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

Take a sample of clay from Melbourne's western basalt plains and one from Adelaide's Keswick Clay, and test both. The Melbourne clay will usually come out more reactive, meaning it swells and shrinks more for the same change in moisture. Yet it is Adelaide's clay that engineers have long known causes more trouble (Cameron 2018). The reason is climate: AS 2870 sets how deep the ground dries and wets each year by where the site is, and in Adelaide that depth is about double Melbourne's.

That one input explains most of why the same soil can earn a different letter in different cities.

What does climate have to do with a site class?

A site classification (A, S, M, H1, H2, E or P) is a prediction of how far the ground surface will rise and fall over the seasons. The calculation multiplies two things together:

• how reactive the clay is, measured in the lab as the shrink-swell index (Iss), and
• how much of the profile takes part, which is the depth the seasonal wet-dry cycle reaches. AS 2870 calls this Hs, the depth of design suction change.

Put the same clay in a profile that dries to 4 m instead of 1.5 m and far more of it goes into the sum. That alone can move a site up one or more classes. Our AS 2870 explainer walks through the classes.

How deep does the ground dry in each city?

AS 2870-2011 lists Hs for the major centres. The lower end of each range applies to the wetter coastal or hill suburbs:

City	Hs (depth of seasonal drying)	Cracked zone used in the calculation
Sydney	1.5 to 1.8 m	0.5Hs
Brisbane / Ipswich	1.5 to 2.3 m	0.5Hs
Melbourne	1.8 to 2.3 m	0.75Hs
Perth	1.8 m	0.5Hs
Adelaide	4.0 m	0.75Hs

Two other details matter. The cracked zone is the top part of the profile where shrinkage cracks open in dry weather, and the calculation lets clay in that zone move more freely, so Melbourne and Adelaide carry an extra allowance there. And where Hs is 3 m or more, as in Adelaide, the class gets a "-D" suffix (H1-D, for example) to flag deep-seated movement. The surface moisture swing itself is the same 1.2 pF in every city in the table, so the difference really is depth.

Outside the listed cities, the engineer works out the zone from the Thornthwaite Moisture Index (TMI), a wetness score built from rainfall and evaporation. Chan and Mostyn (2009) did this for 64 weather stations in NSW and Lopes and Osman (2010) for Victoria. Maps by different authors do not always agree where they meet, so the zone is a judgement, not a lookup, and it should be written on the report.

Why do the three cities behave so differently?

Sydney has the shallowest drying depth of the three, so the letter is driven mostly by which rock you sit on. The Bringelly Shale of Western Sydney weathers to reactive clay; the sandstone suburbs are often close to benign.

Melbourne has the most reactive common clay: the basalt plains to the west and north. Li et al. (2016) tested 47 sites in 37 suburbs and found shrink-swell values above 6 percent per pF were common there, with a wide spread inside the one geological unit. Here the letter is driven by the clay itself.

Adelaide's red-brown clays are less reactive than Melbourne's basalt clays, and their test results fall in a fairly tight band (Cameron 2018). What makes them difficult is the 4 m drying depth. More clay takes part, and the footing has to cope with it, which is why Adelaide slabs look heavy next to Sydney slabs on what sounds like similar ground. More in our Adelaide expansive clay myths piece.

So the builder who says "an H in Adelaide is not the same as an H in Sydney" is half right. The letter means the same predicted movement in both places. The ground and climate that produce it are different, and so is the footing detailing that follows.

Can a drying climate change the class?

It can, and Victoria is the best-documented case. Lopes and Osman (2010) recalculated the state's TMI over three 20-year periods from 1948 and found it has been getting drier since the late 1940s, by more than rainfall alone suggests, because temperature, wind and evaporation all count. They estimated that a 20 percent increase in predicted movement is enough to push a borderline S/M, M/H, H1/H2 or H2/E site up a class. Sun et al. (2017) projected the zones forward to 2070 and found the drying depth moving deeper in parts of the state.

AS 2870 designs a house for a 50-year life. A classification sitting at the top of its band, calculated on older climate figures, is worth a second question.

What should you ask for?

1. The climate zone and Hs the classification used. It belongs on the report. If it is missing, ask.
2. No letters carried between cities. Same clay, different depth, different slab.
3. A second look at borderline results, especially in Victoria.
4. The geology and zone before the soil test. Mapped geology, soils, nearby bore logs and the climate zone give the reactivity range to expect, so the engineer knows what to look for. The site investigation then decides the class.

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, 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
• Chan, I. and Mostyn, G. (2009). Climatic factors for AS2870 for New South Wales. Australian Geomechanics, Vol 44 No 2. 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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