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 for you. 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 commission one.

The parts that go wrong are the order you do things in and what you tell the engineer.

Who writes a geotechnical report?

For a house: a residential soil testing firm or a small geotechnical consultancy. You get an AS 2870 site classification, signed by a suitably qualified engineer, that a certifier will accept for footing design. Most builders have a firm they use, but you can choose your own.

For a development, multi-storey or basement work: a geotechnical consultancy with drilling capacity. You get an investigation report with founding recommendations and the design parameters the structural engineer works from.

Not a geotechnical report: a desktop assessment, including ours. It compiles the records for an address and helps scope and price the investigation, but it is not a site classification and no certifier will accept it as one. The site investigation decides the class. Geotechnical report versus desktop assessment sets out the difference.

When should you commission it?

Stage	What to commission	Why then
Before you buy the block	Desktop assessment	Ground risk can be priced 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	A second mobilisation, so the most expensive way to get there

Order matters more than speed. A classification ordered after the slab is designed means either a redesign or a footing designed for ground the block does not have. Andrews (2006), reviewing 201 Sydney geotechnical reports, made the point from the other side: a designer given thin data designs conservatively, and the cost shows up in construction, not in the consultant's fee.

What should you send with the request?

Consultants price contingency into a vague request. Send:

1. The address and lot on plan, so there is no doubt 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 house and a house with a basement and retaining wall are three different investigations.
4. Planned cuts and fill. AS 2870 measures the minimum hole depth from below any cut known at the time, so a 1 m cut means a deeper hole.
5. Known history. Previous buildings, filling, demolition, flooding, a dam that was filled in, trees that have been or will be removed. Fill is the single most common reason a two-hole job becomes something larger, and tree removal can change the class.
6. Access notes: slope, gates, side setbacks, soft ground, located services.
7. Any existing ground information, such as an old investigation or a desktop assessment for the address.

What should you check before accepting the quote?

The quote should state the number of holes, method, target depth, the field and lab tests included, and exactly what document you get. Ask what happens if a hole hits refusal early, since that decides whether a second visit is billed, and check that an appropriately qualified engineer will sign the classification. The minimum holes and depths AS 2870 asks for are in how many boreholes a house block needs.

Compare scope before price. Ben Collingwood, a piling engineer, advised clients that the cheapest proposals usually rest on a reduced scope, so check the scope is adequate before accepting the price, and if unsure, ask someone who designs piles for a living (Collingwood 2003). On a house, that is the engineer designing your slab. What is in a geotechnical quote breaks down each line item.

How long does it take?

One to three weeks from booking to report for a residential classification, mostly waiting for a rig. Busy periods and wet weather, when rigs cannot get onto soft blocks, stretch it. A full investigation with lab testing runs three to six weeks.

What should you do when the report arrives?

Read the class letter, then read the assumptions behind it.

• Does the site as tested match 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.
• Did any hole finish in fill? That is a reason to ask for more depth, not to proceed.
• What does it say about slope? Hargreaves (2005), looking back on twenty years of AS 2870 in South East Queensland, noted that many classification reports carried a line saying slope stability had not been considered. Most readers took that to mean there was no slope problem. It means the classifier does not know. If the lot is on a slope in an area with landslide mapping, ask for the slope to be addressed.

If the result is Class P, the site sits outside the standard designs and the footing has to be designed specifically. Class P site classification explains the triggers. A neighbouring lot's letter is no 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, Victoria or South Australia.

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
• 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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