Site classification and slab cost: Class M, H1, H2 and P

The AS 2870 site classification decides which slab a house can use: each step from Class M to H1 to H2 means deeper beams, more steel and more concrete, and at Class E or P the standard designs stop and an engineer designs the footing. That is why the site works figure in a building contract moves when the soil test comes back.

The step is sharper than the millimetres suggest. The standard's off-the-shelf slab designs are each sized for the top of their class's movement range (Cameron 2018). A site predicted to move 45 mm gets the same H1 slab as one at 60 mm, and a site at 65 mm gets the H2 slab.

What does each class letter measure?

AS 2870 classifies a residential site by its characteristic surface movement (ys), the amount the ground surface is expected to rise and fall between its driest and wettest design states. Lochaden and Haberfield (2018) set out the bands used for reactive clay sites:

Class	Characteristic surface movement (ys)	Reactivity
S	up to 20 mm	slightly reactive
M	over 20 mm to 40 mm	moderately reactive
H1	over 40 mm to 60 mm	highly reactive
H2	over 60 mm to 75 mm	highly reactive (very high movement)
E	over 75 mm	extremely reactive

Class A (mostly sand or rock with little or no movement) and Class P (a problem site the standard designs do not cover) sit outside that sequence. Where seasonal moisture change reaches 3 m or deeper, as in Adelaide, the reactive classes carry a "-D" suffix, such as H1-D. The class describes the ground, not the house: it is an input to footing design, not the design.

What changes in the slab as the class rises?

AS 2870 gives standard designs for Classes A to H2 and none for E or P. As the class rises:

• Beams get deeper and closer together, with more steel and concrete, so the slab line item rises with them.
• Excavation gets deeper. Stiffened raft beams go between about 0.3 m and 1.1 m into the ground, deeper for higher classes (Lochaden and Haberfield 2018). That matters most where rock or fill sits shallow.
• The choice of footing system narrows. A waffle raft sits its beams on the ground surface over polystyrene void formers, with little excavation. A stiffened raft puts its beams into the ground, uses more steel and concrete, and by Lochaden and Haberfield's estimate is about four times stiffer in bending.
• Above Class H2, standard designs stop. Class E and Class P sites are engineered specifically.

Nobody can put a fair dollar figure on each step without the plan, because it depends on floor area, plan shape and local rates. Our geotechnical report cost breakdown covers what the investigation itself costs.

Are the standard designs enough on highly reactive clay?

Not everyone thinks so. Lochaden and Haberfield, two Melbourne geotechnical engineers, took up the question in 2018, after reports that more than 4,000 homes on the reactive clays west and north of Melbourne had been damaged. They modelled the standard waffle and stiffened raft designs for an H2 site in three dimensions. The waffle raft moved about 50 percent more than the stiffened raft, and neither met the standard's own limits for differential movement. They called the results preliminary and asked for all the standard designs, M and H1 included, to be reassessed.

That is one firm's analysis, not a change to the standard. It is still a fair reason to ask, on an H1 or H2 site, which footing system the design uses and why. AS 2870 lets an owner ask for an upgraded footing design with better expected performance, at extra cost (Cameron 2018). That conversation is easier before the contract is signed.

What puts a site in Class P?

Class P means the site falls outside the standard designs, so an engineer designs the footing from first principles. Triggers include soft or unstable soil, uncontrolled fill, landslip, mine subsidence, cut-and-fill platforms and abnormal moisture conditions such as trees; the full list is in Class P site classification.

Fill is the trigger most often decided by earthworks rather than geology. Under AS 2870-2011, controlled fill (placed and tested to AS 3798) up to 0.4 m deep, or 0.8 m for sand, leaves the site classified as natural ground; deeper controlled fill other than sand makes it P unless an engineer reclassifies it; and uncontrolled fill makes it P unless, where the fill is shallow, every footing is taken through it to natural soil.

That shallow-fill allowance interacts badly with waffle rafts. Delaney (2005), writing about the Newcastle region under the earlier edition, saw subdivision earthworks spread topsoil, mulch and unsuitable material up to about 400 mm thick on the basis that it would not change the class. A trenched slab's beams go through that layer. A waffle raft sits on top of it, and whether the material gets removed often depends on site workers spotting it. A lot with unrecorded fill is therefore not a Class M lot with a contingency; it may need an engineered footing, removal of the fill, or both.

What is a compliant slab designed to deliver?

A footing built to AS 2870 is designed to limit distortion, not to prevent movement. The Foundation and Footing Society Victoria (2022) reminds its members that such footings are an acknowledged compromise between cost and performance. The standard expects damage no worse than Category 0 to 1 (hairline to very slight cracking) in most conditions, and accepts Category 2 (cracks up to about 5 mm) under adverse conditions, although that should be rare.

Reactive clay still drives a large share of damage. After the 1989 Newcastle earthquake, an unpublished review of one major insurer's files found reactive soils were the main cause in 51 percent of claims. Yet Delaney (2005) also observed that damage to houses on AS 2870 footings is rare where the correct site class was adopted and extreme moisture conditions, such as large trees close to the house, were avoided.

What can be established before the soil test?

Before the site investigation, mapped and historical records can narrow the expected range:

• The mapped geological unit under the lot, which sets the plausible reactivity range.
• The AS 2870 climate zone, which sets the design depth of seasonal moisture change (Hs) and so scales the movement.
• Fill history, from historical aerial imagery, contours and terrain: filled gullies, dams, pads and batters.
• Nearby borehole logs: depth to clay, depth to refusal, standing water levels, and how much adjacent logs disagree.
• Slope and fall across the building platform, which drives cut, fill, retaining and drainage.
• Trees on or beside the lot that are staying, since they are a design input under the standard.

That evidence does not produce a class letter. It gives an expected range to price against and a drilling brief that answers the right questions. The site investigation decides.

Common questions

What is a P class slab?

There is no standard P class slab. Class P means the site sits outside the standard designs in AS 2870, so an engineer designs a footing specifically for it. The result may be a stiffened raft, piers to a competent layer, or removal and replacement of the material causing the problem.

What is an H1 class slab?

H1 is a highly reactive site with characteristic surface movement over 40 mm and up to 60 mm. AS 2870 provides standard raft and strip footing designs for it, deeper and more heavily reinforced than the equivalent Class M design.

Does the site classification change the cost of a slab?

Yes. A higher class means more concrete, more steel and deeper beams, and Class P removes the standard design altogether. The size of the difference depends on floor area, plan shape and local rates, not on the letter alone.

Can the site class be known before the soil test?

No. Only a site investigation by a geotechnical engineer produces a classification a certifier will accept. Mapped geology, climate zone, terrain, fill history and nearby bore logs give an expected range, which is enough to price against and to brief the driller.

To see the geology, terrain, fill history and nearby bore records for a lot before the investigation is booked, generate a desktop report for the address, or see a sample report for what it 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

• Andrew L.E. Lochaden and Chris M. Haberfield (2018). Assessment of the AS2870 standard designs for residential rafts on reactive clay. Australian Geomechanics, Volume 53, Number 1 (Mar 2018). geomechanics.org.au
• M. G. Delaney (2005). An overview of engineering geology and geotechnical challenges in the Newcastle Region. Australian Geomechanics, Volume 40, Number 1 (Mar 2005). geomechanics.org.au
• Donald A. Cameron (2018). Dealing with reactive clay soils through a national standard. Australian Geomechanics, Volume 53, Number 1 (Mar 2018). geomechanics.org.au
• Foundation and Footing Society Victoria Inc. (2022). Practice Note No. 8. Australian Geomechanics, Volume 57, Number 2 (Jun 2022). 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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