Geotechnical FAQs

A site classification is a geotechnical assessment of how much the ground under a building site is expected to move, carried out to Australian Standard AS 2870. A geotechnical engineer drills boreholes, tests the soil and assigns the site a class — A, S, M, H1, H2, E or P.

Your structural engineer and builder use this class to design a footing or slab that suits the ground, and most councils and building surveyors in WA require one before approving a new home.

The classes in AS 2870 rank sites by expected ground surface movement:

• Class A — mostly sand or rock; little or no movement (0–10 mm)
• Class S — slightly reactive clay (10–20 mm)
• Class M — moderately reactive clay (20–40 mm)
• Class H1 / H2 — highly reactive clay (40–60 mm / 60–75 mm)
• Class E — extremely reactive (over 75 mm)
• Class P — problem site: soft or uncontrolled fill, landslip risk, trees or other issues needing specific engineering design

Higher classes generally mean stiffer, more expensive footings.

A standard residential site classification (soil test) in Perth typically costs between $500 and $1,500, depending on site access, the number of boreholes and laboratory testing required. Difficult sites — reactive clays, sloping ground or filled sites — can cost more because they need deeper boreholes and additional lab work.

If you just need an early read on ground conditions before you buy or design, our instant preliminary desktop report is $75.

A preliminary desktop report is a rapid assessment of a site using published geological, groundwater and environmental datasets — no drilling. Ours covers the mapped geology, an estimated site classification, groundwater depth, ground level and slope, acid sulfate soil risk and a contaminated sites register check for the exact lot.

It's useful for due diligence before buying land or briefing a designer, but it does not replace an intrusive soil test — councils still require a full AS 2870 classification for building approval.

If a final site classification report is booked post desktop review, we'll absorb the $75 fee for all site classification reports starting from $280.

Yes, in practice. AS 2870 requires footings and slabs for houses to be designed for a classified site, and building surveyors in Western Australia will ask for the site classification report as part of the building permit application.

Even where an "assumed" classification is technically possible, most builders and structural engineers insist on a soil test because guessing wrong risks cracked walls and expensive footing changes during construction.

A field crew drills boreholes (typically two or more for a house site) to around 1.5–3 m or refusal, logs the soil profile, and may run in-situ tests such as Perth sand penetrometer (PSP) or dynamic cone penetrometer (DCP) tests to measure soil density. Samples are taken for laboratory testing where reactive clays are present.

The site visit usually takes under two hours; the engineer then combines the field data, lab results and published geology into a classification report.

Reactive soils are clays that swell when wet and shrink as they dry, moving the ground surface up and down with the seasons. If a footing isn't designed for that movement, the house can crack — doors jam, brickwork cracks and slabs tilt.

Perth's coastal sands are mostly non-reactive, but clay soils in the eastern suburbs, the Darling Scarp foothills and much of the South West can be moderately to highly reactive — which is exactly what a site classification measures.

For most Perth metro sites, allow 1–2 weeks from booking to report. The site visit itself takes a couple of hours; the timeline depends mainly on drilling availability and whether laboratory testing is needed — lab tests for reactive clay sites add several days.

Straightforward sand sites without lab work can often be turned around within a few days of the field visit.

The Perth sand penetrometer test (AS 1289.6.3.3) measures the density of sandy soils by counting the hammer blows needed to drive a standard rod 300 mm into the ground. It's widely used in Perth to confirm that sand — natural or a compacted sand pad — is dense enough to support footings at the assumed bearing pressure.

Low blow counts indicate loose sand that may need compaction before building.

Acid sulfate soils are naturally occurring soils containing iron sulfides that generate sulfuric acid when excavated or drained. Undisturbed they're harmless, but disturbing them can corrode concrete and steel, harm vegetation and pollute waterways.

In WA they're mapped mainly around wetlands, estuaries and low-lying coastal areas. If your lot is in a mapped risk area and you're excavating below the risk depth or dewatering, councils may require an acid sulfate soil investigation and management plan.

Usually yes, but slope changes the engineering. Sloping sites often need cut-and-fill earthworks, retaining walls and footings designed for the retained conditions, and any structural fill must be compacted and certified. Steeper sites in the Perth Hills and South West may also need a slope stability assessment.

A geotechnical investigation identifies the soil and rock profile so the earthworks and retaining design are based on real conditions rather than assumptions.

We service the Perth metropolitan area and South West Western Australia, including Mandurah, Bunbury, Busselton and surrounding regions. Instant preliminary desktop reports are available online for any lot in Western Australia covered by the state's published geological mapping.