Geotechnical desktop study: what it covers and its limits

A geotechnical desktop study is an assessment of the ground under a specific property made from existing information rather than new drilling. Geological mapping, soil surveys, nearby borehole logs, groundwater bore records, elevation data, historical aerial imagery and hazard overlays are pulled together and read by someone who knows what they mean, to answer one question: what is the ground here likely to be doing, and what should we check before we commit?

"Likely" is doing real work in that sentence. No desktop study drills, samples or tests anything. It does something different and, early in a project, often more valuable: it sets the expectation, and it tells you where the expectation could be wrong.

What a desktop study is built from

The Australian Geomechanics Society's own practice note on landslide risk describes the data-gathering stage of any assessment as assembling published papers, geological maps, aerial photographs, regional hazard studies and the local knowledge of practitioners who have worked the area before (Walker et al. 2007). That is a fair description of a desktop study for any ground question, not just slopes. In practice the inputs are:

• Geology. The mapped rock or sediment under the site and how it weathers. Soils that formed in place from weathered rock dominate much of Queensland and grade unevenly into the rock below, while river-laid soils tend to be layered and more uniform. The distinction matters for both reactivity and bearing.
• Soils and reactivity. Regional soil mapping, the climate zone that sets the depth of seasonal moisture change, and published shrink-swell data for the local geological units. From that, a probable AS 2870 site classification.
• Boreholes. Logs from investigations on or near the site. These are the closest thing to ground truth a desktop study has, and their value depends on how near, how deep and how recent.
• Groundwater. Depth to water from the state bore database and nearby monitoring records. We cover reading bore records separately.
• Topography and history. Slope, drainage lines, and the historical aerial record that shows whether a lot was cut, filled, quarried, dammed or built over before.
• Constraints. Acid sulfate soil mapping, flood and landslide overlays, mine subsidence districts, and anything else the planning scheme already knows about the site.

What it resolves, and what it defers

A useful way to think about a desktop study is as two lists.

Things it can usually settle. Which geological unit is under the lot and whether that unit is known for reactivity. Which AS 2870 climate zone applies. Whether the lot sits on a floodplain or a filled gully. Whether groundwater is likely to be shallow. Whether there is an acid sulfate, landslide or subsidence overlay to plan around. Whether the neighbouring bore logs show rock at one metre or soft clay at ten.

Things it must defer to a field investigation. The actual soil profile at the footing locations. The measured shrink-swell index of the clay on this lot. Bearing capacity. The compliant site classification a certifier needs. Anything where the mapping says one thing and the lot may say another.

The second list is not a weakness of the method; it is what the method is for. Li et al. (2016) tested 47 sites across Melbourne and found large scatter in reactivity within a single mapped geological unit, concluding that a blanket value for a soil type is not a safe substitute for testing. The desktop tells you the unit and its published range; only a borehole tells you where this lot sits in that range.

The desktop study as a scoping tool

The most expensive thing in ground engineering is a surprise after money is committed. Kelly et al. (2020) report that recent large Australian infrastructure projects have typically spent between 0.5 and 1.5 percent of estimated cost on site investigation, cite a study of 41 court cases in which more than half the claims arose from changed ground conditions, and note that the benefit of investigation spend shows diminishing returns beyond a few percent of cost. Published case reviews of foundations that went wrong tend to land on the same cause: a minimal investigation in variable ground, where the quality and timing of the investigation mattered more than its price.

Those are big-project numbers, but the logic scales down to a house lot. A desktop study costs a small fraction of a drilling investigation and it determines what the drilling should look for: how many holes, how deep, whether to budget for shrink-swell testing, whether to expect fill, whether groundwater will be met. Hargreaves (2005) describes SEQ lowland estates from Noosa to Coolangatta where soft compressible deposits at depth control performance, and where an initial investigation may need to go beyond 30 metres. You do not want to discover that category of site with a two-metre auger.

A forensic example: why the desktop comes first

Tayler (2009) describes the investigation of a cracked house on the Gold Coast after road construction nearby. Two engineers looked at it. The second began with a desktop study: the geological sheet, the regional land-resource soil map and aerial imagery, which together showed the estate had been built on the Nerang River floodplain and that the natural soils were alluvium. That origin reframed the borehole logs and led to the diagnosis. Tayler's conclusion was that establishing the origin of the soils from existing sources is essential to reading a borehole correctly. The desktop is not a cheaper substitute for the borehole. It is the context that makes the borehole mean something.

When a desktop study is the right tool

• Before purchase. Pricing ground risk into an offer, or walking away, while it still costs nothing.
• Feasibility. Deciding whether a site is likely to carry the development as planned and what footing and earthworks cost to allow for.
• Portfolio and lot-by-lot screening. Ranking many lots to decide which deserve drilling first.
• Briefing the investigation. Giving the geotechnical engineer a head start on geology, fill history, groundwater and constraints, so the field scope is right the first time.
• Disputes and forensics. As Tayler's case shows, establishing what the ground was before anyone touched it.

When it is not enough

A desktop study is not the document a certifier signs off on. It will not replace a field classification for footing design, and it is the wrong tool on its own where the ground is complex or poorly documented, where there is a history of fill or contamination, where the structure is sensitive (multi-storey, basement, long span, steep slope) or where the desktop evidence points to potential Class P conditions, which by definition need site-specific engineering to confirm and design for. In all of those cases the desktop study's job is to say so clearly and to 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? It can give a probable classification from geology, soil mapping, nearby bores and the climate zone. A compliant classification for footing design requires a field investigation.

How much does a geotechnical desktop study cost? An automated desktop report is a small fixed fee; a consultant's desktop assessment is typically in the high hundreds to low thousands of dollars. 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 an indicative site class for any QLD, NSW or VIC address. Generate a desktop report for the lot or see a sample report.

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

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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