Geotechnical report vs desktop assessment: the difference

A geotechnical report is based on drilling, sampling and testing on the lot itself, while a desktop assessment is compiled from existing records such as geological maps, soil surveys, nearby borehole logs and groundwater databases, with no fieldwork. The geotechnical report is what a building certifier accepts for footing design. The desktop assessment is for deciding, pricing and scoping before that point.

Both terms are used loosely. "Geotechnical report" covers anything from a two-hole AS 2870 site classification to a multi-volume basement investigation, and a desktop assessment also goes by desktop study or preliminary geotechnical assessment. What a geotechnical desktop study covers goes into the desktop side in more depth.

How do they compare side by side?

	Desktop assessment	Geotechnical report (site investigation)
Based on	Existing records: geological and soil mapping, nearby borehole logs, groundwater bore records, terrain, historical aerial imagery, hazard overlays	Boreholes or test pits on the lot, in-situ tests such as penetrometer readings, soil samples, often laboratory testing
Site visit	None	Yes, usually a few hours of drilling for a house
What it tells you	The mapped geology and how that unit usually behaves, an indicative reactivity range, likely groundwater depth, slope, and flood, acid sulfate and landslide overlays	The soil profile on the lot, identified or measured reactivity, depth to rock or water where the holes meet them, and any fill the holes find
AS 2870 site classification	No. It can indicate likely reactivity but cannot classify the site	Yes. This is where the class (A, S, M, H1, H2, E or P) comes from
Accepted for footing design	No	Yes, when signed by a suitably qualified engineer
Class P problems (fill, soft ground, landslip)	Flags the risk from records	Investigates it on the lot; a Class P site may need more than the standard scope
Typical cost, 2026	$119 automated; about $800 to $1,200 from a consultant for a residential lot	From about $700 for a two-hole classification, $1,200 to $2,500 with footing design, $3,000+ for a full investigation with laboratory testing
Turnaround	Minutes (automated) to 1 to 2 weeks (consultant)	1 to 3 weeks for a classification, 3 to 6 weeks for a full investigation
Best used	Before purchase, at feasibility, comparing lots, briefing the investigation	Once you own the block and before the footing design is locked

The cost figures match the geotechnical report cost guide, which also explains what moves a quote and why an under-scoped investigation costs more later.

Which one do you need?

It depends on the stage of the project.

• Deciding whether to buy, or comparing several blocks: a desktop assessment. Drilling on land you do not own needs the owner's permission and is rarely practical before settlement, and the records are enough to show which blocks carry ground risk worth pricing in.
• Designing and approving a house: a geotechnical report, in the form of an AS 2870 site classification. The certifier expects the class before the slab is designed.
• A basement, multi-storey building, retaining walls, or a steep or filled lot: a full geotechnical investigation, scoped from what the desktop records show.
• Cracking, a dispute or an insurance claim: usually both. The desktop work establishes what the ground was before anyone built on it, and the fieldwork shows what is there now.

Why are they stages rather than alternatives?

Because each does a job the other cannot. Snowy 2.0 is the extreme case. Kelly et al. (2020) describe an investigation inside a national park, where no new tracks could be cut and boreholes up to a kilometre deep were slow and expensive. So the geological model came first, from a desk study of the original Snowy Mountains Scheme archives, geological mapping and geophysics, and the 53 boreholes were aimed at the risks it identified. Even so, holes at the planned power station found a rock sequence running to 800 or 900 m depth where a thin layer had been expected. The records set the questions. The drilling answered them.

A house lot works the same way, and the records cannot settle the lot itself. Li et al. (2016) built a shrink-swell database from 47 Melbourne sites. Three samples from Wollert, all from the same mapped basalt unit, came back at about 3, 6.4 and 6.9 percent strain per pF. Knowing the unit tells you the range. Only testing the lot tells you where in that range it sits.

The desktop step is what makes the fieldwork targeted: two shallow holes on residual clay, at least three holes to 3 m on Adelaide's reactive clay where AS 2870-2011 requires that minimum, or a deep investigation on soft coastal ground. For a new house, the fieldwork is not optional. The desktop assessment makes it better aimed; it does not replace it.

What do the other names mean?

• Soil test. The everyday name for a residential AS 2870 site classification.
• Preliminary geotechnical assessment. Usually a desktop assessment, sometimes with a site walkover. Check whether any holes are included before treating it as an investigation.
• Factual report. Borehole logs and test results only, without interpretation.
• Interpretive or geotechnical investigation report. The factual results plus the engineer's recommendations for footings, earthworks or retaining.

Common questions

Is a desktop assessment a geotechnical report?

In the broad sense it is a report about the ground, but it is not what a builder or certifier means by a geotechnical report. That term normally means a report based on drilling and testing on the lot.

Can a desktop assessment replace a soil test?

No. It can indicate likely reactivity and scope the investigation, but the AS 2870 site classification used for footing design comes from the site investigation.

Which should come first?

The desktop assessment. It is quicker and cheaper, it can be done before you own the block, and it tells you what the site investigation needs to cover.

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To see what the records show for a block before you commission fieldwork, generate a desktop report for the address, or see a sample report for what it covers.

LayeredGeo compiles geology, soils, groundwater and site data into an automated geotechnical desktop report for any address in Queensland, New South Wales, Victoria or South Australia.

Sources

• Kelly, R. B., Drechsler, M. and Goldsmith, R. (2020). Connecting geotechnical investigations with project risk. Australian Geomechanics, Vol 55 No 1. geomechanics.org.au
• Li, J., Zou, J., Bayetto, P. and Barker, N. (2016). Shrink-swell index database for Melbourne. Australian Geomechanics, Vol 51 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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