Adelaide expansive clay: 5 site classification myths

Adelaide classifies house sites on expansive clay differently from the eastern capitals. Cameron (2018) describes the local practice: three boreholes on every house lot rather than one, the movement worked out for each, and the worst of the three adopted. The reason is gilgai, the mound-and-hollow pattern that shrink-swell clays form, which is common in Adelaide and makes the profile change within a few metres. Add a design drying depth of 4 m and the clay earns its reputation.

AS 2870-2011 calls that depth Hs, and why the same clay classifies differently in Sydney, Melbourne and Adelaide explains what it does to the numbers. It also sits behind five beliefs about Adelaide blocks, each partly right.

Myth 1: a soil or geology map tells you the site class

Mapping tells you which formation to expect. Across much of the plains that is red-brown Pooraka Formation clay over older Hindmarsh Clay, both laid down as alluvial fans off the Mount Lofty Ranges. On the Darlington upgrade of Main South Road, Gibbons and Cowan (2018) describe the Pooraka clay as usually less than 3 to 4 m thick, over Hindmarsh Clay up to 30 m thick.

What a map cannot tell you is how reactive the clay is on your lot. Along that 3.3 km of road, the shrink-swell index (Iss, how much the clay strains for each unit change in soil suction) ran from 0.5 to 5 percent per pF. The authors calculated about 12 mm of surface movement for untreated ground at the low end of that range and about 122 mm at the high end. That is the span from Class S to Class E, on ground the map shows as one formation.

A desktop report sets out the reactivity range to expect. The site investigation decides the class.

Myth 2: every Adelaide block is H2 or E

The class comes from a calculation of characteristic surface movement (ys, the predicted rise and fall of the ground surface), not from the suburb's reputation. The bands are the same in every state. What Adelaide adds is a "-D" suffix for deep-seated movement, because its Hs is 3 m or more:

Class	ys	Written in Adelaide (Hs 4 m)
S	up to 20 mm	S
M	over 20 to 40 mm	M-D
H1	over 40 to 60 mm	H1-D
H2	over 60 to 75 mm	H2-D
E	over 75 mm	E-D

On test, the red-brown clays that cover much of the plains are less reactive than Melbourne's basalt clays, and their results fall in a tight band (Cameron 2018). The high letters come from depth: 4 m of clay taking part in the movement. So a profile that is clay all the way down can reach H2-D or E-D, while a block with sand, calcrete or rock high in the profile can come out M-D or lower.

Myth 3: nothing moves below 4 metres

On an open site, the seasonal cycle does not reach that far. Cameron (2018) puts the depth of seasonal movement in Adelaide at about 2 m, half the 4 m design depth.

Trees change that. At Holden Hill, Jaksa et al. (2002) drilled at increasing distances from a single 20 m spotted gum on black earth and found the soil still drier than normal at 4 m deep, 5 and 10 m from the trunk. Beside a row of 20 to 25 m spotted gums at Netherby, the drying reached 18 m out from the row. The tree method added to AS 2870 in 2011 grew out of this South Australian work (Cameron 2018).

Past land use leaves the same mark. At Darlington, ground beside street trees and on former vineyard land was still dry at 6 to 10 m, and the designers treated those stretches as abnormal sites with their own suction profiles (Gibbons and Cowan 2018). On a house lot, tell the classifier about every tree near the footprint, including the neighbour's and any recently removed.

Myth 4: winter rain keeps the clay damp and stable

Adelaide averages about 530 mm of rain a year, most of it in the cool months, with long dry summers (Jewell and Mitchell 2009). The clay swings rather than settles. The clearest record is a black earth test site near Adelaide, with weathered shale at 4.5 m, that Mitchell monitored in 1978 and 1979. It came out of two drought years grazed bare and deeply cracked. When the rain arrived in April 1978 the ground started to rise, heaved 82 mm over that winter, then shrank quickly from October as the grass drew the water out.

The same site showed what steady moisture does. Once the dead grass was left as mulch instead of being grazed, the ground barely moved the following summer. Around a house the equivalent is paving and fall away from the walls, no watered beds against the slab and drainage finished with the house; building on reactive clay covers those decisions.

Myth 5: the reactivity on the report was measured in a lab

Often it was not. Cameron (2018) notes that very little reactivity testing has been done in South Australia since AS 2870 first appeared in 1986: experienced classifiers estimate the common clays by eye and feel, and AS 2870-2011 requires them to check those estimates against laboratory tests at regular intervals. That works well for the red-brown clays, whose results fall in a narrow band. It is weakest where the profile is unusual or the result sits on a class boundary.

If a block lands near a class boundary, or the footing price turns on one letter, ask whether a lab shrink-swell test on the critical layer is worth the fee. The free AS 2870 calculator shows how lab results turn into movement and a class.

What the class leaves to the engineer

The letter is where footing design starts, not where it ends. Fill, cut and fill on a slope, trees and drainage are dealt with on top of it. A desktop report puts the mapped formation, published soil reactivity, nearby bore records and terrain for an Adelaide address in one place so the investigation is scoped before the rig arrives. It is not a site classification: the site investigation decides the class, and the engineer 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

• Donald A. Cameron (2018). Dealing with reactive clay soils through a national standard. Australian Geomechanics, Volume 53, Number 1 (Mar 2018). geomechanics.org.au
• 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
• Mark B. Jaksa, William S. Kaggwa, John A. Woodburn and R. Sinclair (2002). Influence of large gum trees on the soil suction profile in expansive soils. Australian Geomechanics, Volume 37, Number 1 (Mar 2002). 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

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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