Geotechnical desktop study: what it covers, costs and can't do

A geotechnical desktop study (also called a desk study or desktop geotechnical study) is an assessment of the ground under a property made only from records that already exist: geological maps, soil mapping, nearby borehole logs, groundwater bores, elevation, historical aerial photos and hazard overlays. Nobody drills. Its job is to say what the ground is likely to be, and what to check, before money is committed.

A Gold Coast forensic case shows what that context is worth (Tayler 2009). A man who had owned his early-1980s masonry veneer house for about twenty years found it badly cracked after a road was built out the front with a compaction roller. The contractor's insurer sent a geotechnical engineer, whose soil tester drilled with a power rig and logged residual clay to 2.5 m; the cracking was put down to drought. The owner's second engineer had holes dug by hand auger and found 1.2 m and 2.2 m of sandy and clayey fill over alluvial clay. A desk check of the geological sheet, the regional soil map and Google Earth showed the estate had been built on the Nerang River floodplain, where the natural soil is alluvium, not residual clay. The first logs were wrong, and so was the diagnosis: fill settling under vibration from the roller was judged very likely. Tayler's lesson was that checking the logs against the maps would have caught it.

The desktop study is not a cheaper version of the borehole. It is what tells you whether the borehole makes sense.

What goes into a geotechnical desktop study?

The AGS landslide practice note describes the desk study as gathering published papers, geological maps, aerial photographs, hazard studies and the experience of practitioners who know the area (Walker et al. 2007). That holds for any ground question. In practice:

• Geology. The mapped rock or sediment under the site and how it weathers. Soil formed in place from weathered rock grades unevenly into the rock below; river-laid soil is layered. The difference matters for reactivity and footing depth.
• Soils and reactivity. Regional soil mapping, the climate zone that sets how deep the soil dries and wets each year, and published shrink-swell data for the local geological units. Together they give a reactivity range to expect. The AS 2870 site classification itself comes from the site investigation.
• Boreholes. Logs from investigations on or near the site: the closest thing to ground truth on the desk, worth what their distance, depth and age allow.
• Groundwater. Depth to water from the state bore database and monitoring records. More in reading bore records.
• Terrain and history. Slope, drainage lines, and old aerial photos that show whether the lot was cut, filled, quarried, dammed or built on before.
• Constraints. Acid sulfate soil, flood, landslide and mine subsidence overlays.

What can a desktop study settle without drilling?

Usually: which geological unit is under the lot and whether it is known for reactive clay; which AS 2870 climate zone applies; whether the lot sits on a floodplain or in a filled gully; whether groundwater is likely to be shallow; whether there is an acid sulfate, landslide or subsidence overlay to plan around; and whether nearby logs show rock at one metre or soft clay at ten.

What does it leave to the site investigation?

The soil profile at the footing locations. The measured shrink-swell index of the clay on this lot. Bearing capacity. The site classification a certifier needs. Anything where the mapping says one thing and the lot may say another.

That is not a weakness of the method. It is the reason for it. Li et al. (2016) tested clay at 47 Melbourne sites and found wide scatter in reactivity within a single mapped geological unit, enough that a blanket value for the unit is not a safe stand-in for testing. The desktop tells you the unit and its range. Only a hole on the lot tells you where in that range you are.

Why do the desktop work first?

Because ground problems cost most once the money is committed. Kelly et al. (2020) found recent large Australian infrastructure projects spent about 0.5 to 1.5 percent of cost on site investigation. They cite a study of 41 court cases in which more than half the claims came from ground conditions that turned out different from expected, and note that spending beyond a few percent buys little extra.

On a house lot the logic is the same. A desktop study costs a fraction of a drilling job and decides what the drilling should look for: how many holes, how deep, whether to budget for shrink-swell testing, whether to expect fill or groundwater. Sometimes it changes the job entirely. Hargreaves (2005) describes lowland estates from Noosa to Coolangatta where soft, compressible deposits at depth govern how houses perform, and where the first investigation can need to go deeper than 30 m. That is not a two-metre auger hole, and it is better known before the rig is booked.

When should you commission a geotechnical desktop study?

• Before you buy, to price ground risk into an offer or walk away before settlement.
• At feasibility or when comparing many lots, to judge what footings and earthworks to allow for and which lots deserve drilling first.
• To brief the investigation, so the geotechnical engineer starts with the geology, fill history, groundwater and constraints and gets the field scope right the first time.
• In disputes, to establish what the ground was before anyone touched it, as in Tayler's case.

When is a desktop study not enough?

Whenever a decision needs measured ground. It does not replace the site classification a certifier needs for footing design. On its own it is the wrong tool where the ground is complex or poorly recorded, where there is fill or contamination, where the structure is sensitive (multi-storey, basement, long span, steep slope), or where the evidence points to Class P conditions, which need site-specific engineering by definition. In those cases the desktop study's job is to say so plainly and scope the investigation that follows.

Common questions

What is the difference between a geotechnical desktop study and a geotechnical report? A desktop study uses existing data only; a geotechnical report (site investigation) involves drilling, sampling and laboratory testing on the lot. The desktop comes first and scopes the field work. See geotechnical report vs desktop assessment.

Can a desktop study give an AS 2870 site classification? No. It can indicate likely reactivity from geology, soil mapping, nearby bores and the climate zone, and scope the investigation, but the site classification used for footing design comes from the site investigation.

How much does a geotechnical desktop study cost? In 2026 an automated desktop report is $119, and a consultant's desktop assessment for a residential lot is typically $800 to $1,200, more for development sites. See our geotechnical report cost guide.

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Get the desktop picture before you scope the drilling: geology, soils, nearby bore logs, groundwater, cross-sections, historical imagery and indicative soil reactivity for any QLD, NSW, VIC or SA address. Generate a desktop report for the lot, see a sample report or read what our geotechnical desktop study includes.

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

• Hargreaves, B. (2005). 20 years of AS 2870 in South East Queensland: a personal view. Australian Geomechanics, Vol 40 No 3. geomechanics.org.au
• 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
• Tayler, J. (2009). Forensic investigation of subsidence to dwelling, December 2007. Australian Geomechanics, Vol 44 No 1. geomechanics.org.au
• Walker, B., Davies, W. and Wilson, G. (2007). Practice note guidelines for landslide risk management. Australian Geomechanics, Vol 42 No 1. 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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