Class P site classification: triggers, cost and meaning

Class P is the AS 2870 classification for a site the standard footing designs do not cover. Rather than a class letter that maps to a deemed-to-comply slab, the site gets a footing designed specifically for it by an engineer, on the basis of a site-specific investigation.

It is triggered by fill, soft or unstable ground, landslip, mine subsidence, abnormal moisture conditions and several other situations, and it is more common on the ground than it is on paper. This covers the seven triggers, why P is under-reported, what it changes for the build, and how to see it coming before the soil test.

Where P sits among the classes

Under AS 2870, the Australian Standard for residential slabs and footings, every new home site gets a classification from how much the ground is expected to move as it dries and wets. A is stable ground (sand, rock), then S, M, H1, H2 and E run from slightly to extremely reactive clay, banded by predicted surface movement. Our AS 2870 explainer covers the bands and the calculation.

P is different in kind. Class A is non-reactive ground (sand, rock) with negligible movement; S through E describe how much a reactive clay swells and shrinks under normal conditions. P says the normal conditions do not hold, so a standard design cannot be picked off the shelf. The Standard treats the A to E classes as "normal sites" where moisture change is driven only by season, climate, the building itself and ordinary garden conditions (Hargreaves 2005). Anything outside that is where P begins.

The seven things that trigger a Class P

1. Uncontrolled or deep fill. If the block has been filled and the fill was not placed and compacted to a documented standard, it cannot be relied on to carry a footing. This is the classic trigger on cut-and-fill estates and older subdivisions. Even controlled clay fill changes the sums: Hargreaves (2005) shows that because compacted fill has no cracked zone, the predicted surface movement for the same clay can rise by roughly half to two thirds once it is placed as fill, and warns that the classifier often reports the natural soil while the designer never hears about the heave potential of the fill above it. Is there fill on your block? looks at how to tell from records.

2. Soft or compressible ground. Soft clays, silts and organic deposits that settle under load. Hargreaves (2005) singles out the lowland filled estates along the SEQ coast from Noosa to Coolangatta, where swamp and estuarine deposits at depth keep consolidating and the initial investigation may need to go deeper than 30 metres. He also notes that the Standard is genuinely unclear about where such estates sit once the earthworks are engineered to a target class.

3. Abnormal moisture conditions. AS 2870 lists the situations that count: a recently demolished building that has kept the ground under it dry, drains, channels, ponds, dams or tanks on or near the site, large trees recently removed, trees growing too close to a footing, irregular garden watering, neglected site drainage and unrepaired plumbing leaks (Hargreaves 2005). The first three are usually present before the classifier arrives; the rest develop after construction.

4. Trees. Worth its own line because it is the most common abnormal condition and the one most often waved through. Hargreaves' (2005) illustration is two flat lots with identical Class M clay, one with a large tree in the middle of the footprint removed just before the slab is poured. The soil under the tree is far drier than its neighbour's; after construction both wet up toward an equilibrium over a period of years, but the tree lot moves much further to get there. The Standard acknowledges that quantifying that movement is beyond what current methods allow, and still requires it to be considered. Cameron (2018) notes that tree-drying settlement and reactive fills are now explicit adjustments to the classification method.

5. Sloping sites and cut-and-fill platforms. Where part of the slab sits on natural ground and part on fill, the two halves move differently. Slope instability in its own right is also a P condition.

6. Mine subsidence and other instability. Declared mine subsidence districts, landslip-prone slopes, erosion and collapsing soils all fall outside the normal classes.

7. Conditions the Standard does not cover. Any situation where the assessor cannot confidently place the site in A to E.

Why P is under-reported

Hargreaves (2005) records a consultant admitting that if they applied the abnormal-moisture provisions strictly, around half of their site classifications would be Class P, and his own view was that the true share was higher still. The reluctance is understandable: a P is unwelcome news to a builder on a fixed-price contract. But the conditions do not go away because the letter does. They show up later as the sticking doors and stepped cracking that reactive soil damage is known for, and in the disputes that follow about whether the classification was adequate.

The practical consequence for a buyer or builder is that a clean M or H on a report does not tell you the site is free of P conditions. It tells you the classifier did not call them. Ask what was on the site, what was removed, and what the fill history is.

What a Class P means for the build

The footing is engineered, not standardised. Instead of a builder using a standard waffle or stiffened raft, a structural engineer designs for the specific site: a deeper or stiffer raft, bored piers or screw piles to stable ground, or removal and re-compaction of fill.

The cost usually goes up. Piering, extra excavation and replacing poor fill all add to the slab, sometimes substantially. The figure depends entirely on the cause, which is why the reason for the P matters more than the letter.

Timelines can stretch. Engineered footings mean design time and often further investigation (deeper boreholes or test pits) before anyone can commit.

A P with a simple, shallow cause can be a modest extra. A P caused by deep uncontrolled fill or soft ground can be a major one. The letter alone does not tell you which.

Seeing it coming

Almost every P trigger leaves a trace in existing records before anyone drills. Historical aerial imagery shows the gully that was filled, the dam that was there in 1975, the stand of trees cleared last year. Geological and soil mapping shows floodplain alluvium and swamp deposits. Elevation data shows the cut-and-fill platform. Planning overlays show landslide, flood and mine subsidence areas. Bore logs nearby show soft clay or shallow water. A desktop study that reads those sources sets your expectation before you sign, and it tells the geotechnical engineer where to drill and how deep. See what a geotechnical desktop study covers.

The single best move is still to get the field classification done before you are locked in, ideally as a contract condition or before a fixed-price build is signed. A surprise P discovered after you have signed a standard slab allowance is exactly how budgets blow out.

Common questions

Does a Class P site mean I cannot build? No. It means the footing has to be engineered for the site rather than taken from the standard designs. Almost any block can be built on; P changes how, and usually how much.

What are the AS 2870 abnormal moisture conditions? A recently removed building, nearby drains, channels, ponds, dams or tanks, recently removed large trees, trees too close to the footing, irregular garden watering, poor site drainage and plumbing leaks. Any of these on reactive clay can require a Class P.

Can I find out the Class P risk before paying for a soil test? You cannot get the formal classification without a field investigation, but a desktop review of geology, slope, watercourses, historical imagery and fill history will flag whether a site is higher risk, enough to budget sensibly and order the test early.

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Want to know whether a block is sitting on reactive ground, fill, soft alluvium or a cleared tree line before you commit? Generate a desktop report for the address or see what a sample report contains.

LayeredGeo compiles geology, soils, groundwater and site data into an automated geotechnical desktop report for any address in Queensland, New South Wales or Victoria.

Sources

• Cameron, D. A. (2018). Dealing with reactive clay soils through a national standard. Australian Geomechanics, Vol 53 No 1. geomechanics.org.au
• Hargreaves, B. (2005). 20 years of AS 2870 in South East Queensland: a personal view. Australian Geomechanics, Vol 40 No 3. geomechanics.org.au

About this article. Published by LayeredGeo and written from the published research cited in the Sources section above. It is general information about how property and ground conditions are assessed in Australia, not engineering, planning, legal or financial advice, and it is not specific to any property. Check anything that matters against the source dataset or a suitably qualified professional before you rely on it. If you spot something wrong, tell us at hello@layeredgeo.com.au and we will fix it.

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