Class P site classification: triggers, cost and meaning

Class P is the AS 2870 classification for a problem site: ground that the normal classes and the standard's off-the-shelf footing designs do not cover, so an engineer designs the footing for that site from a site-specific investigation. Under clause 2.1.3 of AS 2870-2011 it is triggered by inadequate bearing ground, soft or unstable soils, fill, landslip, mine subsidence, collapsing or erodible soils, abnormal moisture conditions, and any site that cannot otherwise be classified.

It is more common than the paperwork suggests. A consultant once told Bruce Hargreaves, a Brisbane geotechnical engineer, that if they applied the abnormal-moisture rules of the 1996 edition strictly, about half their site classifications would be Class P. Hargreaves replied that the true figure was probably nearer 70 to 80 percent (Hargreaves 2005).

How is P different from the other classes?

A, S, M, H1, H2 and E describe how much the ground is expected to move under normal conditions: the seasons, the climate, the effect of the house and subdivision, and an ordinarily kept garden. Our AS 2870 explainer covers those bands. The standard deliberately does not design for moisture changes that could be avoided (Cameron 2018). P says something about the site falls outside those conditions, so no standard design fits. That makes the reason more important than the letter: AS 2870 expects a Class P report to say why the site is P and what further investigation is needed.

What puts a site in Class P?

1. Fill. Clause 2.5.3 sets the rules. Controlled fill (placed and tested to AS 3798, the earthworks standard) no deeper than 0.4 m, or 0.8 m for sand, leaves the site classified as the natural ground was before filling. Controlled fill other than sand deeper than 0.4 m makes the site P, although an engineer can reclassify it after assessing how the fill and the ground under it will move. Uncontrolled fill makes the site P; where it is shallow, the only way out is to take every footing through it to natural soil.

"Controlled" does not mean the fill will not move: fill several metres deep still settles, with the worst differential movement near its edge (Cameron 2018). Compacted clay fill can also heave. Hargreaves (2005) described the usual sequence on single lots: the compaction tester will rarely certify the fill, so the classifier calls it uncontrolled and the engineer designs piers, while nobody tells the designer the clay fill can swell. Is there fill on your block? covers what the records can show.

2. Soft, loose or weak ground. Soft clays and silts, loose sands and organic deposits that keep settling under load. The extreme case in Queensland is the lowland estates on filled swamp and estuarine ground from Noosa to Coolangatta, where the first investigation sometimes has to go below 30 m to find the bottom of the compressible layers (Hargreaves 2005). See our SEQ guide.

3. Abnormal moisture conditions. Clause 1.3.3 of AS 2870-2011 lists examples, where the feature is close enough or the event recent enough to affect the ground under the building. Before construction: a removed building or trees, trees on the site or next door, and drains, channels, ponds, dams, pools, effluent areas or tanks. During construction: poor site drainage. After construction: trees too close to a footing, irregular garden watering, neglected drainage, plumbing leaks and loss of vegetation near the building. The classifier can find and call the pre-construction items; the later ones are why the standard puts site maintenance on the owner.

Knock-down rebuilds raise the same problem. Soil under an old timber-floored house is usually dry, soil under an old slab usually moist, and the bigger new house lands partly on the old footprint and partly on garden soil in a different state (Hargreaves 2005).

4. Trees. Hargreaves (2005) set out two flat lots with the same Class M clay, one with a 30 m tree in the middle of the footprint that is cut down days before the slab is poured. The soil under the tree is far drier. Over the next three to five years both lots wet up under their slabs toward the same moisture level, but on the tree lot the moisture roughly triples on the way, against about a 50 percent rise next door, and the ground moves accordingly. The standard then said such movement could not be quantified; the 2011 edition added Appendix H, a method for estimating tree drying and designing for it. In Cameron's (2018) worked example on an H1 site, it added up to 25 mm of settlement for a single tree and 40 mm for a group.

5. Cut-and-fill platforms. Where part of the slab sits on natural ground and part on fill, the two halves move differently, and the fill side is judged under the fill rules above. A cut on a reactive site deeper than the lesser of 0.25Hs or 0.5 m that was not allowed for means the class must be reconsidered.

6. Mine subsidence, landslip and other instability. Mine subsidence, landslip, collapsing soils and coastal or other erosion all fall outside the normal classes. Slope alone is not a trigger; slope instability is.

7. Anything else the classifier cannot place. Any site the assessor cannot confidently put in A to E.

Why do so few reports say P?

Partly because some triggers are easy to leave out. Slope is the clearest example. Hargreaves (2005) found many SEQ classifiers ignored slope stability or added a line saying the report had not considered it. Most readers take that to mean there is no slope problem. It means the classifier does not know, even though published slope stability mapping for the region goes back to 1981.

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 describes. A clean M or H on a report does not tell you the site is free of P conditions, only that the classifier did not call them. Ask what was on the site, what was removed, and what the fill history is.

What does a Class P mean for the build?

The footing is engineered, not standardised. Instead of a standard waffle or stiffened raft, an engineer designs for the site: a deeper or stiffer raft, bored piers or screw piles to stable ground, or removal and recompaction of fill.

Cost and time usually go up. How much depends entirely on the cause. A P with a simple, shallow cause can be a modest extra; deep uncontrolled fill or soft ground can be a major one. Engineered footings also mean design time and often more investigation, such as deeper boreholes or test pits, before anyone can commit.

Can you see a Class P coming?

Often. Historical aerial imagery shows the gully that was filled, the dam that was there in 1975 and the trees cleared last year. Geological mapping shows floodplain alluvium and swamp deposits, elevation data the cut-and-fill platform, planning overlays the landslide, flood and mine subsidence areas, and nearby bore logs soft clay or shallow water. A desktop study that reads those sources sets your expectation before you sign and tells the geotechnical engineer what to look for. It does not classify the site: the site investigation decides, and how much fill a lot carries is for the boreholes to find. See what a geotechnical desktop study covers.

The best protection 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.

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? Clause 1.3.3 gives examples: a removed building, removed trees, trees on the site or next door, drains, channels, ponds, dams, pools, effluent areas or tanks, poor site drainage, trees too close to footings, irregular garden watering, plumbing leaks and loss of vegetation near the building. Where they are present at classification on reactive clay, the site can be 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, Victoria and South Australia.

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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