Do You Need a Geotechnical Report? Scala Tests, CPT and Boreholes Explained

Why What's Under Your Site Matters

Every foundation design starts with one question: what is the ground actually like? Most New Zealand homes are designed to NZS 3604, the standard for timber-framed buildings, which assumes the site sits on what is called "good ground". If your site doesn't clearly meet that bar, a geotechnical engineer needs to be involved before a standard design can go ahead.

What Counts as "Good Ground"?

Under NZS 3604, good ground is soil or rock able to permanently carry an ultimate bearing capacity of 300kPa (an allowable bearing pressure of around 100kPa). It specifically excludes:

• Compressible soils - topsoil, soft clays, loose or uncertified fill
• Expansive soils - clays that swell and shrink with moisture changes
• Unstable ground - land that could move 25mm or more from slips, subsidence or erosion
• Land where a liquefaction risk hasn't been ruled out

If any of these apply, or if your builder's excavations turn up soft clay, peat, buried services or uncertified fill, the standard NZS 3604 details no longer apply and a specific geotechnical assessment is needed.

When Do You Actually Need a Report?

Guidance from MBIE and councils points to a few common triggers:

• Your site's liquefaction vulnerability hasn't been confirmed as low or ruled out
• Excavations or site observations reveal soft ground, fill, old services or signs of past movement
• The site is sloping, has a history of slips or subsidence, or sits near a stream bank or cliff
• You are building something NZS 3604 doesn't cover - for example, some masonry or tilt-slab construction, which some councils require site-specific investigation for regardless of zone

Requirements vary between councils and liquefaction zones, so it's worth asking early - before you've locked in a floor plan - rather than finding out at consent stage.

The Three Main Investigation Methods

Scala Penetrometer (Dynamic Cone Penetrometer)

A hand-held probe, usually paired with a hand auger, driven into the ground to check bearing strength to around 2 metres depth. It's a quick, low-cost way to confirm shallow ground meets "good ground" criteria on lower-risk sites.

CPT (Cone Penetration Test)

A mechanical probe pushed into the ground by a rig, recording cone resistance, sleeve friction and pore-water pressure roughly every 10mm as it goes. This gives a near-continuous profile of the ground and is one of the most commonly used investigation methods across New Zealand, including on sites where liquefaction risk needs to be properly assessed.

Boreholes

Machine-drilled holes where samples and strength tests are taken at intervals (commonly every 1.5 metres). Boreholes help map how soil layers change with depth, their strength, and where groundwater sits. They are often used alongside CPT on more demanding sites, and more than one borehole across a building footprint is common where they're required.

Which method - or combination - your site needs depends on its liquefaction vulnerability and your council's specific rules, which vary. Your geotechnical engineer will recommend the right approach for your site and building type.

Why This Step Is Worth Getting Right

Skipping ground investigation, or relying on assumptions, can mean redesigning foundations mid-build, consent delays, or - in the worst case - a house that settles unevenly or cracks later on. Getting the ground checked early, before plans are finalised, is almost always cheaper than fixing a foundation problem after the fact.

If you're not sure whether your site needs a geotechnical report, get in touch with the team at Fil Engineers before you finalise your plans.

Sources

• MBIE - Ensuring new buildings can withstand liquefaction effects
• BRANZ Level - Site conditions and ground stability
• New Zealand Geotechnical Society - CETANZ best practice for CPT testing in NZ
• Hastings District Council - Geotechnical Site Investigation for Residential Building Consents

This article is general information only and is not a substitute for site-specific engineering advice. Every site is different - talk to a geotechnical or structural engineer about your property before making design decisions.