Adelaide soil types by suburb: what is under your block

Ask an Adelaide builder what the ground is like and you will get an answer by suburb: the plains are clay, the beach suburbs are sand, the Hills are rock. That is broadly right, and the geological mapping backs it up. What it does not do is tell you your site classification, and the reason why is more interesting than it sounds.

This covers which formation sits under which part of Adelaide, what each one means for a build, and why two blocks on the same mapped unit can classify differently.

The one-minute version

• Most of the northern and central suburbs sit on the Pooraka Formation, a thick alluvial fan clay shed off the Mount Lofty Ranges. The inner east and south-east sit on Keswick Clay. Both are reactive.
• The beach suburbs sit on the Saint Kilda Formation and the Semaphore Sand Member: sand, shells and estuarine mud, with a high water table.
• The foothills and the Hills sit on Adelaide Geosyncline rock (Saddleworth Formation siltstone, Tapley Hill Formation, Aldgate Sandstone) with shallow soil over it.
• The formation name tells you the likely character of the ground. It does not set your class, because shrink-swell index measured on Adelaide clay varies far less between formations than most people assume. What varies is how much of the top four metres is clay at all.

What is under each part of Adelaide

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%

The plains clay: Pooraka Formation and Keswick Clay

These are the two units most Adelaide houses sit on. Both are alluvial fan deposits, laid down by creeks running west off the Mount Lofty Ranges, and both are reactive clay. The Pooraka Formation is the younger and more widespread of the two, running the length of the northern plains from Salisbury through to the western suburbs and down the coastal strip behind the dunes. Keswick Clay is the older unit and shows up through the inner suburbs.

What matters for a build is that these can be thick. Deep, continuous reactive clay is the profile that produces the higher site classifications, and Adelaide's climate zone drives seasonal moisture change four metres down into it.

The sand belt: Saint Kilda Formation and Semaphore Sand Member

The beach and estuary suburbs sit on a different world. Saint Kilda Formation is estuarine, with mud, sand and shell, and the Semaphore Sand Member is dune and beach sand over the top. The reactive clay problem largely goes away here. What replaces it is water: a shallow water table, batter stability in excavations, and, in the estuarine ground around the Port, soft compressible layers and the possibility of acid sulfate soils.

The foothills and the Hills: rock

From the foothills east, the mapping changes to Adelaide Geosyncline bedrock: Saddleworth Formation siltstone through Modbury and Golden Grove, Tapley Hill Formation through Happy Valley and Mount Barker, Aldgate Sandstone and the Barossa Complex through Stirling and Aldgate. Soil cover is thinner and often derived from the rock below it.

Different risks follow. Excavation gets expensive rather than easy, cut-and-fill platforms are the norm on sloping blocks, and slope stability and site drainage matter more than reactivity. The hazard checks change too: see what the Adelaide Hills bushfire and flood overlays actually say.

Why the mapped unit does not give you a site class

Here is the part that surprises people. The shrink-swell index, the laboratory measurement of how much a clay swells and shrinks per unit change in soil suction, has been measured across metropolitan Adelaide on hundreds of specimens. Sheard and Bowman (1996) reported it by mapped unit, and the median values sit in a narrow band across every unit in the metropolitan area, including the ones we call sand.

That is not a contradiction. It is telling you something specific: Adelaide clay is Adelaide clay, wherever you dig it up. The reason a Semaphore block behaves nothing like a Salisbury block is not that the clay is different. It is that there is far less clay in the Semaphore profile. What varies across the city is how much of the top four metres is reactive material at all, and that is a property of the profile, not of the formation name on the map.

Two practical consequences:

• A geology map is a starting point, not an answer. It tells you what to expect and where to sample. Nearby bore logs, which record what was actually hit and at what depth, do more work than the unit name.
• The suburb reputation can mislead in both directions. A block on mapped Pooraka clay with rock or sand close to the surface can classify lower than its neighbours. A beachside block with a clay lens in the active zone can classify higher.

The mechanism behind the classification itself is covered in five site classification myths on Adelaide clay, and the climate-zone effect that makes Adelaide harsher than Sydney or Melbourne in why the same clay classifies differently.

What to do with this before you build

1. Find the mapped unit for the lot, not for the suburb. Suburb boundaries cross geological ones routinely: Unley is 61% Keswick Clay and 39% Pooraka Formation.
2. Look for nearby bore logs. They tell you what is actually in the top few metres and how deep the clay runs.
3. Check the fill history. Fill is the most common cause of a Class P result and it is invisible on a geology map. Historical aerials and terrain will show a filled gully or a cut-and-fill platform.
4. On the sand belt, ask about water. Standing water level, not just soil type, drives the cost of footings and any basement or pool.
5. Then brief the soil test. Knowing whether you are on thick plains clay, estuarine sand or shallow rock tells the driller what depth to plan for, and tells you whether to allow for a high-class footing system 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. The foothills and Adelaide Hills are rock with thin soil cover.

Which Adelaide suburbs have the worst reactive clay? The question is better asked of the profile than the suburb. Deep, continuous clay on the alluvial fans of the northern and central plains produces the highest movement. But the class depends on how much of the top four metres is clay at the specific lot, so neighbouring blocks 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 field investigation with laboratory testing of the actual profile.

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See the mapped geology, nearby bore logs, terrain and groundwater for a specific Adelaide address in one document. Generate a desktop report, see a sample, or browse the South Australian suburb profiles.

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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
• 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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