — especially the western and outer suburbs — sit on reactive clay. Two neighbouring blocks can even differ, so you can't assume your site matches the house next door. The classification protects you: it ensures your foundation is engineered for the actual ground, reducing the risk of cracking and movement later.
What does a soil test involve?
A geotechnical engineer drills boreholes within the building footprint, logs the soil profile, takes samples for laboratory testing (moisture, clay reactivity and so on), and considers drainage, slope, fill and nearby trees. They then assign the AS 2870 class and provide the design parameters your structural engineer uses.
We work closely with these reports — our residential structural engineering team turns your site classification into a compliant footing and slab design.
About to build in Melbourne? Talk to a Melbourne structural engineer and we'll design your foundation to suit your site.
Frequently asked questions
What does my site classification mean? It describes how reactive your soil is, from Class A (stable) to Class P (problem). The more reactive, the more robust your footings and slab must be.
What is a Class P (problem) site? A site where conditions such as soft soils, fill, poor drainage or nearby trees mean it can't simply be classified A–E. P sites just need more investigation and engineered design — you can still build.
Can I build on highly reactive soil? Yes. With the correct engineered foundation — such as a stiffened raft or piers — almost any site can be built on safely.
Who carries out the soil test? A geotechnical engineer conducts the test and classifies the site; your structural engineer then uses that classification to design the footings and slab.
