Adelaide expansive clay: 5 site classification myths

Adelaide sits in one of the most reactive clay settings in the country, and AS 2870 assigns it a depth of design suction change (Hs, the depth over which soil moisture varies enough to move a footing) of 4 metres, the highest of the standard's climate zones apart from the arid interior (Gibbons & Cowan 2018). That single number drives a lot of confusion. Below are five beliefs builders, owner-builders and developers hold about classifying an Adelaide block, what is right about each, and where each breaks down.

Myth 1: a desktop soil map tells you the site class

Soil and geological mapping tells you what formation is likely present and roughly how reactive it tends to be. On the Adelaide Plains the near-surface deposits are largely the Hindmarsh and Pooraka Formation clays, alluvial fan sediments shed from the Adelaide Hills, and at the Darlington site on Main South Road the Pooraka Formation clay was found up to 30 metres thick (Gibbons & Cowan 2018).

What the mapping cannot give you is the characteristic surface movement (ys), the estimated vertical movement of the ground surface that AS 2870 uses to set the class. ys depends on the shrink-swell index (Iss, the strain a clay undergoes per unit change in soil suction) measured layer by layer, and on those same Adelaide fan clays the reported Iss range was 0.5 to 5 per cent strain per pF (Gibbons & Cowan 2018). That is a wide spread. A desktop report scopes the likely class and tells you where to sample; it does not replace the field classification, and nothing in it is engineering advice for a specific lot.

Myth 2: Adelaide clay is always Class H2 or E

Adelaide clay is often highly reactive, but the class is set by ys, not by reputation. The AS 2870 bands are fixed regardless of state:

Class	Reactivity	ys (mm)
S	Slightly reactive	0 to 20
M	Moderately reactive	over 20 to 40
H1	Highly reactive	over 40 to 60
H2	Very highly reactive	over 60 to 75
E	Extremely reactive	over 75

(Sun et al. 2017)

Because Adelaide's Hs is 4 metres, a reactive clay profile there generates a larger ys than the identical clay would in a wetter zone, which is why so many Adelaide blocks land in H1, H2 or E. But a shallow clay over rock, a sandy profile, or a well-drained fan margin can classify lower. The number has to be calculated from the profile, not assumed from the suburb. If you want the mechanism behind this, why the same clay classifies differently in Sydney, Melbourne and Adelaide works through the climate-zone effect.

Myth 3: the 4 metre Hs is the whole story on how deep the movement reaches

Hs is a design suction depth, not a fixed physical fact for every lot. AS 2870 defines Hs as 4 metres for Adelaide and treats the cracked zone in existing ground as about 3 metres deep (Gibbons & Cowan 2018). Those are regional design assumptions.

Two things complicate them. First, the Thornthwaite Moisture Index (TMI) map underpinning the climate zones was built from mid-twentieth-century climate data, roughly 1940 to 1960, and researchers have flagged that it may no longer reflect current conditions (Sun et al. 2017). A drying climate deepens the soil drying zone and increases seasonal movement (Lopes & Osman 2010). Second, the standard's exclusions apply where abnormal moisture conditions exist, such as a leaking service or a tree drawing the profile down well past its usual range (Gibbons & Cowan 2018). At a black earth test site near Adelaide, total heave of 82 millimetres was measured during one winter on a highly expansive profile over weathered shale at 4.5 metres (Jewell & Mitchell 2009). Real movement can exceed the tidy design triangle.

Myth 4: a low-suction climate means the ground stays wet and stable

Adelaide's Mediterranean climate averages around 530 millimetres of rain a year, with long, hot, dry summers (Jewell & Mitchell 2009). That produces large seasonal swings in soil suction rather than a stable, damp profile. At the Darlington site, laboratory suctions ranged from 3.3 pF up to and beyond 4.7 pF, which was the apparatus limit, so higher suctions were expected than the instrument could record (Gibbons & Cowan 2018).

Those swings are the problem. Reactive clay moves when its moisture content changes from equilibrium, and a hot dry Adelaide summer followed by winter wetting drives exactly that cycle. Climate is the critical control on the size of seasonal heave in highly expansive profiles (Jewell & Mitchell 2009). Managing moisture around the finished building, through drainage and planting decisions, matters as much as the footing itself. Building on reactive clay covers the design calls that follow.

Myth 5: the site class alone tells the builder what footing to pour

The class is the starting point, not the specification. The same class covers a range of ys, and the required treatment depends on the actual ys and Iss values, not just the letter. Work on Adelaide subgrade treatment showed that for a given target movement, the depth of treatment needed climbs sharply as Iss rises and as Hs deepens, and Adelaide's 4 metre Hs demands markedly deeper treatment than a Victorian profile at 2.3 metres for the same clay reactivity (Gibbons & Cowan 2018).

For a house footing that means an H1 block near the top of its ys band, on a thick Pooraka clay, calls for a more substantial system than an H1 block scraping the bottom of the band. The design also has to account for site-specific factors the class ignores: fill, cut and fill across a sloping lot, trees, and drainage. The site class narrows the options; the engineer sets the footing.

Where a desktop report fits

A GeoReport desktop assessment pulls the geological formation, published soil reactivity, groundwater bore records and terrain for an Adelaide address into one document, so you know before you commission the field investigation whether you are likely looking at H1, H2 or E, and where the profile might surprise you. It scopes the sampling and sets expectations. It does not finalise the class or the footing.

Generate a desktop report for the address, or see a sample report first to check what it covers.

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

• P. Gibbons and S. Cowan (2018). Subgrade treatment design for expansive soils in Adelaide using Australian Standard AS2870 - is this the right approach?. Australian Geomechanics, Volume 53, Number 1 (Mar 2018). geomechanics.org.au
• Sarah A. Jewell and Peter W. Mitchell (2009). The Thornthwaite Moisture Index and seasonal soil movement in Adelaide. Australian Geomechanics, Volume 44, Number 1 (Mar 2009). geomechanics.org.au
• Xi Sun, Jie Li and Annan Zhou (2017). Assessment of the impact of climate change on expansive soil movements and site classification. Australian Geomechanics, Volume 52, Number 3 (Sep 2017). geomechanics.org.au
• D. Lopes and N. Y. Osman (2010). Changes of Thornthwaite’s Total Moisture Indices in Victoria from 1948-2007 and the effect on seasonal foundation movements. Australian Geomechanics, Volume 45, Number 1 (Mar 2010). 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.

← All articles