Adelaide soil types by suburb: what is under your block

Most of Adelaide's plains suburbs sit on reactive alluvial clay, the beach and estuary suburbs sit on sand and estuarine sediment, and the foothills and Hills sit on rock with thin soil over it. The mapped geology draws the lines clearly: Norwood is 90 percent Keswick Clay, Semaphore is 78 percent Semaphore Sand Member, and Aldgate is 92 percent Aldgate Sandstone.

What the map does not give you is a site classification. Within a suburb, what changes from block to block is how much of the top four metres is clay, and which clay it is.

What is under each Adelaide suburb?

Shares below are the mapped geology clipped to each suburb boundary.

Suburb	Dominant unit	Share
Salisbury	Pooraka Formation	100%
Mawson Lakes	Pooraka Formation	97%
Elizabeth	Quaternary alluvial sediments	100%
Munno Para	Quaternary alluvial sediments	88%
Blakeview	Quaternary alluvial sediments	87%
Norwood	Keswick Clay	90%
Unley	Keswick Clay	61%
Prospect	Pleistocene calcrete	84%
Woodville	Pooraka Formation	55%
Brighton	Pooraka Formation	79%
Port Adelaide	Saint Kilda Formation	83%
Henley Beach	Saint Kilda Formation	63%
Glenelg	Saint Kilda Formation	50%
Semaphore	Semaphore Sand Member	78%
Largs Bay	Semaphore Sand Member	64%
Modbury	Saddleworth Formation	43%
Golden Grove	Saddleworth Formation	52%
Aberfoyle Park	Tertiary sediments	64%
Happy Valley	Tapley Hill Formation and Tertiary sediments	mixed
Stirling	Aldgate Sandstone	59%
Aldgate	Aldgate Sandstone	92%
Mount Barker	Tapley Hill Formation	45%

Suburb boundaries cross geological ones all the time. Unley is 61 percent Keswick Clay and 39 percent Pooraka Formation, so look up the unit for the lot, not the suburb.

What is the clay on the Adelaide Plains?

Three units, all laid down as alluvial fans by creeks running west off the Mount Lofty Ranges. The standard reference for them is Sheard and Bowman (1996).

• Pooraka Formation is the youngest and the most widespread, running from Salisbury through the western suburbs and down behind the coastal dunes. On the Darlington upgrade of Main South Road, Gibbons and Cowan (2018) describe it as red-brown clay with limey inclusions and the odd sand or gravel zone, usually less than 3 to 4 m thick.
• Hindmarsh Clay lies beneath it: grey-green to red-brown clay with sand and gravel lenses. At Darlington it was up to 30 m thick.
• Keswick Clay is an older plains clay that comes to the surface in the inner east and south-east, under Norwood and much of Unley.

They do not test the same. Cameron (2018) shows that the red-brown clays fall in a tight band of shrinkage index (the lab measure of how much a clay swells and shrinks per unit change in soil suction), rarely much above 4 percent per pF. The grey-green "gleyed" clays, coloured by long periods of waterlogging, which include the Keswick and Hindmarsh clays, test higher.

Either way, Adelaide's high site classes owe as much to its 4 m design drying depth as to the clay itself, which why the same clay classifies differently explains.

Are the beach suburbs better to build on?

For reactivity, generally yes. Port Adelaide, Henley Beach and Glenelg sit mostly on the Saint Kilda Formation, an estuarine mix of mud, sand and shell, and Semaphore and Largs Bay on the dune and beach sand of the Semaphore Sand Member. The reactive clay problem largely goes away.

Water replaces it: a shallow water table, unstable sides in excavations and, in the estuarine ground around the Port, soft compressible layers and possible acid sulfate soils. Standing water level, more than soil type, sets the cost of footings, basements and pools here.

What changes in the foothills and Hills?

From the foothills east the mapping turns to bedrock: Saddleworth Formation siltstone through Modbury and Golden Grove, Tapley Hill Formation through Happy Valley and Mount Barker, Aldgate Sandstone through Stirling and Aldgate. Soil cover is thinner and often weathered from the rock below.

Excavation gets harder rather than easier, most sloping blocks are built on cut-and-fill platforms, and slope stability and drainage matter more than reactivity. The planning hazards change too: see Adelaide Hills bushfire and flood overlays.

Why can't the geology map give you a site class?

Because the class depends on the profile, and the profile varies inside a mapped unit. A block on mapped Pooraka clay can have calcrete, sand or gravel in the top few metres and classify lower than its neighbours. A block on the sand belt can have a clay lens in the active zone and classify higher.

Adelaide clay also varies over very short distances. Gilgai, the mound-and-hollow pattern that shrink-swell clays form, is common here, and the profile can change within a few metres. That is why Adelaide classifiers drill three holes on a house lot and design to the worst of them (Cameron 2018). Five site classification myths on Adelaide clay shows how wide the reactivity spread can be on one mapped formation.

So the map is a starting point. Nearby bore logs, which record what was hit and at what depth, do more work than the unit name, and the site investigation decides the class.

What should you check before you build?

1. The mapped unit for the lot, not the suburb.
2. Nearby bore logs. They show how deep the clay ran close by. That is a guide to your lot, not a measurement of it.
3. Fill history. Fill is a common cause of a Class P result and does not show on a geology map. Historical aerials and terrain can show a filled gully or a cut-and-fill platform; only the site investigation confirms whether fill is there and how deep. The AS 2870 rules are in uncontrolled fill.
4. On the sand belt, groundwater. Ask for the standing water level, not just the soil type.
5. Then brief the soil test. Knowing whether the lot sits on plains clay, estuarine sand or shallow rock tells the driller what depth to plan for, and tells you whether to allow for a heavier footing before the builder's quote is locked.

Common questions

What type of soil does Adelaide have? Most of the metropolitan plains are reactive alluvial clay, principally the Pooraka Formation and Keswick Clay. The beachside and estuary suburbs are sand and estuarine sediment of the Saint Kilda Formation and Semaphore Sand Member. The foothills and Adelaide Hills are Adelaide Geosyncline rock with thin soil cover.

Which Adelaide suburbs have the worst reactive clay? The largest movements come from profiles that are clay through most of the top four metres, especially where the more reactive grey-green clays such as the Keswick Clay are near the surface. But the class depends on the profile at the specific lot, so neighbouring blocks on the same mapped unit can differ.

Is beachside Adelaide sand better to build on? For reactivity, generally yes. For everything else, not necessarily: a shallow water table, soft estuarine layers and possible acid sulfate soils bring their own costs.

Can I tell my site classification from the geology map? No. The map tells you the likely material and where to sample. The classification comes from a site investigation of the actual profile on the lot.

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Sources

• Sheard, M. J. and Bowman, G. M. (1996). Soils, Stratigraphy and Engineering Geology of Near Surface Materials of the Adelaide Plains. South Australia Department of Mines and Energy, Report Book 94/9. pid.sarig.sa.gov.au
• Cameron, D. A. (2018). Dealing with reactive clay soils through a national standard. Australian Geomechanics, Vol 53 No 1. geomechanics.org.au
• Gibbons, P. and Cowan, S. (2018). Subgrade treatment design for expansive soils in Adelaide using Australian Standard AS2870 - is this the right approach? Australian Geomechanics, Vol 53 No 1. geomechanics.org.au
• Geological mapping: SARIG 1:100,000 surface geology, Geological Survey of South Australia.

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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