Dunedin’s layered ground tells a story of volcanic flows, harbour silts, and wind-laid loess. The city expanded from the flat reclaimed areas around the Octagon onto the steep basalt slopes of the peninsula, creating a patchwork of foundation conditions. Early settlers learned quickly that a site on weathered basalt behaves nothing like one on the alluvial gravels of South Dunedin. Today’s engineers need more than a boring log. The CPT (Cone Penetration Test) provides a continuous soil profile, capturing tip resistance, sleeve friction, and pore pressure every centimetre. This high-resolution data is critical when designing for sites where soft clays underlie stiff crusts, a scenario we encounter frequently across Dunedin. For broader site characterization, the CPT data often guides where to place test pits for visual sampling or to calibrate shear wave surveys for seismic site class determination.
A CPT log reveals what a disturbed sample cannot: the in-situ state of Dunedin’s layered loess and alluvium, centimetre by centimetre.
Methodology applied in Dunedin
Local geotechnical conditions in Dunedin
Dunedin sits in a moderate seismicity zone, and the liquefaction susceptibility of South Dunedin’s hydraulic fills and alluvial sands is well documented. The 1974 earthquake and the more recent Canterbury sequence both showed that continuous stratigraphy is non-negotiable for liquefaction assessment. A CPT (Cone Penetration Test) provides the normalized tip resistance and friction ratio needed for the Idriss-Boulanger triggering method. The water table here is often less than 1.5 metres deep across the Taieri Plain and harbour margins, keeping fine sands in a saturated state. A single SPT blow count cannot capture the subtle interbedding of silts and sands that controls pore pressure redistribution during shaking. The cone data, combined with laboratory grain size analysis of adjacent borehole samples, builds a defensible liquefaction severity map for the site.
Our services
Our geotechnical investigations in Dunedin combine the CPT with targeted sampling and laboratory testing to build a complete ground model for the site.
Piezocone Sounding with Pore Pressure Measurement
Deploy a 20-tonne CPT rig with a piezocone to measure dynamic pore pressure during penetration in Dunedin's harbour silts and alluvial clays. Dissipation tests at target depths provide the coefficient of consolidation for settlement analysis.
Seismic CPT for Shear Wave Velocity
Add a seismic module to the cone string to capture shear wave velocity profiles for site class determination per NZS 1170.5, essential for the hill suburbs where rock depth varies sharply.
CPT-Based Pile Capacity Design
Direct design of driven piles and screw piles from cone resistance data using the LCPC and ICP methods, calibrated for the volcaniclastic soils of the Dunedin region.
Common questions
How much does a CPT test cost in Dunedin?
For sites in Dunedin, a CPT programme typically ranges from NZ$280 to NZ$400 per metre of sounding, depending on site access, depth, and whether seismic or dissipation modules are required. Mobilization costs are quoted separately based on location within the city.
How does a CPT compare to a standard borehole in Dunedin's soils?
A CPT provides a continuous electronic profile without disturbing the soil, while a borehole recovers discrete samples for visual classification. In Dunedin's interbedded alluvial deposits, the CPT detects thin silt seams that a standard split-spoon sampler can miss. We often pair both methods: CPT for stratigraphic detail, and targeted boreholes for index testing.
What depth can a CPT reach in Dunedin's basalt areas?
In the hill suburbs like Roslyn or Mornington, weathered basalt and schist bedrock can cause cone refusal at depths of 2 to 5 metres. On the flat, in South Dunedin or the Taieri Plain, the alluvial sediments allow pushes to 25 or 30 metres before reaching the bearing layer.