Geotechnical assessment cost: what is in the quote

A geotechnical assessment for a house block usually costs from about $700 for a two-borehole AS 2870 site classification, $1,200 to $2,500 with footing design, and $3,000 and up once laboratory testing and a full investigation are involved. Two quotes for the same block can differ by a factor of two, and the difference is almost always scope: how many holes, how deep, what is tested, and what the engineer signs.

This explains what the line items in a geotechnical quote pay for, why quotes for the same address differ, and what a quote has to state before you can compare it with another one. For the price tiers by report type, see the geotechnical report cost guide.

What the quote is made of

Line item	What it pays for	What makes it move
Mobilisation	Getting a rig, an operator and a logger to the block and away again	Distance from the consultant's base, access, whether a truck-mounted rig can get on
Drilling or augering	Time on the hole, by hole or by metre	Number of holes, depth, hard ground, rock, wet ground
In-situ testing	Penetrometer or shear-vane readings taken as the hole goes down	How many, and whether a refusal forces a second hole
Sampling and laboratory testing	Undisturbed samples, shrink-swell index (Iss), moisture, strength	Reactive clay, fill, anything the profile cannot settle by identification alone
Engineering time	Logging, interpretation, the classification or founding recommendations	Complexity, not site area
Reporting	The document your certifier or designer receives	Whether footing design is included
Design (optional)	A footing system designed for your house on your lot	Class, footprint, whether the site is Class P

Field time and mobilisation dominate a small residential job. On a two-hole suburban classification, adding one more hole costs far less than the first hole did, because the rig is already there. That arithmetic is why a second visit after a surprise result is the expensive outcome, and why scoping the investigation before the rig is booked is worth doing.

Why two quotes for the same block differ

Different numbers of holes and different depths. AS 2870 sets a minimum, not a sensible maximum, and consultants read the same lot differently. In the Australian Geomechanics Society's (2000) case study of a University of Adelaide building, the tendered investigations varied in maximum borehole depth between 10 and 22 metres for the same site. The project team took the most expensive tender because of the scope behind it.

Laboratory testing included or excluded. A classification can be made by identifying the profile against known local soils, or by measuring the shrink-swell index in a laboratory. Measuring costs more and is harder to argue with. Li et al. (2016), testing 47 sites across 37 Melbourne suburbs, found wide scatter in Iss within a single geological unit, which is the case for measuring on ground you cannot predict from mapping.

Footing design in or out. A classification tells you the letter. A design tells the builder what to pour. Bundled, the design component is usually cheaper than commissioning it after an unexpected result.

What the consultant takes responsibility for. Andrews (2006) sets out ranks of investigation scope against the risk each one leaves with the developer, from a preliminary walk-over where the consultant stands behind the factual data only, up to a full investigation with testing where design parameters are provided and stood behind. A cheap quote has not made the risk go away; it has left it with you.

What a cheap investigation costs later

The published Australian work on this is consistent. Collingwood (2003) worked through piling examples where sparse boreholes left the pile designer with no choice but to design long: in one case the extra piling cost from inadequate investigation was around $76,000, against roughly $2,500 to have drilled the extra depth in each borehole. Andrews (2006) makes the same argument from the design side, that a small scope produces conservative design and expensive construction.

Kelly et al. (2020) tested the industry saying that a project pays for its site investigation one way or another against case histories, and reported that recent large Australian infrastructure projects spent roughly 0.5 to 1.5 percent of estimated cost on investigation, with the benefit flattening beyond a few percent. The Australian Geomechanics Society (2000) notes investigation spend as low as 0.1 to 0.3 percent of project cost on some projects, against a long-standing recommendation of around 3 percent.

None of that means a house block needs a percentage of the build spent on boreholes. It means the cheap step that comes first, which is understanding the ground from existing records, is what stops the expensive step later.

What a quote has to state before you can compare it

1. Number of holes, method and target depth, and what happens if a hole meets refusal early.
2. Which tests are included, in the field and in the laboratory, and how many.
3. The deliverable: a classification letter, a factual report, or a report with founding recommendations and footing design.
4. Exclusions, including a second mobilisation, service location, traffic control and reinstatement.
5. Turnaround, measured from when the rig can get on the block rather than from the day you accept.

A quote that gives a price and a date but not a scope cannot be compared with anything.

What you can settle before you ask for a quote

Most of what makes a geotechnical quote go up is knowable from records before anyone visits: mapped geology and soils, terrain and slope, nearby borehole logs, groundwater depth, whether the estate is on filled ground, and whether the site sits in a hazard overlay. Handing that to the consultant with the site plan and the proposed footprint removes the contingency they would otherwise price in, and tells you in advance whether you are looking at a standard two-hole job or something deeper.

What it does not do is replace the fieldwork. No desktop assessment, including ours, produces a classification a certifier will accept, because AS 2870 requires site-specific investigation by an engineer who has seen the profile.

Common questions

How much does a geotechnical assessment cost?

For a house block, from about $700 for a standard AS 2870 site classification with two boreholes, $1,200 to $2,500 with footing design, and from about $3,000 for a full investigation with laboratory testing. Commercial and multi-storey work is scoped individually and starts higher.

Why are geotechnical quotes so different for the same block?

Scope. Different consultants propose different numbers of holes, different depths, laboratory testing included or excluded, and different levels of responsibility for the design parameters. Compare the scope line by line before comparing the price.

What is included in a geotechnical investigation quote?

Mobilisation, drilling or augering, in-situ testing, sampling and any laboratory testing, the engineer's logging and interpretation, and the report. Footing design, service location, traffic control and a return visit are commonly excluded and should be checked.

Is the cheapest geotechnical quote a false economy?

Often. Collingwood (2003) documented piling projects where an under-scoped investigation added tens of thousands to construction against a few thousand for the extra drilling, and Andrews (2006) shows the same effect through conservative design. The saving is real, but it is usually smaller than the risk it leaves behind.

To scope the investigation before you ask for quotes, 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 or Victoria.

Sources

• Andrews, P. V. (2006). The hidden cost of geotechnical investigations. Australian Geomechanics, Vol 41 No 4. geomechanics.org.au
• Collingwood, B. (2003). Geotechnical investigations for piling projects: the false economy of a cheap site investigation. Australian Geomechanics, Vol 38 No 1. geomechanics.org.au
• Australian Geomechanics Society (2000). Geotechnical risk and inadequate site investigations: a case study. Australian Geomechanics, Vol 35 No 2. 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

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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