Depth of suction change (Hs) in Adelaide: why it is 4 m

Adelaide sits in AS 2870 climate zone 5, which sets the depth of design suction change (Hs) at 4 metres. That is deeper than anywhere on the east coast: Melbourne works to roughly 2.3 m and Sydney to about 1.8 m. Hs is the depth over which soil suction, and so soil moisture, is assumed to change seasonally. A deeper Hs means more of the clay profile is active, more layers contribute to the characteristic surface movement (ys), and a given clay classifies into a more reactive site class than the identical clay would on the other side of the country. For anyone pricing footings, reading a classification, or scoping an investigation on an Adelaide lot, Hs is the input that explains most of the difference.

This is a technical read for engineers, surveyors and developers who already know the AS 2870 framework and want the detail behind the 4 m figure and its edge cases.

What Hs is and where the 4 m comes from

The characteristic surface movement ys is the sum of the movement of each clay layer down to a limiting depth, and that limiting depth is Hs. AS 2870 assigns Hs by climate zone, and the zones are read off a Thornthwaite Moisture Index (TMI) map (Sun et al. 2017). TMI combines long-run rainfall and evaporation into a single dryness number. The drier the climate, the deeper the ground dries out each year, and the deeper Hs runs.

AS 2870-2011 lists Hs by zone with a fixed surface suction change of about 1.2 pF applied at the ground surface for all zones (Sun et al. 2017):

Climate zone	Typical Hs	Representative capital
Zone 1	1.5 m	Wet tropics
Zone 2	1.8 m	Sydney
Zone 3	2.3 m	Melbourne
Zone 4	3.0 m	Inland temperate
Zone 5	4.0 m	Adelaide
Zone 6	greater than 4.0 m	Arid inland

Adelaide's placement in zone 5 is why it carries the deepest Hs of any major Australian capital. The TMI map in AS 2870 was built from climate data for roughly 1940 to 1960 (Sun et al. 2017), so it is old, and the suction depth it assigns is a convention rather than a site measurement. That distinction matters once you leave the standard behind for a real lot.

Why a deeper Hs raises the site class

For a given clay, ys scales with how much of the profile is treated as active. A 4 m Hs brings deeper clay layers into the summation that a 1.8 m Hs in Sydney would never reach. The same reactive clay therefore returns a larger ys in Adelaide, and a larger ys steps up the AS 2870 class. The class bands are fixed (Nguyen et al. 2024):

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

This is the mechanism behind the common observation that the same clay classifies differently in Sydney, Melbourne and Adelaide. It is not the clay that moved: it is Hs.

The shrink-swell index (Iss), measured in percent strain per pF, is the other half of the calculation. Work on the Darlington Upgrade on Main South Road in Adelaide's southern suburbs showed how wide the Iss range runs in local clays. For untreated cracked subgrade, an Iss of 0.5 percent per pF gave a characteristic surface movement of about 12 mm, while an Iss of 5.0 percent per pF gave about 122 mm (Gibbons & Cowan 2018). The Hindmarsh and Pooraka Formation clays tested there, alluvial fan sediments washed down from the Adelaide Hills, returned Iss values spanning roughly 0.5 to 5 percent per pF, plasticity indices from about 10 to 70 percent, and total suctions that frequently exceeded the apparatus limit of 4.7 pF across more than 250 suction tests (Gibbons & Cowan 2018). Pooraka Formation clay can reach around 30 m thick in that area, so there is plenty of reactive material within the 4 m suction depth.

Sensitivity: how much treatment depth Hs drives

For infrastructure work, where subgrade is treated to control movement, the sensitivity to Hs is direct. Published charts for Adelaide pavements model treatment depth against Iss for different suction depths and suction changes. For an 800 mm expressway pavement designed to a ys of 40 mm, moving from an Hs of 3.3 m to 4 m lifts the required treatment depth, and the effect compounds with higher Iss and larger suction change (Australian Geomechanics 2018). The same logic applies, scaled down, to residential footing systems: the deeper the active zone, the stiffer and deeper the slab or footing has to be to ride out the movement.

This is why reactive clay decisions are best made before, not after, the soil test. The groundwork for those decisions is covered in building on reactive clay.

Edge cases a desktop read should flag

Suction change at the edge versus the centre. Suction change is not uniform under a structure. Field observation reported decades ago, and reflected in current practice, is that suction change is lower directly under a well-built pavement or slab and higher at the verge or edge (Gibbons & Cowan 2018). Edge-heave and edge-dry effects are a real part of residential damage, and a single ys figure does not capture where on the slab the movement concentrates.

The TMI map is old. Because the AS 2870 zones derive from mid-twentieth-century climate data (Sun et al. 2017), a drying trend would push TMI lower and, in principle, deepen Hs further over time. Projection work for other states has modelled TMI shifts to 2070 (Sun et al. 2017). Adelaide's standing as a zone 5 city is unlikely to reverse under a drying climate.

Trees and abnormal moisture. ys from the soil profile is only part of the surface movement. Tree-induced movement is added separately where trees sit near the footprint (Nguyen et al. 2024), and large trees are one of the triggers that push a lot toward Class P.

A desktop report does none of this testing. It reports the climate zone and Hs that apply, names the likely geological unit, and sets out the Iss range reported nearby so you can brief the field work and budget realistically. It scopes the investigation. It is not an AS 2870 site classification, and nothing in it is footing design for a specific lot.

Common questions

What is the depth of suction change Hs in Adelaide?

Hs in Adelaide is 4 metres, because Adelaide sits in AS 2870 climate zone 5. That is the deepest design suction depth of any major Australian capital, and it means more of the clay profile contributes to the characteristic surface movement than in Sydney or Melbourne.

Why does Adelaide clay classify as more reactive than Sydney clay?

Because Adelaide's design suction depth Hs is 4 m against about 1.8 m in Sydney, more clay layers are treated as active in the ys summation. The same clay therefore returns a larger characteristic surface movement and a higher AS 2870 class in Adelaide.

What is a typical shrink-swell index for Adelaide clay?

Testing on Hindmarsh and Pooraka Formation clays in Adelaide's south returned Iss values from about 0.5 to 5 percent strain per pF (Gibbons & Cowan 2018). At the high end that corresponds to a characteristic surface movement of around 122 mm for untreated cracked subgrade, well into the extremely reactive range.

To see the climate zone, Hs and likely geological unit for a specific Adelaide address before you scope the field work, generate a desktop report for the address, or see a sample report first.

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
• Australian Geomechanics Society (2018). 2018 Seminar Design and construction of earthworks and pavements on expansive clay. Australian Geomechanics, AGS Public Resources page. 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
• Jim Nguyen, Lauren Amato, Paul Bailie and Peter Mitchell (2024). A framework for assessing subgrade shrink swell potential for newly constructed pavements on expansive soils in Adelaide, South Australia. Australian Geomechanics, 15th Young Geotechnical Professionals Conference (Nov 2024). 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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