Why the same clay classifies differently in Sydney, Melbourne and Adelaide

A builder who has poured slabs in Western Sydney, then moves to Melbourne's west, then takes a job in Adelaide, learns the same lesson three times: the soil report letters do not travel. Clay that would be a comfortable M in one city can be an H in the next, and the reason is not that one engineer is stricter. It is climate, and it is written into the Standard.

This guide explains, in plain terms, the one input to a site classification that you cannot see by standing on the block: how deep the ground dries out each year where you are. It also covers what a drying climate is doing to that number over time, and why that matters for a slab that has to last fifty years.

The short version

• A site classification (A, S, M, H1, H2, E or P) is worked out from two things: how reactive the clay is, and how deep the seasonal wet-dry cycle reaches. The second is set by climate zone under AS 2870. Our AS 2870 explainer has the detail.
• Wet climates dry the ground to a shallow depth; dry climates dry it deep. In the 1996 Standard that ran from 1.5 m in wet coastal areas up to 4.0 m in semi-arid country (Chan & Mostyn 2009), with an even deeper arid zone added in 2011 (Sun et al. 2017).
• Deeper drying means more clay moving, which means a higher letter for the very same soil.
• Australia's south-east has been drying for decades, which pushes borderline sites up a class (Lopes & Osman 2010).

How the zones are drawn

Engineers use a wetness score called the Thornthwaite Moisture Index (TMI), built from rainfall and evaporation, to place each town in a climate zone, and each zone carries a design drying depth. Chan and Mostyn (2009) worked this out for 64 weather stations across NSW, building on an earlier Sydney map; Lopes and Osman (2010) did it for Victoria, and similar maps exist for Queensland. Maps from different authors use different assumptions and can disagree where they meet, so your engineer's zone call is a judgement informed by a map, not a lookup.

The practical takeaway: the zone for your lot is knowable before anyone visits, and it should be stated on any desktop assessment you get.

Sydney, Melbourne, Adelaide: three different problems

Sydney. The shale country of Western Sydney (the Bringelly Shale around the growth corridors) weathers to reactive clay, while the sandstone suburbs are often benign. The Standard has long given Sydney a larger surface drying allowance than most of the country (Chan & Mostyn 2009), and up the coast the Newcastle coal measures weather to more reactive clay again. Sydney's letters tend to be driven by which rock you are on.

Melbourne. The basalt plains to the west and north are the most reactive ground in the state. Li et al. (2016) tested 47 sites across 37 suburbs and found shrink-swell values above 6 percent strain per pF common in the west and north (roughly three times a typical Sydney residual clay), with huge variation inside the one geological unit. Melbourne's letters are driven by the basalt clay, and its climate zone has been moving (see below).

Adelaide. The Keswick and Hindmarsh clays under much of the metropolitan area are highly expansive and the climate is semi-arid, so the seasonal drying is severe and deep. Jewell and Mitchell (2009) measured how strongly vegetation and ground cover changed the seasonal heave at two reactive sites near the city. Adelaide's letters are driven by deep drying of very reactive clay, which is why its footing practice looks heavier than Sydney's for what sounds like similar ground.

So a builder's instinct that "H in Adelaide is not the same H as in Sydney" is half right: the letter means the same movement, but the ground and climate that produce it differ, and so does the footing that follows.

The zone is not fixed: a drying climate moves the letter

This is the part most buyers never hear. Lopes and Osman (2010) recomputed Victoria's moisture index across three twenty-year periods from 1948 and found the state has been drying since the late 1940s; rainfall alone understates it because temperature, wind and evaporation all dry the ground. Their conclusion for slabs is blunt: a 20 percent rise in predicted movement is enough to tip borderline S/M, M/H, H1/H2 and H2/E sites up a class, and they recommend the drier figures be reflected in classifications. Sun et al. (2017) projected the zones forward to 2030, 2050 and 2070 under climate-model rainfall and temperature changes, with the design drying depth moving deeper in parts of the state.

For a developer the implication is simple. A classification done on a 1990s climate zone may be a class light for a house that will stand until 2075. If a report is old, or borderline, that is a reason to ask the engineer which climate figures it used.

What to do with this

1. Ask which climate zone and drying depth the classification used. It is a line on the report. If it is missing, ask.
2. Do not carry a letter from one city to another. Same clay, different zone, different slab.
3. Treat borderline results as borderline. In a drying climate the next edition of the map may move them up.
4. Get the geology and zone before the soil test. Mapped geology, soils, nearby bore logs and the climate zone together give a probable class and tell the engineer what to brief. The field test then settles it.

Common questions

Why does the same soil get a different site classification in different cities? Because AS 2870 sets the depth of seasonal drying by climate zone. Drier climates dry the ground deeper, more clay moves, and the predicted movement, and therefore the letter, is higher for identical soil.

What is the Thornthwaite Moisture Index? A wetness score from rainfall and evaporation that engineers use to put a town in a climate zone for AS 2870. Each zone carries a design drying depth from 1.5 m in wet coastal areas to 4 m or more in arid country.

Can climate change my site classification? Over time, yes. Published work on Victoria shows a drying trend since the 1940s that can lift borderline sites a class, and projections to 2070 move some zones deeper.

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Find out which climate zone, geological unit and nearby bore profile a lot sits on before you commission the soil test. Generate a desktop report for the address or see a sample report.

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
• Jewell, S. A. and Mitchell, P. W. (2009). The Thornthwaite Moisture Index and seasonal soil movement in Adelaide. Australian Geomechanics, Vol 44 No 1. 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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