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 says nothing about the ground: not the soil reactivity, not whether there is fill, not the AS 2870 site class. The nearest it comes is the buyer's information notice attached to it (Form R3), a list of questions for you to consider. "Has fill been used on the site?" sits there between termites and cooling towers. It is a question for you to follow up, not an answer from the vendor.

On Adelaide's clay that gap matters. Here are seven mistakes buyers and builders make with the ground 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 is the vendor's statement under section 7 of the Land and Business (Sale and Conveyancing) Act 1994. It covers mortgages, easements and other encumbrances, the Planning and Design Code zone, subzone and overlays, council and government charges, building indemnity insurance, and notices or orders registered against the land, such as Environment Protection Authority site contamination orders. A clean Form 1 does not tell you whether the block will come out Class S or Class E.

Instead: read it as the legal and planning picture only. Ask the vendor directly about fill, cracking and drainage problems, and budget for a site classification before you commit to a slab type.

2. Assuming Adelaide clay is only "moderately" reactive

On test, Adelaide's common red-brown clays are less reactive than Melbourne's basalt clays, yet they cause more trouble (Cameron 2018). The reason is depth: AS 2870-2011 sets Adelaide's design depth of seasonal drying at 4 m, about double Melbourne's, so far more clay takes part in the movement. Why the same clay classifies differently in Sydney, Melbourne and Adelaide works through it.

Instead: on the Adelaide Plains, allow for a highly reactive result (H1-D, H2-D or E-D) until the site investigation says otherwise. Building on reactive clay sets out the design decisions this affects.

3. Ignoring the formation on the geological map

The mapped unit under a block is the best early guide to how it will behave. On the Darlington upgrade of Main South Road, the near-surface unit is red-brown Pooraka Formation clay, usually less than 3 to 4 m thick, over Hindmarsh Clay up to 30 m thick, both laid down as alluvial fans off the Adelaide Hills (Gibbons and Cowan 2018). Elsewhere the near-surface ground can be beach sand and estuarine mud near the coast, or rock with thin soil in the foothills.

Instead: find the mapped unit for the lot before you price footings. Adelaide soil types by suburb shows which units sit where.

4. Relying on a neighbour's soil report

Reactive clay varies over short distances even within one formation. Along the 3.3 km Darlington alignment, the shrink-swell index (Iss, how much the clay strains per unit change in soil suction) ran from 0.5 to 5 percent per pF (Gibbons and Cowan 2018). Adelaide practice already allows for this: classifiers drill three holes on every house lot, because the ground's mound-and-hollow gilgai pattern makes the profile change within a few metres, and they design to the worst of the three (Cameron 2018).

Instead: treat a neighbour's report or a published value as one point in a spread. Your lot needs its own investigation; how many boreholes a house block needs covers the minimums.

5. Treating fill as neutral ground

Fill changes the classification, and the Form 1 does not disclose it. Under AS 2870, uncontrolled fill (placed without compaction control or testing) makes a site Class P unless every footing is taken through it to natural ground, and controlled fill deeper than 0.4 m (0.8 m for sand) also makes a site Class P unless an engineer reclassifies it. Cut-and-fill platforms on the foothills fringe and filled gullies or old dams on infill lots are the usual places. Whether a particular block has fill, and how deep, can only be found by drilling on it.

Instead: ask the vendor the Form R3 question in writing, check old aerial photos for earthworks, and read fill on a building block for what the records can show.

6. Assuming trees and past land use do not matter

Trees dry clay well below the seasonal zone, and in Adelaide that drying can reach past 4 m (see Adelaide expansive clay myths). A tree removed before building leaves a dry zone that slowly wets up and swells. AS 2870 treats reactive sites with abnormal moisture conditions, which include trees close enough to matter and recent tree removal, as Class P, needing site-specific design. South Australian engineers worked out rules for designing near trees in the 1990s, and those became the tree method in AS 2870-2011 (Cameron 2018).

The Form 1 is relevant here. Its planning section lists the Regulated and Significant Tree Overlay where it applies and asks whether any significant tree is declared on the land. A regulated tree on or next to the block may not be removable without approval, so the footing may have to be designed for it staying.

Instead: record existing and recently removed trees, including the neighbour's, and give the list to whoever does the site classification.

7. Going straight to the soil test

A classifier who arrives without the background works out the site from the boreholes alone. Mapped geology, nearby bore logs, groundwater records, terrain and historical aerials (old orchards, vineyards, dams, demolished buildings) tell them what to look for and where to drill, and tell you whether to budget for a heavier slab before the builder's quote is locked.

Instead: run a desktop check first. It names the formation, flags likely fill and groundwater, and briefs the engineer. It is not a site classification and not engineering advice for your lot: the site investigation decides the class.

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

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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