The 20-tonne CPT rig pushes a 35.7 mm conical probe into the alluvial and dolerite-derived soils of Launceston at a steady 20 mm/s, recording tip resistance and sleeve friction at every centimetre. This continuous profile is invaluable for the city's mixed geology—soft estuarine clays near the North Esk River and denser colluvium on the hillsides—because it catches thin layers that discrete sampling can miss. The system also measures pore pressure behind the cone, which directly informs drainage assumptions for deep excavations and road embankments. Combining CPT data with a geotechnical instrumentation programme allows real-time verification of design parameters during construction in Launceston's variable ground.
A 20 m CPT profile in Launceston's estuarine clays delivers continuous stratigraphy in under one hour, catching thin layers that SPT sampling routinely misses.
Methodology and scope
The Tamar Valley basin presents a challenging mix: up to 8 metres of soft silty clay over weathered basalt or mudstone, often with perched water tables after winter rains. Standard penetration tests can miss the subtle interbeds that control settlement in multi-storey developments. CPT fills that gap by delivering a near-continuous stratigraphic log—tip resistance (qc), sleeve friction (fs), and pore pressure (u2)—at 1 cm intervals. The friction ratio (Rf = fs/qc) helps distinguish clay from silt or sand without laboratory back-up, though we always confirm with soil classification tests. For Launceston projects, this speed means we can complete a 20 m profile in under an hour, reducing site disruption on busy urban blocks like Brisbane Street or the Invermay precinct.
Technical reference image — Launceston
Local considerations
Consider a recent mixed-use development on the former railyard at the Launceston end of the Tamar River. The geotechnical model, based on limited SPT borings, predicted uniform stiff clay to 12 m. When the contractor started bulk excavation for a two-level basement, a CPT run across the site revealed a 2 m thick loose silt lens at 5.5 m depth—precisely where the crane pad was planned. Without that continuous cone profile, the silt would have gone undetected until the pad settled under load. The CPT allowed the design team to switch to deep strip footings and avoid a costly remedial grouting programme later.
Piezocone (CPTu) Sounding with Pore Pressure Dissipation
The full u2 piezocone records pore pressure in real time, and after a pause in the clay layer we run a dissipation test to estimate consolidation coefficient (ch). Essential for Launceston's soft estuarine clays where drainage assumptions drive foundation costs.
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Seismic CPT (SCPT) for Shear Wave Velocity
A geophone array behind the cone picks up shear waves generated by a surface hammer, giving Vs profiles at 1 m intervals. This data feeds directly into site class assignment (AS 1170.4) for seismic design in Launceston's moderate-hazard zone.
Applicable standards
AS 1726:2017 – Geotechnical site investigations (CPT procedure), AS 1289.6.5.1 – Standard test method for electronic friction cone and piezocone penetration testing, AS 4678:2002 – Earth-retaining structures (uses CPT-derived strength parameters)
Frequently asked questions
How does CPT differ from SPT for Launceston's soft clays?
The SPT hammer collects disturbed samples at 1.5 m intervals, but in Launceston's soft estuarine silts the blow counts are low and the sample often washes out. CPT provides a continuous profile with high resolution—every centimetre—and measures pore pressure directly, which is critical for assessing undrained shear strength in the Tamar Valley basin.
Can CPT data be used to estimate bearing capacity for shallow foundations?
Yes. The tip resistance qc correlates to undrained shear strength (su = qc/Nk) for clay and to relative density for sand. Using methods by Robertson & Campanella (1983) or the Norwegian Geotechnical Institute, we derive allowable bearing pressures that match AS 4678 requirements. For Launceston's alluvial clays we typically apply a cone factor Nk of 14–16 after local calibration.
What is the typical cost range for a CPT survey in Launceston?
For a standard 20 m CPTu sounding with pore pressure dissipation, expect a price between AU$270 and AU$440 per test point, depending on site access, number of points, and whether seismic (SCPT) add-ons are required. Bulk discounts apply for multi-point surveys on larger Launceston subdivisions.
