In Port Macquarie, where development extends from the Hastings River floodplain across Pleistocene dune ridges and into Triassic meta-sedimentary foothills, AS 1726.1 requires site-specific data that standard window sampling often cannot deliver alone. The Cone Penetration Test (CPT) provides continuous soil profiling with depth, logging cone tip resistance (qc), sleeve friction (fs), and dynamic pore pressure (u2) in a single push. For engineers working on the Thrumster expansion or coastal redevelopments near Town Beach, this high-resolution data identifies soft lenses, loose sand layers, and potential paleochannels that could be missed by a sparse SPT grid. When combined with targeted SPT drilling for sample recovery in transitional strata, the CPT becomes the backbone of a solid foundation design that meets Port Macquarie-Hastings Council’s geotechnical submission requirements.
A single CPT sounding replaces multiple SPT borings for stratigraphic profiling, delivering a continuous digital log that reveals thin soft seams invisible to standard 1.5-metre interval sampling.
Scope of work
Area-specific notes
The urban expansion of Port Macquarie over the last two decades, particularly into the low-lying alluvial corridors between the Pacific Motorway and the Hastings River, has placed new structural loads on compressible estuarine clays and loose floodplain sands that were previously agricultural land. These deposits, often containing organic lenses and variable salinity, present a consolidation settlement risk that is notoriously difficult to quantify with disturbed samples alone. A CPT sounding through these layers captures the in-situ pore pressure dissipation curve after penetration arrest, from which the coefficient of consolidation (cv) and hydraulic conductivity (k) are derived for settlement-rate calculations. When the cone encounters a loose, saturated sand layer at depth, the measured qc is used in the Robertson (2009) correlation to evaluate the cyclic resistance ratio (CRR), which is essential for a liquefaction assessment under the AS 1170.4 seismic demand for Port Macquarie’s Class II sites. Without this continuous profile, a designer risks averaging a critical thin soft seam across a 1.5-metre SPT split-spoon interval, potentially underestimating differential settlement beneath a raft or piled foundation.
Standards used
AS 1726.1-2018: Geotechnical site investigations, ASTM D5778-12: Standard Test Method for Electronic Friction Cone and Piezocone Penetration Testing, Robertson & Cabal (2015) Soil Behaviour Type charts, AS 1170.4-2007: Structural design actions – Earthquake actions in Australia, AS 4678-2002: Earth-retaining structures (for lateral load interpretation)
Linked services
Piezocone (CPTu) with Dissipation Testing
Full CPTu with pore pressure measurement at the u2 position, including staged dissipation tests at depths of engineering interest to directly estimate the coefficient of consolidation (cv) for settlement time-rate analysis in estuarine clays.
Soil Behaviour Type Interpretation
Application of the Robertson (1990) and updated Robertson (2015) SBTn charts to the corrected cone data, producing a continuous soil classification log that guides the selection of laboratory testing on targeted samples.
Liquefaction Screening by CPT
Cyclic resistance ratio (CRR) evaluation from corrected cone tip resistance, using the Boulanger & Idriss (2014) CPT-based method, integrated with AS 1170.4 seismic hazard parameters for Port Macquarie.
Typical parameters
Top questions
What depth can a CPT rig reach in Port Macquarie’s coastal soils?
In the alluvial sands and clays along the Hastings River floodplain, a 20-tonne CPT truck typically achieves 20 to 25 metres of penetration before reaching practical refusal. Refusal can occur shallower in areas where Pleistocene dune sands are heavily cemented with iron oxides, such as parts of Lighthouse Beach and Tacking Point. In these zones, pre-drilling through the cemented layer may be required to continue the test to the target depth.
How does the CPT compare to SPT testing for foundation design?
The CPT provides a continuous, high-resolution profile of soil response with depth, whereas the SPT gives a single N-value every 1.5 metres. Thin soft layers of 50–100 mm thickness, which can control settlement behaviour, are often entirely missed by SPT sampling intervals but are clearly visible in the CPT friction ratio and pore pressure traces. However, the SPT recovers a physical sample, so the two tests are complementary: we use the CPT for stratigraphic definition and the SPT for sample recovery where laboratory index testing is required.
What does a CPT test cost in the Port Macquarie region?
For a standard CPT sounding to 20 metres depth, you can expect to budget between AU$240 and AU$430 per linear metre mobilised, depending on the number of soundings, the need for dissipation testing, and the site accessibility for the CPT truck. A full-day mobilisation with multiple soundings typically offers a lower rate per metre than a single isolated test.
Is the CPT suitable for sites with gravelly soils or fill?
The CPT performs best in sands, silts, and clays. In Port Macquarie, many subdivision sites contain a layer of imported gravelly fill for working platforms, which can cause early refusal of the cone. We assess each site individually: if gravel refusal is expected, we may recommend a pre-drilled pilot hole through the fill layer or a complementary test pit investigation to characterise the fill, followed by the CPT from the base of the pre-drilled hole into the natural ground.