How to get a geotechnical report: who to call, what to ask

To get a geotechnical report you engage a geotechnical consultant or a soil testing firm directly, or your builder or designer arranges it on your behalf. For a house, the whole process is a phone call or a web form, a quote, a drilling visit of a few hours, and a report one to three weeks later. There is no council or government body you apply to, and no licence you need to hold to commission one.

This sets out who to call, when in the project to call them, what to send with the request, what to check in the quote, and what to do with the report once it arrives.

Who writes a geotechnical report

For a house. A residential soil testing firm or a small geotechnical consultancy. The deliverable is an AS 2870 site classification, signed by a suitably qualified engineer, that a building certifier will accept for footing design. Most builders have one or two firms they use, which is convenient but not an obligation.

For a development, multi-storey or basement work. A geotechnical engineering consultancy with its own or contracted drilling capacity. The deliverable is an investigation report with founding recommendations, and often the design parameters the structural engineer will work from.

What is not a geotechnical report. A desktop assessment, including ours, compiles the mapped geology, soils, terrain, nearby bore logs, groundwater and hazard mapping for an address. It scopes and prices the investigation. It is not a classification and no certifier will accept it as one, because AS 2870 requires site-specific investigation by an engineer who has seen the profile. The difference is set out in geotechnical report versus desktop assessment.

When to commission it

Stage	What to commission	Why then
Before you buy the block	Desktop assessment	Ground risk is priceable while you can still walk away
After settlement, before design is locked	Site classification, with footing design if the footprint is known	The class changes the slab, so the designer needs it early
Before the earthworks or piling tender	Full investigation	Tenderers who cannot see the data price the uncertainty back as contingency
After a surprise, a dispute or a Class P result	Additional investigation	Second mobilisation, so the most expensive way to get there

The order matters more than the timing. Commissioning the classification after the slab is designed means either a redesign or a footing that was designed for ground the block does not have. Andrews (2006) makes the same point from the other end: a designer working with thin data designs conservatively, and the conservatism appears in the construction budget rather than the consultant's fee.

What to send when you ask

Consultants price contingency into a vague request. Sending the following usually gets a tighter number and a faster turnaround.

1. The address and lot on plan. A lot and plan number removes any doubt about which block.
2. A site plan with the proposed footprint, or the builder's siting plan if there is one.
3. What is being built. A single-storey slab-on-ground, a two-storey with a basement and a retaining wall are three different investigations.
4. Known history. Previous structures, filling, demolition, flooding, dumped material, a dam that was filled in. Fill is the single most common reason a two-hole job becomes something larger.
5. Access notes. Slope, gates, side setbacks, soft ground, established trees, whether services have been located.
6. Any existing ground information, including a neighbour's report, an old investigation, or a desktop assessment for the address.

What to check before you accept the quote

Ask the quote to state the number of holes, the method, the target depth, which field and laboratory tests are included, and exactly what document you receive at the end. Ask what happens if a hole meets refusal early, since that decides whether a second visit is billed. Check that the classification will be signed by an appropriately qualified engineer, because a certifier will look for that.

Compare scope before price. 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, and the project team chose the most expensive tender because of the scope behind it. Collingwood (2003), writing for people engaging site investigation consultants on piling projects, recommends the same discipline: give the consultant a sufficient budget to meet the project requirements, and ask them to demonstrate that the scope they propose is adequate. What the line items in a quote pay for is broken down in what is in a geotechnical quote.

How long it takes

One to three weeks from booking to report for a residential classification is typical, and most of that is waiting for drilling availability rather than the engineering. Turnaround stretches in busy construction periods and after wet weather, when rigs cannot get onto soft blocks. A full investigation with laboratory testing runs three to six weeks. Expedited turnaround is usually available at a premium. Book before the design is finalised rather than after.

What to do when the report arrives

Read the class letter, then read the assumptions behind it. Check that the site as investigated matches the site as it will be built: a classification made on natural ground does not carry over once the block is cut and filled, and a report that assumes the trees stay does not hold if they come out. Check whether the report says the holes terminated in fill, which is a reason to ask for more depth rather than to proceed.

If the result is Class P, the site sits outside the standard designs and the footing has to be designed specifically. The triggers are set out in Class P site classification. Hargreaves (2005), reviewing two decades of AS 2870 practice in South East Queensland, records practitioners' view that Class P conditions are present more often than the letters suggest, which is worth knowing before a neighbouring lot's classification is used as a guide to yours.

Common questions

How do I get a geotechnical report?

Contact a geotechnical consultant or residential soil testing firm directly, or ask your builder or designer to arrange one. Send the address and lot on plan, a site plan with the proposed footprint, what is being built and any known site history. You receive a quote, the drilling visit takes a few hours, and the report follows in one to three weeks.

Who do I call for a soil test on a house block?

A residential soil testing firm or a geotechnical consultancy that does AS 2870 site classifications. Builders usually have firms they work with, and you can also commission one yourself. Check that the classification will be signed by an appropriately qualified engineer, since that is what a certifier looks for.

How long does a geotechnical report take?

One to three weeks for a residential site classification, most of it waiting on drilling availability. A full investigation with laboratory testing takes three to six weeks. Busy construction periods and wet weather extend both.

Do I need a geotechnical report to build a house?

Yes, in practice. A building certifier will expect an AS 2870 site classification before the slab is designed and approved, because the footing system depends on the site class.

To screen a block before commissioning 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 or Victoria.

Sources

• Andrews, P. V. (2006). The hidden cost of geotechnical investigations. Australian Geomechanics, Vol 41 No 4. 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
• 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
• Hargreaves, B. (2005). 20 years of AS 2870 in South East Queensland: a personal view. Australian Geomechanics, Vol 40 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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