Form 1 and site conditions in SA: 7 geotech mistakes

The Form 1 vendor statement in South Australia discloses title, planning and legal matters about a property. It does not certify ground conditions, soil reactivity or the AS 2870 site class. Buyers and builders who treat the Form 1, or the absence of a red flag on it, as evidence that the ground is straightforward are the ones caught out later by reactive clay, fill or a Class P result.

Adelaide sits on some of the most troublesome expansive clay in the country, so the gap between what the paperwork says and what the ground does is wide here. These are the specific mistakes people make with ground conditions and desktop reporting before a soil test in SA, and what to do instead.

1. Reading the Form 1 as a statement about the ground

The Form 1 sets out easements, encumbrances, planning zone and notices under the Planning, Development and Infrastructure Act. It is silent on soil reactivity, fill history and founding conditions. Nothing on a clean Form 1 tells you the block will classify as Class S rather than Class H2 or E under AS 2870.

What to do instead: treat the Form 1 as the legal picture only. Order a separate desktop check of geology, soil mapping and groundwater, and budget for a site classification before you commit to a slab type.

2. Assuming Adelaide clay is only "moderately" reactive

Adelaide's fluvial Pleistocene clays, including the Keswick and Hindmarsh clays, hold less active clay minerals than Melbourne's basaltic clays, yet they cause more building damage. The reason is the depth of soil suction change in Adelaide's semi-arid climate, which drives movement well below the surface (Cameron 2018). Reactivity is not just about the clay mineral; it is about how deep the drying reaches.

What to do instead: expect deeper design moisture change than a wetter-climate reader would assume. On the Adelaide Plains, plan for a highly reactive result until the shrink-swell test says otherwise. Our guide to building on reactive clay sets out the decisions this affects.

3. Ignoring the formation named on the geological map

The geological unit under a block predicts its behaviour. On the Darlington upgrade in Adelaide's southern suburbs, the near-surface deposit is Pooraka Formation clay up to 30 metres thick, an alluvial fan clay derived from the Adelaide Hills with expansive characteristics (Gibbons & Cowan 2018). Two blocks a few streets apart can sit on different formations and classify differently.

What to do instead: identify the mapped formation before you price footings. A desktop report names the unit from the Geological Survey of South Australia mapping and flags whether it is a known reactive clay.

4. Trusting a single number for soil reactivity

Reactive clay is variable, even within one formation. Across the Pooraka and Hindmarsh clays at Darlington, the reported shrink-swell index (Iss, the strain in percent per unit of soil suction change) ranged from about 0.5 to 5 percent per pF, plasticity index from 10 to 70 percent, and linear shrinkage from 5 to 18 percent (Gibbons & Cowan 2018). One borehole result does not describe a whole block.

What to do instead: read published ranges as a spread, not a point value. Where a desktop check shows a formation with wide reactivity, expect the field classification to need enough boreholes to capture it. See how many boreholes a house block needs.

5. Treating fill as neutral ground

Fill changes the classification arithmetic and it does not show on a Form 1. A block that was cut and filled to level a slope, common on Adelaide's foothills fringe, may need a deeper investigation and can push the result toward Class P. Uncontrolled fill has no verified compaction and no cracked drying zone, so it moves differently from natural clay.

What to do instead: check the site history and mapping for filling before you buy. Sloped and infill blocks are the usual culprits. Our guide to fill on a building block covers what the records show.

6. Assuming trees and past land use do not matter

AS 2870 treats seasonal moisture change as only one cause of expansive movement. In Adelaide, vegetation and past drying dominate on many sites. On highly reactive black earth clay near Adelaide, a measured total heave of 82 millimetres was recorded during one winter, with weathered shale reached at 4.5 metres depth (Jewell & Mitchell 2009). Removed or established trees leave a moisture legacy that shifts the class.

What to do instead: note existing and recently removed trees, and long-dry ground, as abnormal moisture triggers. These are among the conditions that make a site Class P and require site-specific design rather than a standard class.

7. Skipping the desktop step and going straight to the soil test

Booking a site classification without any prior desktop check means the driller arrives blind to depth of fill, likely formation and groundwater. The Thornthwaite Moisture Index (TMI), used to estimate the depth of seasonal moisture change (Hs), sits at a mean near minus 17 for Adelaide over a 120-year record, with wide year-to-year variation (Jewell & Mitchell 2009). Knowing the climate driver and the mapped geology before the rig arrives scopes the job properly.

What to do instead: run a desktop report first. It sets expectations, names the formation, flags fill and groundwater, and tells the geotechnical engineer what to expect. It does not replace the field site classification, and nothing in it is engineering advice for your specific lot.

A quick reference

Mistake	The risk it creates
Form 1 read as a ground statement	No warning of reactive clay, fill or Class P
"Only moderate" reactivity assumed	Underestimated deep suction movement
Formation on the map ignored	Wrong footing budget
Single reactivity number trusted	Missed variability across the block
Fill treated as neutral	Deeper investigation, possible Class P
Trees and land use overlooked	Abnormal moisture missed
No desktop before the soil test	Driller works blind, scope wrong

Before you order a soil test in Adelaide, you can generate a desktop report for the address to see the mapped formation, fill and groundwater indicators, or see a sample report to check what it covers first.

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

• 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
• Donald A. Cameron (2018). Dealing with reactive clay soils through a national standard. Australian Geomechanics, Volume 53, Number 1 (Mar 2018). 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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