The contrast between the firm sands of Mount Pleasant and the soft estuarine clays along the Pioneer River tells you everything about Mackay's subsurface complexity. In the river flats near Bakers Creek we routinely encounter undrained shear strengths below 20 kPa in the top 4 metres. That's exactly where the field vane shear test becomes indispensable. It measures the in-situ undrained shear strength of soft to medium clays without disturbing the sample structure. Before designing any shallow foundation in those low-lying areas, we run the VST alongside a calicata exploratoria to verify stratigraphy and confirm the weakest layer. The vane gives us direct su values that laboratory tests on disturbed samples would underestimate.
In Mackay's soft clays, sensitivity values often exceed 8 — the vane captures that loss of strength better than any other field test.
Methodology and scope
Mackay sits on the floodplain of the Pioneer River, with annual rainfall exceeding 1,600 mm and a water table often less than 2 m deep. This saturated environment creates soft, compressible clays that demand careful strength profiling. The field vane shear test follows AS 1726 — we use a 65 mm diameter vane with a height-to-diameter ratio of 2:1. The procedure involves pushing the vane to test depth, rotating it at 6° per minute, and recording the peak torque. We also perform a remoulded test immediately after to calculate sensitivity — the ratio of peak to remoulded strength. For projects requiring continuous strength profiles, we combine the VST with ensayo CPT to correlate cone resistance with undrained shear. In Mackay's soft clays, sensitivity values often exceed 8, indicating highly structured soils that lose most of their strength upon disturbance.
Technical reference image — Mackay
Local considerations
A 6-storey residential development on Gordon Street was designed using laboratory triaxial data that showed su values of 35 kPa. We ran a field vane shear test at 5 m depth and found the actual undrained strength was only 18 kPa. That discrepancy would have led to a bearing capacity failure within the first year of loading. The vane caught it. In Mackay, where soft estuarine deposits alternate with sandy lenses, relying solely on lab data is a gamble. The vane measures what the soil actually offers in place — no sample disturbance, no stress relief. That single difference can save a project from catastrophic settlement or slope failure.
Full profile testing with peak and remoulded measurements at 1 m intervals. Includes torque calibration, sensitivity calculation, and Bjerrum correction for high-plasticity clays. Suitable for multi-storey residential and commercial projects on Mackay's river flats.
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Rapid Assessment VST
Single-depth vane tests at critical horizons identified by CPT or test pits. Ideal for preliminary slope stability checks or quick confirmation of design su values in homogeneous soft clay layers. Reduced reporting format with same-day results.
Applicable standards
AS 1726-2017 Geotechnical Site Investigations, AS 1289.6.2.1 Standard Test Method for Field Vane Shear Test, AS 4678-2002 Earth Retaining Structures (reference for strength parameters)
Frequently asked questions
What is the difference between field vane shear test and laboratory triaxial test for undrained strength?
The field vane shear test measures undrained shear strength in situ, preserving the natural fabric, water content, and stress state of the soil. Laboratory triaxial tests require sampling, transportation, and reconsolidation, which can disturb soft clays and reduce measured strength. In Mackay's sensitive estuarine clays, VST results are typically 20-40% lower than triaxial values, reflecting the true in-situ condition. We recommend VST as the primary method for soft soils and use triaxial tests to cross-check specific layers or to obtain effective stress parameters.
How much does a field vane shear test cost in Mackay?
A standard VST profile with measurements every metre to 10 m depth typically costs between AU$1,020 and AU$2,000, depending on site access, number of test holes, and reporting detail. The rapid single-depth test is around AU$600-AU$900. Prices may vary with mobilisation distance and required turnaround time.
When should I use a field vane shear test instead of a cone penetration test?
Use VST when you need direct undrained shear strength values for soft clays — for example, in slope stability analysis, bearing capacity of shallow foundations on soft ground, or embankment construction on compressible soils. CPT gives continuous profiles but requires empirical correlations to estimate su. VST provides direct measurement at discrete depths. In Mackay, we often run both: CPT for stratigraphy and VST at critical depths for calibration. For pure undrained strength determination in soft clays, VST is the most reliable field method.
