AS 2870 site classification: classes A to P explained

An AS 2870 site classification is a single letter. Class A is non-reactive ground, such as sand or rock; S, M, H1, H2 and E rank how much the ground under a house is expected to move as reactive clay wets and dries. P marks a problem site outside that scale, needing its own site-specific engineering. Either way, it tells a footing designer what the slab is being designed against. It is the first thing a certifier wants to see before a slab is designed in Queensland, New South Wales or Victoria, and it is also one of the most misunderstood lines on any soil report. The letter looks simple. What sits behind it is a climate model, a laboratory test and a calculation, and each of those can shift the result.

This guide explains what the classes mean, how the letter is produced, why the same clay can classify differently in Brisbane, Sydney, Melbourne and Adelaide, and what a desktop assessment can and cannot tell you before anyone drills.

The classes, and the movement behind them

AS 2870, Residential Slabs and Footings, has governed footing design on reactive clay in Australia since 1986, with the current edition published in 2011. The Standard works from one predicted quantity: the characteristic surface movement, written ys, which is the vertical movement the ground surface is expected to make between its driest and wettest design states, with no building on it.

The 2011 edition bands ys into classes: up to 20 mm is slightly reactive (S), 20 to 40 mm moderately reactive (M), 40 to 60 mm highly reactive (H1), 60 to 75 mm very highly reactive (H2), and anything over 75 mm extremely reactive (E) (Sun et al. 2017). Class A is non-reactive ground such as sand or rock. Class P is different in kind: it means the site has a problem the normal classes do not cover (fill, soft ground, abnormal moisture, instability) and needs specific engineering. We cover Class P triggers in detail separately.

Moving up the scale means a stiffer, deeper and more expensive footing. On a borderline site, one class can be the difference between a standard raft and a piered design.

How the letter is produced

Three inputs go into ys, and it helps to know their names because they are the words you will see on a report.

The shrink-swell index, Iss. A laboratory measurement of how much a clay sample changes volume per unit change in soil suction, reported in percent strain per pF. It is the main measure of how reactive a particular clay is. The Standard also accepts loaded-shrinkage and core-shrinkage tests as routes to the same shrinkage index (Hargreaves 2005). Li et al. (2016) note that the shrink-swell test is preferred in practice because it captures both swell and shrink behaviour in one test.

The depth of design suction change, Hs. How deep the seasonal wetting and drying reaches. This is set by climate, not by the clay. AS 2870 assigns Hs by climate zone using the Thornthwaite Moisture Index (TMI), a wetness index built from rainfall and evaporation: the wetter the climate, the shallower the depth of moisture change, and the drier the climate the deeper it goes. In the 1996 edition the zones ran from 1.5 m in alpine and wet coastal areas, through 1.8 m (wet temperate), 2.3 m (temperate) and 3.0 m (dry temperate), to 4.0 m in semi-arid country (Chan & Mostyn 2009). The 2011 edition added a sixth, arid zone deeper than 4.0 m (Sun et al. 2017).

The suction change at the surface. A design value for how much drier the surface gets in the worst season. It is prescribed by region, and Sydney carries a larger allowance than most of the country (Chan & Mostyn 2009).

The calculation integrates the reactivity of each clay layer over the depth Hs, with an allowance for the upper "cracked zone" where the clay is already fissured. Change any input and ys changes. If you hold laboratory Iss values, the free AS 2870 site classification calculator runs the full AS 2870-2011 calculation with a per-layer breakdown.

Why the same clay classifies differently in different cities

Because Hs is climatic, identical soil profiles can land in different classes depending on where they are. The published record on this is worth knowing:

• South East Queensland. For years Brisbane practice used a 1.5 m depth of moisture change. The 1996 edition then gave a 1.5 to 2.3 m range for Brisbane and Ipswich and 1.8 to 2.3 m for Toowoomba without a map of where the boundaries fell, and Hargreaves (2005) describes consultants in the Brisbane-Ipswich growth corridor adopting values between the two with little consistency. Later statewide climate mapping suggests only a small part of SEQ sits in the shallowest zone, with the 3.0 m zone reaching the country just west of Toowoomba (Hargreaves 2005). Engineers who work the region expect H and E results across it, coast included, and more of them inland toward Ipswich; see our SEQ site classification guide.
• Sydney and NSW. Chan and Mostyn (2009) mapped the moisture index across 64 NSW weather stations, building on their earlier Sydney work, so that the climate zone for a NSW lot can be read from a published contour map. Sydney's letters are driven largely by which rock the lot sits on: shale country weathers to reactive clay, sandstone suburbs are often benign, and the Newcastle coal measures are more reactive again.
• Melbourne and Victoria. The basaltic clays of the Newer Volcanics west and north of Melbourne are the state's most reactive ground. Li et al. (2016) tested 47 sites across 37 Melbourne suburbs and found shrink-swell indices above 6 percent strain per pF common in the west and north, with the Newer Volcanics alone ranging from about 1.1 to 11. They also found large scatter within a single geological unit, and their correlations showed that cheap index tests (liquid limit, linear shrinkage and the like) cannot stand in for a measured Iss. Lopes and Osman (2010) showed Victoria's TMI has trended drier since the late 1940s, and that a 20 percent rise in ys is enough to push borderline sites up a class.
• Adelaide. The Keswick and Hindmarsh clays under much of metropolitan Adelaide are highly expansive, and the semi-arid climate drives deep seasonal drying, which is why Adelaide footing practice looks heavier than Sydney's for what sounds like similar ground.

Two cautions come with all of this. Climate maps from different authors use different assumptions and can disagree where they meet, so a zone call is an engineer's judgement informed by a map, not a lookup. And Sun et al. (2017) have projected how Victoria's climate zones may shift by 2030, 2050 and 2070, which matters for a footing meant to last fifty years.

How a classification is obtained

In practice there are two routes.

Field and laboratory. A geotechnical engineer or technician drills the lot (typically two boreholes on a standard residential block), logs the profile, and either tests the clay for Iss or assigns reactivity from experience of the local geology. This is the classification a certifier needs for footing design, and the only route that can produce a compliant classification for construction.

Desktop. Using geological mapping, soil surveys, nearby bore logs and the regional climate zone, an engineer or an automated service derives a probable class without drilling the lot. This is not a substitute for the field classification. It is the right tool before purchase, at feasibility, and when you are deciding how much investigation to commission. A desktop assessment that says "probable H, reactive basaltic clay mapped, 1.8 m climate zone" changes how you brief the drilling and what you budget for the slab.

The gap between the two is exactly the scatter Li et al. (2016) measured: within one mapped unit, individual lots vary. The desktop sets the expectation; the borehole settles it. See our guide to what a geotechnical desktop study covers for the boundary in detail.

Getting it right is cheaper than getting it low

An under-classified site (the ground is more reactive than the letter says) risks a slab designed for less movement than it will see. The damage pattern is familiar: cracked masonry, doors that stick seasonally, separated cornices, and in worse cases a distressed slab. Reactive soils and building damage explains the mechanism. An over-classified site simply costs more in every footing.

Hargreaves (2005) makes a further point that is often missed: compacted clay fill behaves differently from natural ground because it has no cracked zone, and in his worked examples for SEQ a cut-and-fill platform raised ys by roughly 50 to 66 percent over the same clay in place. Fill is not a footnote to classification; it can be the classification.

Common questions

What do the AS 2870 site classes mean? A is non-reactive, S slightly, M moderately, H1 highly, H2 very highly and E extremely reactive clay, banded by predicted surface movement (S to 20 mm, M 20 to 40 mm, H1 40 to 60 mm, H2 60 to 75 mm, E over 75 mm). P means a problem site that needs specific engineering.

Can a site classification be done without a soil test? A compliant classification for footing design needs a field investigation. A desktop assessment can give a probable class from geology, soil mapping, nearby bores and the climate zone, which is useful before purchase and for scoping the investigation.

Why is Brisbane's classification different from Melbourne's for similar clay? The depth of seasonal moisture change (Hs) is set by climate zone. A drier climate means deeper moisture change, more of the clay profile contributing to movement, and a higher ys for the same soil.

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Before you commission drilling, pull the desktop picture for the lot: mapped geology, soils, nearby bore logs, groundwater and an indicative site class in one report. Generate a desktop report for your 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

• Chan, I. and Mostyn, G. (2009). Climatic factors for AS2870 for New South Wales. Australian Geomechanics, Vol 44 No 2. 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
• Li, J., Zou, J., Bayetto, P. and Barker, N. (2016). Shrink-swell index database for Melbourne. Australian Geomechanics, Vol 51 No 3. geomechanics.org.au
• Lopes, D. and Osman, N. Y. (2010). Changes of Thornthwaite's total moisture indices in Victoria from 1948-2007 and the effect on seasonal foundation movements. Australian Geomechanics, Vol 45 No 1. geomechanics.org.au
• Sun, X., Li, J. and Zhou, A. (2017). Assessment of the impact of climate change on expansive soil movements and site classification. Australian Geomechanics, Vol 52 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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