A site classification is a single letter that tells your engineer how much the ground under your house is expected to move, and it drives more of your build cost than almost any other line in the documentation.
What It's Actually Measuring
Clay soils change volume with moisture. They swell when wet and shrink as they dry, and in a climate with distinct wet and dry seasons that movement is both seasonal and repeated. A site classification under AS 2870, the Australian Standard for residential slabs and footings, quantifies that expected movement as a value called the characteristic surface movement, usually written as ys, and converts it into a class.
It is not a measure of how strong your soil is, or how good your block is. A perfectly sound site can be highly reactive. It's a measure of how much the ground will move up and down over the years, which is what a footing has to be designed to tolerate.
The Classes
- Class A, most sand and rock sites. Little or no ground movement from moisture change.
- Class S. Slightly reactive clay. Only slight movement expected.
- Class M. Moderately reactive clay or silt.
- Class H1 and H2. Highly reactive clay, with H2 the more reactive of the two.
- Class E. Extremely reactive.
- Class P. Problem sites. Not a reactivity rating at all: P covers sites where movement is driven by something other than normal moisture change, such as uncontrolled fill, soft or unstable ground, erosion, slope instability, or abnormal moisture conditions.
Classes M, H1, H2 and E can also carry a -D suffix, indicating deep-seated moisture movement where the ground dries out to greater depth. That changes the design as much as moving up a class does.
Class P Is the One to Pay Attention To
People often read P as “the worst one”, but it's a different kind of answer entirely. A P classification means the standard tables in AS 2870 can't be applied to your site, and the footing system has to be specifically engineered for whatever the actual problem is.
Uncontrolled fill is the most common cause we see. A block that was levelled at some point with material that was never compacted in controlled layers. It doesn't make the site unbuildable. It means the footing design has to get through the fill to something reliable, and that costs more than a standard raft.
Why the Classification Has to Come Before the Design
The classification determines the footing system. Not influences it. Determines it. A Class A site and a Class H2 site with the same house on top need materially different footings, different reinforcement, different beam depths and different costs.
Which is why ordering the soil test after the footing design has been drawn is a false economy. Either the design gets redone, or it was drawn conservatively enough to cover the worst case, in which case you've paid for concrete you didn't need.
Where the Test Is Taken Matters
A classification is derived from boreholes. If those boreholes are taken well away from the proposed building footprint, or on a block with cut and fill they sit on the wrong side of the change in level, the classification may not describe the ground your house will actually sit on.
On sloping or previously modified sites, it's worth making sure the investigation is positioned against the proposed footprint rather than wherever the drilling rig could most easily park.
What a Classification Does Not Promise
Footings designed to AS 2870 are designed to a performance criterion, not to a guarantee of no cracking. The standard sets out an acceptable low probability of foundation damage under normal moisture conditions.
Abnormal conditions are excluded, and they're the ones homeowners control: over-watering next to the slab, leaking stormwater or plumbing, poor site drainage, and large trees planted close to the house all change the moisture regime the design assumed. Most of the reactive-soil damage we're asked to look at has one of those four things behind it rather than a deficient footing.
This article is general information about structural engineering practice in South Australia. It isn't advice for a specific site, and it doesn't replace confirming requirements with your local council or a registered building certifier. Requirements vary between council areas and change over time.