Site classification in Brisbane and South East Queensland

Site classifications across Brisbane and South East Queensland commonly come back M, H1, H2 or E, because much of the region sits on clay weathered in place from the rock below, and the climate dries that clay to a depth that varies across the region. Coastal sand and shallow rock classify low. Filled gullies and cut-and-fill platforms often classify as P.

Climate matters as much as the clay. When the 1996 edition of AS 2870 changed the depth of seasonal drying for Ipswich and the size of the cracked zone, Bruce Hargreaves worked through what it did to a highly reactive clay site there. A site that calculated at 65 mm of movement under the old settings came out at 115 mm under the new ones: same clay, same method, a jump from the old Class H well into E (Hargreaves 2005). Nothing in the ground had changed; the inputs had. (If the letters are new to you, our AS 2870 explainer covers them.)

What ground is under South East Queensland?

Thorley (2001), writing a guide for engineers new to the region, describes three kinds of ground:

Soft recent clays along the coast and rivers. Gold Coast to Sunshine Coast, strongest near the shore and along the big river channels. They settle under load, and in low-lying areas below about 10 m AHD (Australian Height Datum, roughly sea level) they and the coastal sands can be acid sulfate soils. This is the ground under many canal and lowland estates.

Coastal sands. The Gold and Sunshine Coast dunes and the islands, often with a cemented "coffee rock" layer and a high water table. Usually kind to slabs, harder on excavations.

Older alluvium and residual clays everywhere else. Residual clay formed in place from weathered rock, plus older river deposits. These produce the M, H and E results. Thorley expected Class H and E sites anywhere in the region, coast included, and more of them inland toward Ipswich. Look (2016) cites figures putting residual soils at 84 percent of SEQ by area, and Look (2005) puts reactive clays across more than a third of Queensland.

Residual clay is also hard to judge from a quick lab test. It is gritty, and the standard plasticity test throws away everything coarser than 0.425 mm, often a large share of a residual sample. Look (2016) found this makes the plasticity result overstate how reactive these clays are. The AS 2870 calculation runs on the shrink-swell index, and mapped geology, which gives you the parent rock, is a better first guide than a single index test.

Ipswich has one more check: old underground coal workings, with a history of surface collapses and sinkholes (Thorley 2001). The government bought back many unsuitable lots in the early 1990s and made them parks, and newer estates leave such areas out of the buildable land (Hargreaves 2005).

Why do Brisbane, Ipswich and Toowoomba get different letters for similar clay?

The depth that seasonal wetting and drying reaches, which AS 2870 calls Hs, is set by climate zone, not by your block. Deeper means more clay moving and a higher letter.

AS 2870-2011 gives Brisbane and Ipswich an Hs of 1.5 to 2.3 m and Toowoomba 1.8 to 2.3 m, with the lower values for wetter areas near the coast or in the hills. Those ranges date from the 1996 edition, and Hargreaves (2005) describes how they played out. Before 1996, Brisbane's 1.5 m was used across SEQ because consultants had no way to work out anything else. The 1996 edition gave ranges but no map of the boundaries. Toowoomba consultants agreed a local boundary. In Brisbane there was no consensus: most treated Brisbane City as 1.5 m and the Ipswich area as 2.3 m, and the new estates in the corridor between them got whatever value the consultant chose.

Mapping followed. Fox (2002) calculated the Thornthwaite Moisture Index, a wetness score from rainfall and evaporation, across Queensland from weather records averaging 88 years, and showed how far a short record can mislead. Reading the later maps, Hargreaves concluded that only a small part of SEQ sits in the shallowest 1.5 m zone, and that the deeper 3.0 m zone reaches the country just west of Toowoomba.

So the same clay profile can be an M in inner Brisbane and an H further out, and two consultants can differ on a corridor estate. The report should state the Hs it used. If it does not, ask.

Why can a cut-and-fill pad classify worse than the natural ground?

Most SEQ estates are earthworked into level pads. Natural clay has a cracked upper zone that takes up some of the movement; freshly compacted clay fill does not. AS 2870-2011 takes the cracked zone as zero in controlled clay fill placed less than five years before the house is built. In Hargreaves' worked example, a site cut 1.2 m and filled 1.2 m with the same clay, the predicted movement of the fill came out 50 to 66 percent higher than the natural ground across SEQ's climate zones.

Cuts carry their own warning. Hargreaves argued that where a cut is planned, the minimum hole depth should be measured from the finished pad level, not the original surface, because some SEQ profiles get more reactive with depth and a cut can bring that clay up to slab level.

Fill also has its own rules. Controlled fill (placed and tested to AS 3798) deeper than 0.4 m, other than sand, makes a site P unless an engineer reclassifies it. Uncontrolled fill (an old gully or dam filled without records) makes it P unless it is shallow enough for every footing to be taken through it to natural ground. It is a common reason an SEQ block comes back P rather than a clean letter. We cover uncontrolled fill in detail.

How are lowland estates from Noosa to Coolangatta classified?

Hargreaves (2005) singles out the coastal estates on filled swamp and estuarine ground, where soft layers at depth keep settling for years. These estates are usually designed up front: investigation sometimes deeper than 30 m, preloading, controlled fill depths, and a target such as Class M or H slabs.

He wrote that the standard is unclear on how these lots should be classified. The "suitable for Class H" wording on such a lot is a performance target that holds only up to an assumed house-and-landscaping load, not a measured reactivity. Build heavier than that, or landscape with more fill, and the long-term settlement can exceed what the slab was designed for. Canal lots add another limit: the batters down to the revetment wall are generally not preloaded, so in some cases they can carry little more than the weight of about half a metre of compacted fill, which rules out high-level footings and sometimes even a pool. If you are buying in one of these estates, ask for the estate geotechnical report and the house load it assumed.

What should you check before you sign?

1. The mapped geology and the climate zone for the lot. Residual clay inland of Brisbane on a 2.3 m zone is a different budget from dune sand at the coast.
2. The fill history. Historical aerials and elevation show cut-and-fill platforms, filled gullies and old dams. Nearby bore logs show what the natural profile is.
3. Floodplain, acid sulfate, landslide and mine subsidence overlays. All are mapped and all change the investigation.
4. Then the soil test. A desktop picture does not replace it, but it tells you what to brief (how many holes, how deep, whether to look for fill or soft ground) and whether to allow for an H slab before the quote is locked. AS 2870 asks for at least one borehole per site, and three where Hs is 3 m or more, as it is west of Toowoomba, or where the profile is known to vary. The site investigation decides the letter.

Common questions

What site classification is typical in Brisbane? There is no typical. Engineers working in the region report that H and E results occur across SEQ including the coast and become more common inland toward Ipswich, while coastal sands can be A or S. The mapped geology and the climate zone for the lot are the best early guide.

Why did my neighbour's block get a different classification? Different natural profile, a fill edge running through one block, a different climate-zone call by a different consultant, or trees and drainage that triggered a P. Adjacent lots often match, but the edges of estates and filled gullies are where they diverge.

Can I find out my likely site class before a soil test? You can get an expected range from the mapped geology, soils, nearby bore logs and the climate zone. Only a field investigation gives the classification a certifier will accept.

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See which geological unit, climate zone, fill history and overlays a SEQ lot sits on before you price the slab. Generate a desktop report for the address or see a sample report.

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

• Fox, E. (2002). Development of a map of Thornthwaite Moisture Index isopleths for Queensland. Australian Geomechanics, Vol 37 No 3. geomechanics.org.au
• Hargreaves, B. (2005). 20 years of AS 2870 in South East Queensland: a personal view. Australian Geomechanics, Vol 40 No 3. geomechanics.org.au
• Look, B. G. (2005). Equilibrium moisture content of volumetrically active clay earthworks in Queensland. Australian Geomechanics, Vol 40 No 3. geomechanics.org.au
• Look, B. G. (2016). The weighted plasticity index in road design and construction. Australian Geomechanics, Vol 51 No 3. geomechanics.org.au
• Thorley, C. (2001). Welcome to geotechnical engineering in South East Queensland: soft clay one day, hard rock the next. Australian Geomechanics, Vol 36 No 3. 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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