Geotechnical desktop study mistakes: 7 NSW pitfalls

A geotechnical desktop study in New South Wales reads the published record for an address - geology, soil landscapes, groundwater bores, acid sulfate soil risk mapping and the planning certificate - and sets expectations before anyone drills. It scopes the field investigation. It does not replace a site classification under AS 2870, and nothing in it is engineering advice for a specific lot. The mistakes below are the ones that turn a useful desktop study into a misleading one, drawn from how NSW ground and NSW paperwork behave.

Here are seven that recur on NSW sites.

1. Treating a section 10.7 certificate as a geotechnical clearance

A section 10.7 planning certificate lists the hazards a council has mapped for a lot: landslip, flooding, acid sulfate soils, bushfire. It does not report soil reactivity, bearing capacity or fill depth, and a blank line is not a statement that the ground is sound. Councils map at different resolutions and update on different cycles, so absence on the certificate means not mapped, not safe. Read the certificate for what it flags, then go to the soil landscape and geology mapping for the ground itself. Our guide to reading a section 10.7 certificate for ground hazards walks through which clauses matter.

2. Assuming fill keeps the natural classification

Fill is the most common reason a Sydney or Newcastle infill lot ends up as Class P. AS 2870 only lets controlled fill, placed and tested to the earthworks standard, keep the natural site classification where it is shallow, and the allowance differs by material: a thin layer for clays and a thicker one for sand. Anything deeper, or any fill that was not placed and documented as controlled, puts the burden on the engineer to assess movement or to take footings through to natural ground.

The trap is that "compacted" does not equal "controlled", and bearing values recover slowly. Recent testing presented at the AGS Sydney symposium found that fill soaked for several days gave markedly lower strength than the same material tested dry (Australian Geomechanics 2025), which matters on lots that sit wet for part of the year. Uncontrolled fill: how to tell if a block has it covers what the records reveal before you drill.

3. Skipping acid sulfate soils on coastal lots

Acid sulfate soils sit across the NSW coastal lowlands - the Shoalhaven, Hunter and Richmond floodplains are classic examples - and disturbing them below the water table can release acid that attacks concrete and steel footings. A desktop study should check the acid sulfate risk mapping and the likely depth, because the problem only appears when excavation or dewatering exposes the layer. Work on the Shoalhaven floodplain has modelled treating acidic groundwater in place with permeable reactive barriers (Australian Geomechanics 2015), which gives a sense of how persistent the chemistry is once the ground is drained. If you plan a basement or deep footings near the coast, flag it in the desktop stage, not on site.

4. Reading bore logs as if they describe your lot

Groundwater bores in the NSW records tell you what one driller logged at one point, often decades ago and sometimes hundreds of metres away. A standing water level recorded in 1985 is not today's level, and the lithology in a neighbour's bore can change across a single block where an old creek line runs. Use bore logs to build a geological picture and to spot a shallow water table, not to fix the water level under a footing. We set out the specific traps in reading groundwater bore logs: 7 common mistakes.

5. Ignoring reactive clay because the suburb "isn't known for it"

Shrink-swell clays appear across Sydney, from the shale-derived soils of the Wianamatta Group on the Cumberland Plain to pockets of weathered material elsewhere. Reactivity is driven by the clay mineralogy and the depth of seasonal moisture change (Hs), not by a suburb's reputation. The shrink-swell index (Iss) from a remoulded sample also carries real uncertainty: laboratory work reported to the ANZ geomechanics conference found that test method affects the measured result enough to shift a site classification (Australian Geomechanics 2015). Treat a desktop estimate of Class M or H as a prediction to be confirmed by field sampling, not a settled answer.

6. Forgetting that flooding changes the ground, not just the floor level

A flood study tells you the water level. It does not tell you what the inundation does to the soil beneath a slab. Expansive clays, soft alluvium and uncontrolled fill can behave differently after a major flood event, with increased differential settlement and greater than expected shrink-swell movement (Australian Geomechanics 2025). On a Hunter or Hawkesbury-Nepean floodplain lot this is a foundation question as much as a habitable-floor question, and it belongs in the desktop study alongside the depth and velocity figures.

7. Expecting the desktop study to be the site classification

The desktop study sets expectations and scopes the drilling. It cannot assign a final AS 2870 class, because that needs samples and in-situ testing from the lot itself. A desktop review that points to Class M clay and possible edge fill tells the field crew where to put the boreholes and how deep to take them; it does not let anyone sign a classification certificate. Geotechnical report versus desktop assessment explains where the line sits and why both stages exist.

Getting the desktop stage right

Used properly, the desktop study saves money by scoping the field investigation instead of pretending to replace it. You start drilling knowing the likely geology, the fill risk, the acid sulfate flag and the groundwater picture, so the field work confirms or overturns a stated expectation rather than starting blind.

For a NSW address, you can generate a desktop report for the address or see a sample report to check what the compiled record shows before you commission the drilling.

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

• Australian Geomechanics Society (2015). Proceedings of the 12th Australia New Zealand Geomechanics Conference. Australian Geomechanics, 12th Australia and New Zealand Conference on Geomechanics (Feb 2015). geomechanics.org.au
• Australian Geomechanics Society (2025). Proceedings of the 2025 Australian Geomechanics Society Sydney Chapter Symposium. Australian Geomechanics, 2025 Sydney Symposium (Nov 2025). 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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