Reactive clay and subsidence risk by Australian city

Grounded in AS 2870 site classification as published by QBCC, plus state geoscience sources. General information, not advice for your specific property.

How Australia's AS 2870 reactive-clay classification works, and what the published geology of each major capital means for the chance of footing movement where you live.

Not every Australian capital carries the same subsidence risk, because the risk comes down to what is actually in the ground. Site classification under Standards Australia's AS 2870 is the same national system everywhere, grading a block from Class A (stable sand or rock, effectively no moisture-driven movement) up to Class E (extremely reactive clay). What differs city to city is how much of that city sits toward the reactive end of the scale, and which particular clay or sand formation is underneath it. The table below, published by the Queensland Building and Construction Commission as a plain-language guide to those same AS 2870 classes, shows roughly how much ground movement each class can produce.

Sydney: sandstone near the harbour, shale-derived clay to the west

Sydney's risk is uneven across the metro area. Much of the west and north-west, running from around Campbelltown and Liverpool through Parramatta and into parts of the inner west, sits on weathered Wianamatta Group shale, principally Ashfield and Bringelly shale, one of the more extensively studied reactive soils in the country. Closer to the harbour, sandstone footings are generally stable, so the concrete cancer common in older brick apartment blocks and balconies there owes more to waterproofing and reinforcement detailing than to the ground beneath them.

Melbourne: basalt-derived clay across the west and growth corridors

Melbourne sits on a mix of low-lying sedimentary ground and extensive basalt flows from the Newer Volcanics, a period of volcanic activity the Geological Survey of Victoria dates to roughly the last two million years, when hundreds of eruption points built up a basalt layer generally less than 50 metres thick across much of western Victoria. As that basalt weathers it forms residual clay that geotechnical literature classes as typically highly reactive, and it is concentrated across Melbourne's western suburbs, the city's western growth corridors, and out toward Geelong and the Western Freeway corridor, where engineers routinely apply AS 2870's full A to E scale (plus Class P, for problem sites such as filled or unstable ground). That distribution is a large part of why restumping, reblocking and underpinning work concentrates in those areas rather than spreading evenly across the city.

Brisbane and South East Queensland: reactive clay is the regional norm

The Queensland Building and Construction Commission's own consumer guide states plainly that the majority of Queensland homes are built on reactive clay soils. It is also the only regulator among the states whose published homeowner guide gives a concrete regional figure: on Class E sites, the most reactive, it notes that movement of 100 to 150mm has been recorded in South East Queensland, well beyond the general 75mm-plus definition of that class nationally. Combined with Brisbane's large stock of older high-set homes on individual timber or concrete stumps, and the cut-and-fill blocks common on the city's hills, that regional intensity is a big part of why restumping, reblocking and underpinning are such routine jobs here.

Perth: mostly stable sand, with clay increasing toward the Darling Scarp

Perth is the outlier among the mainland capitals, because most of its metropolitan footprint does not sit on reactive clay at all. The Western Australian government's own soil survey of the region, the Department of Primary Industries and Regional Development's Soils of the Swan Coastal Plain, describes the coastal belt as a series of dune systems built from sand eroded off the Yilgarn Block and reworked by wind and sea, with the ground further inland, the Pinjarra Plain, made up of material washed down off the Darling Scarp, where soils pick up more clay in a sand-over-clay, or duplex, profile. Free-draining sand does not shrink and swell the way clay does, so footing movement traced to soil reactivity is less common on the coastal sand belt than in the eastern capitals. When it happens there, it is more often linked to a specific cause, such as a leaking pipe washing sand out from under a footing, than to broad seasonal ground movement. That balance shifts further east, where the more clay-influenced soils near the scarp warrant the same AS 2870 assessment as anywhere else.

Adelaide: some of the country's most reactive metropolitan clay

Adelaide is frequently cited as sitting on some of the most reactive clay of any Australian capital. The Keswick and Hindmarsh Clays that underlie much of the Adelaide Plains, named for the suburbs where they were first studied in detail, push large areas of the city into the H1, H2 and E classes under AS 2870, and published local reporting on the geology puts ground movement on the most reactive of those sites at over 75mm in extreme drought or rainfall conditions, consistent with the general Class E threshold used nationally. Older bluestone and sandstone homes on shallow strip footings, common across Adelaide's established inner suburbs, tend to show that movement as cracking sooner than a newer home built to a modern engineered slab.

Wherever you are, the site classification of your specific block, not just the city-wide average, is what determines the real risk, and only a soil test or a structural engineer's report establishes that classification for your address. Treat everything above as context for that conversation, not a substitute for it.

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